The Messenger No. 126 - December 2006 - ESO.org
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The Messenger
No. 126 – December 2006
Reports from Observers
Mapping the Properties of SDSS Galaxies
with the VIMOS IFU
Joris Gerssen 1 tunately, mass cannot be measured di- imum equivalent widths of 2 nm). This
Lise Christensen 2 rectly from the SDSS data and the could potentially bias us toward selecting
David Wilman 3 derived metallicities could be affected by objects with strong nuclear emission
Richard Bower 4 aperture bias. such as AGN. However, it ensures that
each galaxy requires only 60 minutes
Another essential ingredient of galaxy of observing time to build a detailed map
1
strophysikalisches Institut Potsdam,
A evolution, intimately connected to feed- of spatially resolved star formation and
Germany back, is the star-formation history. metal abundance. We use the MR mode
2
ESO Quantifying the star-formation rate from of VIMOS as its wavelength coverage
3
Max-Planck-Institut für Extraterres- the past to the present is therefore an (~ 450 to 900 nm) and spectral resolution
trische Physik, Garching, Germany active area of research. The largest study closely match the SDSS fibre observa-
4
Durham University, United Kingdom to date (Brinchmann et al. 2004) uses tions.
~ 10 5 galaxies in the SDSS database.
They conclude that the present-day star- The sample was constructed to uniformly
We present initial results from our formation rate is now at about a third of cover the redshift range up to 0.1. Above
VIMOS IFU study of galaxies selected the average value over the lifetime of these redshifts aperture effects become
from the Sloan Digital Sky Survey. the Universe. As the SDSS apertures typ- less important. We visually inspected
Large fibre-based surveys like SDSS ically sample less than half of a galaxy’s the SDSS images of candidates to assess
have made a major contribution to size, they need to correct their results for their morphology and inclination and se-
our understanding of processes that this missing information using resolved lected a total of 24 galaxies to guarantee
shape galaxies. The SDSS results, images and procedures based on nuclear that after binning the data in a variety
however, are derived from integrated correlations between SFR and colour. of ways (in redshift, size, or luminosity) we
properties over the area of the fibre. still obtain statistically significant results
As the angular extent of galaxies is usu- for each bin.
ally considerably larger than the fibre Aperture bias
diameter, the SDSS results are biased The selected galaxies are all at intermedi-
toward the nuclear properties of galax- Large surveys such as the SDSS provide ate inclination. While not the main goal
ies. By contrast, data obtained with the statistically most complete samples of our project, this allows us to constrain
an Integral Field Unit (IFU) are free of of fundamental galaxy properties. How- the velocity fields and, hence, the en-
aperture bias. ever, the SDSS properties represent inte- closed mass profiles of the galaxies in our
grated quantities derived over the cen- sample. The mass distribution as a func-
tral three arcsec only. Consequently, the tion of radius is a key prediction of hierar-
In the increasingly well-defined cosmo- results suffer from highly significant aper- chical galaxy formation scenarios. Ob-
logical framework, the broad outline ture effects (Brinchmann et al. 2004, servational constraints on velocity fields
of galaxy formation is thought to be well Wilman et al. 2005, Kewley et al. 2005) are scarce even in the local Universe.
understood. Briefly, galaxies form in the that bias the results toward the bulge and The SDSS database itself contains no kin-
gravitational wells of dark matter halos nuclear emission properties. Galaxies, ematical information other than the re-
from gas that got trapped there after los- however, can exhibit strong colour gra- cessional velocity of a system. The total
ing kinetic energy through cooling or dients. Correcting emission line strengths masses of SDSS galaxies are normal-
dissipative shocks. However, galaxy-for- for aperture effects when gradients are ly estimated indirectly, usually from their
mation models generally overpredict present is uncertain at best, and com- total magnitude.
the fraction of gas that is locked up by a pounded by unknown contributions from
factor of about five compared to observa- variations in metallicity and age. With IFU In Period 76 we obtained data for 12 of
tions. To overcome this problem a feed- observations the bright emission lines the galaxies in our sample. A further
back mechanism is needed to remove are spatially resolved and can be traced 12 systems are scheduled for observation
gas from galaxies. The detailed physical over the whole galaxy. These data in Period 78. The VIMOS IFU provides
processes that govern this are not well are therefore free of aperture effects. data sets of the form (RA, DEC, l). Four
known and are at present hard to con- examples of our data are shown in Fig-
strain observationally (Wilman et al. 2005, ure 1. For each galaxy we show an image
Bower et al. 2006). This project slice (i.e. a cut in l through a data set) in
the light of Ha and a composite broad-
The vast database accumulated by the To quantify internal variations in the emis- band image.
SDSS survey (York et al. 2000) is ideally sion line properties of SDSS galaxies
suited to constrain many of the funda- we have begun a project to map a num-
mental physical processes that drive gal- ber of them with the VIMOS IFU. In order Preliminary results
axy evolution. For example, Tremonti to build up a sample of galaxies in a
et al. (2004) find evidence for stellar-wind modest amount of observing time we se- The emission line properties are derived
feedback in the SDSS data from the ob- lected galaxies from the SDSS database by fitting Gaussian profiles simultaneously
served mass – metallicity relations. Unfor- with moderately strong Ha emission (min- to the Balmer lines (Ha, Hb) and strong
The Messenger 126 – December 2006
Figure 1: Examples from our sample of
SDSS galaxies observed with the
VIMOS IFU. Shown from left to right in
ascending redshift order are sdss6,
sdss13, sdss22 and sdss9 (the names
simply reflect the RA ordering in our
selected sample) at redshifts of 0.028,
0.034, 0.074 and 0.106 respectively.
In the top panels composite colour im-
ages derived from the VIMOS IFU
data extracted over the SDSS r and i
bands are shown. The corresponding
Ha images are shown in the bottom
panels. Panels measure 27 by 27 arc-
sec and each pixel is 0.67 arcsec.
For comparison the SDSS fibre size is
indicated by the red circle in the top
left panel.
forbidden transition lines ([O iii], [N ii]) after 10 Figure 2: Cumulative quantities de-
rived using a software aperture with in-
removing the continuum using a sliding sdss6
8 creasing radius and centred on the
median. In our full analysis we will follow sdss13
nucleus of each galaxy. The cumula-
sdss22
Tremonti et al. (2004) and fit the continu- tive Ha line flux (arbitrarily normalised)
Hα line flux
6 sdss9
um with an optimal stellar template mod- shown in the top panel grows mono-
tonically as the galaxies in our sample
el. Subtracting this model will correctly 4
are larger than the radius of the SDSS
take any underlying absorption into ac- fibre (dashed line). The continuum
count that may otherwise significantly af- 2 flux does not necessarily follow the
fect our results (in this article we assume same trend. This is illustrated in the
bottom panels where the cumulative
an average correction for absorption of 0
line strength of the Ha emission line
EW = 0.2 nm). This model also provides a is shown. This can lead to strong
5
handle on the stellar kinematics. aperture bias when extrapolating the
Hα line strength (nm)
4
SDSS results to larger radii.
To quantify aperture effects we examine
the cumulative line flux and line strength 3
of the Ha lines in the four galaxies used
2
in this article (Figure 2) as a function of
aperture size. Not surprisingly, the cumu- 1
lative flux grows systematically beyond
0 2 4 6 8
the radius of the SDSS aperture. When Aperture (arcsec)
extrapolating to larger radii it is frequently
assumed that the line flux and continu- 1.5 Figure 3: To quantify the effect of
varying aperture size on the derived
um properties follow the same trend. But
metallicities we plot the VIMOS IFU
as the bottom panel illustrates the line- results as ‘tracks’ in a BPT diagram.
flux to continuum-flux ratio (that is, equi- 1.0 The starting point (i.e. smallest radius)
valent width or line strength) is not always of each track is highlighted by the
open squares. The crosses mark
constant. Extrapolating quantities de-
where the radius is equal to the SDSS
Log [O III]500.7 / HB
rived from the SDSS database to larger 0.5 fibre radius. The underlying gray-
radii is therefore fraught with difficulties. scale image shows the ‘raw’ emission
line measurements by Brinchmann et
al. (2004, see also http://www.mpa-
Systems harbouring an AGN such as
0.0 garching.mpg.de/SDSS/#dataprod) of
sdss22 display the strongest variation in some 500 000 SDSS galaxies. The
cumulative line strength. A useful way lines divide the sample into star form-
to classify the activity level of a galaxy is ing (left), hybrid (centre) and AGN
– 0.5 sdss6 (right).
by determining its location in a diagnos-
sdss13
tic BPT diagram (Baldwin, Phillips and sdss22
Terlevich 1981). In Figure 3 we reproduce sdss9
the BPT diagram derived by Brinchmann –1.5
et al. (2004) using ~ 10 5 SDSS galaxies. –1.5 –1.0 – 0.5 0.0 0.5
This diagram of emission line ratios has a Log [N II]658.4 / HA
The Messenger 126 – December 2006 
Reports from Observers
sdss6 – Hα velocities
characteristic ‘double-wing’ shape. Nor- Figure 4: Together with metallicity,
25 mass is another key observable used
mal galaxies are found on the left branch
to quantify galaxy evolution. However,
while active systems occupy the top right it can only be estimated indirectly
part. Overplotted on this diagram are from the SDSS database using mag-
the results of our cumulative emission line 20 nitude as a proxy. Our VIMOS IFU
data analysis yields emission line
analysis. Varying the size of the aperture
velocity maps with which the circular
can have an impact on the location of velocity, and hence the enclosed
∆ dec (arcsec)
a galaxy within this diagram. But, as the mass, can be constrained accurately.
15
four examples used here show, it would In the preliminary example shown
here the velocity field is derived from a
not necessarily change the classification
three-component Gaussian fit to the
of a system. Ha + [N ii] lines.
10
Apart from the large variation in the single
AGN system, all systems show at least
0.2 dex change in their line ratios as a 5
function of radius. This translates roughly
into 0.1 dex in metallicity, a value that is
– 75.0 km/s 75.0
not inconsistent with the 0.13 dex aver- 0
age difference of Kewley et al. (2005) for 0 5 10 15 20 25
large galaxies and which they claim is ∆ RA (arcsec)
substantial. At this preliminary stage it
should be kept in mind that different Flores et al. (2006) recently demonstrated the data-space covered by SDSS re-
methods to estimate metallicities from the power of IFU observations to con- quires a much larger sample. As our re-
strong emission lines can yield values strain internal kinematics at intermediate sults show, such a sample can be ob-
that differ considerably. redshifts. They used the Flames IFU but- tained efficiently with the VIMOS IFU even
tons to reach the striking conclusion that in relatively poor atmospheric conditions.
Our project aims to quantify the internal only one in three galaxies is dynamically
variations of emission line properties in unperturbed at redshifts of ~ 0.5 and thus
a self-consistent manner. A by-product of presumably undergoing rapid evolution. References
these observations are emission line ve- It will be very interesting to compare this Baldwin J. A., Phillips M. M. and Terlevich R. 1981,
locity fields. The data analysis yields to the kinematical properties derived from PASP 93, 5
mean line positions for every spatial loca- our lower redshift galaxies. Bower R. et al. 2006, MNRAS 370, 645
tion in our data sets. An example is shown Brinchmann J. et al. 2004, MNRAS 351, 1151
Flores H. et al. 2006, A&A 455, 107
in Figure 4 where the velocities are de- Aperture effects are important. To inves- Kewley L. J. et al. 2005, PASP 117, 227
rived from the mean positions of the Ha tigate the accuracy of the various cor- Tremonti C. A. et al. 2004, ApJ 613, 898
and [N ii] lines. rection methods we are observing a small Wilman D. et al. 2005, MNRAS 358, 88
sample of SDSS galaxies. To fully probe Wilman R. et al. 2005, Nature 436, 227
York D. G. et al. 2000, AJ 120, 1579
The barred spiral galaxy NGC 613 was imaged with
the FORS1 and FORS2 multi-mode instruments
(at VLT MELIPAL and YEPUN, respectively) in De-
cember 2001. The images were taken by Mark
Neeser (Universitäts-Sternwarte München, Germany)
and Peter Barthel (Kapteyn Astronomical Insti-
tute, the Netherlands) during twilight. The galaxy was
observed in three different wavebands for up to
300 seconds per waveband, and the image obtained
in each waveband was associated to a colour:
B (blue), V (green) and R (red). The full-resolution
version of this photo retains the original pixels. Note
the many arms and the pronounced dust bands.
North is up and East is left. Neeser and Barthel also
performed the first stage of the image processing;
further processing and colour-encoding was made
by Hans Hermann Heyer and Henri Boffin (ESO).
(From ESO Press Photo 33a/03)
The Messenger 126 – December 2006
Reports from Observers
The ARAUCARIA Project –
First Observations of Blue Supergiants in NGC 3109
Chris Evans 1
Fabio Bresolin 2
Miguel Urbaneja 2
Grzegorz Pietrzyński 3,4
Wolfgang Gieren 3
Rolf-Peter Kudritzki 2
1
nited Kingdom Astronomy Technology
U
Centre, Edinburgh, United Kingdom
2
Institute for Astronomy, University of
Hawaii, USA
3
Universidad de Concepción, Chile
4
Warsaw University Observatory, Poland
NGC 3109 is an irregular galaxy at the
edge of the Local Group at a distance
of 1.3 Mpc. Here we present new VLT
observations of its young, massive star
population, which have allowed us to
probe stellar abundances and kinemat-
ics for the first time. The mean oxygen
abundance obtained from early B-type
supergiants confirms suggestions that NGC 3109 is a large Magellanic Irregular Figure 1: Part of the V-band FORS pre-image of our
most western field, with the targets encircled.
NGC 3109 is very metal poor. In this at 1.3 Mpc, which puts it at the outer edge
NGC 3109 is approximately edge-on and the FORS
context we advocate studies of the stel- of the Local Group. Using FORS2 in the targets are well sampled along both the major
lar population of NGC 3109 as a com- configurable MOS (multi-object spectros- and minor axes.
pelling target for future Extremely Large copy) mode, we have observed 91 stars
Telescopes (ELTs). in NGC 3109. These were observed in Example spectra are shown in Figure 2.
4 MOS configurations, using the 600 B Of our 91 targets, 12 are late O-type stars,
grism (giving a common wavelength cov- ranging from O8 to O9.5 – such high-
The ARAUCARIA Project is an ESO Large erage of l3900 to l4750 Å). The cumula- quality observations of resolved O-type
Programme using FORS2 on the VLT. tive exposure time for each field was stars (note the He ii emission ‘bump’ at
Its principal motivation is to provide im- roughly 3 hours. Part of our most western l4686 Å in the spectrum of star #33) be-
proved distances to galaxies in the Local field is shown in the FORS pre-image in yond 1 Mpc are really quite remarkable.
and Sculptor Groups, via the period-lu- Figure 1, with our targets encircled. From
minosity relationship of Cepheid variables published photometry it has been sug-
(Gieren et al. 2005). A secondary com- gested that red giants in NGC 3109 have #33 09 If V = 19.6
ponent of the project is to characterise metal abundances that are similar to
tens of blue supergiants (typically B- and those found in stars in the Small Magel-
A-type stars) in each of the target gal- lanic Cloud (SMC), i.e. very metal poor #09 B0.5 Ia V = 18.8
axies. Blue supergiants are the most vis- when compared to the solar neighbour-
ually luminous ‘normal’ stars, thereby hood. With this in mind, we classified the
Normalised flux
enabling direct studies of stellar popula- FORS spectra using criteria that have #37 B2.5 Ia V = 19.7
tions in galaxies that are otherwise already tackled the issue of low metallic-
unreachable with 8-m telescopes. From ity (e.g. Evans et al. 2004). Our sample
#05 B8 Ia V = 18.5
comparisons with theoretical spectra, is primarily composed of late-O, B and A
we can investigate physical parameters spectral types – this is the first spectral
such as temperatures and chemical exploration of this galaxy. As an aside, we #01 A2 Ia V = 17.8
abundances of our targets, obtaining es- note that the first large-scale CCD sur-
timates of the metallicity of the host vey of NGC 3109 was reported in this
systems. Moreover, blue supergiants have publication by Bresolin et al. (1990) – the
also been advanced as an alternative acquisition of high-quality spectroscopy 3 800 3 900 4 000 4 100 4 200 4 300 4 400 4 500 4 600 4 700 4 800 4 900
Wavelength (Å)
method of distance determination via the in this galaxy some 16 years later illus-
flux-weighted gravity luminosity relation- trates the considerable advancement in
Figure 2: FORS spectra of five of our targets in
ship (Kudritzki et al. 2003). studies of extragalactic stellar popula- NGC 3109. The quality of the data is particular-
tions over that period. ly impressive when one remembers that the stars
are at distances of over 1 Mpc.
The Messenger 126 – December 2006 
Reports from Observers Evans C. et al., The ARAUCARIA Project
However, in terms of quantitative analy- 1.6 Figure 3: FORS spec-
trum (black line) of
sis, the early B-type supergiants in our NGC 3109 #22 B1 Ia
star #22, classified as
sample are of more immediate interest – B1 Ia. A FASTWIND
these stars have a wide variety of strong 1.4 model spectrum (Teff =
metallic lines in their absorption spectra, FASTWIND model 22 000 K, logg = 2.60) is
shown above in red,
providing an excellent tool for investigat-
smoothed to the same
ing chemical abundances of young stellar 1.2 resolution as the FORS
populations. data.
VLT-FORS
We have analysed a subset of eight of our 1.0
early B-type spectra using the FAST-
WIND model atmosphere code (Puls et
al. 2005). From comparisons with theo-
0.8
retical spectra we can obtain physical
parameters such as temperatures, gravi-
ties, and, of most interest in a broader
0.6
context, chemical abundances. An ex- 4 000 4 100 4 200 4 300 4 400 4 500 4 600 4 700 4 800 4 900
ample FASTWIND model matched to one Wavelength (Å)
of the observed spectra is shown in Fig-
ure 3. The mean oxygen abundance in
our eight stars is found to be log(O/H)
+ 12 = 7.76 ± 0.07, in excellent agreement – 200 Figure 4: Differential ra-
dial velocities as a func-
with results from H ii regions. This is only
–150 tion of radius along the
~ 12 % of the oxygen abundance found major axis of NGC 3109
in the solar neighbourhood, and is lower –100 – typical uncertainties
than the oxygen abundances found in the are of order ± 20 km/s.
Also shown are rotation
SMC (cf. log(O/H) + 12 = 8.13, Trundle
∆v(vr –vsys ) [kms –1]
– 50
curves from H i (solid line)
and Lennon, 2005). We also obtain upper and Ha (dotted line).
limits to the magnesium and silicon abun- 0
dances, which are comparable to those
50
found for stars in the SMC – the exact
abundance of the alpha-elements will re- 100
quire higher-resolution spectroscopy, but
it is clear that stars in NGC 3109 have 150
metal abundances that are very deficient
200
when compared to the solar neighbour- 400 300 200 100 0 –100 – 200 – 300 – 400
hood, and likely even lower than in the Radius [arcsec]
SMC.
We have also used our FORS spectra velocities of the young population largely Meanwhile, lower-resolution spectros-
to investigate the stellar rotation curve of trace those of the gas, with a fair amount copy could trace the kinematics of the
NGC 3109. H i observations suggest of scatter. Further observations of this non-supergiant population (e.g. via the
a dominant dark-matter halo (Jobin and sort would be of value to ascertain wheth- Calcium Triplet), probing the outer struc-
Carignan 1990), that cosmological N- er the stellar results are revealing genuine ture of this dark-matter dominated
body cold dark matter simulations have sub-structures in the disc, or whether dwarf and providing crucial input for cos-
struggled to reproduce (Navarro et al. we are simply limited by the small sample/ mological simulations.
1996). The spectral resolution from FORS spectral resolution.
(R ~ 1,000) is somewhat limiting for stud-
ies of stellar kinematics, but from simple Plans for the next generation of large References
measurements of line-centres of hydro- ground-based telescopes, the so-called Blais-Ouellette S., Amram P. and Carignan C. 2001,
gen and helium lines, we estimated radial Extremely Large Telescopes (ELTs), are AJ 121, 1952
velocities for the majority (84) of our stars. now gaining momentum. In this context Bresolin F., Capaccioli M. and Piotto G. 1990,
The mean 1-sigma (internal) uncertainty we suggest NGC 3109 as an exciting op- The Messenger 60, 36
Evans C. J. et al. 2004, MNRAS 353, 601
is of order 20 km/s. Figure 4 shows differ- portunity to study many stages of stel- Gieren W. et al. 2005, The Messenger 121, 23
ential radial velocities for each of our lar evolution in a very metal poor environ- Jobin M. and Carignan C. 1990, AJ 100, 648
stars, compared with published results ment. A large primary aperture would Kudritzki R.-P., Bresolin F. and Przybilla N. 2003,
from H i radio maps and Ha imaging enable high-resolution spectroscopy of ApJ 582, 83L
Navarro J. F. et al. 1996, ApJ 462, 563
(Jobin and Carignan 1990, Blais-Ouellette the young, massive population, and Puls J. et al. 2005, A&A 435, 669
et al. 2001). As one might expect, the of stars on the asymptotic giant branch. Trundle C. and Lennon D. J. 2005, A&A 434, 677
The Messenger 126 – December 2006
Reports from Observers
Early Science Results
from the UKIDSS ESO Public Survey
Steve Warren 1 Lawrence et al. (2006). This first release is priority access to the data. The science
Andy Lawrence 2 an important milestone on the route to described here is some of the work
Omar Almaini 3 completion of UKIDSS, as it marks the with which we have been involved. We
Michele Cirasuolo 2 point where the survey surpassed 2MASS look forward to hearing about work
Sebastien Foucaud 3 as the largest near-infrared survey, quan- being undertaken by other ESO astrono-
Nigel Hambly 2 tified by the product P = AΩt. Here A mers who have not been involved in the
Paul Hewett 4 is the telescope collecting area, Ω is the implementation of the surveys.
Richard Jameson 5 solid angle of the camera field, and t is
Sandy Leggett 6 the summed integration time. The symbol
Nicolas Lodieu 7 P stands for photons, since, for the same High-redshift galaxies in the
Phil Lucas 8 field, and other things being equal (such Ultra Deep Survey
Ross McLure 2 as camera throughput), the quantity P
Richard McMahon 4 is proportional to the number of source The deepest, and narrowest, element of
Daniel Mortlock 1 photons collected. UKIDSS is the Ultra Deep Survey (UDS).
David Pinfield 8 The final goal of the UDS is to cover
Bram Venemans 4 UKIDSS is an ESO public survey (see 0.8 deg2 to 5s depths of K = 23.0,
The Messenger 108, 31), with equal data H = 23.8, J = 24.6. The aim of the UDS is
access rights to all astronomers at insti- to produce a deep, large-scale map
1
Imperial College, London, United tutions in ESO member states. The data of a representative volume of the distant
Kingdom are available from the WFCAM Science Universe, 1 < z < 6, providing large sam-
2
University of Edinburgh, United Archive at http://surveys.roe.ac.uk/wsa/ ples with which to directly test models for
K ingdom index.html. The procedure for archive galaxy formation and evolution. The
3
University of Nottingham, United registration is described in a previous ar- depths reached in DR1 are K Q 21.6 and
Kingdom ticle (see The Messenger 119, 56), as well J Q 22.7, over the full field, based on
4
Institute of Astronomy, Cambridge, as on the UKIDSS web site (at http:// 86 hours of observations (the results re-
United Kingdom www.ukidss.org). The UKIDSS programme ported here in fact use the shallower
5
University of Leicester, United Kingdom comprises five surveys covering com EDR data set). The area also benefits from
6
Gemini North, Hawaii, USA plementary combinations of area, depth, public deep optical data obtained
7
Instituto de Astrofísica de Canarias, Galactic latitude, and filter coverage, with the Subaru instrument SuprimeCam.
Tenerife, Spain from the full ZYJHK set of the camera.
8
University of Hertfordshire, Hatfield, Table 1 summarises the contents of Although the UDS campaign is in its in-
United Kingdom DR1 for each of the five surveys, in terms fancy, the DR1 data set is already the larg-
of area and depth over regions with est existing near-infrared survey to these
coverage by the full filter set for that sur- depths. This enables surveys for rare ob-
The first large release of data from the vey. DR1 contains substantial additional jects. For example, McLure et al. (2006)
UKIDSS ESO public survey took place data in fields where the filter coverage have reported the discovery of nine of the
in July 2006. The size of the data set is is so far incomplete. The contents of DR1, most luminous candidate Lyman-break
about 7 % of the size of the final survey including maps of the areas surveyed, galaxies at redshifts 5 < z < 6. These ap-
data set. Early science results are pre- are detailed in a submitted paper (Warren pear to be relatively massive stellar
sented here, ranging from the nearest et al. 2006). The median seeing across systems (M stars > 5 × 1010 MA) already in
coolest brown dwarfs, to the most lu- the data set is 0.82 arcsec. place < 1.2 Gyr after the Big Bang. Be-
minous, rarest, galaxies at 5 < z < 6. cause they are so rare, these luminous
Progress on the headline science goals Although DR1 only appeared at the end objects are particularly useful for testing
of UKIDSS, such as the determination of July, some interesting science is theories of galaxy formation. Another
of the faint end of the stellar IMF, and already emerging. In this article we publi- galaxy population of current interest are
the discovery of quasars beyond z = 6, cise some of the early results of which the Distant Red Galaxies (DRGs), objects
is in line with expectation at this stage we are aware. The authors of this article selected with (J − K) AB > 1.3, which are
of the surveys. are members of the UKIDSS Consor- believed to be the most massive galaxies
tium, which designed and is implement- at z ~ 2. Foucaud et al. (2006) used
ing the surveys. This explains the UK the UDS EDR to produce a sample of 239
The UKIDSS First Data Release (DR1) bias, but we emphasise that we have no bright DRGs. This sample is an order of
took place on 21 July 2006 (as an-
nounced on the ESO web pages), follow- Survey Area Filters K 5s depth Table 1: Depth and
deg2 (Vega) coverage in fields
ing on from the small Early Data Release
Large Area Survey 190 YJHK 18.2 with the filter comple-
(EDR), in February (The Messenger 123, ment in UKIDSS DR1.
Galactic Clusters Survey 52 ZYJHK 18.2
67). DR1 is a much larger data set than
Galactic Plane Survey 77 JHK (+ H2 ) 18.1
the EDR, and marks completion of 7 % of
Deep ExtraGalactic Survey 3.1 JK 20.7
the survey programme. The programme
Ultra Deep Survey 0.8 JK 21.6
and the goals of UKIDSS are set out in
The Messenger 126 – December 2006 
Reports from Observers Warren S. et al., Early Science Results from the UKIDSS ESO Public Survey
magnitude larger than existing samples Figure 1: The 2-point angular corre-
lation function determined for a sam-
of bright DRGs, allowing a first look at 1.2
ple of bright Distant Red Galaxies
their clustering properties. The computed 1 1.0 (DRGs), measured by Foucaud et al.
2-point angular correlation function is (2006) from the UDS EDR.
δ
reproduced in Figure 1. Full circles rep- 0.8
resent DRGs, while open circles mark the 0.6
correlation function for the parent sample 0.001 0.01
Aω
of K-selected field galaxies, from which 0.1
ω (θ)
the DRG sample is drawn. The inferred
correlation length of r0 ~ 12 h−1 Mpc, con-
firms that DRGs are hosted by massive
dark matter halos.
0.01
At somewhat lower redshifts, Cirasuolo
et al. (2006) have used the UDS EDR UDS-DRGs
to chart the evolution of the K-band lumi- UDS-Field – K AB < 20.7
nosity function (LF) over the redshift 0.001
range 0.25 < z < 2.25; the first time this 0.01 0.1
has been achieved to such high statistical θ (deg)
accuracy. Galaxy colours were also used
to separate systems with blue/red rest- Figure 2: Rest-frame K-band luminosi-
ty function from Cirasuolo et al. (2006),
frame optical colours. The results are il- –3
based on the UDS EDR. The red and
lustrated in Figure 2. It was found that red blue symbols and lines plot the LF for
galaxies dominate the bright end of the –4 galaxies with red/blue rest-frame opti-
LF at z < 1, with bright blue galaxies dom- cal colours. The solid line is the LF fit-
ted to the combined sample. For ref-
inating at z > 1. –5
erence the dashed line shows the local
K-band LF from Kochanek et al. (2001).
Log φ(M)(Mpc – 3 mag –1 )
–6
Rare objects in the Large Area Survey I: 0.25 < z < 0.75 0.75 < z < 1.00 1.25 < z < 1.00
High-redshift quasars
–3
One of the main factors that influenced
the design of the LAS was the opportu- –4
nity to search for rare objects, extending
the work of 2MASS in finding very cool –5
brown dwarfs, and of SDSS in finding
quasars of very high redshifts, as well as –6
cool brown dwarfs. These goals are 1.25 < z < 1.50 1.50 < z < 1.75 1.75 < z < 2.25
described in Lawrence et al. (2006), and – 20 – 22 – 24 – 26 – 20 – 22 – 24 – 26 – 20 – 22 – 24 – 26
Hewett et al. (2006). UKIDSS DR1 pro- M K (AB) M K (AB) M K (AB)
vides the first opportunity for teams to
exploit a data set sufficiently large to be The search for high-redshift quasars ex- Rare objects in the Large Area Survey II:
of interest. ploits the UKIDSS Y-band (0.97−1.07 μm). Cool brown dwarfs
Quasars at z > 6.4 will be very red in i-Y
SDSS has been highly successful in dis- or z-Y, but bluer in Y-J than the more com- The coolest brown dwarfs are the
covering quasars beyond z = 6. The most mon L and T brown dwarfs, and there- T dwarfs, of which 99 are known, all dis-
distant quasar at z = 6.4, found by SDSS, fore distinguishable from them. So far we covered since 1995. The main samples
lies near the observable limit of the sur- have searched some 140 deg 2, and have have come from SDSS and 2MASS. The
vey. Due to absorption by intervening neu- found a single high-redshift quasar, at classification scheme of Burgasser et al.
tral hydrogen, at higher redshifts a quasar z = 5.86. The spectrum is shown in Fig- (2006) defines nine spectral classes from
would be extremely faint in z, the long- ure 3, and shows the characteristic very T0 to T8. The primary spectral stand-
est-wavelength SDSS band. Yet analysis strong break in the continuum across ard for the coolest class, T8, is the object
of the very strong absorption in the Lya Lya. To a limit Y = 19.5 we expect to find 2MASS 0415-09. There are only six
forest of the highest redshift quasars has about one quasar z > 6.0 in 150 deg 2, T8 dwarfs known. These are the coolest
yielded tantalising evidence that at z = 6 so our results so far are consistent with brown dwarfs and have temperatures
we have reached the tail-end of the epoch this expectation. The discovery of this ~ 700 K. Jupiter has a temperature
when the Universe was reionised. There- high-redshift quasar is extremely encour- ~ 150 K. What lies in between? One of
fore there is strong motivation for extend- aging for the future of the search, as the the goals of UKIDSS is to explore this
ing the redshift limit of quasar surveys. LAS database expands. temperature range. Ultracool dwarfs are
The Messenger 126 – December 2006
expected to be extremely red in z-J, and 2 Figure 3: The discovery spectrum of
the first very high redshift quasar
f(λ) 10 –17 erg s –1 cm – 2 Å –1
so difficult to detect in z. Therefore the
from UKIDSS (from Venemans et al., in
Y-filter is again expected to play an impor- Lyα ULAS J0203+0012
1.5 prep.). This 1200 sec spectrum
tant role. At some point a new spectral z = 5.86 was taken on the night of 1 September
feature is expected to emerge, possibly NV
2006, with FORS2 on the VLT.
NH3 absorption, defining a new spec- 1
tral class, for which (coincidentally) the
letter Y has been suggested. 0.5
One brown dwarf discovered in DR1, 0
ULAS J0034, is extremely cool, and has
proven particularly interesting. The 7500 8 000 8 500 9 000 9 500
spectrum is plotted in Figure 4, where it Wavelength (Å)
is compared against the T8 standard
2MASS 0415-09. There are some minor Figure 4: Spectrum of the cool T dwarf
3 ULAS J0034, the coolest brown
differences, for example, the sugges-
ULAS J0034 dwarf found so far in DR1. The colours
tion of excess absorption in the blue wing correspond to different orders of
of the 1.5−1.6 μm emission peak – a T8
Relative f(λ)
this cross-dispersed spectrum which
2
wavelength region where NH3 may ap- was a 60 min exposuretaken with
GNIRS on Gemini South. The black
pear – as well as the enhanced flux in
line plots the spectrum of the T8
the Y-band. These hint that ULAS J0034 1 standard 2MASS 0415-09, the coolest
may be even cooler than 2MASS 0415- T dwarf known, for comparison.
09, and they warrant deeper spectros-
copy. Nevertheless, because the principal 0
molecular absorption bands, due to wa-
ter and methane, are practically saturated 1 1.5 2
at these cool temperatures, it may be that Wavelength (µm)
it will become necessary to obtain pho-
tometry and spectroscopy at mid-infrared Figure 5: Z-J versus Z colour-magni-
12 tude diagram for 6 deg2 in the Upper
wavelengths of candidates such as this, Upper Sco
Scorpius assocation. The cluster
in order to delineate the development of 0.200 M � sequence stands out clearly from field
the spectral sequence beyond T8. stars all the way down to 10 M J, ac-
0.100 M � cording to theoretical models.
14
0.075 M �
0.050 M �
The substellar initial mass function
0.030 M �
below 30 Jupiter masses, from the
Galactic Clusters Survey 16
0.020 M �
Z
The aim of the Galactic Clusters Survey
(GCS) is to investigate the substellar initial 0.015 M �
mass function (IMF) in a number of open 18
clusters and star-forming regions, to shed
light on the formation of brown dwarfs.
The survey will cover 1000 deg2 in ZYJHK, 0.010 M �
20
in 10 clusters, to uncover low-mass
0.008 M �
brown dwarfs. A second epoch coverage
will be conducted in a few years time to
derive proper motions over a large mass 0.0 0.5 1.0 1.5 2.0 2.5 3.0
range. One of the regions covered in DR1 Z-J
is the young (age = 5 Myr) and nearby
(d = 145 pc) OB association Upper Scor- ter members is straightforward. We have as first epoch. Preliminary optical spec-
pius. Over 6 deg2 have been covered increased significantly the number of troscopy of the bright members reveals
in the central part of the association. The known substellar members in Upper Scor- signs of chromospheric activity and weak
Z-J versus J colour-magnitude diagram pius, and uncovered over a dozen new gravity features, characteristics of young
for stellar sources is striking (Figure 5). brown dwarfs below 20 MJ , the limit of stars. The inferred cluster IMF keeps ris-
The cluster sequence stands out clearly previous studies in the region. Further- ing across the hydrogen-burning limit and
from the field stars over the 0.3−0.01 MA more, we have confirmed all candidates is best fit by a single power law index
mass range, i.e. right down to 10 Jupiter more massive than 15 MJ as proper mo- a = 0.6 ± 0.1 down to 10 MJ . This result is
masses (MJ ), and the selection of clus- tion members using the 2MASS database in agreement with previous IMF estimates
The Messenger 126 – December 2006 
Reports from Observers Warren S. et al., Early Science Results from the UKIDSS ESO Public Survey
Figure 6: The synergy of UKIDSS-GPS and Spitzer- for sources with GLIMPSE 4.5 μm detections.
GLIMPSE data. Upper: K-band image of the central Candidate YSOs are sources with K-4.5 μm excess,
parts of a star-formation region in the mid-plane: and are cleanly separated in this diagram. In the
G28.983-0.603 from Bica et al. (2003). Lower left: K-band image, black triangles mark GLIMPSE mid-
The J-H versus H-K two-colour diagram, used to IR detections, and red squares mark candidate
establish A(V). Lower right: The K-4.5 μm versus YSOs.
A(V) diagram, combining UKIDSS and Spitzer data,
in open clusters but extends our knowl-
edge to lower masses.
Stellar clusters in the Galactic Plane
Survey
The Galactic Plane Survey is a legacy
survey designed to be useful for all areas
of Galactic astronomy. It consists of a first
epoch of JHK photometry at longitudes
l = − 2˚ to 107˚ and l = 142˚ to 230˚, and
latitudes |b| < 5 degrees, followed by two
additional epochs of K-band photome-
try to provide proper motion data and to
detect rare, high amplitude variable stars.
One of the principal science goals is
to search for any variation of the IMF over
different star-forming environments, by
studying a larger sample of young clus-
ters than any previous survey. To detect
Young Stellar Objects (YSOs), the com-
bination of Spitzer-GLIMPSE mid-IR data
with UKIRT JHK is much more effective
than the use of the mid-IR or near-IR data
alone. This is illustrated in Figure 6. The 4.5
5.5
image at the top shows a UKIDSS K‑band 4.0
5.0
image covering 3; × 3;, of a star-formation
region in the mid plane. The GLIMPSE 4.5
3.5
data on its own in this region can be used 4.0
3.0
to identify YSOs – but there are only 3.5
128 four-band IRAC detections, and nine 3.0 2.5
K-4.5
YSOs identified in a (3.6–4.5) versus
J-H
2.5
2.0
(5.8–8.0) μm two-colour diagram for the 2.0
field. Alternatively the UKIDSS data alone 1.5 1.5
may be used to select YSOs. The 1.0
lower-left diagram plots J-H versus H-K 0.5
1.0
for 2326 sources, with uncertainties 0.5
0.0
< 0.1 mag on each axis, in the field. We
– 0.5
see a well-defined reddening sequence 0.0 0.5 1.0 1.5 2.0 2.5
0.0
–10 0 10 20 30 40 50
from lower left to upper right. Candi- H-K A(V) = 9.8 (J-H-0.6)
date YSOs are objects with infrared ex-
cess to the right of this sequence.
Combining the UKIDSS and GLIMPSE Timetable for future releases References
data gives a much cleaner separation.
Bica E. et al. 2003, A&A 404, 223
The lower left-hand diagram may be used The next release, DR2, is planned for the Burgasser A. et al. 2006, ApJ 637, 1067
to estimate A(V). In the lower right-hand end of February 2007, and will include Cirasuolo M. et al. 2006, MNRAS, submitted,
diagram the K-4.5 μm colour is plot- new data obtained in the period May to astro-ph/0609287
Foucaud S. et al. 2006, MNRAS, submitted,
ted against A(V) for the 1084 sources with July 2006. Note that the UDS was not
astro-ph/0606386
GLIMPSE 4.5 μm detections. Candidate observable in this block. A new very large Hewett P. et al. 2006, MNRAS 367, 454
YSOs are identified by their K-4.5 μm WFCAM block began at the end of Oc- Kochanek C. et al. 2001, ApJ 560, 566
colour excess. These are plotted as red tober 2006, and runs through to mid-May Lawrence A. et al. 2006, MNRAS, submitted,
astro-ph/0604426
open squares in the upper figure, and 2007. By the end of this block UKIDSS
Lodieu N. et al. 2006, MNRAS, in press,
show a concentration towards the cluster will be about 20 % complete. These data astro-ph/0610140
centre. will be released in DR3, intended to take McLure R. et al. 2006, MNRAS 372, 357
place late in 2007. Warren S. et al. 2006, MNRAS, submitted,
astro-ph/0610191
10 The Messenger 126 – December 2006Reports from Observers
Starburst Galaxies Under the Microscope:
High-Resolution Observations with VISIR and SINFONI
Paul P. van der Werf, Leonie Snijders, an enormous boost from technical de- gion, for which the youngest regions
Liesbeth Vermaas, Juha Reunanen and velopments in ground-based and space- have to be isolated. A second example is
Marten Hamelink (Leiden Observatory, based infrared astronomy. The infrared the origin of the PAH emission in star-
the Netherlands) regime in fact offers two advantages. In burst galaxies, which can be studied if
the first place, reduced extinction offers the emission regions and the local
the opportunity to see through the obs- sources of excitation can be spatially re-
Infrared observations of starburst gal- curing dust, and to probe the active star- solved. Both of these require high spa-
axies not only enable penetration of forming complexes directly. Secondly, tial resolution and will be discussed
the obscuring veil of dust, but also pro- a number of unique diagnostics are avail- in some detail in the following sections.
vide unique diagnostics in the form able in the infrared in the form of highly
of nebular emission lines and emission diagnostic nebular emission lines, H2 vi-
from dust and polycyclic aromatic hy- brational lines which provide a kinematic A case study: superstarclusters in the
drocarbons (PAHs). Here we describe probe of the molecular gas at high spa- Antennae (NGC 4038/4039)
some first results of our ongoing study tial resolution, and emission and absorp-
of starburst galaxies with VISIR and tion features of the dust itself, includ- The Antennae system (NGC 4038/4039)
SINFONI at the VLT. ing those attributed to polycyclic aromatic is the nearest major merger of two large
hydrocarbons (PAHs). spiral galaxies. Since the beginning of
the interaction the system went through
Starburst galaxies We have recently embarked on an obser- several episodes of violent star forma-
vational study of nearby starbursts with tion, of which the last one is probably still
Starburst galaxies are unique laborato- two new VLT instruments: SINFONI and ongoing.
ries. Starburst episodes are phases in the VISIR, and here report some first results.
evolution of galaxies that are by defini- The resulting star clusters have been
tion transient, and during which they con- studied extensively. Radio and mid-IR ob-
vert a significant fraction of their gas res- The importance of spatial resolution servations show that the region between
ervoirs into stars. During a starburst the two remnant nuclei (usually referred
phase a galaxy thus evolves rapidly in stel- The study of starburst galaxies through to as the overlap region) hosts spectacu-
lar, gas, dust and metal content, colour, infrared techniques has benefited signi- lar obscured star formation. The brightest
luminosity and morphology. Starburst ficantly from observations with the Infra- mid-IR component produces 15 % of
galaxies also cover an enormous range in red Space Observatory (ISO) and the the total 15 μm luminosity of the entire
luminosity. At the low luminosity end the Spitzer Space Telescope. Yet, while these system (Mirabel et al. 1998). This region is
small star-forming dwarf galaxies such as space-based observations have provided covered by a prominent dust lane and
the Small and Large Magellanic Clouds unmatched sensitivity and wavelength may be associated with a faint, red source
have infrared luminosities L IR = 7 10 7 L A coverage, they cannot provide the spatial in Hubble Space Telescope (HST) im-
and L IR = 7 10 8 L A. More distant infra- resolution enabled by ground-based ages, illustrating how optical data alone
red-bright dwarf galaxies typically have telescopes. VISIR at the VLT has opened are insufficient to identify and study
L IR = 3 10 9 L A. Well-studied nearby up the ground-based mid-infrared (mid- the youngest star-forming regions. Such
starbursts such as NGC 253 and M82 IR) spectral region for routine imaging superstarclusters are of interest as poten-
have L IR = 3 10 10 L A and 6 10 10 L A. and spectroscopy at an angular resolu- tially the youngest simple coeval stellar
At higher luminosities, we have the lumi- tion of 0.3? (essentially the diffraction limit populations in starbursts and thus furnish
nous infrared galaxies (LIRGs) with of the VLT). For comparison, the resolu- excellent tests for the properties of the
L IR > 10 11 L A. (e.g., the Antennae, NGC tion of Spitzer at 8 μm is 2.5?. Thus VISIR most massive stars formed in these sys-
4038/4039), the ultraluminous infrared gains over Spitzer in spatial resolution by tems. For sufficiently massive and young
galaxies (ULIRGs) with L IR > 10 12 L A (e.g. a factor of eight in two dimensions. As superstarclusters, they may offer the
Arp 220), and the hyperluminous infra- we will show, this gain in spatial resolu- opportunity of directly measuring a pos-
red Galaxies (HyLIRGs) with L IR > 10 13 L A tion is fundamentally important for study- sible upper mass cutoff of the stellar
While the luminosity range spanned is ing the anatomy of starburst galaxies in Initial Mass Function (IMF). Mid-IR nebu-
more than five decades, the starbursts detail. The VISIR data are complemented lar fine-structure lines are excellent
that are most amenable to detailed study with SINFONI near-infrared (near-IR) in- probes of such systems, since they are
are obviously the nearest ones, which tegral field spectroscopy at a similar reso- relatively unaffected by dust and can be
have only moderate luminosity. It is there- lution. High spatial resolution allows us used to measure the temperature of
fore important to understand how these to isolate active star-forming regions from the ionising radiation field, and hence the
nearby starbursts relate to their more dis- diffuse extended emission and thus masses of the most massive stars pres-
tant and spectacular cousins. provides a more secure diagnostic of the ent.
conditions in the star-forming regions
Since stars form in dusty molecular themselves (e.g., local densities and radi- We used VISIR to study the most promi-
clouds, it is no surprise that (most) star- ation fields). An application of this is nent clusters at 0.3? resolution (30 pc
bursts are also dusty. The study of the determination of the mass of the most at the assumed distance of 21 Mpc for
starburst galaxies has therefore received massive star in a young star-forming re- the Antennae). Our data set consists of
The Messenger 126 – December 2006 11Reports from Observers van der Werf P. P. et al., Starburst Galaxies Under the Microscope
1000
Source 1a
[Ne II]
[S IV]
100 [Ar III]
Flux (in mJy)
10
1000
Source 2
Figure 1 (above): VISIR image of the Figure 2 (right): VISIR spectra, tak-
[Ne ii] 12.8 μm emission from the most en with a 0.75? slit, of the two promi- PAH
prominent superstarclusters in the nent superstarclusters in the Anten-
Antennae (right panel). The diameter of nae seen in Figure 1. Source 1a is the 100
the VISIR field shown here is 9?. Its brightest part of the Eastern source,
location is indicated in the left panel, while Source 2 is the Western source.
which shows a composite of data ob- The apparently enhanced noise from
tained with Spitzer (Wang et al. 2004). 9 to 10 μm results from the log scale of
The inset in the right panel shows these plots (from Snijders et al. 2006).
the contours of the dust emission at
11.3 μm overlaid on the [Ne ii] image
(from Snijders et al. 2006). 10
8 9 10 11 12 13
Lambda (in micron)
imaging in a number of narrow-band fil- tra (Brandl, priv. comm.) with a 5? slit, The low equivalent width of the PAH emis-
ters in the N-band, and long-slit spec- revealing that approximately 75 % of the sion indicates that either the PAHs are
troscopy with a 0.75? slit, covering the 12 μm continuum is detected in the destroyed in the direct environment of the
two most prominent clusters. Some key 0.75? VISIR slit; however, the equivalent superstarclusters, or that the PAH emis-
results are shown in Figures 1 and 2 width of the 11.3 μm PAH feature in sion is not preferentially excited by the su-
(Snijders et al. 2006), which show a num- the VISIR data is much smaller than in perstarclusters, but is dominated by more
ber of surprising results. In the first place, the larger aperture Spitzer spectra. diffuse emission, excited by the softer
the Eastern cluster is separated into two UV radiation from more widespread young
components, separated by approximately Both clusters exhibit emission in the stars of slightly later type. Understand-
0.5? (50 pc). The brightest of these two 10.5 μm [S iv] line, an ionisation stage re- ing which of these explanations is correct
(cluster 1a) is slightly resolved. This result quiring 34.8 eV (while the 12.8 μm [Ne ii] is important for the interpretation of the
immediately shows that any attempt to line requires only 21.6 eV); in particular in PAH emission. In order to study this issue
model this region as a single coeval stel- cluster 2 the [S iv]/[Ne ii] ratio in our data further, we now turn to a more nearby
lar population is flawed. Cluster 1b has no is higher than in larger aperture Spitzer starburst, where much higher linear reso-
counterpart in any other available data data, significantly affecting the interpreta- lution is obtained.
set; from the available upper limits, we de- tion of the results, and indicating that
rive a visual extinction A V > 72 m towards VISIR closes in on the regions of most in-
this cluster. Remarkably, the 11.3 μm tense star formation, while larger ap- The resolved starburst in M83
emission shows a different morphology, erture data are significantly affected by
suggesting a common envelope of emis- more diffuse emission. M83 is a nearby (D = 4.5 Mpc) grand-
sion from hot dust and polycyclic ar- design barred spiral with a nuclear region
omatic hydrocarbons (PAHs). Cluster 2, A detailed analysis of the fine-structure that is sometimes described as ‘amor-
which is optically complex, is a simple line ratios in the two clusters indicates phous’. It has a prominent optical peak,
and compact object at 10 μm; presum- conditions similar to those in Galactic ul- which is however not at the centre of
ably the N-band emission is dominat- tracompact Hii regions (but extended the fainter isophotes and therefore proba-
ed by a single (obscured) object within over tens of parsecs). This is an important bly not the dynamical centre. The star-
the general complex. result, since it would affect the interpreta- burst in M83 is not centred on this optical
tion of results at other wavelengths (from peak, but displaced significantly towards
An even more surprising result comes radio to near-IR) as well (Snijders et al., in the West. The situation is illustrated in
from comparison with Spitzer-IRS spec- preparation). Figure 3. Here the K-band continuum is
12 The Messenger 126 – December 2006Figure 3: The off-nuclear starburst in M83. Each of
6 6
these frames shows a 13.6? × 13.6? (300 pc diameter)
region. Top-left panel: K-band continuum (from
SINFONI); top-right panel: Brg 2.17 μm (from SIN-
4 4 FONI); lower-left panel: [Fe ii] 1.26 μm (from SIN-
FONI). These data result from three overlapping SIN-
2 2 FONI exposures, with on-source integration times
Dec (arcsec)
Dec (arcsec)
of 10 minutes per frame and per spectral band (from
0 0
Vermaas et al., in preparation); lower-right panel:
11.3 μm PAH emission obtained with VISIR (from
Snijders et al., in preparation).
−2 −2
−4 −4
−6 −6
6 4 2 0 −2 −4 −6 6 4 2 0 −2 −4 −6
RA (arcsec) RA (arcsec)
and excited by the supernova blast wave
6 6 shock. Since the [Fe ii] 1.64 μm and
1.26 μm lines originate from the same up-
4 4 per level, their intrinsic ratio is fixed and
the observed line ratio can thus be used
2 2
as an independent extinction measure-
Dec (arcsec)
Dec (arcsec)
ment. Furthermore, fainter [Fe ii] lines can
0 0
be used to constrain temperature and
−2 −2
density of the emitting material.
−4 −4 Also commonly detected are the rovi-
brational lines of H2. Arising from levels
−6 −6 about 6 000 K above the ground state,
these lines trace hot molecular gas. While
6 4 2 0 −2 −4 −6 6 4 2 0 −2 −4 −6 the diagnostic use of these lines is com-
RA (arcsec) RA (arcsec) plicated by the fact that multiple excita-
tion mechanisms can play a role (and
dominated by the underlying bulge popu- tions). These lines trace the helium-ionis- probably do play a role), such as fluores-
lation, with emission from red supergiants ing continuum and therefore the most cence following UV-absorption, shock
formed in the starburst superimposed. massive stellar population in the starburst. waves and X-ray excitation, they provide
The full spectral data cubes produced by In principle helium/hydrogen recombi- a unique high-resolution probe of mo-
SINFONI provide a wealth of detail on nation line ratios can be used to measure lecular gas in galactic nuclei which can
the morphology and spatial and temporal the relative volumes of the helium and often be used for gas-dynamical studies.
evolution of this starburst, which is illus- hydrogen Strömgren spheres and thus
trated by the spectra shown in Figure 4. the hardness of the ionising radiation In addition to these emission features,
field. In practice, collisional excitation ef- there are photospheric absorption fea-
Key spectral features which are evident in fects from metastable levels in the heli- tures arising in the cool atmospheres
these spectra include lines of ionised um atom make this procedure complicat- of red supergiants created in the star-
hydrogen (Brg 2.17 μm in the K-band, the ed, and in particular the use of the bright burst. These include the CO first overtone
Brackett series in the H-band and Pab He i 2.06 μm line is fraught with difficul- absorption bands at 2.30 μm and long-
1.28 μm in the J-band). These lines trace ties; however, the He i 1.70 μm line is quite er wavelengths, as well the second over-
the distribution of the young massive suitable for this purpose. tone absorptions in the H-band; there
stars (spectral types B3 and earlier) and are also atomic absorption lines (e.g., Si i
(under the usual assumption of dust- In addition we find forbidden fine-struc- 1.59 μm, Na i 2.21 μm and Ca i 2.27 μm)
free and ionisation-bounded H ii regions) ture lines of singly ionised iron, principally which together with the CO bands can be
provide a direct measurement of the the [Fe ii] 1.64 μm line in the H-band and used for stellar dynamical studies as
Lyman continuum output of these stars; the 1.26 μm line in the J-band. The [Fe ii] well as for spectral typing of the dominant
in addition, combining these lines gives emission results from strong shocks stellar population.
a direct measurement of the extinction from supernovae, which destroy the dust
towards the ionised gas. In addition we grains, thus raising the gas-phase iron Finally, high-excitation so-called coronal
observe He i lines (principally the 2.06 μm abundance by a large factor. The result- lines (named after their detection from
and 1.70 μm lines, but also fainter transi- ing iron atoms are then easily ionised the solar corona) require a hard radiation
The Messenger 126 – December 2006 13Reports from Observers van der Werf P. P. et al., Starburst Galaxies Under the Microscope
2.0 8 Figure 4: SINFONI spectra taken at
[Fe II]
H2 two positions in M83. Shown are
Flux (*10 –13 ergs/s*cm 2 * µm)
SINFONI K-band spectra (left column)
Flux (*10 –13 ergs/s*cm 2 * µm)
1.5 6
and partial J-band spectra (right col-
umn) on the K-band continuum peak
1.0 4 (upper panels), on the peak of the Brg
emission (lower panels). These spectra
0.5 CO-absorption 2 are from Vermaas et al. (in prepara-
tion).
0.0 0
2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40 1.25 1.26 1.27 1.28 1.29 1.30
Wavelength (µm)
Wavelength (µm)
2.0 8
Flux (*10 –13 ergs/s*cm 2 * µm)
Flux (*10 –13 ergs/s*cm 2 * µm)
1.5 6
1.0 Brγ 4
He I
[Fe II] Paβ
0.5 H2 2
0.0 0
2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40 1.25 1.26 1.27 1.28 1.29 1.30
Wavelength (µm)
Wavelength (µm)
field that cannot be produced by nor- Since the supernova rate is dominated by by these is unlikely, given the lack of
mal young stars and therefore reveal the stars with a mass of about 8 MA (the detailed morphological agreement. The
presence of an active galactic nucleus most numerous stars still producing su- presence of supernova remnants how-
(AGN); these lines include [Si vi] 1.96 μm, pernovae), which have a lifetime of about ever indicates a radiation field dominated
[Ca viii] 2.32 μm and [S ix] 1.25 μm. 3 10 7 years, the Brg and [Fe ii] emission by the most massive stars that do not
trace phases of the starbursts that are end as supernovae, i.e., mid-B-type stars
With the exception of the coronal lines, temporally separated by this amount of or later. This result confirms earlier claims
all of these lines are evident in the spec- time. In principle, one could use these re- that PAHs can be excited by a fairly soft
tra shown in Figure 4. Given that these sults then to calculate the speed at which radiation field (e.g., Li and Draine 2002). A
tracers probe different temporal phases the star formation propagates through quantitative analysis will be able to show
of the starburst, they can be used as an the nuclear region. Remarkably however, what fraction of the PAH emission is ex-
age indicator. For instance, the Brg equiv- there is no pattern in the derived ages. cited by stars of various types, which will
alent width EW(Brg) can be formed by Instead, the results point to a situation in ultimately lead to a more secure calibra-
dividing the Brg emission by the underly- which a large area becomes globally un- tion of PAH emission as a star formation
ing continuum, thus measuring the rel- stable, after which individual star-form- indicator.
evant importance of young O-stars and ing complexes form stochastically. There
their direct descendants, the red super- is thus no evidence for propagating star
giants, which is time-dependent and can formation in this region. However, a glob- Mid-IR emission as a star formation
thus be used to determine the age of al trigger is still needed. Presumably this indicator
the stellar population. Age determinations may be found in the accumulation of gas
may also be obtained by comparing in the barred potential in the M83 nucle- A fundamental result from Spitzer is the
Brg flux to CO absorption bands (again a us, which continues until a critical value is use of 24 μm dust emission (well away
comparison of O-stars with red super- reached, after which star formation is ig- from solid state spectral features) as a
giants) or [Fe ii] flux (O-stars compared to nited stochastically. star-formation indicator (e.g., Calzetti et
supernova remnants). Turning again to al. 2005, Pérez-González et al. 2006).
Figures 3 and 4, it is seen that at the Inspection of Figure 3 also reveals that Ground-based imaging in the Q-band
K-band nucleus the EW(Brg) is very low, the PAH emission traces star formation spectral window (17–26 μm) allows us to
but that the CO bands are quite prom- only approximately. Clearly the brightest examine the dust emission in this spec-
inent, showing that this region is domi- PAH emission traces the brightest Brg tral region in detail in spatially resolved
nated by an evolved stellar population. In emission, and there is therefore no evi- starbursts.
contrast, the bright Brg region seen in dence for PAH destruction by the hottest
Figure 3 has essentially no counterpart in stars. However, diffuse PAH emission An example is presented in Figure 5,
the K-band continuum and its high is present also where no Brg emission is where we present images of Brg emission
EW(Brg) thus indicates a very young age. found, e.g., in the region of the K-band (from SINFONI) together with the Q-
This is also seen in its J-band spectrum, nucleus. The presence of [Fe ii] emission band dust emission imaged with VISIR
where the Pab line is much stronger than in this area indicates the presence of (Snijders et al., in preparation). It is evi-
the [Fe ii] line at 1.26 μm. supernova remnants, but direct excitation dent that these two match quite well.
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