Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L72 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014554 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Radial distribution of gas and dust in spiral galaxies![[*]](/icons/foot_motif.png)
The case of M 99 (NGC 4254) and M 100 (NGC 4321)
M. Pohlen1 - L. Cortese1 - M. W. L. Smith1 - S. A. Eales1 - A. Boselli3 - G. J. Bendo2 - H. L. Gomez1 - A. Papageorgiou1 - R. Auld1 - M. Baes4 - J. J. Bock5 - M. Bradford5 - V. Buat3 - N. Castro-Rodriguez6 - P. Chanial7 - S. Charlot8 - L. Ciesla3 - D. L. Clements2 - A. Cooray9 - D. Cormier7 - E. Dwek10 - S. A. Eales1 - D. Elbaz7 - M. Galametz7 - F. Galliano7 - W. K. Gear1 - J. Glenn11 - M. Griffin1 - S. Hony7 - K. G. Isaak1,12 - L. R. Levenson5 - N. Lu5 - S. Madden7 - B. O'Halloran2 - K. Okumura7 - S. Oliver13 - M. J. Page14 - P. Panuzzo7 - T. J. Parkin15 - I. Perez-Fournon6 - N. Rangwala11 - E. E. Rigby16 - H. Roussel8 - A. Rykala1 - N. Sacchi17 - M. Sauvage7 - B. Schulz18 - M. R. P. Schirm15 - M. W. L. Smith1 - L. Spinoglio17 - J. A. Stevens19 - S. Srinivasan8 - M. Symeonidis14 - M. Trichas2 - M. Vaccari20 - L. Vigroux8 - C. D. Wilson15 - H. Wozniak21 - G. S. Wright22 - W. W. Zeilinger23
1 - School of Physics and Astronomy, Cardiff University, Queens
Buildings The Parade, Cardiff CF24 3AA, UK
2 -
Astrophysics Group, Imperial College, Blackett Laboratory, Prince
Consort Road, London SW7 2AZ, UK
3 -
Laboratoire d'Astrophysique de Marseille, UMR6110 CNRS, 38 rue F.
Joliot-Curie, 13388 Marseille France
4 -
Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9,
9000 Gent, Belgium
5
- Jet Propulsion Laboratory, Pasadena, CA 91109, United States;
Department of Astronomy, California Institute of Technology, Pasadena,
CA 91125, USA
6 -
Instituto de Astrofísica de Canarias, vía Láctea S/N, 38200 La
Laguna, Spain
7 -
CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
8
- Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre
& Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
9 -
Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
10 -
Observational Cosmology Lab, Code 665, NASA Goddard Space Flight
Center Greenbelt, MD 20771, USA
11 -
Department of Astrophysical and Planetary Sciences, CASA CB-389, University of Colorado, Boulder, CO 80309, USA
12 -
ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
13 -
Astronomy Centre, Department of Physics and Astronomy, University of Sussex, UK
14 -
Mullard Space Science Laboratory, University College London,
Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
15 -
Dept. of Physics & Astronomy, McMaster University, Hamilton,
Ontario, L8S 4M1, Canada
16 -
School of Physics & Astronomy, University of Nottingham, University
Park, Nottingham NG7 2RD, UK
17 -
Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
18
- Infrared Processing and Analysis Center, California Institute of
Technology, Mail Code 100-22, 770 South Wilson Av, Pasadena, CA 91125,
USA
19 -
Centre for Astrophysics Research, Science and Technology Research
Centre, University of Hertfordshire, College Lane, Herts AL10 9AB, UK
20 -
University of Padova, Department of Astronomy, Vicolo Osservatorio
3, 35122 Padova, Italy
21 -
Observatoire Astronomique de Strasbourg, UMR 7550 Université de
Strasbourg - CNRS, 11, rue de l'Université, 67000 Strasbourg
22 -
UK Astronomy Technology Center, Royal Observatory Edinburgh, Edinburgh, EH9 3HJ, UK
23 -
Institut für Astronomie, Universität Wien, Türkenschanzstr. 17,
1180 Wien, Austria
Received 30 March 2010 / Accepted 19 April 2010
Abstract
By combining Herschel-SPIRE data with archival Spitzer, I, and
CO maps, we investigate the spatial distribution of gas and dust in the two
famous grand-design spirals M 99 and M 100 in the Virgo cluster. Thanks to
the unique resolution and sensitivity of the Herschel-SPIRE photometer,
we are for the first time able to measure the distribution and extent of cool,
submillimetre (submm)-emitting dust inside and beyond the optical radius. We
compare this with the radial variation in both the gas mass and the
metallicity. Although we adopt a model-independent, phenomenological approach,
our analysis provides important insights. We find the dust extending to at
least the optical radius of the galaxy and showing breaks in its radial
profiles at similar positions as the stellar distribution. The colour indices
f350/f500 and f250/f350 decrease radially consistent with the temperature
decreasing with radius. We also find evidence of an increasing gas to dust
ratio with radius in the outer regions of both galaxies.
Key words: galaxies: structure - galaxies: individual: M 99 - galaxies: individual: M 100 - infrared: galaxies - ISM: dust, extinction - submillimetre: galaxies
1 Introduction
![]() |
Figure 1:
The matched SDSS, MIPS 70 |
Open with DEXTER |
The study of the gas and dust distribution in galaxies is essential to
understanding their formation and evolution. The rate at which gas is accreted
and converted into stars regulates not only the star formation history of
galaxies but also their chemical evolution. Dust is supposed to play a key
role in this process. Dust grains are the main coolant in star-forming
galaxies, shielding the gas from the UV radiation and representing the site at
which I is converted into
, and then collapses into stars (see e.g., reviews by Calzetti 2001; Draine 2003).
To understand the dust, we need to map the cold component, which does not
dominate the energy but dominates the mass. However, using previously existing
facilities, our knowledge of the interplay between gas and dust and their
radial distribution have remained highly uncertain, being based only on
m space observations (e.g. Bendo et al. 2010a; Muñoz-Mateos et al. 2009a), and challenging ground-based submm observations that were of
optimal quality only at significantly longer wavelength. For many galaxies,
only integrated quantities have been derived because of the poor resolution of
previous satellites. Resolved submm studies from the ground remain limited to
the very nearby universe and large surveys of more distant galaxies are
unfeasible.
The SPIRE instrument (Griffin et al. 2010) on-board Herschel
(Pilbratt et al. 2010) now bridges this gap observing in the range of
250 m-500
m. With the benefit of the stable conditions of a space
observatory, it is much more sensitive to the cold component and provides
excellent maps at high resolution, so is ideal for large surveys of many
galaxies. With Herschel, we are now able to tackle the problem of the
interplay between gas and dust, and combined with the number of recent high
resolution surveys tracing the gas mass of local galaxies
(e.g. Kuno et al. 2007; Chung et al. 2009), we can study the distribution of gas,
metals, and dust on a kpc scale for hundreds of galaxies.
Here, we discuss first results for two famous grand-design spirals M 99 and
M 100 (see Fig. 1) in the Virgo cluster. We explore the distribution
of the cool dust traced with Herschel by inspecting their radial profiles from
mid-infrared to submm wavelengths. We then attempt to correlate this with the
observed gas and metallicity distributions and search for temperature
variations. We use the Herschel-SPIRE maps taken during Herschel's science
demonstration phase. The two galaxies are part of the Herschel Reference
Survey (Boselli et al. 2010a). This guaranteed time key project will provide
maps for a statistically-complete sample of 323 nearby galaxies in all three
SPIRE bands. In the RC3 (de Vaucouleurs et al. 1991), the classification and optical radius
R25 of M 99 and M 100 is given as SA(s)c, 2.69(
12.9 kpc),
and SAB(s)bc, 3.71
(
17.8 kpc), respectively. We assume a
distance for the Virgo Cluster of 16.5 Mpc (Mei et al. 2007).
2 Data
2.1 Herschel-SPIRE
The SPIRE photometer (Griffin et al. 2008,2010) data were processed up to
Level-1 (i.e., to the level where the pointed photometer time-lines were
derived) with a customised pipeline script adapted from the official
pipeline (POF5_pipeline.py, dated 27 Nov. 2009) provided by the SPIRE
Instrument Control Centre (ICC). This Jython script was run in the Herschel interactive processing
environment (HIPE Ott 2010) coming with continuous integration build
number 3.0.327, which is the current developer's branch of the data
reduction software. However, in terms of the SPIRE scan-map pipeline up to
Level-1, this is in principle identical to the Herschel common science
system/standard product generation v2.1, even down to the
calibration files
associated
with the individual pipeline modules. This version of the pipeline is used
at ESA to produce the standard products that will be available from the
Herschel Science Archive once they become public.
Currently, the Level-1 photometer time-lines still requires a residual baseline subtraction to be made. However, instead of subtracting the median of the time-line for each bolometer per scanleg (the default), we subtracted the median of the time-lines for each bolometer over the whole observation. This circumvents shadow artefacts caused in cases where the signal time-lines in individual scanlegs are dominated by structured emission e.g. a large, extended galaxy or a strong cirrus component.
This baseline-subtracted Level-1 data were then fed through an iterative de-striper, which minimises the difference between the signal in individual detector time-lines and the final map (see Bendo et al. 2010b, for a longer description). At the end of this process, the signal time-lines were then mapped into a final image using the Naive Mapper available in HIPE.
For M 99, the de-striping approach left some residual large-scale gradients. In this case, we resorted back to an initial baseline subtraction on a scan by scan basis. However, instead of a median, we used a robust linear fit with outlier rejection to the first and last fifty sample points, thus avoiding the galaxies in the centre of the time-lines.
According to the ICC, the uncertainty in the flux calibration is of the order
of 15% (Swinyard et al. 2010) and is currently based on a preliminary
calibration. However, the ICC has released some interim small correction
factors to improve this calibration. All flux values derived using the current
standard calibration file for the flux conversion, are multiplied by 1.02,
1.05, and 0.94, for the 250 m, 350
m, and 500
m,
respectively
.
The full widths at half maximum (FWHM) of the SPIRE beams are 18.1
,
25.2
,
and 36.9
,
the pixel sizes are 6
,
10
,
and
14
at 250, 350, and 500
m, respectively.
For both, M 99 and M 100, we observed a 12
field
doing three repetitions of a cross-linked scan-map at nominal detector
settings and nominal scan speed (30
/s). The M 100 observation was
carried out twice. Both were treated independently here and used to verify the
consistency of our results.
2.2 MIPS, CO, and HI
![]() |
Figure 2:
Left: Radial surface brightness profiles for M 99 ( top) and
M 100 ( bottom) obtained from the smoothed and matched maps
(see text). The profiles are from the bottom up: MIPS 24, 70, 160 |
Open with DEXTER |
The 24, 70, and 160 m images were part of the SINGS survey
(Kennicutt et al. 2003) and were processed using the MIPS data analysis tools
(Gordon et al. 2005) along with techniques described by Bendo et al. (2010a) and
Clements et al. (2010, in prep.). The FWHM of the MIPS beams are about
6
,
18
,
and 38
,
the pixel sizes are 1.5
,
4.5
,
and 9
per pixel at 24, 70, and 160
m, respectively.
The CO(J=1-0) maps, used as the tracer of the molecular hydrogen dominating
the molecular mass, are taken from the Nobeyama CO Atlas of Nearby Spiral
Galaxies (Kuno et al. 2007). The FWHM is 15
and the pixels are
1
per pixel. For the
I , we used the zeroth moment maps from the VIVA
survey (VLA Imaging of Virgo spirals in Atomic Gas; Chung et al. 2009). The
FWHM and pixel sizes are
37
and 5
per pixel for
M 99 and
30
and 10
per pixel for M 100.
2.3 Analysis
The SPIRE maps were first converted into Jy per pixel assuming a Gaussian beam
(of the above quoted sizes). For the I , we used the elliptical beam sizes
given in Chung et al. (2009). In all maps, we masked strong sources and
artefacts. However, in the case of the SPIRE maps, being confusion limited,
all faint sources are unmasked and part of the background. The residual
background on each map was subsequently determined using IRAF's
ELLIPSE task
as described in Pohlen & Trujillo (2006). The background-subtracted maps of all
wavelengths were thereafter smoothed to the MIPS 160
m resolution of
40
using custom convolution kernels derived as described in
Bendo et al. (2010a) and then matched to the SPIRE 500
m pixel size of 14
.
The SPIRE 250 m map was chosen to derive the final set of ellipse-fitting
parameters (i.e., ellipticity, position angle, and centre). To ensure that our
results are independent of the particular ellipse geometry selected, we
applied four different fixed ellipse fits to each map. For example, M 99
being a one-armed spiral, is slightly asymmetric (cf. Fig. 1) so we
selected one set of ellipse parameters derived in the outer parts and one in
the inner parts, which each have slightly different centres. The final
radial profiles, obtained using a combined mask on the smoothed and matched
maps, are shown in Fig. 2 out to where we can trace signal on
the map. The error bar in each measured point is a combination in quadrature
of the uncertainty in the overall absolute calibration (currently for SPIRE
the dominating source), the error in the ellipse intensity from the
ELLIPSE task, the uncertainty in the estimate of the background, and
an additional uncertainty calculated by comparing the results from
different versions of the pipeline. This last, very conservative uncertainty, is
responsible for the currently rather large error bars in the measured flux
ratios in Fig. 2.
3 Results
We detect dust emission traced by all three SPIRE bands out to at least
the optical radius defined by R25 for both galaxies (see Fig. 1 for
detections in the nominal maps, and Fig. 2
for the deeper, radially averaged profiles from the smoothed maps). Compared
to
I , the dust can be found almost out to the
I -edge of the regular disk
for M 100. This is not entirely surprising, since M 100 is an intermediate
I -deficient (Haynes & Giovanelli 1984; Cayatte et al. 1994) galaxy (
I -def = 0.35) and
thus its outer
I -disk has probably been already stripped by the interaction
with the cluster environment (Boselli & Gavazzi 2006). A similar extension of the
dust and
I -disk is observed for this range of
I deficiencies in other
Virgo cluster galaxies (Cortese et al. 2010).
The situation is different for M 99, which is not
I -deficient
(
I -def = -0.1). Figure 1 clearly shows that the
I emission is
much more extended than the submm at least to the north. Interestingly, this
extended
I halo (Chung et al. 2009) however might be barely detected, but at
the moment we cannot exclude that this is caused by residual background
inhomogeneities coupled with a cluster of unresolved background sources. We
can however exclude the presence of submm emission corresponding to the giant
I tail of M 99 (Haynes et al. 2007) to the southwest and we also find no
measured flux associated with the extended low surface brightness feature to
the southwest of M 100 (Chung et al. 2009).
The left column in Fig. 2 shows the radial profiles obtained by
the ellipse fitting to the smoothed maps. The MIPS and I profiles agree
with the published ones by Muñoz-Mateos et al. (2009b) and Chung et al. (2009),
respectively. For M 99, the MIPS and SPIRE profiles follow similar trends
including a weak radial break in the profile, i.e., a change in the slope, at
,
and a broken exponential is a more accurate fit than
a single exponential (e.g. Pohlen & Trujillo 2006, for more background on
breaks). This break is also visible in the optical profile shown
by Muñoz-Mateos et al. (2009b). The same is true for M 100, which also exhibits a more
obvious break at around the same distance (it is even more striking in the profile at native resolution presented by Sauvage et al. 2010).
This is the first time we see these breaks clearly in the dust distribution,
while they are well-known at optical wavelengths. None of the so far presented
hypotheses for the origin of these breaks have addressed this before
(see e.g. the recent review by Vlajic 2010, for references) and it will
be another pice of the puzzle to be explained by the various proposed
models. The rising profile in the inner part of M 100 is related to the more
prominent bulge, bar, or inner-disk component, which is discussed in more
detail in Sauvage et al. (2010).
To investigate the variation in submm colours as a function of radius we plot in the middle column of Fig. 2 the ratio of the SPIRE bands f350/f500 to f250/f350. These are colour temperature indices. The advantage of using these instead of a derived dust mass is that they are independent of the specific, not yet well studied, model assumptions in this new wavelength range. They both decrease with radius, which suggests that the dust in the outermost regions is colder than in the centre of the galaxies. This is naturally explained by an interstellar radiation field becoming less intense in the outskirts. Our profiles are very similar to those of M 81 presented in Bendo et al. (2010b), who argue that the radial variation is driven by heating from the evolved stars in the galaxy. The observed range of flux ratios along the galactic radii is the same as found for a sample of galaxies with a wide variety of morphologies using integrated SPIRE fluxes (Boselli et al. 2010b). Interestingly, the agreement between their integrated and our resolved analysis extends beyond the colour profiles as shown in the right panels of Fig. 2, where we couple our colour gradients to the metallicity gradient published by Skillman et al. (1996) renormalised to the [OIII]/[NII] base of Pettini & Pagel (2004). The trend we observe radially for M 99 and M 100 matches the fit to the integrated properties of the Boselli et al. (2010b) sample very well. Both f350/f500 and f70/f160 (albeit only very weakly) decrease with the radially decreasing metallicity. This is again expected since a lower activity of star formation in the outer parts, as observed by Wilson et al. (2009), consequently entails lower metallicities.
In Fig. 3 we finally show the ratio of the total gas mass
( I plus
)
to 500
m flux ratio for the two galaxies. Since the
500
m flux is a proxy of the dust mass, this provides a
``model-independent'' indication of the radial evolution of the dust-to-gas
ratio. There is a clear trend visible with the gas-to-dust ratio
increasing radially, which is consistent with earlier results
(Bendo et al. 2010a; Muñoz-Mateos et al. 2009a), but the exact shape needs to be revised once a
proper SED dust modelling including the new SPIRE bands is available.
In conclusion, we have found that the dust emission can be traced by the SPIRE
bands at least out to the optical radius and beyond. The dust shows the same
breaks in the radial profile as seen in the optical. The I is only slightly
more extended but this needs to be regarded here in the context of the cluster
environment. The SPIRE colour temperature indices decrease with radius
following the measured trends in metallicity, and the extent of the measured
values along the galaxies' radii is consistent with the integrated properties
of galaxies with a variety of morphologies. We have shown evidence of a
radially rising gas-to-dust ratio. These results provide the first indication
of the improved capabilities Herschel can offer for studying the
resolved dust distribution in galaxies.
![]() |
Figure 3:
Total gas mass ( |
Open with DEXTER |
SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). Thanks to Tom Hughes for providing the recalibrated metallicities. The SDSS jpg was taken from http://www.sdss.org using the Finding Chart tool.
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Footnotes
- ... galaxies
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA
- ... (ICC)
- See ``The SPIRE Analogue Signal Chain and Photometer Detector Data Processing Pipeline'' (Griffin 2009) for a more detailed description of the pipeline and a list of the individual modules.
- ... files
- Aprt from the BsmPos file, for which we use an updated version that should improve the absolute astrometry.
- ...
respectively
- See http://herschel.esac.esa.int/SDP_wkshops/presentations/IR/3_Griffin_SPIRE_SDP2009.pdf.
- ... IRAF's
- Image Reduction and Analysis Facility (IRAF) http://iraf.noao.edu/
- ...
)
- We use a radially constant X-factor as given in Kuno et al. (2007).
All Figures
![]() |
Figure 1:
The matched SDSS, MIPS 70 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Left: Radial surface brightness profiles for M 99 ( top) and
M 100 ( bottom) obtained from the smoothed and matched maps
(see text). The profiles are from the bottom up: MIPS 24, 70, 160 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Total gas mass ( |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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