Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L67 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014613 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
PACS and SPIRE photometer maps of
M 33: First results of the HERschel M 33
Extended Survey (HERM33ES)![[*]](/icons/foot_motif.png)
C. Kramer1 - C. Buchbender1 - E. M. Xilouris2 - M. Boquien3 - J. Braine4 - D. Calzetti3 - S. Lord5 - B. Mookerjea6 - G. Quintana-Lacaci1 - M. Relaño7 - G. Stacey8 - F. S. Tabatabaei9 - S. Verley10 - S. Aalto11 - S. Akras2 - M. Albrecht12 - S. Anderl12 - R. Beck9 - F. Bertoldi12 - F. Combes13 - M. Dumke14 - S. Garcia-Burillo15 - M. Gonzalez1 - P. Gratier4 - R. Güsten9 - C. Henkel9 - F.P. Israel16 - B. Koribalski17 - A. Lundgren14 - J. Martin-Pintado18 - M. Röllig19 - E. Rosolowsky20 - K. F. Schuster21 - K. Sheth22 - A. Sievers1 - J. Stutzki19 - R. P. J. Tilanus23 - F. van der Tak24 - P. van der Werf16 - M. C. Wiedner13
1 - Instituto Radioastronomía Milimétrica, Av. Divina Pastora 7, Nucleo
Central, 18012 Granada, Spain
2 - Institute of Astronomy and Astrophysics, National Observatory of
Athens, P. Penteli, 15236 Athens, Greece
3 - Department of Astronomy, University of Massachusetts, Amherst, MA
01003, USA
4 - Laboratoire d'Astrophysique de Bordeaux, Université Bordeaux 1,
Observatoire de Bordeaux, OASU, UMR 5804, CNRS/INSU, BP 89, Floirac
33270
5 - IPAC, MS 100-22 California Institute of Technology, Pasadena, CA
91125, USA
6 - Department of Astronomy & Astrophysics, Tata Institute of
Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
7 - Institute of Astronomy, University of Cambridge, Madingley Road,
Cambridge CB3 0HA, England
8 - Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
9 - Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
10 - Dept. Física Teórica y del Cosmos, Universidad de
Granada, Spain
11 - Department of Radio and Space Science, Onsala Observatory,
Chalmers University of Technology, 43992 Onsala, Sweden
12 - Argelander Institut für Astronomie. Auf dem Hügel 71, 53121 Bonn,
Germany
13 - Observatoire de Paris, LERMA, CNRS, 61 Av. de l'Observatoire,
75014 Paris, France
14 - ESO, Casilla 19001, Santiago 19, Chile
15 - Observatorio Astronómico Nacional (OAN) - Observatorio de Madrid,
Alfonso XII 3, 28014 Madrid, Spain
16 - Leiden Observatory, Leiden University, PO Box 9513, 2300 RA
Leiden, The Netherlands
17 - ATNF, CSIRO, PO Box 76, Epping, NSW 1710, Australia
18 - Centro de Astrobiología (INTA-CSIC), Ctra de Torrejón a Ajalvir,
km 4, 28850 Torrejón de Ardoz, Madrid, Spain
19 - KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher
Straße 77, 50937 Köln, Germany
20 - University of British Columbia Okanagan, 3333 University Way,
Kelowna, BC V1V 1V7, Canada
21 - IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
22 - California Institute of Technology, MC 105-24, 1200 East
California Boulevard, Pasadena, CA 91125, USA
23 - JAC, 660 North A'ohoku Place, University Park, Hilo, HI 96720, USA
24 - SRON Netherlands Institute for Space Research, Landleven 12, 9747
AD Groningen, The Netherlands
Received 31 March 2010 / Accepted 3 May 2010
Abstract
Context. Within the framework of the HERM33ES key Aims.
We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 m
wavelength, to study the variation of the spectral energy distributions
(SEDs) with galacto-centric distance.
Methods. Detailed SED modeling is performed using
azimuthally averaged fluxes in elliptical rings of 2 kpc
width, out to 8 kpc galacto-centric distance. Simple
isothermal and two-component grey body models, with fixed dust
emissivity index, are fitted to the SEDs between 24 m and
500
m
using also MIPS/Spitzer data, to derive
first estimates of the dust physical conditions.
Results. The far-infrared and submillimeter maps
reveal the branched, knotted spiral structure of M 33. An
underlying diffuse disk is seen in all SPIRE maps (250-500 m). Two
component fits to the SEDs agree better than isothermal models with the
observed, total and radially averaged flux densities. The two component
model, with
fixed at 1.5, best fits the global and the radial SEDs. The cold dust
component clearly dominates; the relative mass of the warm component is
less than 0.3% for all the fits. The temperature of the warm component
is not well constrained and is found to be about 60 K
10 K.
The temperature of the cold component drops significantly from
24 K
in the inner 2 kpc radius to 13 K beyond
6 kpc radial distance, for the best fitting model. The
gas-to-dust ratio for
,
averaged over the galaxy, is higher than the solar value by a factor of
1.5 and is roughly in agreement with the subsolar metallicity of
M 33.
Key words: galaxies: individual: M 33 - galaxies: evolution - Local Group - galaxies: ISM - galaxies: evolution - dust, extinction
1 Introduction
In the local universe, most of the observable matter is contained in stellar objects that shape the morphology and dynamics of their ``parent'' galaxy. In view of the dominance of stellar mass, a better understanding of star formation and its consequences is mandatory. There exists a large number of high spatial resolution studies related to individual star forming regions of the Milky Way, as well as of low linear resolution studies of external galaxies. For a comprehensive view onto the physical and chemical processes driving star formation and galactic evolution it is, however, essential to combine local conditions affecting individual star formation with properties only becoming apparent on global scales.
At a distance of 840 kpc (Freedman et al. 1991), M 33 is the only nearby, gas rich disk galaxy that allows a coherent survey at high spatial resolution. It does not suffer from any distance ambiguity, as studies of the Milky Way do, and it is not as inclined as the Andromeda galaxy. M 33 is a regular, relatively unperturbed disk galaxy, as opposed to the nearer Magellanic Clouds, which are highly disturbed irregular dwarf galaxies.
M 33 is among the best studied galaxies; it has been observed extensively at radio, millimeter, far-infrared (FIR), optical, and X-ray wavelengths, ensuring a readily accessible multi-wavelength database. These data trace the various phases of the interstellar medium (ISM), the hot and diffuse, the warm and atomic, as well as the cold, dense, star forming phases, in addition to the stellar component. However, submillimeter and far-infrared data at high angular and spectral resolutions have been missing so far.
In the framework of the open time key program ``HERschel
M 33
Extended Survey (HERM33ES)'', we use all three
instruments
onboard the ESA Herschel Space
Observatory (Pilbratt
et al. 2010) to
study the dusty and gaseous ISM in M 33.
One focus of HERM33ES is on maps of the FIR
continuum observed
with PACS (Poglitsch
et al. 2010) and SPIRE (Griffin
et al. 2010),
covering the entire galaxy. A second focus lies on observing
diagnostic FIR and submillimeter cooling lines [C II],
[O I], [N II],
and
H2O, toward a
strip along the major axis with PACS
and HIFI (de Graauw
et al. 2010).
In this first HERM33ES paper, we use
continuum maps covering the
full extent of M 33, at 100, 160, 250, 350, and m. These
data
are an improvement over previous data sets of M 33, obtained
with ISO
and Spitzer (Hippelein et al. 2003;
Hinz
et al. 2004; Tabatabaei et al. 2007),
in
terms of wavelength coverage and angular resolution.
The total bolometric luminosity of normal galaxies is only
about a
factor of 2 larger than the total IR continuum emission
(Hauser & Dwek 2001),
which in turn accounts for more than
98% of the
emission of the ISM (dust+gas)
(e.g. Malhotra
et al. 2001; Dale et al. 2001).
Massive star formation heats the dust mainly via its far-ultraviolet
(FUV) photons and the absorbed energy is then reradiated in the IR.
FIR continuum fluxes are therefore often used as a measure of the
interstellar radiation field (ISRF) (e.g. Kramer
et al. 2005) and the
star formation rate (SFR) (e.g. Schuster
et al. 2007). However, a
number of authors have suggested that half of the FIR emission or more
is due to dust heated by a diffuse ISRF, and not directly linked to
massive star formation (Verley et al. 2009; Israel
et al. 1996).
Another disputed topic is the evidence for a massive, cold
dust
component in galaxies. The SCUBA Local Universe Galaxy Survey
(Dunne & Eales 2001)
identified a cold dust component at an average
temperature of 21 K. A number of studies of the millimeter
continuum
emission of galaxies found indications for even lower temperatures
(Liu
et al. 2010; Misiriotis et al. 2006;
Weiß
et al. 2008).
In order to estimate the amount of dust at temperatures below about
20 K, and to improve our understanding of the physical
conditions of
the big grains, well calibrated observations longward of
150
m wavelength
are needed.
2 Observations
M 33 was mapped with PACS & SPIRE in parallel mode in
two orthogonal
directions, in 6.3 h on January 7, 2010. Observations
were
executed with slow scan speed of 20''/s, covering a region of
about .
Data were taken simultaneously with the PACS
green and red channel, centered on 100 and 160
m. SPIRE
observations were taken simultaneously at 250, 350, and 500
m.
The PACS and the SPIRE data sets were both reduced using the Herschel
interactive processing environment (HIPE) 2.0,
with in-house reduction scripts based on the two standard reduction
pipelines.
2.1 PACS data
The maps are produced with ``photproject'', the default map maker of
the PACS data processing pipeline, and a two-step masking technique.
First we generate a ``naive'' map, i.e. not properly taking into
account partial pixel overlaps and geometric deformation of the
bolometer matrix, and build a mask considering that all pixels above a
given threshold do not belong to the sky. Then we use this mask to run
the high-pass filter (HPF) taking into account this map. The mask
helps to preserve the diffuse component to some extent. With new HIPE
tools becoming available, we will try improving data processing to
fully recover the diffuse emission in the PACS maps (cf.
Fig. 1).
The final map is built using the filtered,
deglitched frames. They have a pixel size of 3.2
at 100
m
and 6.4
at 160
m.
The spatial resolutions of the PACS
data are
at 100
m
and
at
160
m.
The pipeline processed data were divided by 1.29 in the
red band and 1.09 in the green one, as this correction is not yet
implemented in HIPE 2.0.
The rms noise
levels of the PACS maps are 2.6 mJy pix-2
at 100
m
and
6.9 mJy pix-2 at 160
m. The
background of the PACS maps of
M 33 shows perpendicular stripes in each scanning direction
due to
1/f noise.
2.2 SPIRE data
A baseline fitting algorithm (Bendo
et al. 2010) was applied to every
scan of the maps. Next, a ``naive'' mapping projection was applied to
the data and maps with pixel size of 6'', 10'', and 14'' were
created for the 250, 350, and 500 m data, respectively.
Calibration correction factors of 1.02, 1.05, and 0.94 were applied to
the 250, 350, and 500
m maps, as this is not yet implemented in
HIPE 2.0. The spatial resolutions are
,
,
and
at 250, 350, and
500
m,
respectively. The calibration accuracy is
15%
. The rms noise levels of
the SPIRE maps of M 33 are 14.1, 9.2, and 8 mJy/beam,
at 250, 350,
500
m.
![]() |
Figure 2:
Ratio map of the 250 |
3 Results
3.1 Maps
Figure 1
shows a composite image of the 160 m,
250
m,
and 500
m
PACS and SPIRE maps. All data sets show
the flocculent and knotted spiral arm structure, extending slightly
beyond 4 kpc radial distance. The PACS 160
m map
provides the
most detailed view, thanks to its unprecedented linear resolution of
50 pc, allowing to resolve individual giant molecular clouds
(GMCs)
over the entire disk of M 33. A large number of distinct
sources
delineates the spiral arms. The properties of these sources are
studied by Verley et al.
(2010) and Boquien
et al. (2010). The SPIRE data
show a faint, diffuse disk, extending out to
7 kpc. Outside of
8 kpc, both maps show some weak emission.
Galactic cirrus is evident only in the outermost part of the
galaxy
beyond 6 kpc radial distance, showing an average contamination
of the
order of 2% which can go up to 8% at the very faint levels at
500 m.
This is still below the 15% calibration error, which is
the dominant part of the uncertainty. We did not correct the
M 33 data
for Galactic Cirrus emission.
The
ratio of flux densities
(Fig. 2)
drops from about 6 in the inner spiral arms,
to
4 at
4 kpc
radius, continuing to less than
3 at
more than 6 kpc radial distance. This drop is also seen in the
radially averaged spectral energy distributions (Fig. 3,
Table 1).
In addition, the inner spiral arms and a
couple of prominent H II regions (cf.
Fig. 1),
out to about 5 kpc radius, show an enhanced ratio of
6
relative to the inter-arm ISM, exhibiting a ratio of typically
4. This
shows that dust is mainly heated by the young massive
stars rather than the general interstellar radiation field in
M 33.
This is in agreement with a multi-scale study of MIPS data
(Tabatabaei et al.
2007), where the 160
m emission was found
to be well correlated with H
emission.
![]() |
Figure 3:
Spectral energy distributions (SEDs) of M 33 at wavelengths
between 24 |
Table 1: Results of fits of one and two emission components to the measured spectral energy distributions (SEDs) of the MIPS, PACS, SPIRE data of M 33 shown in Fig. 3.
3.2 Spectral energy distributions (SEDs)
Figure 3
shows the total flux densities of M 33 and
radially averaged SEDs. The SEDs at different annuli were created by
smoothing all data to a common resolution of 40'', and averaging the
observed flux densities in radial zones of 2 kpc width:
with ri=0,
2, 4, 6 kpc (cf.
Fig. 2).
The Herschel data agree in general well
with
the data from the literature. The MIPS data at 160
m agree
within 20% with the corresponding PACS data, for all radial zones.
The 100
m
PACS flux density, measured in the outermost annulus, is
far below the expected value, indicating that extended, diffuse
emission is at present lost by the data processing. We do not use
these data for the fits.
Figures 3c,d shows the drop of emission by almost two orders of magnitude between the center and the outskirts at 8 kpc radial distance. One striking feature of the radially averaged SEDs is the change of the 160/250 PACS/SPIRE flux density ratio (color), which drops systematically with radial distance, from 1.7 in the inner zone, to 0.5 in the outer zone. At the same time, the slope of the SPIRE data turns shallower with distance, as already seen in Fig. 2.
We fit simple isothermal and two-component grey body models to
the
data. Each component is described by ,
assuming optically thin emission, with
the flux
,
the Planck function
,
the opacity
,
the dust mass
,
the distance D, and the dust absorption
coefficient
cm2 g-1
(Kruegel
& Siebenmorgen 1994; Kruegel 2003),
is the dust
emissivity index. The fit minimizes the function
using
the Levenberg-Marquardt algorithm (Bevington
& Robinson 1992), with
the assumed calibration error
.
The fits
are conducted at 7 wavelengths using the SPIRE, PACS, and the MIPS
data at 70 and 24
m.
The 24
m
data helps in constraining
the warm component, though its emission partly stems from
stochastically heated small grains, not only from grains in thermal
equilibrium. To maintain at least two degrees of freedom
(Bevington & Robinson 1992)
in the 2-component fit, we kept
fixed to
values between 1 and 2. These values are typically found in
models
and observations of interstellar dust (see literature compiled
by Dunne & Eales 2001).
The fits of isothermal models do not reproduce the data well,
the
values of the reduced
are very high. To a large extent, this
is because of the 24
m
points, which clearly require a second,
warm dust component. Two-component grey body models result in a much
better agreement with the data. The best fitting model is the
two-component model with
.
The
values
are better than 0.2 for all annuli out to 6 kpc, and 1.8 for
the
outermost annulus. However, these values are only slightly better or
equal to the
values of the two other two-component
models.
We find higher temperatures of the cold component for lower
values of
,
rendering it difficult to determine both parameters at the same
time. This degeneracy between dust temperature and dust emissivity is
a common problem (e.g. Kramer
et al. 2003). Note, however, that the
total mass of the cold component, is rather well determined. For
-values
varying between 1 and 2, this mass only varies by
20%. The
masses in each annulus were determined by fitting the
observed SEDs. As the fitted temperatures are slightly different, the
sum over the four annuli does not exactly agree with the fitted total
cold dust mass in Table 1.
The cold dust component dominates the mass for the galaxy for
all
annuli. Though the warm component is needed to reproduce the data
shortwards of m, its
relative mass is less than 0.3%
for all cases. Therefore, its temperature is not well constrained. It
is found to be about 60 K
10 K. The temperature of the
cold
component is determined to an accuracy of about 3 K, as
estimated
from a Monte Carlo analysis using the observed data with the estimated
accuracies. It drops significantly from
24 K in the inner
2 kpc radius to 13 K beyond 6 kpc radial
distance, using the best
fitting model.
Table 1
also gives the total gas masses
of
the entire galaxy, and of the elliptical annuli. These are calculated
from H I and CO data presented in Gratier et al. (2010).
They assume a
constant CO-to-H2 conversion factor. But note
that this X
factor does not strongly affect the total gas masses, as the
H I is dominating. The gas-to-dust mass
ratio for the entire galaxy,
using the best fitting dust model with
,
is
200,
about a factor of 1.5 higher than the solar value of 137 (cf.
Table 2 in Draine
et al. 2007), and a factor of 2 higher than
recent dust
models for the Milky Way (Draine et al. 2007; Weingartner
& Draine 2001). A
factor of about 2 is expected, as the metallicity is about half solar
(Magrini et al. 2009).
The gas-to-dust ratio for
varies
between 200 and 120 in the different annuli. Within our errors this is
consistent with the shallow O/H abundance gradient found by
Magrini et al. (2009).
The gas-to-dust ratios found in M 33 are similar to the
typical values
found in nearby galaxies (e.g. Draine et al. 2007; Bendo
et al. 2010).
Braine et al. (2010)
combine the dust and gas data of M 33 to study the
gas-to-dust ratios in more detail and derive dust cross sections.
HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia. We would like to thank all those who helped us processing the PACS and SPIRE data. In particular we would like to acknowledge support from Pierre Royer, Bruno Altieri, Pat Morris, Bidushi Bhattacharya, Marc Sauvage, Michael Pohlen, Pierre Chanial, George Bendo. MR acknowledges the MC-IEF within the 7th European Community Framework Programme.
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Footnotes
- ... (HERM33ES)
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
- ... 2.0.
- PACS photometer - Prime and Parallel scan mode release note. V.1.2, 23 February 2010.
- ...%
- SPIRE Beam Model Release Note V0.1, SPIRE Scan-Map AOT and Data Products, Issue 2, 21-Oct.-2009.
All Tables
Table 1: Results of fits of one and two emission components to the measured spectral energy distributions (SEDs) of the MIPS, PACS, SPIRE data of M 33 shown in Fig. 3.
All Figures
![]() |
Figure 1:
A composite 500 |
In the text |
![]() |
Figure 2:
Ratio map of the 250 |
In the text |
![]() |
Figure 3:
Spectral energy distributions (SEDs) of M 33 at wavelengths
between 24 |
In the text |
Copyright ESO 2010
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