Abstract We present ISOCAM images of the Andromeda galaxy center. They display an elliptical stellar bulge and a more complexe dust distribution. The spatial dust distribution follows a mini-spiral structure identical to the distribution of absorption in optical wavelengths. A discussion on the nature of the dust from CVF observations is also presented. The central part and the molecular ring of M31 are excellent targets for FIRST

A.-L Melchior

J Lequeux

M Hanus

M.-C Willaime

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ISOCAM OBSERVATIONS OF M31
M.-C. Willaime, J. Lequeux, A.-L. Melchior, and M. Hanus
DEMIRM, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, FRANCE.
Abstract
We present ISOCAM images of the Andromeda galaxy
center. They display an elliptical stellar bulge and a more
complexe dust distribution. The spatial dust distribution
follows a mini-spiral structure identical to the distribu-
tion of absorption in optical wavelengths. A discussion on
the nature of the dust from CVF observations is also pre-
sented. The central part and the molecular ring of M31
are excellent targets for FIRST.
Key words: Galaxies: individual: M31 - Galaxies: ISM
1. Introduction
Star formation in the Andromeda galaxy M31 (Hubble
type: Sb ; inclinaison = 77◦; position angle = 38◦; 1′=
200 pc) presents many differences from that in our Galaxy.
M31 total star formation rate is only 1/10 of that in the
Galaxy in spite of a higher total mass (Xu & Helou 1996)
and the ratio of the far UV flux to the visible flux is quite
smaller than in our Galaxy. The spatial distribution is also
quite peculiar : star formation is only active in a ring of 10
kpc in radius, the molecular ring. The metallicity in M31
is roughly similar to that in our Galaxy.
From ISOPHOT observations (Haas et al. 1998), the
far-IR luminosity of M31 is only 20% of that of our Galaxy.
The bulk of the dust is cold with a temperature of 16K,
mostly located in the molecular ring and in two central
areas. Haas et al. (1998) has found a dust/gas ratio of
1/130 (1/170 in our Galaxy).
Mid-IR data from ISO give the best unbiased view of
the distribution of dust in the bulge of M31. First, we will
report conclusions on the dust properties from the CVF
observations. Then we will present images from ISOCAM.
After removal of the stellar bulge contribution, we will
discuss the spatial dust distribution and its consequences.
2. Nature and properties of the dust
Spectral data have been obtained with a cicular variable
filter (CVF mode) on ISOCAM (covered wavelength : 5.5
to 16.5 µm). For more details , see Cesarsky et al. 1998
and Pagani et al. 1999.
The spectrum of the central 3 arcminutes (as well as
spectra in the molecular ring) is characterized by a strong
broad band at 11.3 µm while other PAHs are absent or
faint. Such spectra differ completely from usual interstellar
dust spectra.
Neutral PAHs are known to emit strongly in the 3.3
and 11.3 µm bands but faintly in the 6.2 7.7 and 8.6 µm
ones (Allamandola et al. 1999) and are possible candi-
dates for M31 dust. However they also emit a strong band
at 13.5 µm which is not seen in M31. However neutral
PAHs do not absorb photons at wavelengths longer than
5000Å, and we do not understand how they could be ex-
cited so efficiently in M31 given its UV flux (5% or less of
the local galactic value see Cesarsky et al. 1998).
SiC emits a band near 11.3 µm and might be a bet-
ter candidate. SiC emission is seen in carbon rich AGB
stars and SiC coming from such stars is present in mete-
orites, so that this material should exist in the interstellar
medium. Unfortunately the position and the shape of the
emission band when emitted transiently by nanometer size
SiC grains are not known (Mutschke et al. 1999).
3. ISOCAM observations
The observations have been obtained with the 32 x 32 ele-
ments mid-infrared camera ISOCAM on board of the ISO
satellite (see Cesarsky et al. 1996, for a complete descrip-
tion).
The central region was mapped in 4 broad filters : LW1
centered at 4.5 µm, LW2 at 6.75 µm, LW3 at 15 µm and
LW6 at 7.7 µm. The emission in the LW1 filter is essen-
tially due to the red giants of the bulge. LW6 includes the
7.7 µm PAHs emission and has a shorter bandwith than
LW2.
For each filter, several 2.1 s exposures (around 12) have
been taken at different positions on the sky so that it is
possible to rebuilt a larger field image of the object (raster
mode).
For the data reduction we used CIA software (Starck
et al. 1999) . Some square patterns still remain at the end
of reduction and this is partly due to the fact that the
data reduction are not yet completely finalized.
Proc. Symposium ‘The Promise of the Herschel Space Observatory’ 12–15 December 2000, Toledo, Spain
ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris
520 M.-C. Willaime et al.
Figure 1. Image in LW6 of M31 central part. The elliptical con-
tribution of the stellar bulge is clearly detected. The pixel size is
6′′and the total field is about 21′. Coordinates are J2000. From
Willaime et al., in preparation.
4. Spatial distribution of the dust
The LW1 image corresponds to the stellar emission from
the bulge. It presents a very smooth regular elliptical shape
that can be recognized also in the other filters (Fig. 1).
Since the image is very noisy, we decided to build a bulge
model by fitting ellipses to the LW1 isophotes. This model
was assumed to represent the spatial distribution of the
stellar emission in all the mid-infrared wavelengths (with
only a change in flux scale).
After subtraction of the stellar bulge, the residuals in
LW6 image show the distribution of insterstellar matter
at 7.7 µm. It displays an excentered elliptical ring with
a diameter of around 1.5 kpc. Moreover, it seems that a
straight structure connects the location of the bulge center
and the ring (Fig. 2). Two or three faint spiral arms seem
to begin at two opposite side of the ring. All the structure
resemble to a mini-spiral in the center of the galaxy. The
emission at the image corners comes from the molecular
ring at 10 kpc. All these structures are also present in LW2
and LW3 images.
5. Comparison with other observations
5.1. Optical extinction
The extinction maps are based either on visual examina-
tion of dark lanes projected against the bulge light (Hodge
1992) or on multicolor surface photometry (Melchior et al.
2000). In both cases, they are biased against detection of
dust located on the far side of the bulge. However, Hodge
(1992) underlined the fact that the absorbing clouds in
the center of M31 form a small spiral. Direct comparison
Figure 2. Result of the subtraction of the precedent figure with
the bulge model (see text for details). The residual structures
follow the spatial dust distribution. The nucleus location corre-
spond to the black dot at the center of the image.
with the extinction map of Melchior et al. (2000) gives a
qualitatively excellent correspondence (Fig.3).
Figure 3. LW6 contours superimposed to the extinction map
from Melchior et al. (2000)
521
5.2. Direct observations
The comparison with direct observations of the gas is quite
difficult since there is little HI in the bulge and the existing
CO data are sparse and not sensitive enough to display
all the molecular emission (except in the observations of
Melchior et al. 2000 but it concerns only few locations).
The brightest region in the ISOPHOT image (at 175
µm) is a supergiant cold molecular cloud with no sign of
star formation (Schmidtobreick et al. 2000). It corresponds
to a bright structure in LW6 located at the beginning of
one of the ”spiral arm” (Fig. 4).
Figure 4. Contours of the emissiom at 175 µm superimposed
to LW6 map in greyscale. The FIR image was taken through
the C200-160 ISOPHOT filter and has an angular resolution
of 1.3′(Haas et al. 1998).
5.3. Kinematical data
Kinematical studies of the center have been made using ra-
dial velocities. Jacoby et al. 1985 interpreted their Hα ob-
servations as a bar + ring structure. Boulesteix et al. 1987
find an overall rotation pattern with low velocities (around
100 km/s). They concluded that most of the gas is rotat-
ing in a strongly tilted plane. The emission is very faint,
but the structure observed in Hα corresponds roughly to
what we see in the LW6 filter (size and orientation of the
”ring”, straight structure at the same location, etc.).
The CO radial velocities from Melchior et al. (2000)
seem to correspond to the Hα velocities, suggesting that
the molecular clouds lie in the same plane as the ionized
gas.
6. Conclusion
Most of the dust in the Andromeda galaxy is located in
the center and the 10 kpc molecular ring. It corresponds
to the star formation regions in the molecular ring, the
origin of dust in the center is more puzzling. But the pres-
ence of a mini-spiral in the center of M31 will have direct
consequences in dynamical studies of the galaxy, and more
precisely on the existence of a bar.
FIRST is the best instrument to perform a complete
study of the interstellar matter in our nearest neighbour.
For example, a [CII] survey of the center with HIFI should
complement the Hα observations. Far-IR observations at
higher resolution than with ISOPHOT will be of high in-
terest.
References
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ISOCAM OBSERVATIONS OF M31

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ISOCAM OBSERVATIONS OF M31

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