Annual Report 2011 Max Planck Institute for Astronomy

2.3 kpc
Fig. III.4.1: A herschel far-infrared image of the Andromeda
galaxy showing 70 µm (blue), 100 µm (green), and 250 µm
(red). The bluer colors indicate hotter dust.
CO heracles (Leroy et al., 2009) large surveys. Along
with ancillary Galex-, optical- and radio data these surveys
provide resolved ultraviolet to radio imaging of
50 nearby galaxies, and the ISM gas traced via HI and
CO line emission and spectral maps of other optical to
far-IR emission lines. These data provide the best opportunity
to link resolved galaxy ISM and stellar properties
with the global galaxy type and environment. In the following
we highlight some of our recent findings from
these projects.
Dust heating: Andromeda’s Hot Dust
The dust in the ISM of galaxies is strongly associated
with the gas, with the total dust column being a function
of the gas column. However, the IR emission from dust
in galaxies is also dependent upon the dust temperature,
which is determined by the interstellar radiation field.
As dust opacity is strongly biased to the UV, massive
stars tend to dominate the heating of dust. Due to this,
IR emission, either alone or in association with another
tracer such as Hα or UV emission, is used as a measure
of the current star formation rate (SFR).
The center of Andromeda presents a difficulty in this
standard paradigm of dust IR emission. As seen in Figure
III.4.1, the center is particularly bright in IR, with the blue
colors indicating warm (30 K) dust. Yet high resolution
imaging (e.g. from hubble; Rosenfeld et al., in prep.)
reveal no massive stars and no ongoing star formation.
However, the heating mechanism of the dust is revealed by
the steep radial profile in dust temperatures at the center,
determined from simple fits to the herschel far-IR bands
in each pixel with emissivity modified-blackbodies, shown
in Figure III.4.2 (from Groves et al., 2012). This dust temperature
distribution is overlaid with a scaled version of
III.4 The Interstellar Medium of Nearby Galaxies 65
the expected dust temperature from heating by the diffuse
radiation field arising from the old stellar population of the
bulge of M 31, with the profiles showing a close match.
This match indicates that it is the bulge stars that are
heating the dust, with the high density of stars in this
region providing a sufficiently strong radiation field to
heat the dust to warm temperatures. This is significant
for two reasons; it demonstrates that warm dust emission
is not always associated with young, massive stars, and
that, given the very red optical spectra of the bulge, optical
light, not UV, can dominate the heating of the dust.
Given the “early-type” nature of Andromeda’s bulge,
such heating may also be occurring in other early-type
galaxies that also show warm dust emission.
Fig. III.4.2: The distribution of dust temperatures within the inner
2 kpc (530) of M 31. The colors show the number density
of the 23 pc pixels dust temperature (T d ) and distance from
the center, as labeled by the color bar. Overlaid is a curve
showing a scaled version of the expected dust heating given the
bulge radiation field from old stars.
T dust [K]
40
35
30
25
20
aT d, bulge
15
0 0.5
0 26
1
Radius [kpc]
#pixels
N
E
53 80 107 134
1.5 2
Credit: Groves et al. (2012)
Credit: Groves et al. (2012)