Friday, June 15th

The heating of dust by old stellar populations in the Bulge of M31

Brent Groves, Oliver Krause, Karin Sandstrom, Anika Schmiedeke, Adam Leroy, Hendrik Linz, Maria Kapala, Hans-Walter Rix,Eva Schinnerer, Fatemeh Tabatabaei, Fabian Walter, Elisabete da Cunha
(Submitted on 13 Jun 2012)

We use new Herschel multi-band imaging of the Andromeda galaxy to analyze how dust heating occurs in the central regions of galaxy spheroids that are essentially devoid of young stars. We construct a dust temperature map of M31 through fitting modified blackbody SEDs to the Herschel data, and find that the temperature within 2 kpc rises strongly from the mean value in the disk of 17 pm 1K to \sim35K at the centre. UV to near-IR imaging of the central few kpc shows directly the absence of young stellar populations, delineates the radial profile of the stellar density, and demonstrates that even the near-UV dust extinction is optically thin in M31’s bulge. This allows the direct calculation of the stellar radiation heating in the bulge, U\ast(r), as a function of radius. The increasing temperature profile in the centre matches that expected from the stellar heating, i.e. that the dust heating and cooling rates track each other over nearly two orders of magnitude in U\ast. The modelled dust heating is in excess of the observed dust temperatures, suggesting that it is more than sufficient to explain the observed IR emission. Together with the wavelength dependent absorption cross section of the dust, this demonstrates directly that it is the optical, not UV, radiation that sets the heating rate. This analysis shows that neither young stellar populations nor stellar near-UV radiation are necessary to heat dust to warm temperatures in galaxy spheroids. Rather, it is the high densities of Gyr-old stellar populations that provide a sufficiently strong diffuse radiation field to heat the dust. To the extent which these results pertain to the tenuous dust found in the centres of early-type galaxies remains yet to be explored.

Comments: 11 pages. Accepted for publication in MNRAS
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1206.2925v1 [astro-ph.CO]

Spatially resolved stellar, dust and gas properties of the post-interacting Whirlpool Galaxy system

Erin Mentuch Cooper, Christine D. Wilson, Kelly Foyle, George Bendo, Jin Koda, Marten Baes, Médéric Boquien, Alessandro Boselli, Laure Ciesla, Asantha Cooray, Steve Eales, Maud Galametz, Vianney Lebouteiller, Tara Parkin, Hélène Roussel, Marc Sauvage, Luigi Spinoglio, Matthew W. L. Smith
(Submitted on 14 Jun 2012)

Using infrared imaging from the Herschel Space Observatory, observed as part of the VNGS, we investigate the spatially resolved dust properties of the interacting Whirlpool galaxy system (NGC 5194 and NGC 5195), on physical scales of 1 kpc. Spectral energy distribution modelling of the new infrared images in combination with archival optical, near- through mid-infrared images confirms that both galaxies underwent a burst of star formation ~370-480 Myr ago and provides spatially resolved maps of the stellar and dust mass surface densities. The resulting average dust-to-stellar mass ratios are comparable to other spiral and spheroidal galaxies studied with Herschel, with NGC 5194 at log M(dust)/M(star)= -2.5+/-0.2 and NGC 5195 at log M(dust)/M(star)= -3.5+/-0.3. The dust-to-stellar mass ratio is constant across NGC 5194 suggesting the stellar and dust components are coupled. In contrast, the mass ratio increases with radius in NGC 5195 with decreasing stellar mass density. Archival mass surface density maps of the neutral and molecular hydrogen gas are also folded into our analysis. The gas-to-dust mass ratio, 94+/-17, is relatively constant across NGC 5194. Somewhat surprisingly, we find the dust in NGC 5195 is heated by a strong interstellar radiation field, over 20 times that of the ISRF in the Milky Way, resulting in relatively high characteristic dust temperatures (~30 K). This post-starburst galaxy contains a substantial amount of low-density molecular gas and displays a gas-to-dust ratio (73+/-35) similar to spiral galaxies. It is unclear why the dust in NGC 5195 is heated to such high temperatures as there is no star formation in the galaxy and its active galactic nucleus is 5-10 times less luminous than the one in NGC 5194, which exhibits only a modest enhancement in the amplitude of its ISRF.

Comments: 26 pages, 24 figures, Accepted for publication in ApJ
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1206.2989v1 [astro-ph.CO]
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