Thursday June 7th, 2012

The GALEX view of the Herschel Reference Survey – Ultraviolet structural properties of nearby galaxies

Authors: L. Cortese, S. Boissier, A. Boselli, G. J. Bendo, V. Buat, J. I. Davies, S. Eales, S. Heinis, K. G. Isaak, S. C. Madden
(Submitted on 6 Jun 2012)

Abstract: We present GALEX far-ultraviolet (FUV) and near-ultraviolet (NUV) as well as SDSS g, r, i photometry and structural parameters for the Herschel Reference Survey, a magnitude-, volume-limited sample of nearby galaxies in different environments. We use this unique dataset to investigate the ultraviolet (UV) structural scaling relations of nearby galaxies and to determine how the properties of the UV disk vary with atomic hydrogen content and environment. We find a clear change of slope in the stellar mass vs. effective surface brightness relation when moving from the optical to the UV, with more massive galaxies having brighter optical but fainter UV surface brightnesses than smaller systems. A similar change of slope is also seen in the radius vs. surface brightness relation. By comparing our observations with the predictions of a simple multi-zone chemical model of galaxy evolution, we show that these findings are a natural consequence of a much more efficient inside-out growth of the stellar disk in massive galaxies. We confirm that isophotal radii are always a better proxy for the size of the stellar/star-forming disk than effective quantities and we show that the extent of the UV disk (normalized to the optical size) is strongly correlated to the integrated HI gas fraction. This relation still holds even when cluster spirals are considered, with HI-deficient systems having less extended star-forming disks than HI-normal galaxies. Interestingly, the star formation in the inner part of HI-deficient galaxies is significantly less affected by the removal of the atomic hydrogen, as expected in a simple ram-pressure stripping scenario. These results suggest that it is the amount of HI that regulates the growth of the star-forming disk in the outskirts of galaxies.

Comments: 32 pages, 8 figures, 5 tables. Accepted for publication in A&A. Data are available at this http URL
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1206.1130v1 [astro-ph.CO]

Herschel / HIFI observations of CO, H2O and NH3 in Mon R2

Authors: P. Pilleri, A. Fuente, J. Cernicharo, V. Ossenkopf, O. Berné, M. Gerin, J. Pety, J.R. Goicoechea, J.R. Rizzo, J. Montillaud, M. González-García, C. Joblin, J. Le Bourlot, F. Le Petit, C. Kramer
(Submitted on 6 Jun 2012)

Abstract: Context. Mon R2 is the only ultracompact HII region (UCHII) where the associated photon-dominated region (PDR) can be resolved with Herschel. Due to its brightness and proximity, it is the best source to investigate the chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to estimate the abundance of H2O and NH3 in this region and investigate their origin. Methods. We present new observations obtained with HIFI and the IRAM-30m telescope. Using a large velocity gradient approach, we model the line intensities and derive an average abundance of H2O and NH3 across the region. Finally, we model the line profiles with a non-local radiative transfer model and compare these results with the abundance predicted by the Meudon PDR code. Results. The variations of the line profiles and intensities indicate complex geometrical and kinematical patterns. The H2O lines present a strong absorption at the ambient velocity and emission in high velocity wings towards the HII region. The spatial distribution of the o-H2^18O line shows that the its emission arises in the PDR surrounding the HII region. By modeling the o-H2^18O emission we derive a mean abundance of o-H2O of ~10^-8 relative to H2. The ortho-H2O abundance is however larger, ~1×10^-7, in the high velocity wings. Possible explanations for this larger abundance include an expanding hot PDR and/or an outflow. Ammonia seems to be present only in the envelope with an average abundance of ~2×10^-9 relative to H2. Conclusions. The Meudon PDR code can account for the measured water abundance in the high velocity gas as long as we assume that it originates from a <1 mag hot expanding layer of the PDR, i.e. that the outflow has only a minor contribution to this emission. To explain the abundances in the rest of the cloud the molecular freeze out and grain surface chemistry would need to be included.

Comments: 12 pages, 7 figures, 3 tables. Accepted for publication in A&A
Subjects: Galaxy Astrophysics (astro-ph.GA)
Cite as: arXiv:1206.1249v1 [astro-ph.GA]
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