Monday 12/10/12

Title: The excitation of near-infrared H2 emission in NGC 253
Authors: Rosenberg, M. J. F.; van der Werf, P. P.; Israel, F. P.
Publication: eprint arXiv:1212.1567
Publication Date: 12/2012
Origin: ARXIV
Keywords: Astrophysics – Cosmology and Extragalactic Astrophysics, Astrophysics – Galaxy Astrophysics
Bibliographic Code: 2012arXiv1212.1567R


Because of its large angular size and proximity to the Milky Way, NGC 253, an archetypal starburst galaxy, provides an excellent laboratory to study the intricacies of this intense episode of star formation. We aim to characterize the excitation mechanisms driving the emission in NGC 253. Specifically we aim to distinguish between shock excitation and UV excitation as the dominant driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line tracers. Using SINFONI observations, we create linemaps of Br\gamma, [FeII]_{1.64}, and all detected H_2 transitions. By using symmetry arguments of the gas and stellar gas velocity field, we find a kinematic center in agreement with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H_2 transitions can be used as a diagnostic to discriminate between shock and fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as several other H_2 line ratios and the morphological comparison between H_2 and Br\gamma and [FeII], we find that excitation from UV photons is the dominant excitation mechanisms throughout NGC 253. We employ a diagnostic energy level diagram to quantitatively differentiate between mechanisms. We compare the observed energy level diagrams to PDR and shock models and find that in most regions and over the galaxy as a whole, fluorescent excitation is the dominant mechanism exciting the H_2 gas. We also place an upper limit of the percentage of shock excited H_2 at 29%. We find that UV radiation is the dominant excitation mechanism for the H_2 emission. The H_2 emission does not correlate well with Br\gamma but closely traces the PAH emission, showing that not only is H_2 fluorescently excited, but it is predominately excited by slightly lower mass stars than O stars which excite Br\gamma, such as B stars.

Title: The global gas and dust budget of the Small Magellanic Cloud
Authors: Matsuura, M.; Woods, Paul M.; Owen, P. J.
Publication: eprint arXiv:1212.1468
Publication Date: 12/2012
Origin: ARXIV
Keywords: Astrophysics – Galaxy Astrophysics, Astrophysics – Cosmology and Extragalactic Astrophysics
Comment: MNRAS accepted
Bibliographic Code: 2012arXiv1212.1468M


In order to understand the evolution of the interstellar medium (ISM) of a galaxy, we have analysed the gas and dust budget of the Small Magellanic Cloud (SMC). Using the Spitzer Space Telescope, we measured the integrated gas mass-loss rate across asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the SMC, and obtained a rate of 1.4×10^-3 Msun yr-1. This is much smaller than the estimated gas ejection rate from type II supernovae (SNe) (2-4×10^-2 Msun yr-1). The SMC underwent a an increase in starformation rate in the last 12 Myrs, and consequently the galaxy has a relatively high SN rate at present. Thus, SNe are more important gas sources than AGB stars in the SMC. The total gas input from stellar sources into the ISM is 2-4×10^-2 Msun yr-1. This is slightly smaller than the ISM gas consumed by starformation (~8×10^-2 Msun yr-1). Starformation in the SMC relies on a gas reservoir in the ISM, but eventually the starformation rate will decline in this galaxy, unless gas infalls into the ISM from an external source. The dust injection rate from AGB and RSG candidates is 1×10^-5 Msun yr-1. Dust injection from SNe is in the range of 0.2–11×10^-4 Msun yr-1, although the SN contribution is rather uncertain. Stellar sources could be important for ISM dust (3×10^5 Msun yr-1) in the SMC, if the dust lifetime is about 1.4 Gyrs. We found that the presence of poly-aromatic hydrocarbons (PAHs) in the ISM cannot be explained entirely by carbon-rich AGB stars. Carbon-rich AGB stars could inject only 7×10^-9 Msun yr-1 of PAHs at most, which could contribute up to 100 Msun of PAHs in the lifetime of a PAH. The estimated PAH mass of 1800 Msun in the SMC can not be explained. Additional PAH sources, or ISM reprocessing should be needed.

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