Tuesday, 2/5/13

Two papers today, the first (Horiuchi et al.) on how wrong SFR calibrations might be, based upon the new Geneva rotating models. This idea is somewhat tempered by the conclusion that such lower SFRs are in tension with the local Type II SN rate, so maybe we can’t ascribe so much extra UV and ionizing radiation to young stars as the rotating models do after all.

The second paper (Tacconi-Garman & Sturm) shows ground-based imaging of the 3.3 micron PAH feature in the central regions of nearby active radio galaxy, Cen A. The feature to continuum ratio does drop towards the AGN, but my impression is that they see the band emission all the way in. – Alison

Title: Effects of stellar rotation on star formation rates and comparison to core-collapse supernova rates
Authors: Horiuchi, Shunsaku; Beacom, John F.; Bothwell, Matt S.; Thompson, Todd A.
Bibliographic Code: 2013arXiv1302.0287H

Abstract

We investigate star formation rate (SFR) calibrations in light of recent developments in the modeling of stellar rotation. Using new non-rotating and rotating stellar tracks of Ekstrom et al. (2012), we study the integrated properties of synthetic stellar populations and find that the UV to SFR calibration for the rotating stellar population is 30% smaller than for the non-rotating stellar population, and 40% smaller for the Halpha to SFR calibration. These reductions translate to smaller SFR estimates made from observed UV and Halpha luminosities. Using the UV and Halpha fluxes of a sample of ~300 local galaxies, we derive a total (i.e., sky-coverage corrected) SFR within 11 Mpc of 120-170 Msun/yr and 80-130 Msun_yr for the non-rotating and rotating estimators, respectively. Independently, the number of core-collapse supernovae discovered in the same volume requires a total SFR of 270^{+110}_{-80} Msun/yr, suggesting a mild tension with the SFR estimates made with rotating calibrations. More generally, when compared with the estimated SFR, the local supernova discoveries strongly constrain any physical effects that might increase the energy output of massive stars, including, but not limited to, stellar rotation. The cosmic SFR and cosmic supernova rate data on the other hand show the opposite trend, with the cosmic SFR almost double that inferred from cosmic supernovae, constraining a significant decrease in the energy output of massive stars. Together, these lines of evidence suggest that the true SFR calibration factors cannot be too far from their canonical values.

Title: 3.3 {\mu}m PAH observations of the central kiloparsecs of Centaurus A
Authors: Tacconi-Garman, L. E.; Sturm, E.
Bibliographic Code: 2013arXiv1302.0767T

Abstract

Aims. The aim of this work is to further investigate the nature of PAH excitation and emission especially in the context of tracing star formation in a variety of extragalactic environments. Here we turn our attention to the energetic environment of the closest AGN in our sample, Centaurus A. Methods. Using ISAAC on the ESO VLT UT1 (Antu) we have made high spatial resolution 3.3 {\mu}m imaging observations of the central kiloparsec of CenA. These observations have been compared with star formation tracers in the near- and mid-infrared, as well as with mid-infrared tracers of nuclear activity. Results. The nucleus is not devoid of PAH emission, implying that the PAH particles are not destroyed in the nucleus as might be expected for such a harsh environment. However, we see the feature to continuum ratio decrease towards the AGN. As well, the 3.3 {\mu}m PAH feature emission generally traces the sites of star formation in Cen A, but in detail there are spatial offsets, consistent with an earlier study of the starburst galaxies NGC 253 and NGC 1808. However, the feature-to-continuum ratio does not drop at the positions of star formation as was previously seen in that earlier study. The cause for this difference remains uncertain. Finally, our data reveal possible evidence for a nearly face-on, circular or spiral, dust structure surrounding the nucleus.

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