8 March, 2013 06:11

[Tuesday, 03/05/13]

Two papers from Tuesday. The first works through what various star-formation proscriptions (such as a constant density threshold, a KS-type law, a Jeans mass criterion, etc.) actually should look like in simulations (Hopkins et al.). The second paper analyses the winds from two Hickson Compact Group galaxies (Vogt et al.). They obtained optical IFU data and argue that in one galaxy the starburst-driven wind is predominantly photoionized at its base, with more and more contribution from slow shocks as the wind gets further out.

Title: The Meaning and Consequences of Star Formation Criteria in Galaxy Models with Resolved Stellar Feedback
Authors: Hopkins, Philip F.; Narayanan, Desika; Murray, Norman
Bibliographic Code: 2013arXiv1303.0285H


We consider the effects of different star formation criteria on galactic scales, in high-resolution simulations with explicitly resolved GMCs and stellar feedback. We compare: (1) a self-gravity criterion (based on the local virial parameter and the assumption that self-gravitating gas collapses to high density in a free-fall time), (2) a fixed density threshold, (3) a molecular-gas law, (4) a temperature threshold, (5) a Jeans-instability requirement, (6) a criteria that cooling times be shorter than dynamical times, and (7) a convergent-flow criterion. We consider these both MW-like and high-density (starburst) galaxies. With feedback present, all models produce identical integrated star formation rates (SFRs), in agreement with the Kennicutt relation. Without feedback all produce orders-of-magnitude excessive SFRs. This is totally dependent on feedback and independent of the SF law. However, the spatial and density distribution of SF depend strongly on the SF criteria. Because cooling rates are generally fast and gas is turbulent, criteria (4)-(7) are ‘weak’ and spread SF uniformly over the disk (above densities n~0.01-0.1 cm^-3). A molecular criterion (3) localizes to higher densities, but still a wide range; for metallicity near solar, it is similar to a density threshold at n~1 cm^-3 (well below mean densities in the MW center or starbursts). Fixed density thresholds (2) can always select the highest densities, but must be adjusted for simulation resolution and galaxy properties; the same threshold that works in a MW-like simulation will select nearly all gas in a starburst. Binding criteria (1) tend to adaptively select the largest over-densities, independent of galaxy model or resolution, and automatically predict clustered SF. We argue that this SF model is most physically-motivated and presents significant numerical advantages in large-dynamic range simulations.

Title: Galaxy Interactions in Compact Groups I : The Galactic Winds of HCG16
Authors: Vogt, Frédéric P. A.; Dopita, Michael A.; Kewley, Lisa J.
Bibliographic Code: 2013arXiv1303.0290V


Using the WiFeS integral field spectrograph, we have undertaken a series of observations of star-forming galaxies in Compact Groups. In this first paper dedicated to the project, we present the analysis of the spiral galaxy NGC838, a member of the Hickson Compact Group 16, and of its galactic wind. Our observations reveal that the wind forms an asymmetric, bipolar, rotating structure, powered by a nuclear starburst. Emission line ratio diagnostics indicate that photoionization is the dominant excitation mechanism at the base of the wind. Mixing from slow shocks (up to 20%) increases further out along the outflow axis. The asymmetry of the wind is most likely caused by one of the two lobes of the wind bubble bursting out of its HI envelope, as indicated by line ratios and radial velocity maps. The characteristics of this galactic wind suggest that it is caught early (a few Myr) in the wind evolution sequence. The wind is also quite different to the galactic wind in the partner galaxy NGC839 which contains a symmetric, shock-excited wind. Assuming that both galaxies have similar interaction histories, the two different winds must be a consequence of the intrinsic properties of NGC838 and NGC839 and their starbursts.

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