The role of AGN in the migration of early-type galaxies from the blue cloud to the red sequence

We present a general picture of the ongoing formation and evolution of early-type galaxies via a specific evolutionary sequence starting in the blue cloud and ending in the low-mass end of the red sequence. This evolutionary sequence includes a Seyfert AGN phase in the green valley, but this phase occurs too late after the shutdown of star formation to be responsible for it. Thus, the bulk of black hole accretion in low-redshift early-type galaxies occurs in post-starburst objects, and not concurrent with star formation. On the other hand, a low-luminosity AGN phase switching on at an earlier stage when some star formation activity remains may be responsible for destroying the molecular gas reservoir fueling star formation.Comment: 4 pages, 2 figures. To appear in proceedings of "The Monster's Fiery Breath", Madison, Wisconsin, 1-5 June 2009, Ed. Sebastian Heinz & Eric Wilcot

Globular cluster formation efficiencies from black-hole X-ray binary feedback

We investigate a scenario in which feedback from black-hole X-ray binaries (BHXBs) sometimes begins inside young star clusters before strong supernova feedback. Those BHXBs could reduce the gas fraction inside embedded young clusters whilst maintaining virial equilibrium, which may help globular clusters (GCs) to stay bound when supernova-driven gas ejection subsequently occurs. Adopting a simple toy model with parameters guided by BHXB population models, we produce GC formation efficiencies consistent with empirically-inferred values. The metallicity dependence of BHXB formation could naturally explain why GC formation efficiency is higher at lower metallicity. For reasonable assumptions about that metallicity dependence, our toy model can produce a GC metallicity bimodality in some galaxies without a bimodality in the field-star metallicity distribution.Comment: Accepted to ApJ Letters on 19th July. 6 pages. The definitive version is available from: http://iopscience.iop.org/2041-8205/809/1/L16

Exploring galaxy evolution with generative models

Context. Generative models open up the possibility to interrogate scientific data in a more data-driven way. Aims: We propose a method that uses generative models to explore hypotheses in astrophysics and other areas. We use a neural network to show how we can independently manipulate physical attributes by encoding objects in latent space. Methods: By learning a latent space representation of the data, we can use this network to forward model and explore hypotheses in a data-driven way. We train a neural network to generate artificial data to test hypotheses for the underlying physical processes. Results: We demonstrate this process using a well-studied process in astrophysics, the quenching of star formation in galaxies as they move from low-to high-density environments. This approach can help explore astrophysical and other phenomena in a way that is different from current methods based on simulations and observations.Comment: Published in A&A. For code and further details, see http://space.ml/proj/explor

Improved and Quality-assessed Emission and Absorption Line measurements in Sloan Digital Sky Survey galaxies

We present a new database of absorption and emission-line measurements based on the entire spectral atlas from the Sloan Digital Sky Survey (SDSS) 7th data release of galaxies within a redshift of 0.2. Our work makes use of the publicly available penalized pixel-fitting(pPXF) and gas and absorption line fitting (gandalf) IDL codes, aiming to improve the existing measurements for stellar kinematics, the strength of various absorption-line features, and the flux and width of the emissions from different species of ionised gas. Our fit to the stellar continuum uses both standard stellar population models and empirical templates obtained by combining a large number of stellar spectra in order to fit a subsample of high-quality SDSS spectra for quiescent galaxies. Furthermore, our fit to the nebular spectrum includes an exhaustive list of both recombination and forbidden lines. Foreground Galactic extinction is implicitly treated in our models, whereas reddening in the SDSS galaxies is included in the form of a simple dust screen component affecting the entire spectrum that is accompanied by a second reddening component affecting only the ionised gas emission. In order to check for systematic departures, we provide a quality assessment for our fit to the SDSS spectra in our sample. This quality assessment also allows the identification of objects with either problematic data or peculiar features. For example, based on the quality assessment, approximately 1% of the SDSS spectra classified as "galaxies" by the SDSS pipeline do in fact require additional broad lines to be matched, even though they do not show a strong continuum from an active nucleus, as do the SDSS objects classified as "quasars". Finally, we provide new spectral templates for galaxies of different Hubble types, obtained by combining the results of our spectral fit for a subsample of 452 morphologically selected objects.Comment: Accepted for publication in ApJS. 23 pages, 14 figures, 4 tables. A version with high-resolution figures is available at http://gem.yonsei.ac.kr/~ksoh/ossy/arXiv/Oh_11_OSSY.pd