Galaxy formation and evolution

Abstract:

We present a new model for the infrared emission of the high redshift hyperluminous infrared galaxy IRAS F10214+4724 which takes into account recent photometric data from Spitzer and Herschel that sample the peak of its spectral energy distribution. We first demonstrate that the combination of the AGN tapered disc and starburst models of Efstathiou and coworkers, while able to give an excellent fit to the average spectrum of type 2 AGN measured by Spitzer, fails to match the spectral energy distribution of IRAS F10214+4724. This is mainly due to the fact that the ν S ν distribution of the galaxy falls very steeply with increasing frequency (a characteristic of heavy absorption by dust) but shows a silicate feature in emission. We propose a model that assumes two components of emission: clouds that are associated with the narrow-line region and a highly obscured starburst. The emission from the clouds must suffer significantly stronger gravitational lensing compared to the emission from the torus to explain the observed spectral energy distribution. © 2012 International Astronomical Union.

Abstract:

We present an exploration of the integrated stellar populations of early-type galaxies (ETGs) from the ATLAS3D survey. We use two approaches: firstly the application of line-indices interpreted through single stellar population (SSP) models, which provide a single value of age, metallicity and abundance ratio. And secondly, by fitting a linear combination of SSP spectra to our data, smoothly weighted in the free parameters of age and metallicity, thereby inferring a star-formation history of these galaxies. Despite the significant differences in these approaches, we obtain generally consistent results, such that galaxies that are more massive appear older with enhanced abundance ratios using line indices, and have shorter star-formation histories weighted to early times. We highlight two limitations of the index-SSP approach. Firstly the SSP-equivalent ages belie the fact that ETGs are overwhelmingly composed of ancient stars. Secondly, the young stellar contributions implied in our star formation histories are required to obtain realistic UV-optical colours. We remark that, even fitting solar-abundance models, we can recover a star-formation duration that correlates with the measured alpha-enhancement, in agreement with other recent work. © 2012 International Astronomical Union.

Abstract:

Recent studies from the Galaxy Evolution Explore (GALEX) ultraviolet (UV) data have demonstrated that the recent star formation is more common in early-type galaxies (ETGs) than we used to believe. The UV is one order of magnitude more sensitive than the optical to the presence of young stellar populations. The near-ultraviolet (NUV) lights of ETGs, especially, are used to reveal their residual star formation history. Here we used the GALEX UV data of 34 nearby early-type galaxies from the SAURON sample, all of which have optical data from MDM Observatory. At least 15% of the galaxies in this sample show blue UV-optical colours suggesting recent star formation (Jeong et al. 2009). These NUV blue galaxies are generally low velocity dispersion systems and change the slopes of scaling relations (colour-magnitude relations and fundamental planes) and increase the scatters. To quantify the amount of recent star formation in our sample, we assume two bursts of star formation, allowing us to constrain the age and mass fraction of the young component pixel by pixel (Jeong et al. 2007). The pixel-by-pixel SED fitting based on UV and optical imaging reveals that the mass fraction of young (< 1 Gyr old) stars in ETGs varies between 1 and 3% in the nearby universe (Jeong et al. in prep.). We will compare our results with the prediction from the hierarchical merger paradigm to understand the mechanism of low-level recent star formation observed in early-type galaxies. © 2012 International Astronomical Union.