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TOPIC: Galaxy Colour Gradients

Blobrana

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Posts: 131433

Date: Apr 25 01:50 2013

Galaxy Colour	Permalink

Title: The Dark Side of Galaxy Colour Authors: Andrew P. Hearin (Fermilab), Douglas F. Watson (KICP/U.Chicago) We present age distribution matching, a new theoretical formalism for predicting how galaxies of luminosity L and colour C occupy dark matter halos. Our model supposes that there are just two fundamental properties of a halo that determine the colour and brightness of the galaxy it hosts: the maximum circular velocity Vmax, and the redshift at which the galaxy will be starved of cold gas z_starve. The halo property z_starve is intended to encompass physical characteristics of a halo's mass assembly history (MAH) that may deprive the galaxy of its cold gas supply and, ultimately, quench its star formation. These include the epochs when: (a) a halo accretes onto a larger halo, z_acc, (b) a halo reaches a characteristic mass (~10^12 solar masses/h), z_char, and (c) a halo transitioned from the fast- to slow-accretion regime, z_form. A halo's z_starve value is determined by whichever of these events happens first in its MAH. The new, defining feature of the model is that, at fixed luminosity, galaxy colour is in monotonic correspondence with z_starve with the larger values of z_starve being assigned redder colours. We populate the Bolshoi N- body simulation with a mock galaxy catalogue based on age distribution matching, and show that the resulting mock galaxy distribution accurately describes the luminosity- and colour-binned two-point correlation function of galaxies in the Sloan Digital Sky Survey (SDSS), as well as a variety of low-redshift galaxy group statistics. Our study of the property z_starve has important implications for how halo MAH influences stellar mass assembly. For example, the significance of the epoch z_form suggests a new, independent channel for so-called "mass quenching" based on the rate of halo mass accretion. We make publicly available our low-redshift, SDSS Mr<-19 mock galaxy catalogue, and main progenitor histories of all z=0 Bolshoi halos, at this http URL Read more (599kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Feb 4 23:07 2012

Galaxy Colour Gradients	Permalink

Title: Colour Gradients in Galaxies Out to z~3: Dependence on Galaxy Properties Authors: Niraj Welikala (IAS, Orsay), Jean-Paul Kneib (LAM) Using HST/ACS observations, we measure the colour gradients of 3248 galaxies in the GOODS-South field out to z~3 and i_{AB}<25.5 and characterize their dependence on galaxy properties (luminosity, apparent magnitude, galaxy size, redshift and morphological type). The colour gradient is measured by the difference of v-i colour outside (R_{50}<r<2R_{50}) and inside the half light radius. The gradient shows little evolution with redshift up to z~1 but increases from z~1 to z~2 before flattening out. It also increases with apparent magnitude, with a median value of 0.24 magnitudes at i_{AB}~25.5. It has a strong colour dependence, with the bluest galaxies (in terms of observed colour) having cores that are bluer relative to their outskirts. We probe the redshift evolution by stacking galaxies and measuring the radial variation of v-i colour within them. At low redshifts (z<0.5), the centres of galaxies (r<R_{50}) are slightly redder than their outskirts (1.5R_{50}<r<2R_{50}). Galaxies at z~1 and -22.0<M_I<-21.0 are bluer in their cores by 0.1 magnitudes, on average, compared to their outskirts. For z>1, galaxies show increasingly bluer cores while the colour of the outskirts does not change as rapidly. At z~2.5 and -22.0<M_I<-21.0, we observe a difference, on average, of 0.4 magnitudes between the centre and the outskirts. The observed colour gradients may indicate that strong star formation in galaxies at z>=2 is concentrated in their central regions. These colour gradients and their dependence on galaxy properties could also have a significant impact on shear measurements in upcoming weak lensing cosmological surveys. Read more (760kb, PDF) __________________

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