Shedding light on the growth of galaxies through dark matter halo occupation

Abstract

Galaxies can be separated into two broad classes; those actively forming stars (star-forming) and those with little-to-no ongoing star formation (passive). A key question of galaxy evolution is, how is star formation shut down in galaxies? As passive galaxies are predominantly more massive than their star-forming counterparts, and contain ≈ 50% of the stellar mass in the present Universe, their formation history gives insight into the formation mechanisms for all galaxies. How did they form? How and why was their star formation shut down? Colour can be used as a rough proxy for these two classes of galaxy. Galaxies that largely emit red light are made up of an older stellar population and are thereby passive. Whereas galaxies that largely emit blue light contain newly forming massive blue stars and are thereby star-forming. Galaxies separated by colour tend to occupy different environments, where red galaxies are found in very dense clustered environments and blue galaxies are generally not. Consequently, environment must play some role in the truncation of star formation in galaxies. To understand the nature of this connection, in this work I measure the clustering properties of galaxies as a function of their physical properties. Using ≈ 180 deg² of the Galaxy And Mass Assembly (GAMA) survey, I measure the projected clustering and dark matter halo occupation statistics for 172,377 galaxies as a function of rest-frame r-band luminosity, stellar mass and colour, over the redshift (z) range 0.05 < z < 0.4. This clustering analysis is then extended over the redshift range 0.2 < z < 1.0 in 8.262 deg² of the NDWFS Boötes field. In Boötes, galaxy clustering statistics are measured for 319,420 galaxies as a function of their rest-frame B-band luminosity, colour and redshift. Analysis in Boötes is extended to z>1 using a sample of Extremely Red Objects (EROs), galaxies at ≥ 1.0 with star-forming and passive sub-populations that are selected using very red optical to infra-red colours (e.g., (R − K)Vega > 5.0). The clustering strength of all galaxies increases with increasing luminosity and stellar mass at all redshifts. Red galaxies have stronger clustering at all scales than blue galaxies to the same stellar mass and luminosity. The small scale clustering strength of red galaxies increases with decreasing stellar mass, opposite to that of the blue galaxy population. The fraction of red galaxies that are satellites therefore increases with decreasing stellar mass. Blue galaxy stellar mass is found to be closely tied with their halo mass, and the majority of blue galaxies are centrally located within their halos. The fraction of all satellite galaxies that are red is ≥ the fraction of all central galaxies that are red at all stellar masses and luminosities. This means that star formation quenching of in-falling blue galaxies occurs across the entire stellar mass and luminosity range analysed in this work. At low stellar masses and faint luminosities, the majority of centrals are blue so these must be quenched as satellites. At high stellar masses and bright luminosities, the majority of centrals are already red so these must be quenched before they become satellites.