Evolution of Radio Galaxies Across Cosmic Time

Despite being a minority among the total population of galaxies, radio galaxies have gained increasing attention, because of the energetic feedback they can provide to the surrounding environment. These systems host an active nucleus that produces large amounts of energy in the form of radio/X-ray emitting jets, injecting energy that in some cases can balance the radiative loses of the gas that cools and condenses within massive halos of dark matter. Therefore, if we want to understand galaxy formation and evolution, we have to understand the role of active galactic nuclei (AGN) in the galaxy assembly process. My work concentrated, first, on constructing a complete sample of radio galaxies. This is not an easy task as they have very different morphologies in different surveys, and ofter break up into separate components. These components must be correctly associated with a unique optical galaxy. We cross-correlated two different radio surveys (NVSS and FIRST) with a sample of luminous red galaxies derived from the Sloan Digital Sky Survey (SDSS). The NVSS provides accurate flux measurements for extended sources, while the angular resolution of FIRST allows the host galaxy to be identified accurately. We also improved the matching of sources below the nominal 1 mJy detection limit of FIRST, to increase the reliability and completeness of the final catalogue. These techniques allowed us to assemble the largest radio galaxy catalogue to date, consisting of around 14,000 radio–loud AGN at intermediate redshifts (0.4 < z < 0.8), with 1.4 GHz fluxes above 3.5 mJy. The matching criteria were tested and refined using Monte–Carlo simulations, leading to an estimated reliability of ∼98.3% and completeness level of about 95% for our catalogue. With this catalog in hand, we were able to compare radio galaxies at z∼0.55 with similar samples in the local universe. We studied the evolutionary properties of radio galaxies, how their spatial density changes as function of time and how the fraction of radio emitting sources varies with galaxies properties such as stellar mass, radio luminosity and redshift. We present a new determination of the luminosity function of radio AGN at z∼0.55 and compare this to the luminosity function of nearby (z∼0.1) radio sources from the SDSS main survey. The comoving number density of radio AGN with luminosities less than 10^25 W/Hz increases by a factor ∼1.5 between z=0.1 and z=0.55. At higher luminosities, this factor increases sharply, reaching values of more than 10 at radio luminosities larger than 10^26 W/Hz. We then study how the relation between radio AGN and their host galaxies evolves with redshift. Our main conclusion is that the fraction of radio–loud AGN increases towards higher redshift in all massive galaxies, but the evolution is particularly strong for the lower mass galaxies in our sample. These trends may be understood if there are two classes of radio galaxies (likely associated with the “radio” and “quasar mode” dichotomy) that have different fuelling/triggering mechanisms and hence evolve in different ways. We conclude that stellar mass seems to be the a very important factor in deciding whether a galaxy develops bright, powerful radio jets. There is also the suggestion that the environment of a galaxy is also crucial in deciding whether it becomes radio-loud. To address this question, we studied how radio galaxies are clustered in the universe and quantified the clustering dependence on stellar mass and radio power. We do this by computing the cross-correlation function between radio galaxies and the parent LRG population. In order to isolate the true clustering of RLAGN, we compare respect to control radio-quiet galaxies selected with the same properties as radio AGN. The main result is that RLAGN are significantly more clustered than radio-quiet objects, particularly below ∼1 Mpc/h, indicating that the gaseous environment of a radio sources at the scale of its dark matter halo is important in modulating the observed output power and determining its radio loudness. Unification models predict that the environments of radio-loud galaxies and radio-loud quasars should be equivalent, as they represent the same object that is observed at different orientations with respect to the line of sight. We have compared our clustering measurements for these two types of objects, setting important restrictions on the conditions that must be met if the models are valid. We find evidence that the idea of unification can hold only for the most luminous radio galaxies in our sample, with radio power above ∼10^26 W/Hz. Finally, we have also modeled the radio-loud population of AGN at z=0.5 by applying a simple prescription for the distribution of radio galaxies in dark matter haloes extracted from N-body simulations. This proves the applicability at high redshift of models that have been shown to work well in the local universe and test directly the radio-mode heating recipe implemented in semianalyitcal models of galaxy formation. By combining an estimate for the mechanical power of radio sources with our determination of the luminosity function we have been able to estimate the heating effect that RLAGN produce in haloes of different mass. The heating power in haloes of ∼10^15 M_sun/h, the most massive in our sample, is a factor of 6-7 larger than in less massive systems of ∼10^13 M_sun/h, and a factor of ∼2.5 larger at z=0.5 than at in the local universe. This result can be used to improve the implementation of radio feedback in models of galaxy formation.