Viktor Ambartsumian: "It is easy to show applying statistical
considerations, thta the galaxies NGC 5128 (Centaurus A) and Cygnus A
cannot be the result of the collisions of two previously independent
Rudolf Minkowski "Results of a detailed investigation of NGC 1275 (Perseus A), reported at this symposium, admit to no other interpretation than that this system consists of two colliding galaxies", IAU Symposium No. 5, Dublin, 1955
Having considered the various building blocks of HzRGs in detail, we now address the nature of HzRGs and how these objects fit into general schemes of galaxy evolution.
There are several reasons for concluding that HzRGs are massive forming galaxies. First, we have seen that their large near-IR luminosities imply that HzRGs are amongst the most it massive galaxies in the early Universe (Section 4.1). Secondly, HzRG hosts have clumpy UV continuum morphologies (Section 4.3), as expected from galaxies forming through mergers and in accordance with hierarchical models of massive galaxy evolution (Larson 1992, Dubinski 1998, Gao et al. 2004, Springel et al. 2005). Thirdly, the rest-frame UV spectra and millimeter SEDs indicate that HzRGs are undergoing copious star formation (e.g. Dey et al. 1997) (Section 4.2).
Not only are HzRGs massive forming galaxies, but there is also strong evidence that they are the progenitors of the most massive galaxies in the local Universe, i.e. the giant galaxies that dominate the central regions of rich local galaxy clusters. We have seen that HzRGs are generally embedded in giant (cD-sized) ionized gas halos (Section 3.2. In (Section 7) we shall show that they are often surrounded by galaxy overdensities, whose structures have sizes of a few Mpc (Section 7). For all these reasons, HzRGs are the probable ancestors of brightest cluster galaxies (BCGs) and cD galaxies.
What can studying the development of HzRGs tell us about the evolution of massive galaxies in general? Recently, two processes have been invoked to explain discrepancies between the observed colour and luminosity statistics of faint galaxies and the model predictions. In contrast with what might be expected from simple hierarchical merging scenarios, the data imply that star formation occurs earlier in the history of the most massive galaxies than for the less massive ones. This process is called galaxy "downsizing" (Cowie et al. 1996, Heavens et al. 2004, Thomas et al. 2005, Panter et al. 2007). AGNs are preferentially found in the most massive galaxies. To quench star formation in these galaxies, it has been proposed that there is some sort of feedback mechanism, in which the AGN influences the star formation history of the galaxy. This AGN feedback (Di Matteo et al. 2005, Best et al. 2005, Springel et al. 2005, Croton et al. 2006, Hopkins et al. 2006, Best 2007) provides modelers with additional parameters that, not surprisingly, gives a better fit to the data. The potential importance of AGN feedback for galaxy formation processes was only realised after it was shown that every galaxy may host a supermassive black hole (Section 5.2), establishing a general link between galaxies and AGN.
As the most massive known galaxies in the early Universe and the seat of powerful AGN, HzRGs are important laboratories for studying the processes responsible for downsizing and AGN feedback. In practice, feedback scenarios are complicated and involve several physical processes that occur simultaneously. We have seen in Sections 2 to 5 that there is considerable evidence for physical interaction between the various building blocks of HzRGs - relativistic plasma, gas, dust, stars and the AGN. As they merge, the satellite galaxies will interchange gas with the ambient medium in the system. The gas will move inwards through cooling flows and accretion and provide fuel for the supermassive black hole. The SMBHs generate quasars and relativistic jets. Hidden and/or dormant quasars heat the dust. The jets together with superwinds from starbursts (Armus et al. 1990, Zirm et al. 2005) drive gas outwards (Nesvadba et al. 2006) and can trigger star formation. Shocks will be rampant in the chaotic environments in which these processes are competing with each other.
Two modes have been distinguished in theoretical models for coupling of the energy output of the AGN to the surrounding gas, with rather confusing names. "Quasar-mode" feedback is defined as the situation when radiative coupling occurs that expels the gas and thereby quenches star formation in the forming galaxy (Hopkins et al. 2006). In "radio-mode" coupling (Best 2007), star formation is slowed down due to the mechanical energy of the synchrotron radio jets that inhibits cooling of the ambient gas. However, such rigid distinction between feedback processes are highly simplified (e.g. Fu and Stockton 2007).
It is impossible to understand how these complicated AGN feedback processes influence the evolution of massive galaxies from a consideration of statistical data alone.
We shall now illustrate some of these processes and their relevance to galaxy formation by considering a specific example of a HzRG in more detail.
6.1. The Spiderweb Galaxy - a case study
The Spiderweb Galaxy (MRC 1138-262) at a redshift of z = 2.2 is one of the most intensively studied HzRGs (Miley et al. 2006). This object provides a useful case study for illuminating several important physical processes that may occur generally in the evolution of the most massive galaxies. Because the Spiderweb Galaxy is (i) relatively close-by, (ii) one of the brightest known HzRGs and (iii) is the HzRG with the deepest HST optical image, it is an important laboratory for testing simulations of forming massive galaxies at the centers of galaxy clusters.
This large galaxy has several of the properties expected for the progenitor of a dominant cluster galaxy (Pentericci et al. 1997, 1998, 2000, 2001). The host galaxy is surrounded by a giant Ly halo (Pentericci et al. 2000, Kurk et al. 2004) and embedded in dense hot ionized gas with an ordered magnetic field (Carilli et al. 1998). The radio galaxy is associated with a 3 Mpc-sized structure of galaxies, of estimated mass > 2 × 1014 M, the presumed antecedent of a local rich cluster (Section 7).
The beautiful ACS Hubble image of the Spiderweb Galaxy is shown in Figure 16, with the radio source and Ly halo superimposed. The figure illustrates the structures of the radio, warm gas and stellar components in a relatively nearby HzRG.
Figure 16. The Spiderweb Galaxy. Deep Hubble image of the core of the MRC 1138-262 protocluster at z = 2.2 obtained with the Advanced Camera for Surveys. [From Miley et al. (2006)]. Superimposed on the HST image are contours of Ly (blue, resolution ~ 1") obtained with ESO's very Large Telescope (VLT), delineating the gaseous nebula and radio 8GHz contours (red, resolution 0.3") obtained with NRAO's VLA, delineating the non-thermal radio emission. The gaseous nebula extends for > 200 kpc and is comparable in size with the envelopes of cD galaxies in the local Universe.
It also provides dramatic evidence that tens of satellite galaxies were merging into a massive galaxy, ~ 10 Gyr ago. The morphological complexity and clumpiness agrees qualitatively with predictions of hierarchical galaxy formation models (Larson 1992, Kauffmann et al. 1993, Baugh et al. 1998, Dubinski 1998, Gao et al. 2004, Springel et al. 2005), and illustrates this process in unprecedented detail. Ly spectroscopy shows relative velocities of several hundred km s-1, implying that the satellite galaxies ("flies") will traverse the 100 kpc extent of the Spiderweb many times in the interval between z ~ 2.2 and z ~ 0, consistent with the merger scenario.
An intriguing aspect of the Spiderweb Galaxy is the presence of faint diffuse emission between the satellite galaxies (Hatch et al. 2007). Approximately 50% of the ultraviolet light from the Spiderweb Galaxy is in diffuse "intergalactic" light, extending over about 60 kpc diameter halo. The luminosity in diffuse light implies that the emission is dominated young stars with a star formation rate of > 80 M yr-1. Under reasonable assumptions, the diffuse emission seen in the Spiderweb Galaxy could evolve into the CD envelopes seen in many dominant cluster galaxies seen at low redshifts.
The total mass of all the flies. in the Spiderweb, derived from their UV luminosities (assuming 1 Gyr starbursts), is less than a tenth of the mass of the whole galaxy obtained from its IR luminosity (Miley et al. 2006). Because the UV emission is produced by ongoing star formation and the IR emission by old stars, this implies that most of the galaxy mass may already have assembled by z ~ 2.2, consistent with downsizing scenarios.
Merging, downsizing and feedback are all likely to be occurring simultaneously in the Spiderweb Galaxy. Merging is a plausible fueling source for the nuclear supermassive black holes that produce the radio sources. Pressure from these radio sources is sufficient to expel a large fraction of gas from the galaxies (Nesvadba et al. 2006), thereby quenching star formation (Croton et al. 2006). Because radio lifetimes are relatively short (few × 107 yr), all massive ellipticals may have gone through a similar short but crucial radio-loud phase during their evolution.
An unexpected feature of the HST image is that there is a significant excess of faint satellite galaxies with linear structures (Miley et al. 2006). These galaxies (linear "flies") have similar morphologies (e.g. chains and tadpoles) to the linear galaxies that dominate resolved faint galaxies (i775 > 24) in the Hubble Ultra Deep Field (UDF) (Elmegreen et al. 2005, Straughn et al. 2006). Although linear galaxies must be an important constituent of the earliest galaxy population, their nature is poorly understood. Their presence in a merging system is relevant for theories of their formation. In the Spiderweb Galaxy the motions of the flies with velocities of several hundred km s-1 through the dense gaseous halo, perturbed by superwinds from the nucleus (Armus et al. 1990, Zirm et al. 2005) and the radio jet, would result in shocks. The shocks would then lead to Jeans-unstable clouds, enhanced star formation along the direction of motion and to chain and tadpole morphologies (Taniguchi and Shioya 2001, Miley et al. 2006).