CENTRO DE INTERÉS “SALIR DE JUERGA O FIESTA”

Powerful radio galaxies inhabit, almost universally, elliptical galaxies (though there are exceptions, seeLedlow et al. 1998;Keel et al. 2006). Seyferts are often found in spiral galaxies, some of which have small jets and lobes (like Circinus, see Chapter

4, and NGC 6764, see Hota & Saikia 2006; Croston et al. 2008b), but they are not generally classified as radio-loud due to their small R factors (see eq.1.1). The reasons behind the correlation between the presence of radio jets and lobes and the host galaxy morphology are not entirely clear. It has been suggested that the richer environments in which ellipticals are generally found are needed to confine the jet and lobes, but some radio galaxies (Best 2004), even some LERGs, are found in sparse environments (e.g. 3C 15, see Chapters5and6). It has also been suggested that the correlation has its origin in the larger black hole masses found in elliptical galaxies (e.g. Sabater et al. 2013;Alexander & Hickox 2012), or in the higher cold gas content in spirals, which may interfere with the formation or propagation of the jet (e.gLedlow et al. 2001).

Evidence supporting the hypothesis that mergers trigger AGN activity (Heckman et al. 1986) can be found in the presence of disks, dust lanes (e.g. Cen AKraft et al. 2003; Croston et al. 2009) and bluer colours than expected for quiescent ellipticals (Govoni et al. 2000). However, it is possible that heavy merger activity, such as that found in superclusters, can suppress radio source formation (Venturi et al. 2000), per- haps due to ram pressure stripping. This is perhaps the reason why powerful radio galaxies are generally found in groups and sparse clusters (Hill & Lilly 1991;Belsole et al. 2007). There is also an environmental dependence of morphology, with FR I sources (especially those with bent or distorted features) being found in clusters more often than FR II (e.g. Wing & Blanton 2011), but this may be caused by the fact that most LERG are FR I, and they are found in richer environments (Hardcastle 2004), and it may not be universal (see e.g.Ineson et al. 2012).

1.2. ENVIRONMENTS OF RADIO-LOUD AGN 25

Environments are also potentially important in the fuelling process for the AGN. In the models ofHardcastle et al.(2007a), low-excitation sources, in particular, need a reservoir of hot gas, which could originate in a rich ICM or in winds caused by star formation. The former seems to be supported by the fact that LERG are more predominantly found in red galaxies compared to HERG (Janssen et al. 2012).

Dense environments are characterised by very hot ICM gas, with temperatures of

∼ ₁₀7−₁₀8 _{K, and are very bright in X-rays (thermal bremsstrahlung emission being}

the primary contribution to their luminosity, as well as line emission from collisional excitation and de-excitation of the hot gas). Where no ICM is detected in X-rays, it is still possible to have a sparser group where some of the galaxies are interacting (see e.g. the 2Jy hosts in Ramos Almeida et al. 2010, and Chapters 5 and 6). Optical observations are helpful in these cases to determine the presence of bridges and tidal features.

The most efficient way in which radio-loud AGN release energy into their sur- rounding environment is through shocks driven by the lobes. A shock happens when a medium, gas in this case, moves at a bulk velocity that is greater than the local sound speed. This creates a sharp discontinuity in the medium, which reflects in an abrupt change in temperature, density and pressure (and magnetic field density, if one is present and favourably aligned with the shock front). For the cases studied in this thesis shocks can be approximated with the simplest conditions, since there are no relativistic effects involved (and we assume no magnetic fields either). To do this, the Rankine-Hugoniot jump conditions are applied (Landau & Lifshitz 1987), impos- ing mass, momentum and energy conservation at the front, and assuming a perfect monatomic gas (with polytropic index Γ = 5/3). This yields an expression for the temperature ratio between the gas in the shocked shells and the external medium:

Tshell

Tout

= [2ΓM + (1 − Γ)] [Γ − 1 + (2/M)]

(Γ + 1)2 (1.27)

where M is the Mach number. This equation can be rewritten for density (or the pressure), using the ideal gas law:

pout

ρoutTout

= pshell ρshellTshell

so that the density ratio is

ρshell

ρout

= Γ + 1

Γ − 1 + (2/M) (1.29)

We can immediately see that for strong shocks, where the Mach number is large, the last term in the denominator of eq.1.29becomes negligible. For a perfect monatomic gas this implies that the density ratio for a strong shock cannot be greater than 4. This condition is used in Chapters3and4to constrain the temperature of the external medium, given that the statistics are too low to obtain a fit to a thermal model. Once the parameters of the gas and the shock velocity are known, the relative contributions from thermal energy (3₂NkT ), mechanical work (PV) and kinetic energy (1₂mgas

₁

4vshell

2

, as- suming the velocity ratios for a strong shock) can be determined, as well as which is the dominant emission process (thermal or non-thermal).

While radio-quiet AGN, even the most luminous ones, restrict their effect to their immediate surroundings, radio-loud objects transport the energy from the central source to much larger scales, affecting the host galaxy in the less powerful cases, and the en- tire cluster in the most powerful ones. The amount of energy released into the external gas in this manner is not negligible: even for small sources it is comparable to that of 104−₁₀6 _{supernova explosions (Croston et al. 2007, 2008b;Kraft et al. 2003, and}

Chapters3 and4), enough to have an effect on the host galaxy’s star formation pro- cesses, and even on its dynamical properties.

The relationship between AGN activity and star formation is not entirely clear, in part because most of the studies focus on radio-quiet AGN, but it is clear that there must be a co-dependence (see e.g. Best & Heckman 2012;Hardcastle et al. 2010b). The key may lie in the timescales involved in both processes (e.g. Wild et al. 2010) and how they relate to merger events. It is also possible that for radio-loud sources the effect may be delayed even longer, since the gas from the lobes needs time to mix with the ISM, making it impossible to estimate its effect directly (by then the radio structures would no longer be detectable).

For very powerful sources the energetic impact is even more dramatic (e.g. 3C 444

Croston et al. 2011). The inner regions of dense clusters are expected to cool very rapidly through X-ray emission, but the observations show that this is not happening as fast as expected (e.gPeterson & Fabian 2006). The energy input from radio-loud AGN is very likely to be one of the main causes behind this ‘quenched cooling’, by