The Link Between Extended Line Emission and AGN Feedback In

Chapter 1.

Introduction

1.1 Feedback and galaxy formation

The role of energy feedback in galaxy evolution is a central problem in extra-galactic astronomy today. In the currently-favoured Cold Dark Matter (CDM) cosmology, galaxies form through hierarchical merging of dark matter halos (White & Rees, 1978). Bary- onic matter cools and condenses within these structures to form the gaseous and stellar components of the galaxies. Although the results of CDM cosmological simulations can match observations of large-scale structure in the universe remarkably well, until recently, the distributions of galaxy masses and morphologies were not as closely described (e.g., Cole et al., 2000; Benson et al., 2003; Croton et al., 2006). For example, semi-analytical merger simulations struggled to reproduce the observationally-derived time-scales and ages of formation for early-type galaxies, predicting an excess of the most luminous and massive systems. It was recognised that powerful sources of energy must act in galaxies to reheat the cold gas that fuels star formation. This heating prevents star formation from continuing at late times in early-type galaxies and slows or regulates growth. Fig- ure 1.1 shows an example of how including heat sources in galaxy formation models improves the match between the high end of the predicted and observed luminosity functions (e.g., Benson et al., 2003). Galaxy feedback refers to the processes that return or recycle energy and mass between a galaxy and its environment, and potentially affect the growth of the galaxy itself. Such feedback can provide energy to suppress the cooling of accreting gas or to remove material from the inner regions of the galaxy altogether. This limits the material available for star formation and the buildup of galaxy stellar mass. These effects can provide both an explanation for the deficit of massive galaxies that is observed relative to the predictions of hierarchical merging, and the means to better reproduce the early and rapid formation of the stellar mass in early-type galaxies. Supernovae and starbursts are an important channel for feedback in galaxies (e.g., White & Rees, 1978; White & Frenk, 1991), and are a critical component of galaxy formation models (e.g., see Cole et al., 2000; Benson et al., 2003). They enrich the intergalactic medium with metals and produce ultraviolet radiation that heats and ionises the local environment. Starburst winds transport mass and energy, and entrain and uplift material from galaxies that might otherwise form stars (e.g., Chevalier & Clegg, 1985; Efstathiou, 2000). However, the predominant source of feedback and heating that

1 The many lives of AGN 23

V vir new fuel for star formation must come from cooling flows which 0.4). Our morphological resolution limit is marked by the dashed are affected by ‘radio mode’ heating. line at a stellar mass of 4 109 M ; this corresponds approxi- The effect of ‘radio mode’ feedback is clearly substantial. Sup- mately to a halo of 100 particles∼ × in the$ Millennium Run. Recall that pression of condensation becomes increasingly effective with in- the morphology of a galaxy depends both on its past merging history creasing virial temperature and decreasing redshift. The effects are and on the stability of its stellar disc in our model. Both mergers 1 6 large for haloes with V vir 150 km s− (T vir 10 K) at z 3. Con- and disc instabilities contribute stars to the spheroid, as described densation stops almost completely between z 1 and the present in Section 3.7. The build-up of haloes containing fewer than 100 1 =6 in haloes with V vir > 300 km s− (T vir > 3 10 K). Such systems particles is not followed in enough detail to model these processes correspond to the haloes of groups and clusters× which are typically robustly. observed to host massive elliptical or cD galaxies at their centres. A number of important features can be seen in Fig. 9. Of note Our scheme thus produces results which are qualitatively similar is the bimodal distribution in galaxy colours, with a well-defined to the ad hoc suppression of cooling flows assumed in previous red sequence of appropriate slope separated cleanly from a broader models of galaxy formation. For example, Kauffmann et al. (1999) ‘blue cloud’. It is significant that the red sequence is composed 1 switched off gas condensation in all haloes with V vir > 350 km s− , predominantly of early-type galaxies, while the blue cloud is com- while Hatton et al. (2003) stopped condensation when the bulge posed mostly of disc-dominated systems. This aspect of our model mass exceeded a critical threshold. suggests that that the physical processes that determine morphology (i.e. merging, disc instability) are closely related to those that control star formation history (i.e. gas supply) and thus determine 4.2 Galaxy properties with and without AGN heating galaxy colour. The red and blue sequences both display a strong The suppression of cooling flows in our model has a dramatic effect metallicity gradient from low to high mass (c.f. Fig. 6), and it is this on the bright end of the galaxy luminosity function. In Fig. 8 we which induces a ‘slope’ in the colour–magnitude relations which present K- and bJ-band luminosity functions (left- and right-hand agrees well with observation (e.g. Baldry et al. 2004). panels respectively) with and without ‘radio mode’ feedback (solid By comparing the upper and lower panels in Fig. 9 we can see how and dashed lines respectively). The luminosities of bright galaxies ‘radio mode’ feedback modifies the luminosities, colours and mor- are reduced by up to two magnitudes when the feedback is switched phologies of high-mass galaxies. Not surprisingly, the brightest and on, and this induces a relatively sharp break in the luminosity func- most massive galaxies are also the reddest and are ellipticals when tion which matches the observations well. We demonstrate this by cooling flows are suppressed, whereas they are brighter, more mas- overplotting K-band data from Cole et al. (2001) and Huang et al. sive, much bluer and typically have discs if cooling flows continue (2003) in the left-hand panel, and bJ-band data from Norberg et al. to supply new material for star formation. AGN heating cuts off the (2002) in the right-hand panel. In both bandpasses the model is quite gas supply to the disc from the surrounding hot halo, truncating star close to the data over the full observed range. We comment on some formation and allowing the existing stellar population to redden. of the remaining discrepancies below. However, these massive red galaxies do continue to grow through Our feedback model also has a significant effect on bright galaxy merging. This mechanism allows the dominant cluster galaxies to colours, as we show in Fig. 9. Here we plot the B V colour dis- gain a factor of 2 or 3 in mass without significant star forma- tribution as a function of stellar mass, with and without− the central tion, in apparent agreement with observation (Aragon-Salamanca, Baugh & Kauffmann 1998). This late-stage (i.e. 1) hierarchi- heating source2 (top and bottom panels respectively). In both panels 1. Introductionz we have colour-coded the galaxy population by morphology as es- cal growth moves objects to higher mass without changing their timated from bulge-to-total luminosity ratio (split at L /L colours. bulge total =

Figure 8. FigureGalaxy luminosity 1.1 functionsThis ﬁgure, in the K (left) from andCrotonbJ (right) photometric et al. (2006 bands,), plotted compares with and without observed ‘radio (blue mode’ feedback points) (solid galaxy and long-dashed lines respectivelyluminosity – see Section functions 3.4). Symbols (the indicate volume observational number results density as listed of in galaxies each panel. vs.As can luminosity) be seen, the inclusion in the of AGNK and heatingbJ produces a good ﬁt tophotometric the data in both colours. bands Without with this those heating predicted source our model by overpredicts galaxy formation the luminosities models of massive that galaxies omit by about (dashed two magnitudes black andfails to reproduce the sharp bright-end cut-offs in the observed luminosity functions. lines) and include (solid black lines) parameterisations of AGN feedback. Without AGN heating these models over-predict the population of massive galaxies by up to two orders of magnitude. C C % 2005 The Authors. Journal compilation % 2005 RAS, MNRAS 365, 11–28

shapes massive galaxies is generally ascribed to active galactic nuclei (AGN), which form when super-massive black holes (SMBHs) in the cores of galaxies actively accrete mass. Several lines of observational evidence indicate that AGN participate in self-regulating cycles of mass and energy feedback in galaxies. For example, it has been found that SMBH masses are tightly correlated with the large-scale properties of the host galaxy, including the host galaxy luminosity (Kormendy & Richstone, 1995), stellar velocity dispersion (Ferrarese & Merritt, 2000; Gebhardt et al., 2000), and bulge mass (Häring & Rix, 2004). These relationships suggest that the growth of the SMBH is coupled to that of the host in some, still uncertain, way (e.g., Kauffmann & Haehnelt, 2000). SMBHs grow by accreting matter from their environment and in the process can output power 46 1 exceeding 10 erg s− (Osterbrock & Ferland, 2006). They also generate relativistic bipolar jets that drive expanding lobes of radio-emitting plasma containing energies of up to 1060 1061 erg (Willott et al., 1999). These can extend over several megaparsecs − from the central galaxy (e.g., Machalski et al., 2008; Palma et al., 2000), and certainly across much larger distances than the sphere of influence of the black hole. The nuclear activity in galaxies, then, depends on the availability of material in the environment for fuel and can affect the surrounding environment on large scales. Semi-analytical models have successfully employed prescriptions of galaxy feedback to reproduce the observed luminosity function of galaxies (e.g., Figure 1.1). In general, these parameterise processes that suppress star formation to simulate the expected con- sequences of feedback physics (see e.g., Benson et al., 2003; Springel et al., 2005; Bower et al., 2006; de Lucia & Blaizot, 2007). However, accurate physical descriptions of how the cycles of mass and energy operate in galaxies are needed to realistically incorporate feedback into galaxy formation models. AGN feedback appears to be actively operating 1.2 Feedback in massive galaxy clusters 3 in the cores of massive galaxy clusters, even at low redshifts where the systems can be resolved and studied in some detail and help to constrain the physics of the feedback processes.

1.2 Feedback in massive galaxy clusters

Galaxy clusters can consist of hundreds to thousands of member galaxies. However, the majority of the baryons bound to a cluster are not found within the galaxies themselves, but in the diﬀuse hot gas of the intra-cluster medium (ICM) between the galaxies. This gas has been heated to temperatures of 107 108 K through shocks driven by the ∼ − virial motion of the galaxies in the cluster. The gas cools from these high temperatures as interactions between the electrons and ions radiate energy, mainly through thermal bremsstrahlung emission at X-ray wavelengths (Sarazin, 1988). Massive clusters produce X-ray luminosities of 1042 1045 erg s 1. − − The cores of rich galaxy clusters are complex and active regions. Recent X-ray imaging reveals a wealth of structure in the ICM, including cavities, shock fronts, metal- enriched plumes, and ﬁlaments (see Peterson & Fabian, 2006; McNamara & Nulsen, 2007, for reviews). Much of this activity is associated with the massive elliptical galaxies that are found in the cluster centres.

1.2.1 Brightest cluster galaxies

The cores of most massive clusters are dominated by one or two giant elliptical galaxies. These are often referred to as brightest or central cluster galaxies, and the term brightest cluster galaxy (BCG) will be adopted here. As a class, BCGs are among the most massive and most luminous galaxies in the universe, with masses above 1012 M . They are ∼ ⊙ usually the most massive galaxy in the host cluster, and often have cD morphology – characterised by an extended, low-surface-brightness stellar halo with excess light in the outer envelope over a de Vaucouleurs surface-brightness proﬁle ﬁt to the central bulge (Matthews et al., 1964). BCGs are located at, or close to, the central peak of the X-ray emission from the ICM, and so are situated at the bottom of the cluster’s gravitational potential well. They have most likely evolved through a long history of accretion and merger activity (e.g., see Gallagher & Ostriker, 1972; Merritt, 1983; de Lucia & Blaizot, 2007).

1.2.2 Cooling ﬂows in galaxy clusters

Massive galaxy clusters can be broadly divided into two classes based on the density profile of the X-ray-emitting halo. The cooling-flow or cool-core clusters have an X-ray surface-brightness distribution that is sharply peaked within several hundreds of parsecs of the cluster centre, whereas non-cool-core systems have a flatter central profile (e.g., Forman & Jones, 1982). In cool-core clusters, the bright core emission implies high 4 1. Introduction central gas densities, and the radiative cooling time of the hot gas can therefore be as low as 3 108 yr, significantly shorter than the lifetime of the cluster (e.g., Peres et al., ∼ × 1998). In the absence of a reheating energy source, this cooling gas would form a large- scale, pressure-driven cooling flow into the cluster centre (Cowie & Binney, 1977; Fabian & Nulsen, 1977; Mathews & Bregman, 1978). Cooling-flow models of X-ray clusters 1 predict mass inflow rates of up to 1000 M yr− in the most massive cool-core clusters ⊙ (e.g., see Fabian et al., 1984, for a review). However, these high accretion rates are not reflected in the results from recent observations of cluster cores. Over the last decade, a range of empirical results, in particular those from X-ray spectroscopy, have modified the picture of strong cooling flows in cluster centres (see Peterson & Fabian, 2006). For example, the cooling and mass deposition rates derived from models fit to X-ray emission-line spectra are substantially lower than pure cooling model predictions (e.g., Peterson et al., 2001; Tamura et al., 2001; Peterson et al., 2003). The data indicate that the actual level of cooling to low temperatures in cluster cores is 1 on the order of 1 100 M yr− , approximately a factor of ten times less than in classical − ⊙ cooling-flow models (e.g., Voigt & Fabian, 2004; Peterson & Fabian, 2006, and references therein). For this to be the case, there must be a mechanism operating in cluster cores that can produce sufficient heat to offset most of the radiative energy loss from the halo gas. The principal candidates for this process are thermal conduction from the outer cluster halo (e.g., Bertschinger & Meiksin, 1986; Narayan & Medvedev, 2001; Zakamska & Narayan, 2003; Voigt & Fabian, 2004), and energy output from AGN in brightest cluster galaxies (see the review by McNamara & Nulsen, 2007, and references therein). Although the relative contributions of these processes are not yet well constrained, there is ample evidence that outflows from active nuclei in BCGs interact strongly with the surrounding medium ICM. They are currently favoured to be a dominant source of the energy that moderates cooling flows.

1.2.3 AGN feedback in brightest cluster galaxies

Most BCGs are observed to host some level of radio activity in the core. Surveys indicate that at least 70% of cool-core BCGs are associated with a radio source (Burns, 1990; Ball et al., 1993; Mittal et al., 2009). The radio sources are generally low-power, 32 1 1 Fanaroff-Riley type I radio galaxies (L1.4GHz < 10 ergs s− Hz− ; Fanaroff& Riley, 1974; Bridle & Perley, 1984), with diffuse, double-lobed structures that are often irregular or disturbed. Detailed X-ray imaging has shown that in cluster cores the extended lobes often coincide with cavities or depressions in the emission from the hot cluster gas. These cavities appear to have been excavated as the radio lobes expand and displace the thermal ICM gas, as illustrated in Figure 1.2 (McNamara & Nulsen, 2007). In some systems, multiple cavities are observed at increasing distance from the nucleus (e.g., Fabian et al., 2002; Wise et al., 2007). These include so-called ghost cavities that lack accompanying high- 1.2 Feedback in massive galaxy clusters 5

Figure 1.2 The left panel shows a Chandra X-ray Observatory image of the core of the Hydra Cluster, which has two distinct cavities in the ICM. In the right image, an optical image of the cluster from the Canada-France-Hawaii Telescope and Digitized Sky Survey is superposed with the radio image from the Very Large Array in red and the X-ray image in blue. The X-ray gas appears to have been displaced and the cavities are filled with radio-emitting plasma driven by the AGN. Image source: http://chandra.harvard.edu/photo/2009/hydra/more.html. frequency radio emission. They are assumed to represent older generations of cavities formed by previous episodes of nuclear activity, and to reveal a history of intermittent nuclear activity and interactions between the radio outflows and hot gas in the cluster cores. Surveys of BCGs and their environments provide evidence that there are strong links between the cooling cluster gas, star formation and radio activity in BCGs. Higher rates of star formation occur in systems that have central cooling times below 0.5 Gyr and ∼ are located within 20 kpc of the gravitational centre of the cluster (Rafferty et al., ∼ 2008). In addition, radio and Hα emission are more pronounced in the BCGs of clusters with short central cooling times (Cavagnolo et al., 2008; Edwards et al., 2007; McDonald et al., 2011a). These findings suggest that AGN activity is linked to the cooling of the surrounding medium and could be responsible for regulating the rate of ICM cooling and star formation in the central galaxy. McNamara & Nulsen (2007) provide a comprehensive review of the prevailing model for heating through the interaction of radio bubbles and X-ray cavities in cluster halos. In this model, the AGN jets drive expanding lobes of low-density, relativistic plasma outward through the inter-galactic medium (e.g., Sutherland & Bicknell, 2007; Wagner &Bicknell, 2011). These bubbles rise buoyantly through the thermal gas of the cluster, and heat the surrounding gas through the transfer of kinetic energy to the material as it is displaced, which is then dissipated in the wake of the bubble (e.g., Brüggen & Kaiser, 2002; Churazov et al., 2002; Bˆırzan et al., 2004). The estimated mechanical energy required to form the bubbles, and the power of the AGN and radio jets, are found to be consistent with the energy needed to quench cooling flows in many systems. Certainly in a population-averaged sense AGN in BCGs have been found to be energetically capable 6 1. Introduction of offsetting cooling in clusters (e.g., Rafferty et al., 2006; Dunn & Fabian, 2006; Best et al., 2006; Allen et al., 2006; McNamara et al., 2009). Models of episodic AGN heating and feedback provide at least a qualitative explanation for how the heating and regulation of cooling flows may occur (e.g., Binney & Tabor, 1995; Jones et al., 1997; Tucker & David, 1997; Ciotti & Ostriker, 2001; Pizzolato & Soker, 2005). In this paradigm, the BCG and its environment experience cyclic periods of cooling and inflow that feed nuclear activity, and heating and outflow powered by the AGN. In the cold feedback model described by Pizzolato & Soker (2005), feedback occurs throughout the cool inner region of a cluster core, over radii of 5 30 kpc. An out- ∼ − burst from the AGN in the central galaxy injects energy into the surrounding medium. Bipolar jets form and drive expanding plumes of hot, radio-emitting plasma that rise buoyantly in the cluster atmosphere. These interact with the surrounding galaxy and displace and heat the cooling cluster gas. This heating slows fuel supply to the AGN and, as outflow activity decreases, material in the cluster core begins to cool. Clumps that become sufficiently dense fall inward and, with some initial angular momentum, may form an accretion disk. A reservoir of cold gas accumulates in the central galaxy. Some of this cooling material will contribute to star formation, while some is accreted into the nucleus and fuels the SMBH. A large mass of accreting material will trigger another burst of nuclear activity and continue the feedback cycle. The diversity of properties of the multi-wavelength emission from cluster cores might be explained, at least in part, because they are observed while undergoing different phases of such feedback cycles (e.g., Wilman et al., 2009). We investigate this idea using the sample of galaxies studied in this work.

1.3 Extended emission in brightest cluster galaxies

This thesis focuses on the extended, filamentary, line-emitting nebulae that often accom- pany BCGs in cool-core clusters. These can extend over tens of kiloparsecs around the host galaxy. A famous example is found in the luminous and relatively nearby Perseus Cluster, where the central galaxy, NGC 1275 (Perseus A), is surrounded by a complex network of filaments that extends over 100 kpc, shown in Figure 1.3. Since the discovery of extended emission-line nebulae in BCGs and other elliptical galaxies (e.g., Heckman, 1981; Hu et al., 1985), there has been growing interest in the role and nature of ionised gas in early-type galaxies. Systematic spectroscopic studies (e.g., Phillips et al., 1986; Shields, 1991; Goudfrooij et al., 1994; Macchetto et al., 1996; Sarzi et al., 2006), have shown that ionised gas is prevalent in elliptical galaxies, including radio galaxies (Baum & Heckman, 1989; Baum et al., 1990, 1992). In deep, integral-field spectroscopy (IFS) of 48 elliptical and lenticular galaxies within the recent SAURON project, Sarzi et al. (2006) find that up to 75% of early-type galaxies may harbour ∼ large quantities of ionised gas. Schawinski et al. (2007) compiled 16,000 early-type Astron. Nachr. / AN 330,No.9/10,1040–1042(2009)/DOI 10.1002/asna.200911290

NGC 1275: Baryonic cycling in the Perseus cluster of galaxies

J.S. Gallagher

Dept Astronomy, University of Wisconsin, 475 N. Charter St., Madison, WI 53706, USA

Received 2009 Sep 29, accepted 2009 Sep 29 Published online 2009 Oct 20

Key words cooling flows – galaxies: clusters: individual (Perseus) – galaxies: individual (NGC 1275) NGC 1275, the central galaxy of the Perseus cluster, contains a remarkable cool ISM. This gas is arrayed in a series of filaments extending over 30 kpc with an extension of 70 kpc to the north. These emit substantial cooling line radiation from H II,ahotH2 zone, and CO in a cool molecular region which contains most of the gas mass. This contribution briefly discusses some of the issues raised by the existence of this remarkable ISM system, including its connections to galactic feedback processes. 1.3 Extended emission in brightest cluster galaxies 7 c 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1IntroductionFigure 1.3 Hα imaging of NGC 1275 in N the Perseus Cluster shows the spectacular NGC 1275 is a nearbysystem example of line-emitting of a central filaments cluster that galaxy sur- z = 0.018 E located in a cool core.rounds It thiscontains BCG an (Conselice extraordinary et al. array, 2001 of). optically visible structures,This figure including is taken from filamentsGallagher of gas(2009 and). young stars, shells driven by young stars, extensive dust obscuration, and young massive star clusters. It also hosts apowerfulAGNresponsiblebothforadoubleloberadio source and Seyfert nucleus (Pedlar et al. 1990). The co- existence of these features suggests this galaxy is in the midst of a period of rapid evolution, and thus offers an op- portunity to explore the operation of feedback processes in a highly dissipative environment (e.g. McNamara, O’Connell &Sarazin1996). Abell 426, the Perseus cluster of galaxies, is a large sys- 1 tem with a 1-D velocity dispersion of σ =1030km s− .It is at D =75Mpc and has a dynamical mass of 1015 M . ∼ ⊙ In the absence of additional power sources, the central ICM 30" would cool in less than a Hubble time, depositing several 10 kpc z = 0.018 hundred solar masses per year of gas into the central galaxy. As in other cases, this is notobserved,and is evidenceforef- Fig. 1 The NGC 1275 galaxy showing the filament system in the fective feedbackgalaxies mechanisms from theto limit Sloan the Digital cooling Sky rate Surveyof the (SDSS;emissionYork lines of et H al.α,+2000[N II)]. that This image, were visually covering 100 kpc in ∼ ICM (see Donahueselected & Voit to have 2004; morphologies Peterson & Fabian of S0 2006; or laterthe into vertical a much direction, larger, is but from shallower, the WIYN 3.5-m sample, telescope (Con- McNamara & Nulsen 2007 for reviews). selice, Gallagher & Wyse 2001). and found that nebular line emission is detected in over 20% of the SDSS spectra. The One immediate signature of feedback comes from the extended emission-line regions of early-type galaxies are seen in both cluster and isolated complex brightness structure of the core X-ray emission. emission are interpreted as relic features where the thermal X-ray imagesenvironments show that the and core have contains a variety large amounts of morphologies,gas has beenkinematics, evacuated and by spectral radio emitting properties bubbles produced of substructure(e.g., (seeBaum Churazov et al. et, al.1990 2003),; Sarzi which et al.Chandra, 2006). by the powerful AGN (e.g. Heinz, Reynolds & Begelman shows includes several X-ray dark voids, areas of reduced Galactic emission-line spectra can be classified1998; Reynolds in a useful et al. way 2001). using In the the optical emission- NGC 1275 is sur- X-ray intensity, as well as a number of shell-like features rounded by its famous spider web of gas filaments (Fig. 1), line flux ratio diagnostic diagrams developed by Baldwin et al. (1981) and Veilleux & (Fabian et al. 2003, 2006). The dark zones of low X-ray which is the focus of this short contribution. Osterbrock (1987), sometimes referred to as BPT diagrams. These utilise the ratios Based in part on observations made with the NASA/ESA Hubble Space Telescope,of obtained flux at from the Space strong Telescope emission Science Institute,lines in which the optical2 Nature spectrum of the that giant are close gas filaments together in is operated by thewavelength, Association of Universities to minimise for Research the e inffect Astronomy, of reddening. Empirical and theoretical divisions Inc., under NASA contract NAS 5-26555. These observations are associated with programhave GO-10546. been defined (e.g., Kewley et al., 2001The; Kau extensiveffmann system et al., of2003 gas; filamentsKewley in et NGC al., 1275 with Corresponding2006 author:; Schawinski [email protected] et al., 2007) that separate star-formingtheir H II spectrum or H ii has-region-like been known spectra for more from than 50 years. AGN-dominated spectra, and low-ionisation LINER1-type emission.

c 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Schawinski et al. (2007) found that early-type galaxies exhibit largely LINER-like or composite/star-forming integrated emission-line ratios, as illustrated in Figure 1.4,which is taken from their paper (see also Best et al., 2007). This figure shows the distribution of the integrated emission-line flux ratios from their large early-type galaxy sample on the BPT diagnostic diagrams. Sarzi et al. (2010) provide a comprehensive investigation and discussion of the emission mechanisms that contribute to nebular ionisation in early- type galaxies. They note that there is a significant variation in the [O iii]/Hβ flux

1Low-Ionisation Nuclear Emission Region; named for the nuclear phenomenon that is observed in many galaxy cores, including spiral galaxies (e.g., see Heckman, 1980; Filippenko, 2003; Ho, 2008), the term LINER is now used more generally to refer to emission-line spectra with similar strong low-ionisation features. 8 1. Introduction 4 Kevin Schawinski et al.

Figure 1.4 Integrated line flux ratios for a large sample of early-type galaxies are plotted on opticalFigure 2. The emission-line BPT line diagnostic diagnostic diagrams for the diagrams. early-type galaxie ThesoftheMOSESsample.Eachgalaxyiscolouredbyitsopticalu coloured markers represent galaxies-r in colour a large(see colour bar in left-hand panel). Point sizes scale with the galaxy velocity dispersion as an indicator of mass (see legend in left-hand panel). In each diagram, we sampleindicate the of demarcation early-type lines used galaxies in our classification from sc theheme. SDSS, The dashed selected line in the [NII]/H byα visuallydiagram (left-hand inspecting panel) is the the empirical morphology star formation line inof SDSS Kauffmann imaging et al. (2003) ( (labelledSchawinski Ka03), while et the al. solid, 2007 curve). (in all The three colour panels) is the denotes theoretical the maximumu starbur colourrst model from of the Kewley galaxy, et al. (2001) and(labelled the Ke01). marker The galaxies size in represents between these two the lines central are SF-AGN velocity composites dispersion. or ’transition region We objects’. overplot Galaxie− sbelowtheKa03linearedominated data from Crawford by star formation. The division line between Seyferts and LINERs is shown as the straight line following Kewley et al. (2006) for the middle and right-hand etpanel al. and(1999 our own) fordefinition the (seeROSAT text) for theBrightest left-hand panel. Cluster Sample (black points), an X-ray-selected sample of BCGs. 2004) and Gaussian emission-line templates to the data. In these fits Phillips, & Terlevich 1981, hereafter BPT). They allow the sep- we account also for the impact of diffuse dust in the galaxy of dust aration of galaxies into those dominated by ongoing star forma- ratiosin the emission-line that are regions,observed which in allows the to extended obtain a decre emission-linement tion and non-stellarregions processes; of early-type and with sufficient galaxies, informa bothtion can 2 withinon the strength individual of the Balmer nebulae lines that and is at leastbetween what expec ditedfferentfurther galaxies, split those and into Seyfertthat aAGN range and LINERs of ionisation.Thediagrams by recombination theory. From the fit to the stellar continuumand also contain a transition region, where the emission lines indicate a mechanismsabsorption features, are welikely measure theto line-of-sight contribute. velocitydisper- blend of star formation and AGN activity. sions. From subtraction of the emission-line spectrum from the ob- We use the four optical line ratios [OIII] λ5007/Hβ,[NII] 2 servedThe one, integrated we get the clean flux-ratio absorption line measurements spectrum free from forλ a6583/H sampleα,[SII] ofλ6583/H X-ray-selectedα and [OI] λ6583/H BCGsα.Wefollowthefrom emission line contamination. We then use this cleaned spectrum to signal-to-noise criterion of Kauffmann et al. (2003) and classify themeasure long-slit the stellar spectroscopic absorption indices. The survey physical of constCrawfordraints on etall al. galaxies(1999 that) have are a S/N also> 3detectionofH overlaid onα,H theβ,[OIII] plotsλ5007 the emission from high-order Balmer lines ensures the strength of and [NII] λ6583. The two low ionisation species [SII] and [OI] are intheFigure corresponding 1.4 absorption(blackpoints). features is correctly These estima emission-lineted, which used nebulae when detected, fall largely as they are in usually the weaker LINER than the region other four. ofisthe crucial diagrams for constraining and the ages show of stellar generally populations lower. [O iii]/Hβ ratios among the early-type galaxy 2.5.1 Diagnostic diagrams sample.2.4.2 Lick absorption line indices The corresponding three diagnostic diagrams are shown in Fig. 2. OnSarzi each spectrum et al. we( measure2010) the emphasise 25 standard Lick that absorption the line presenceEach of galaxy a hot is coloured phase by of its interstellar optical u − r colour. or intra- Point sizes clusterindices (Worthey medium et al. 1994; may Worthey contribute & Ottaviani to 1997) producing follow- scale the with line the galaxyemission velocity in dispersion BCGs. as an indicator They of alsomass. In ing the most recent index definitions of Trager et al. (1998). For each diagram, we indicate the demarcation lines used in our clas- observethis purpose LINER the spectral emission resolution is reduced and associated to the wavelength- dustsification features scheme. in Inthe the left-handmostmassive column, we show ellipticals the [NII]/Hα dependent Lick resolution (Worthey & Ottaviani 1997). The mea- diagram used to separate star forming objects (blue) by meansof insurements their are sample then corrected – those for velocity that dispersion can retain broadening. a significantthe demarkation hot line gas by Kauffmann halo. However, et al. (2003, dashed there line) is.We The correction factor is evaluated using the best fitting stellar tem- verified that the somewhat more restrictive separation between star onlyplate and one velocity BCG dispersion (M87/NGC obtained previously. 4486) Errors amongare deter- their observedforming and galaxies. AGN suggested Many by Stasińska of these et al. (2006) massive does not mined by Monte Carlo simulations on each spectrum individually alter the results of this work. The remaining objects are divided in ellipticalsbased on the signal-to-noise also exhibit ratios soft provided X-ray by the emissionSDSS.Possi- thatcomposite is coincident Transition Region with objects the optical (purple) and filaments AGN using the (Youngble discrepancies et al. between, 2002 the; flux-calibratedSparks etSDSS al.spectral, 2004 system; Trinchieritheoretical & maximum Goudfrooij starburst, 2002 model; fromWerner Kewley et et al. al. (200, 1, and the Lick system are negligible, as shown in a detailed analysis solid lines). 2011of Lick), standard as is starsseen observed in BCGs with SDSS (see(CarsonSection 2007). 1.3.1). The AGN are then further sub-classified into Seyferts (green) and LINERs (red) by the straight solid lines. The more indica- 2.5Though Emission line not diagnostics unique to BCGs, the characteristictive low low-ionisation ionisation species (middle emission-line and right-hand panel) regions are used areEmission observed line diagnostic in at diagrams least 45% are a powerful of central way to cluster probe galaxies, and nearly all cool-core systems the nature of the dominant ionising source in galaxies (Baldwin, 2 LINER: low ionisation nuclear emission line region (Crawford et al., 1999; Edwards et al., 2007). In addition, a number of surveys show that presence of extended LINER-like emission in BCGs is strongly correlated!c 2007 RAS, with MNRAS short000,1–18 cooling times in the cluster core and closer proximity of the BGC to the peak of the X- ray surface brightness, suggesting a link between the hot and warm gas phases (Edwards et al., 2007; Rafferty et al., 2008; Cavagnolo et al., 2008; McDonald et al., 2010). Emis- sion from radio to X-ray wavelengths is detected from the extended filaments of BCGs,

2 From the ROSAT (ROentgen SATellite) Brightest Cluster Sample (BCS; Ebeling et al., 1996) 1.3 Extended emission in brightest cluster galaxies 9 revealing the presence of multi-phase gas and dust, and, in some cases, signiﬁcant star formation within the galaxies and their nebulae.

1.3.1 Emission properties

At radio frequencies, CO line emission that is detected in a number of BCGs (e.g., Edge, 2001; Edge & Frayer, 2003; Salomé& Combes, 2003) indicates the presence of large masses (109 1011.5 M ) of molecular gas at cool temperatures (10 100 K). In several − ⊙ − systems, the cold gas has been detected over extended regions that coincide with the optical filaments (e.g., NGC 1275; Braine et al., 1995; Salomé& Combes, 2004; Salomé et al., 2006; Ho et al., 2009; Saloméet al., 2011). Recent detections of far-infrared atomic cooling lines in line-luminous BCGs with the Herschel Space Observatory corroborate the presence of over 109 M of cold molecular gas in the galaxies (Edge et al., 2010a). ⊙ These data also show some evidence that the gas may be irradiated by a strong ultraviolet field that is probably produced by young, hot stars (Edge et al., 2010a). The molecular line measurements from radio and infrared observations indicate that large reservoirs of cold, molecular gas exist in BCGs, presumably comprising (at least in part) gas cooled from the surrounding hot interstellar and intra-cluster media. As more BCGs are observed, Herschel will provide further insight into the nature and extent of the cold gas, particularly in observations of nearby systems where it may be possible to trace the kinematics of the gas for comparison with the optical emission properties. In the future, the Atacama Large Millimeter/submillimeter Array3 (ALMA) will significantly advance our understanding of the cold phases of material in the filaments. It will enable sensitive, high-resolution interferometry of CO, C ii, and HCN lines to trace the kinematics, structure and extent of the coldest, most dense material, and to determine whether these correlate with the warm, optically-emitting gas. Luminous molecular hydrogen lines are detected from the filaments at mid- and near- infrared wavelengths (e.g., Falcke et al., 1998; Edge et al., 2002). These observations show that molecular material is present wherever there is ionised gas in the nebulae and suggest that the material in the extended filaments is predominantly molecular (e.g., Jaffe et al., 2005; Johnstone et al., 2007; Saloméet al., 2011). Rotational transitions of H arise in material at 300 400 K. The relative intensities of these lines are consistent 2 − with collisional excitation of the gas (Johnstone et al., 2007). Rovibrational lines of H2 emission, seen in the near-infrared, are produced in gas with temperatures of 2000 K ∼ (e.g., Edge et al., 2002; Egami et al., 2006a). This H2 line emission has similar extent, morphology and kinematics as the optical emission-line gas in the filaments (e.g., Wilman et al., 2002; Jaffe et al., 2005; Hatch et al., 2005; Wilman et al., 2009; Oonk et al., 2010). These correlations imply that the molecular and atomic ionised emission lines are excited by a common or related mechanism.

3 Early science observations with an array of 16 antennas began late in 2011 and the full facility, comprising ﬁfty 12m antennas, is due to be completed in 2013. 10 1. Introduction

In the optical waveband, the filaments produce strong emission lines from ionised and partially-ionised gas at temperatures on the order of 104 K. The spectrum is dominated by low-ionisation species, and includes strong lines of [N ii], [S ii], [O i], and [O ii], in addition to Balmer hydrogen recombination lines (e.g., Heckman, 1981; Johnstone et al., 1987). The mass of ionised gas in the filaments amounts to only a small fraction of the total estimated mass of molecular material, suggesting that the filaments may consist largely of dense, cool molecular cloud cores, surrounded by layers of warmer, increasingly-ionised material (e.g., O’Dea et al., 1994; Johnstone et al., 2007; Salomé et al., 2011). At higher temperatures, the presence of gas cooling through 3 105 Kisin- ∼ × ferred from measurements of [O vi] emission from the filaments of several BCGs in Far-Ultraviolet Spectroscopic Explorer (FUSE) observations (Oegerle et al., 2001; Breg- man et al., 2006). Canning et al. (2011b) also report a detection of the coronal emission line [Fe x] λ6374, emitted by gas at approximately (1 5) 106 K, in the filaments of − × NGC 4696 in the Centaurus cluster. Soft X-ray emission from gas at temperatures of 106 107 K also traces regions of the filamentary structures in some systems, for ex- − ample: NGC 4696 (Fabian et al., 2005), NGC 1275 (Fabian et al., 2006), Abell 1795 (Fabian et al., 2001), and M 87 (Sparks et al., 2004), though there is not generally a close correlation between the surface brightness of the X-ray and optical line emission (e.g., in M 87 and Abell 1795). There is evidence for a significant amount of dust associated with the filaments in BCG nebulae (e.g., Sparks et al., 1989; Donahue & Voit, 1993), despite the high- temperature ambient environment of the ICM ( 107 K), in which unshielded dust grains ∼ are easily destroyed through collisional sputtering by hot ions (e.g., Dwek & Arendt, 1992). Dust continuum emission is seen at sub-millimetre and mid-infrared wavelengths (Edge et al., 1999; Egami et al., 2006b). Recent Herschel observations (Edge et al., 2010b) imply masses of dust greater than 3 107 M , at low temperatures of 20 28 K, × ⊙ − in several luminous cool-core BCGs. Dust lanes are also seen in absorption, in some cases with features clearly tracing the optical filaments (e.g., Sparks et al., 1989; McNamara et al., 1996; O’Dea et al., 2004; Crawford et al., 2005; Werner et al., 2011). At least half of BCGs in cool-core clusters show evidence for significant star formation (e.g., Johnstone et al., 1987; McNamara & O’Connell, 1989; Quillen et al., 2008), from 1 rates of a few solar masses per year to up to approximately 100 M yr− (Crawford et al., ⊙ 1999; McNamara et al., 2006; Rafferty et al., 2006; O’Dea et al., 2008). Excess mid- infrared continuum emission is detected in the most line-luminous BCGs, over similar extents as the filamentary optical nebulae (Quillen et al., 2008; O’Dea et al., 2008). Several BCGs (Egami et al., 2006b; Quillen et al., 2008) produce sufficient infrared 11 luminosity to be classified as luminous infrared galaxies, or LIRGs, with LIR > 10 L ⊙ (Sanders & Mirabel, 1996). The infrared emission is generally attributed to emission from young stars, reprocessed by dust in the filaments (e.g., O’Dea et al., 2008; Donahue 1.3 Extended emission in brightest cluster galaxies 11 et al., 2011). Blue and ultraviolet excess continuum emission is also observed in some BCG spectra, consistent with young stellar populations photoionising gas in the filaments (e.g., Allen et al., 1992; Crawford & Fabian, 1993; Koekemoer et al., 1999; O’Dea et al., 2004; Hicks & Mushotzky, 2005; O’Dea et al., 2010; Hicks et al., 2010). That far- ultraviolet, molecular, and optical emission are observed to be closely associated in the nebulae suggests that star formation occurs in molecular gas within the optical emission- line filaments. The estimated star formation rates in the galaxies (assuming a Salpeter initial mass function) are consistently less than the rates of gas cooling that are inferred from X-ray spectroscopy, by factors of 3 10 (O’Dea et al., 2008; Rafferty et al., ∼ − 2006). So, just as only a fraction of the hot gas in the cluster core is cooling, not all of the cooling gas is immediately converted to stars in these galaxies. In the context of feedback models, it is therefore of interest to determine how closely the level of AGN feedback is associated with star-formation activity, and the timescales over which this energy output can offset the progression of gas cooling and condensation to form stars. Though the morphology and kinematics of the emission-line nebulae in different systems are diverse, there are some systematic trends that have been observed in the bulk properties of BCGs. In particular, surveys show that BCGs with more luminous line emission have:

shorter central cluster cooling times (i.e., higher central X-ray luminosity and · inferred mass deposition rates); increased blue or ultraviolet excess emission (e.g., Heckman et al., 1989; Allen · et al., 1992; Crawford et al., 2005); greater infrared luminosity and mid-infrared excesses (Egami et al., 2006a; O’Dea · et al., 2008; Quillen et al., 2008); larger masses of molecular gas associated with the filaments and central galaxy · (e.g., O’Dea et al., 2008); more extensive emission-line regions (Crawford et al., 1999); · decreasing flux ratios of collisionally-excited [N ii] and [S ii]linestoHα emission, · as shown in Figure 1.5 (Crawford et al., 1999); and approximately increasing flux ratios of [O i], [N i] relative to hydrogen emission · (Heckman et al., 1989; Crawford et al., 1999).

Despite the rapidly growing body of observational data, two principal questions about the line-emitting filaments in BCGs still need to be answered: i) what is the origin of the material in the filaments, and ii) what are the physical mechanisms that excite the peculiar emission spectrum? The first relates to the formation, support, and fate of the extended structures, and the second has particular implications for the energetics of the system. Both are important in understanding the feedback cycles in which the emission mechanism and the gas may play a part. The ROSAT Brightest Cluster Sample ± III 879

Figure 6. Ha surface brightness plotted against the diameter of the nebula for all galaxies with spatially resolved spectra. High-Ha-luminosity systems [L(Ha)> 1041 erg sÀ1] are shown by solid circles, and the lower luminosity line emitters by open circles. The crosses shown at an arbitrary value of Ha surface brightness of 1038:2 erg sÀ1 kpcÀ2 indicate the diameters of the [N ii]- only emitters. this galaxy was not observed at the parallactic angle (and so Hb may have been preferentially been lost from the slit relative to Ha).

3.10 Relation of the stellar indices to line luminosity

We plot the values of Mg2 and D4000 for the newly observed galaxies (IDS spectra) in Fig. 12, excluding spectra marked as noisy in Table 4. The indices are all measured from a central ,10-kpc aperture, and the galaxies are marked by symbols dependent on both the presence and strength of Ha line emission in the spectrum. The high-luminosity systems are marked by solid circles, and the Figure 8. Plot of the line intensity ratios [O iii]l5007/Hb (top) and lower luminosity systems by open circles of the same size. [N ii]- [S ii]l6717/Ha (bottom) against [N ii]l6584/Ha. Symbols as in Fig. 7. only12 and non-line emitters are also marked by open circles, but of a 1. Introduction smaller size as shown in the key to the ®gure. Observations not taken at the parallactic angle (which may affect the measured value of D4000) are marked also by a cross. Fig. 13 shows the same data, but now including stellar indices measured from the spectra pre- Figuresented 1.5 in A92Crawford and C95, et al. and(1999 again) excluding spectra marked as observednoisy an in Table anti-correlation 4. The larger between scatter within the plot is mainly due to the integratedthe variety of [N projectedii]/Hα flux apertures ratios from which the FOS and WHT andspectra the Hα areluminosities extracted. None of the the less, fil- the same trend as in Fig. 12 is amentsapparent; in BGCs the stronger from the lineROSAT emitters show a signi®cantly bluer Brightestspectrum, Cluster i.e., Sample a lower (BCS).D4000 for a given Mg2 index. The lower luminosity line emitters show stellar indices little different from the general population of BCG. The association of high-luminosity line emitters with the excess blue continuum is also apparent from a plot of Ha line luminosity against dBR for all apertures (Fig. 14), whether these quantities are corrected for internal reddening or not. The distribution of dBR observed in the non-line-emitting BCG in the sample is also plotted by diamond markers at the arbitrary value of L(Ha) 3 ´ 1038 erg sÀ1 , using only spectra from the ,10-kpc apertures. The size of diamond marker plotted is directly propor- tional to the number of non-emitting galaxies with that value of ii Figure 7. Plot of the Ha slit luminosity against the [N ]l6584/Ha line dBR, where the majority lie in the range 0:08 < dBR < 0:11. Of the intensity ratio, for all emission-line objects in this paper. High-Ha-lumin- three non-emitting systems with dBR 0:05, two are not at the osity1.3.2 systemsOrigin are marked of by the solid filaments circles, and lower luminosity ones by open circles. Outliers from the general trend are marked by stars (at higher parallactic angle. There are a total of six non-line-emitters with 0.14< dBR<0.20, and only one above dBR of 0.20 (A1366 at dBR of ionization)The material or by open in trianglesthe filaments (at lower could ionization; originate two markers from which either the intra-cluster medium that overlap). 0.23). Similar values are found for the dBR values for the data from surrounds the BCG, from other galaxies of the cluster through tidal interactions and qstripping,1999 RAS, MNRAS or from306 within, 857±896 the central galaxy itself. The contributions from each source and the processes that form the filamentary nebula are difficult to identify quantitatively, even in detailed studies of individual systems. The longevity of the extended filaments poses a challenge for models of their origin. High-angular-resolution observations of NGC 1275 suggest that the filamentary structures consist of threads that are only 70 pc wide ( 0.2 ; Fabian et al., 2008). Yet ∼ ∼ some of these filaments appear to stretch continuously over several kiloparsecs (Fabian et al., 2008). High velocity dispersions of 100 200 km s 1 (e.g., Hatch et al., 2007) are ∼ − − measured in the gas of the filaments, suggesting turbulent motions that would disperse the gas in timescales on the order of only 107 years. The line-of-sight velocities in the fil- 1 aments have similar magnitudes to the line-of-sight velocity dispersions (< 500 km s− ), so, if unconfined, the filaments should disperse before the gas can extend over kiloparsec lengths. Consequently, Fabian et al. (2008) suggest that magnetic fields provide support and are critical in forming the narrow, coherent shapes and dynamics of the filaments. There is some indication from observations of Faraday rotation that magnetic fields are enhanced in the centres of clusters (e.g., see Carilli & Taylor, 2002; Vogt & Enßlin, 2005; Taylor et al., 2006a), but without knowledge of the magnetic field structures in cluster cores, magnetic confinement remains speculative, and how the filaments are stabilised is an ongoing debate. Integral-field spectroscopic studies provide kinematic information over a wider area of the filaments than previous long-slit observations. However, a principal difficulty remains in identifying whether the observed line-of-sight motions are due to infall or outflow, as it is often difficult to establish the orientation of the filaments. Observations of different galaxies reveal a diverse range of kinematic properties. In some systems, there is clear 1.3 Extended emission in brightest cluster galaxies 13 component of rotation in the nebula, or the motions are consistent with free-fall of the gas within the BCG gravitational potential (e.g., Wilman et al., 2009). Others have less- ordered velocity fields, and in some cases filaments have been observed to exhibit both redshifted and blueshifted motions along the length of apparently continuous structures (Hatch et al., 2007), complicating the simpler question of either infall or outflow. Arguably, the two most popular models that explain the presence of the filaments in BCGs are cooling from the X-ray-emitting ICM, and entrainment by buoyant radio bubbles produced by the AGN in the central galaxy. As mentioned previously, the model of expanding AGN radio lobes has received much attention as a method for reheating cooling flows in cluster cores (e.g., Churazov et al., 2001; Reynolds et al., 2005; Vernaleo & Reynolds, 2007; Revaz et al., 2008). In this scenario, the radio-emitting bubbles contain low-density plasma and rise buoyantly, transporting energy from small to large radii in the cluster core. As the lobes are inflated, they entrain cool gas from the central regions of the BCG. The filaments may be formed from material that remains in the wake of the bubbles, as cool gas condenses from dense clumps formed by thermal instabilities around the lobes (e.g., Churazov et al., 2001; Fabian et al., 2003; Reynolds et al., 2005; Revaz et al., 2008). Simulations of this process can produce structures that resemble the shapes of filaments observed in the system of NGC 1275 (e.g., Reynolds et al., 2005; Revaz et al., 2008), and the process may be able to explain observations of both infalling and outflowing velocities along the filaments as a stretching of these structures (Hatch et al., 2006). There is evidence, however, that the emission nebulae are directly linked to the cooling ICM in clusters. For example, the luminosity of line emission in BCGs is well- correlated with the X-ray cooling luminosity in cluster cores, and only weakly, if at all, with the strength of the radio emission from the central galaxy (Edwards et al., 2007; Cavagnolo et al., 2008; McDonald et al., 2010, 2011a). In addition, the line-emitting gas is confined to the centre of the cluster, within radii where the cooling time is less than several gigayears (McDonald et al., 2010). Early cooling-flow models predicted that Hα-emitting filaments would directly trace a cooling flow (e.g., Cowie et al., 1980; Fabian et al., 1984; Heckman et al., 1989), and recent hydrodynamic simulations show that filamentary structures can form in cool gas flows (e.g., Hattori et al., 1995; Pope et al., 2008a,b; Ceverino et al., 2010). Sharma et al. (2010) suggest that the cooling gas experiences thermal instabilities along magnetic field lines, which can cause the material to coalesce into dense filaments. McDonald et al. (2010, 2011a) argue that the data best support this scenario, but that a homogenous sample of kinematic observations of the filaments in BCGs is needed to constrain the formation mechanisms. Cold gas and star formation in BCGs are found almost exclusively in cool-core clusters. Even if not linked directly to the cooling ICM, this may be related to the presence of the dense, cool cores of these clusters through ram-pressure stripping from other cluster galaxies (Kirkpatrick et al., 2009). This effect would be enhanced by the high-density 14 1. Introduction core in the ICM and may be sufficient to remove cold gas from cluster galaxies that traverse the core, which will be deposited onto the BCG. Reynolds et al. (2008) propose that a plume of X-ray emission in Abell 4059, in which they measure anomalously high metallicities, consists of gas stripped from a starburst galaxy that plunged through the core of the cluster. However, they acknowledge that this would be a rare event. Kirk- patrick et al. (2009), too, find that it is unlikely that the amount of gas observed in BCGs could have been accumulated through this mechanism. Mergers and tidal stripping are also likely to contribute, but are not unique to cool cores and so probably do not generate the bulk of the gas reservoirs in cool-core BCGs (Kirkpatrick et al., 2009). It has also recently been suggested that normal stellar mass loss within the BCGs may be an important source of the dusty, star-forming gas in BCGs (Voit & Donahue, 2011). Gas that is entrained from the central galaxy by buoyant bubbles produced in the AGN may originate in the atmospheres of evolved red giant stars and planetary nebulae (e.g., Canning et al., 2011a). Voit & Donahue (2011) find that in many cases the stellar mass-loss rate in BCGs is comparable to the inferred star formation rates, and that the gas shed by the galactic stellar population may remain cool and distinct from its hot surroundings, so that it maintains its dust content. The close association between the dust features and extended emission regions observed in BCGs is not expected in models of ICM cooling. There is not yet a consensus on the predominant source, or sources, of the filament gas or the mechanism of formation. The processes that contribute may also depend on the state of heating and cooling, and the phase of feedback that is active in a cluster core.

1.3.3 Excitation of the extended emission

The source of energy and the mechanisms that excite the emission from the filaments in BCGs are also not yet completely understood. A variety of models have been considered, but have difficulty matching the emission-line spectra and energetics of the systems. Mechanisms that potentially contribute to powering the nebulae include photoionisation by radiation from stars, the central AGN, or X-rays from the ICM; conduction of heat between the ICM and cool filament material; shock excitation; or collisional heating by high-energy particles (see e.g., Johnstone & Fabian, 1988; Crawford & Fabian, 1992; Donahue et al., 2000; Sabra et al., 2000; Wilman et al., 2002; Hatch et al., 2007; Johnstone et al., 2007; Ferland et al., 2009, and references therein). There is general agreement that multiple excitation mechanisms participate in powering the emission from the filaments and contribute to varying degrees among different clusters, and perhaps within individual nebulae (e.g., Voit & Donahue, 1997; Hatch et al., 2007; Edwards et al., 2009; Ogrean et al., 2010; Donahue et al., 2011). The importance of star formation and stellar radiation in exciting the line emission in BCGs is much discussed in the literature. As has often been noted, the low-ionisation, 1.3 Extended emission in brightest cluster galaxies 15 or LINER-like, optical spectrum (in particular very high ratios of [N ii]toHα flux, strong lines of [O i], and relatively weak [O iii] lines) and strong molecular hydrogen line emission are not reproduced by pure photoionisation models (e.g., Voit & Donahue, 1997; Jaffe et al., 2001, 2005). However, observations of blue and ultraviolet excess emission indicate that star formation is common in BCGs, occurring in systems with low central cooling times (e.g., Rafferty et al., 2008; Hicks et al., 2010), at rates that 1 generally increase, up to 100 M yr− , in more X-ray-luminous clusters (Crawford ∼ ⊙ et al., 1999; Egami et al., 2006b; O’Dea et al., 2008, 2010; McDonald et al., 2011b). In some cases the star formation is clearly extended and associated with the filaments, though there is not a consistent correspondence between the presence of extended line emission and far-ultraviolet emission in BCGs. Some systems exhibit coincident Hα and far-ultraviolet emission (e.g., O’Dea et al., 2010; McDonald et al., 2011b), while others have optically-emitting filaments that lack far-ultraviolet emission, or extended structures of far-ultraviolet emission with no detected Hα emission (e.g., Abell 1837, Abell 2029). This reinforces that a single excitation mechanism, such as photoionisation by hot stars, is not solely responsible for the emission. However, young stars may contribute significantly to the ionisation of gas in the filaments in the more luminous BCGs. It has been proposed, then, that photoionisation by hot young stars becomes increasingly dominant in high-luminosity systems over an underlying source that excites the low-ionisation optical spectrum and the H2 line emission (e.g., Voit & Donahue, 1997; Hatch et al., 2006). In the low-luminosity systems, where the rate of mass cooling tends to be lower, this underlying mechanism may dominate the excitation. Star formation rates are reduced so there are fewer hot stars to photoionise the gas. This leads to a question of whether such a change in the relative importance of stellar photoionisation and the underlying heating mechanism(s) could explain trends in the emission properties such as the observed anti-correlation between Hα luminosity and [N ii]/Hα line flux ratio (Figure 1.5). The model is consistent with some of the observations that resolve spatial variations in the emission within individual nebulae. In several cases it has been found that the emission-line ratios are locally more LINER-like in dusty regions without blue excess emission, and more HII-region-like in regions associated with excess blue continuum emission that are presumably dominated by stellar photoionisation (Hatch et al., 2007). Molecular hydrogen line emission is observed in many BCG systems, including the most luminous sources, at a level that is not consistent with that expected in star-forming systems of similar infrared luminosities (Jaffe & Bremer, 1997; Jaffe et al., 2005; Egami et al., 2006a; Donahue et al., 2011). The H2 luminosity is only weakly correlated with the infrared luminosity and derived star formation rates, and so is probably independently excited by a supplementary heating mechanism (Donahue et al., 2011). The nature of the underlying source that can produce the low-ionisation emission in 16 1. Introduction the absence of stellar photoionisation is a subject of some debate. As noted by Don- ahue et al. (2011), non-radiative agents that can penetrate the molecular gas clouds are required. Ferland et al. (2008, 2009) suggest that cosmic rays provide such a mechanism. They have produced comprehensive models of the emission produced by collisional heating of dense molecular clouds by high-energy particles and dissipative magnetohy- drodynamic (MHD) waves. The models reproduce the H2 emission-line spectrum in dusty gas that is well shielded from external sources of ionising photons (Ferland et al., 2008). The cosmic-ray heating models can reproduce the infrared and optical emission- line fluxes (Ferland et al., 2008, 2009) in the outermost filaments in NGC 1275 to within a factor of two. However, we note that the cosmic-ray flux that occurs in BCG systems is largely unknown and these models require a range of ionisation rates that is approximately 102 106 times the Galactic background cosmic-ray energy density (Ferland et al., − 2009). The sources of the required population of suprathermal particles are uncertain. The mechanism that excites LINER spectra in general is still a subject of debate. The emission has been attributed to low-level AGN activity when observed as a nuclear phenomenon (e.g., Veilleux & Osterbrock, 1987; Filippenko, 2003), and to shock excitation in extended emission regions (e.g., Dopita & Sutherland, 1995, 1996; Clark et al., 1998; Monreal-Ibero et al., 2006). Extended LINER emission is observed in a variety of galaxy environments, including in regions of gas collision in galaxy mergers and interactions (e.g., Monreal-Ibero et al., 2006; Rich et al., 2011), accretion flows (Dopita et al., 1997), and starburst winds (e.g., Rich et al., 2010; Sharp & Bland-Hawthorn, 2010). Shock excitation of BCG nebulae has been considered in some circumstances (e.g., Heckman et al., 1989), and requires further investigation and modelling. Egami et al.

(2006a) have proposed that the near-infrared H2 emission may be consistent with being shock excited in the very line-luminous BCG of ZwCl 3146. Shock waves are likely to be prolific in the cluster environment, and the expansion of the radio lobes is an obvious source of mechanical energy to drive shocks. In simulations, Sutherland & Bicknell (2007) find that AGN jets are efficient at driving shocks into and crushing clouds in an inhomogeneous intergalactic medium, and that this can result in disruption of the jet. Low-density gas clouds in the medium are shock-heated and ablated, whereas high- density clouds remain in place and cool. Such processes are consistent with the model of cold feedback in cluster cores.

1.4 Thesis summary

This thesis presents a set of detailed studies of the extended optical emission structures in a sample of brightest cluster galaxies. We endeavour to understand the physical processes that occur in the warm, ionised gas and how they are linked to AGN feedback in these systems. It is important that we develop such an understanding if, as it is believed, galaxy feedback dominates the mass and energy balance in the cores of these 1.4 Thesis summary 17 massive clusters. In this case, what we learn about the physical mechanisms in low- redshift BCGs will aid in studies of high-redshift systems and the feedback processes that formed galaxies in the early universe. In particular, studies of low-redshift BCGs, where the emission structures may be spatially resolved, will improve our understanding of the physics of feedback and assist in the interpretation of observations of other galactic and cluster systems. Through this investigation of the optical emission in a small sample of brightest cluster galaxies, we address questions about the emission that is detected and the structures from which it originates such as:

How do the morphologies and kinematics of the optical nebulae compare with the · emission observed at other wavelengths in these systems, in particular the X-ray and radio emission? Is there evidence for interaction between AGN outflows and the emission-line gas? Is gas in the filaments infalling, and perhaps largely associated with the cooling · and accretion processes of feedback, or outflowing, and influenced by the AGN jets and heating? Do the diagnostic emission-line flux ratios vary across the nebulae, such that dif- · ferent excitation mechanisms may dominate in different regions? How is the optical emission excited? To what extent does the emission spectrum · match models of shock excitation? Does star formation dominate on small scales even in systems with low Hα lumi- · nosities? How do the overall trends in emission properties with total luminosity relate to · the model of feedback in clusters?

To study these issues, we have conducted deep integral-field spectroscopic observations to measure the optical emission spectrum and emission-line ratios. Our optical data allow us to study the properties of the warm, atomic component of the line-emitting gas that traces the filamentary nebulae surrounding the BCGs. We directly obtain line-of- sight velocities and velocity dispersions of the warm gas in a sample of ten BCGs with a range of total emission-line luminosities. By employing an integral-field spectrograph we can measure the emission across most of the emission regions in the galaxies. We combine these data with previous observations from the literature, including X-ray and radio data, to investigate the dynamics and emission properties of these systems. Studies of the multi-phase gas surrounding BCGs, particularly including integral-field spectroscopic data that provide kinematic information about the emission-line regions, will help us to understand which, if any, properties of the extended emission systems provide evidence of feedback activity. Further, this may allow us to definitively place individual BCG systems within the sequence of a heating/cooling cycle, and potentially to constrain the physics of feedback models in clusters. 18 1. Introduction

1.4.1 Brightest cluster galaxy sample

A sample of BCGs was chosen for study from previous optical and X-ray surveys. These galaxies reside in clusters with short central cooling times and were known to exhibit strong low-ionisation emission lines in their spectra. The targets were selected to be accessible from the southern observing site at Siding Spring Observatory in Australia, and from a range of right ascension to facilitate observations spread over one year, the time available for data collection. The galaxies were observed with the Wide-Field Spectrograph (WiFeS), a new optical integral-field spectrograph that was installed at the Australian National University (ANU) 2.3 m telescope in April 2009. A set of relatively low-redshift targets with low-to-intermediate emission-line luminosities were chosen for detailed study with WiFeS. These are objects that were known to have line emission extended over an angular extent similar to the 25 38 field of × view of the spectrograph. The relatively poor seeing conditions of the observing site at Siding Spring Observatory also limit the spatial resolution of the observations. We have therefore selected relatively nearby galaxies in which the filament structures can be resolved. These targets have also been previously studied at other wavelengths, and radio and X-ray data are available for comparison. The galaxies have emission-line luminosities that fall in the lower half of the range of luminosity covered in the BCS survey by Crawford et al. (1999), so that the LINER emission mechanism likely dominates the excitation, and a contribution from star formation is minimised. This set of targets is listed in the top part of Table 1.1, with the total on-source exposure times that were obtained with WiFeS. NGC 4696 in the Centaurus Cluster, a well-known cool-core system, was selected as the prototype target of these studies. Deep optical imaging has revealed a complex system of optical filaments surrounding the galaxy (Crawford et al., 2005) that has an extent on the sky that is similar to the field of view of WiFeS. In addition to the low and intermediate-luminosity targets, a sample of BCGs with emission-line luminosities at the high end of the luminosity range were selected for a comparative study. These systems cover a redshift range of z 0.08 0.3 and include two ∼ − of the most line-luminous BCG nebulae known: those of ZwCl 3146 and RXC J1504.1- 0248. The galaxies have high line luminosity, but are more distant, so that the emission is less extended on the sky and therefore less well-resolved spatially in our observations. Thus, shorter total integration times were obtained for most of these objects, with the aim of measuring the bright emission from the principal, large-scale structures in the nebulae and the integrated emission properties. The observed targets are shown in the lower section of Table 1.1. We have also obtained long-slit spectra of the BCGs in ZwCl 3146 and ZwCl 348 using the Gemini Multi-Object Spectrograph (GMOS). These provide spectra at higher spatial resolution than the WiFeS data across a restricted regions of these galaxies. In addition, archival GMOS IFS data, are available for ZwCl 3146 and RXC J1504.1- 1.4 Thesis summary 19

Table 1.1 Brightest cluster galaxy sample

Cluster ID BCG ID zL(Hα) Integration 41 1 (BCG) (10 erg s− ) time (hr)

Abell 3526 / Centaurus NGC 4696 0.010 0.18 18.5 Abell 3581 IC 4374 / PKS 1404-267 0.022 0.47 11.5 2A 0335+096 2MASX J03384056+0958119 0.035 4.9 11.0 Sersic 159-03 ESO 291-9 0.056 2.3 11.5 High-luminosity systems Abell 2597 PKS 2322-12 0.082 18 10.5 RXC J1524.2-3154 2MASX J15241295-3154224 0.102 12 4.5 PKS 0745-19 PKS 0745-19, PGC 021813 0.103 33 6.0 RXC J1504.1-0248 SDSS J150407.51-024816.5 0.217 130 4.0 ZwCl 348 RX J0106.8+0103 0.254 63 4.0 ZwCl 3146 SDSS J102339.64+041110.7 0.290 170 2.0

0248, and these have also been analysed. These data cover a 3 5 ﬁeld of view with × higher spatial resolution than the WiFeS observations, but with wavelength coverage that includes only the blue end of the optical spectrum.

1.4.2 Outline

This thesis work is presented with the following structure. Chapter 2 presents a study of the emission-line gas in NGC 4696, the prototype target of the project. A deep optical IFS study of the extended emission was conducted with WiFeS. Material along the principal bright ﬁlament is found to be infalling and most likely the result of a recent minor merger with a gas-rich cluster companion. In Chapter 3,wedescribetheresults of a similar study of the three other BCGs from the sample that produce emission with relatively low line luminosities and are at similarly low redshifts as NGC 4696. We discuss the state of feedback in these systems as revealed by the results of our optical study in conjunction with data from radio and X-ray observations. Chapter 4 presents the results of the study of the sample of more line-luminous BCG systems. We discuss the excitation properties of the nebulae and present evidence that shocks play a signiﬁcant role in exciting the gas in these galaxies. Detailed descriptions of the individual galaxies, and the observed morphologies and kinematics derived from the WiFeS and GMOS data are provided in Appendix A. Finally, a summary of the results and the conclusions of the work is provided in Chapter 5, with comments on the direction of future research on these topics. 1 1 Throughout this work we adopt the cosmological parameters H0 = 71 km s− Mpc− ,

ΩM =0.27 and ΩΛ =0.73, based on the ﬁve-year Wilkinson Microwave Anisotropy Probe (WMAP)results(Hinshaw et al., 2009). 20 1. Introduction