Galaxy evolution as a function of mass and environment: giant and dwarf galaxies in superclusters and in the field

It has been known for decades that local galaxies can be broadly divided into two distinct populations (e.g. Hubble 1926, 1936; Morgan 1958; de Vaucouleurs 1961). The first population consists in red, passively-evolving, bulge-dominated galaxies mainly populated by old stars that, in the colourmagnitude diagram, makes up the “red sequence”, while the second population makes up the “blue cloud” of young, star-forming, disk-dominated galaxies (e.g. Strateva et al. 2001; Kauffmann et al. 2003a,b; Blanton et al. 2003a; Baldry et al. 2004; Driver et al. 2006; Mateus et al. 2006). It has also long been known that the environment in which a galaxy inhabits has a profound impact on its evolution in terms of defining both its structural properties and star-formation histories (e.g. Hubble & Humason 1931). In particular, passively-evolving spheroids dominate cluster cores, whereas in field regions galaxies are typically both star-forming and diskdominated (Blanton et al. 2005a). These differences have been quantified through the classic morphology–density (Dressler 1980) and star-formation (SF)–density relations (Lewis et al. 2002; G´omez et al. 2003). However, despite much effort (e.g. Treu et al. 2003; Balogh et al. 2004a,b; Gray et al. 2004; Kauffmann et al. 2004; Tanaka et al. 2004; Christlein & Zabludoff 2005; Rines et al. 2005; Baldry et al. 2006; Blanton, Berlind & Hogg 2007; Boselli & Gavazzi 2006; Haines et al. 2006a; Mercurio et al. 2006; Sorrentino, Antonuccio-Delogo & Rifatto 2006a; Weinmann et al. 2006a,b; Mateus et al. 2007), it still remains unclear whether these environmental trends are: (i) the direct result of the initial conditions in which the galaxy forms, whereby massive galaxies are formed earlier preferentially in the highest overdensities in the primordial density field, and have a more active merger history, than galaxies that form in the smoother low-density regions; or (ii) produced later by the direct interaction of the galaxy with one or more aspects of its environment, whether that be other galaxies, the intracluster medium, or the underlying dark-matter distribution. Several physical mechanisms have been proposed that could cause the transformation of galaxies through interactions with their environment such as ram-pressure stripping (Gunn & Gott 1972), galaxy harassment (Moore et al. 1996), and suffocation (also known as starvation or strangulation), in which the diffuse gas in the outer galaxy halo is stripped preventing further accretion onto the galaxy before the remaining cold gas in the disk is slowly consumed through star-formation (Larson, Tinsley & Caldwell 1980). The morphologies and star-formation histories of galaxies are also strongly dependent on their masses, with high-mass galaxies predominately passivelyevolving spheroids, and low-mass galaxies generally star-forming disks. A sharp transition between these two populations is found about a characteristic stellar mass of ∼3 × 1010M, corresponding to ∼M+ 1 (Kauffmann et al. 2003a,b). This bimodality implies fundamental differences in the formation and evolution of high- and low-mass galaxies. The primary mechanism behind this transition appears to be the increasing efficiency and rapidity with which gas is converted into stars for more massive galaxies according to the Kennicutt-Schmidt law (Kennicutt 1998; Schmidt 1959). This results in massive galaxies with their deep potential wells consuming their gas in a short burst (2 (Chiosi & Cararro 2002), while dwarf galaxies have much more extended star-formation histories and gas consumption time-scales longer than the Hubble time (van Zee 2001). In the monolithic collapse model for the formation of elliptical galaxies this naturally produces the effect known as “cosmic downsizing” whereby the major epoch of star-formation occurs earlier and over a shorter period in the most massive galaxies and progressively later and over more extended timescales towards lower mass galaxies. This has been confirmed observationally both in terms of the global decline of star-formation rates in galaxies since z∼1 (Noeske et al. 2007a,b) and the fossil records of low-redshift galaxy spectra (Heavens et al. 2004; Panter et al. 2007). Finally, in analyses of the absorption lines of local quiescent galaxies, the most massive galaxies are found to have higher mean stellar ages and abundance ratios than their lower mass counterparts, confirming that they formed stars earlier and over shorter time-scales (Thomas et al. 2005; Nelan et al. 2005). In this scenario, the mass-scale at which a galaxy becomes quiescent should decrease with time, with the most massive galaxies becoming quiescent earliest, resulting in the red sequence of passively-evolving galaxies being built up earliest at the bright end (Tanaka et al. 2005). However, the standard paradigm for the growth of structure and the evolution of massive galaxies within a CDM universe is the hierarchical merging scenario (e.g. White & Rees 1978; Kauffmann, White & Guideroni 1993; Lacey & Cole 1993) in which massive elliptical galaxies are assembled through the merging of disk galaxies as first proposed by Toomre (1977) (see also Struck 2005). Although downsizing appears at first sight to be at odds with the standard hierarchical model for the formation and evolution of galaxies, Merlin & Chiosi (2006) are able to reproduce the same downsizing as seen in the earlier “monolithic” models in a hierarchical cosmological context, resulting in what they describe as a revised monolithic scheme whereby the merging of substructures occurs early in the galaxy life (z > 2). Further contributions to cosmic downsizing and the observed bimodality in galaxy properties could come from the way gas from the halo cools and flows onto the galaxy (Dekel & Birnboim 2006; Kereˇs et al. 2005) and which affects its ability to maintain star-formation over many Gyrs, in conjunction with feedback effects from supernovae and AGN (e.g. Springel et al. 2005a; Croton et al. 2006). These mechanisms which can shut down star-formation in massive galaxies allow the hierarchical CDM model to reproduce very well the rapid early formation and quenching of stars in massive galaxies (e.g. Bower et al. 2006; Hopkins et al. 2006a; Birnboim, Dekel & Neistein 2007). In particular, the transition from cold to hot accretion modes of gas when galaxy halos reach a mass ∼1012M (Dekel & Birnboim 2006) could be responsible for the observed sharp transition in galaxy properties with mass. If the evolution of galaxies due to internal processes occurs earlier and more rapidly with increasing mass, then this would give less opportunity for external environmental processes to act on massive galaxies. Moreover, lowmass galaxies having shallower potential wells could be more susceptible to disruption and the loss of gas due to external processes such as ram-pressure stripping and tidal interactions. This suggests that the relative importance of internal and external factors on galaxy evolution and on the formation of the SF-, age- and morphology-density relations could be mass-dependent, in particular the relations should be stronger for lower mass galaxies. Such a trend has been observed by Smith et al. (2006) who find that radial age gradients (out to 1Rvir) are more pronounced for lower mass (σ<175kms−1) cluster red sequence galaxies than their higher mass subsample. With all this in mind, we undertook the work presented in this thesis studying galaxy evolution as a function of mass and environment (chapter 1). To this aim, we investigate the evolution of giant and dwarf galaxies in cluster environment (Part I) through the analysis of i) luminosity function and colour distribution (chapter 3), and ii) the fundamental plane of early-type galaxies (chapter 4). We extend, then, our analysis to a wide spread of environments, from the rarefied field to the high density regions, (Part II, chapters 6 and 7). This analysis allowed us to discriminate among the possible physical mechanisms which, driving the star-formation of giant and dwarf galaxies, are able to reproduce the observed bimodal galaxy distribution. Technical aspects of the dataset used throughout the present analysis are presented in chapters 2 and 5

Evolutionary Group Dynamics in Stephan's Quintet. Optical spectroscopy & radio observations with the LBT & IRAM 30m

One of the most fundamental questions in astronomy is that of the evolution of galaxies. Ever since the quantum fluctuations present at the era of recombination, structures have evolved in the Universe. Dark matter halos facilitate congregation of baryonic matter and the gravitational attraction creates groups and clusters of galaxies. Galaxies evolve through both external and internal processes. Internal processes are driven by instabilities in the galactic disk, spiral arms, bars and oval distortions, while external processes are driven by outside forces, such as galaxy mergers and harassment. External processes can have an immense impact on the galaxies involved, by altering the galaxies' morphology and content by moving large amounts of gas and inciting starbursts and active galactic nuclei (AGN). Due to the abundance of gas and the proximity of the galaxies in the early Universe, it is accepted that galaxy mergers/interactions occurred often and were vital in driving galaxy evolution. The environment in which a galaxy resides plays an important role in its formation and evolution, and more than half of the galaxies in the Universe reside in galaxy groups. Compact galaxy groups are perfect laboratories for studying galaxy evolution through extreme galaxy interactions due to their galaxy density and activity. These groups can also reveal key information regarding the connections between galaxies and their environment, as well as details regarding galaxy evolution at high redshift. Low-redshift compact galaxy groups allow us to study the impact of galaxy interactions and mergers on galaxy evolution at high-resolution, thereby providing an insight into the conditions of the galaxies in the early Universe when such interactions were more common. Stephan's Quintet is a nearby compact galaxy group of 5 galaxies with a rich and intriguing history of interactions. The past interactions can be traced via the tidal tails coursing through the group, while the current interaction is causing a galaxy-wide ridge of shock-driven star formation. This shocked star-forming ridge is enabled by the large amount of intergalactic gas present in the group, deposited in the intergalactic medium (IGM) during the previous galaxy interactions. Stephan's Quintet is one of the most well-studied groups in our Universe and every time the group has been observed in a new wavelength window or with higher resolution and sensitivity, new fascinating features have been revealed and our understanding of the processes and structures increased. Multi-wavelength analyses of galaxies are essential, since it is only through comparison and combination of the tracers of galaxy dynamics (i.e., stars, atomic gas and molecular gas) that we can truly study the evolution of galaxies. To fill in blanks in the wavelength ranges covering Stephan's Quintet and contribute to increased understanding of our Universe, I have carried out optical spectroscopy and radio observations. The optical wavelength regime provides information regarding the stellar population as well as the atomic gas, while spectroscopy enables spatial and spectral information to be gathered simultaneously. This facilitates studies of the abundances and kinematics as well as the excitation mechanisms across the group. Covering a part of Stephan's Quintet in multiple slits with the Multi-Object Double Spectrograph at the Large Binocular Telescope in Tucson, Arizona, USA, I am able to achieve a pseudo-Integral Field Spectroscopy observation of the main part of, and the most intense, interactions and activity in group. Focusing on the nucleus of the galaxies and the large-scale dynamics, I detail the kinematics of the IGM and the galaxies. I present an extensive analysis of the mapped area, including fluxes, velocity dispersions, line-of-sight velocities and excitation mechanisms in NGC7319, NGC7318A and B, NGC7317, the bridge, the west ridge and the star-forming ridge. NGC7319 shows a disturbed galaxy, where the gaseous disk is decoupled from the stellar disk. I find a broad line region component in the nucleus, revealing for the first time the Seyfert 1 nature of this galaxy, and I confirm the presence of a blue outflow to the south-west of the nucleus at an average of 476±13.8 km/s. The stellar and gaseous disks are approximately perpendicular to each other and the gas is excited by AGN radiation, indicating that the gas is present in a large-scale nuclear wind. The data further reveal extensive gas emission in the shocked star-forming ridge as well as in the west ridge (south-west of the NGC7318 pair) and the bridge connecting the NGC7318 pair and NGC7319. I confirm dual velocity components (as suggested by Duarte Puertas et al. (2019)) in several parts of the IGM and note that the shock increases the ionisation to LINER-like emission-line ratios in several regions along the star-forming ridge and the west ridge. Furthermore, the multiple velocity components present in many parts of the IGM and galaxies, spanning 5600-7000 km/s, coincide with that of other galaxies, revealing the potential origin. Cold molecular gas, best traced by CO emission detected in radio wavelengths, is a key ingredient in star formation and one of the main ingredients in galaxies. Analysing the behaviour of molecular gas is vital in determining the morphology and understanding the evolution of galaxies. I have, therefore, observed Stephan's Quintet using the IRAM 30m telescope in Sierra Nevada, Spain. Adopting an on-the-fly mapping technique I observed the 12CO(1-0), (2-1) and the 13CO(1-0) emission in a 5.67 arcmin^2 area covering the group. I present maps and spectra of the emission, including abundances and velocities of the respective three CO lines, as well as molecular hydrogen gas masses. I further discuss the line ratios together with the excitation temperatures and the optical depth. I find that the brunt of the CO emission is in/near NGC7319, extending towards and into the bridge and the star-forming ridge. 52-56% of the molecular hydrogen gas mass in Stephan's Quintet is in/near NGC7319, while 38-40% is in the star-forming ridge, the final 4-10% is spread out across the NGC7318 pair and their surroundings. The distribution of 12CO(2-1), however, favours NGC7317, the NGC7318 pair, SQ-A and the star-forming ridge, which retain approximately half of the 12CO(2-1) emission, while NGC7319 contains less than 20%. This highlights the increased temperatures present in the shocked star-forming ridge. The data confirms the presence of multiple velocity components in the group, spanning 5600-7200 km/s. Up to 4 clearly distinguishable velocity components can be found, with NGC7319 and the star-forming ridge showing the highest number of components. Again the velocity components often coincide with that of the other galaxies, tracing the complex history of the group. Furthermore, the CO line ratios indicate optically thick gas at low temperature in NGC7319 and the bridge. While in the star-forming ridge and SQ-A, the gas is found to be dense, optically thick and warm, as expected considering the current interaction of the IGM and NGC7318B. The gas surrounding NGC7318B at a line-of-sight velocity of ~5800 km/s shows an inclination towards being warm, dense and optically thin. This work favours a group evolution scenario of Stephan's Quintet that includes previous interactions of both NGC7319-NGC7317 and NGC7319-NGC7320C, and a scenario in which NGC7318B has passed through and is currently located in front of the group, supported by the multi-component IGM and the tidal streams that connect NGC7318B to the IGM and the galaxies. In addition, NGC7318B increases the group's energy and adds to the IGM gas content which stalls the gas depletion required for aging the group further. The enhancements of the passages of NGC7320C and NGC7318B are expected to be vital in hindering the group's imminent merger into a final fossil state. NGC7318B shows us the impact of diffuse IGM, while NGC7319 reveals a fascinating case of AGN feeding and feedback in a decoupled stellar/gas disk. NGC7319 shows lack of ongoing star formation while still appearing to contain molecular gas, although likely off-nuclear, with an outflow impacting the surrounding gas and structures - raising questions regarding the feeding mechanisms and lifetime of this AGN. Stephan's Quintet differs from other groups due to the prominent extended tidal features and the currently occurring collision with NGC7318B. It is possible that these structures are short-lived and that all compact groups exhibit this kind of variety of galaxy interaction indicators, stellar and gaseous tidal features and galaxy-wide shock structures at some point during their evolution. Understanding these processes in Stephan's Quintet sheds light on the evolution of galaxies at a time in the history of the Universe when gas was abundant and interactions were common

Outflows in Infrared-Luminous Galaxies: Absorption-Line Spectroscopy of Starbursts and AGN

Large-scale galactic outflows, better known as superwinds, are driven by the powerful energy reservoirs in star forming and active galaxies. They play a significant role in galaxy formation, galaxy evolution, and the evolution of the intergalactic medium. We have performed a survey of over 100 infrared-luminous galaxies in order to address the exact frequency with which they occur in different galaxy types, the dependence of their properties on those of their host galaxies, and their properties in the most luminous starburst and active galaxies. Most of our sample consists of ultraluminous infrared galaxies (ULIRGs), and we use moderate-resolution spectroscopy of the NaI D interstellar absorption feature (which directly probes the neutral gas phase). We find superwinds in the majority of these galaxies at typical maximum, de-projected velocities of 500-700 km/s. The detection rate increases with star formation rate (SFR) in starbursts, while the mass outflow rate appears constant with SFR, contrary to theoretical expectations. The resulting mass entrainment efficiencies in ULIRGs are quite low, of order a few percent of the star formation rate. There is some dependence of outflow velocity on host galaxy properties; the outflow velocities in LINERs are higher than those in HII galaxies, and the highest column density gas in each galaxy may have an upper envelope in velocity that increases with SFR. Outflows in most galaxies hosting a dominant AGN have very similar properties to those in starbursts, so discerning their power source is difficult. The velocities in Seyfert 2 outflows may be slightly higher than those in starbursts, and the fraction of neutral gas escaping Seyfert 2s is higher than that in starbursts (~50% vs. &lt;=20%). The outflows in our Seyfert 1 galaxies have extreme velocities of up to ~10^4 km/s, and two of three Seyfert 1s with outflows possess broad absorption lines. Finally, we find that spectroscopy of a few galaxies at very high spectral resolution does not reveal unresolved narrow components. The mass outflow rates at very high resolution are thus comparable to those from our large sample of moderate-resolution spectra, demonstrating the reliability of our moderate-resolution data

The Evolution of Massive Star-forming Galaxies: Energetics and the Interstellar Medium

Over the last ~20 years, the importance of dusty star-forming galaxies in contributing approximately half the energy density of the Universe has been realised. Much research in this field has focused on the subset of submillimetre bright galaxies (SMGs). Submillimetre Astronomy has recently seen major advances due largely to huge developments in the available instrumentation. In this thesis I present the first spectroscopic redshift distribution of unambiguously-identified SMGs, targeted with ALMA. The redshift distribution is shown to peak at z~2.4. The next step to understanding the SMG population is to use their redshifts to facilitate high-resolution follow-up observations, probing the conditions and physical structure within the interstellar medium (ISM) of these systems. I present the detailed observations of the ISM within the gravitationally lensed SMG, SMMJ2135. In particular, the spectral line energy distributions of 12CO, 13CO and C18O are measured and used to infer the temperature, densities and chemical abundances within this intrinsically representative SMG, with strong variation found between the multiple kinematic components in the galaxy. Furthermore, an unusually high abundance of C18O is measured, implying the presence of preferentially massive stars, perhaps highlighting some differences between star formation locally and at high-redshift. The cosmic star-formation rate density has rapidly declined since z~2 and there is much evidence to suggest that massive star-forming galaxies at z~2 may evolve into massive passive elliptical galaxies at z=0. I investigate the potential influence of active galactic nuclei (AGN) on the suppression of star formation within massive elliptical galaxies over z=0.1-1.2. I determine that the hot gas within these evolved systems does not cool as rapidly as expected and demonstrate that heating due to mechanical feedback from radio AGN is more than sufficient to balance the X-ray cooling of hot gas, thus suppressing further star formation