Nuclear star clusters (NSCs) are stellar systems similar in size to globular clusters (GCs) but extremely dense, comparable only to some GCs and ultra-compact dwarfs. They are present in galaxies with a wide range of masses, morphologies and gas content. There are several formation scenarios proposed for the formation of such objects, such as the merger of GCs or extreme star formation caused by the inflow of gas. Recent studies show that the presence of an NSC is related to galaxy stellar mass. Moreover, it has been suggested that NSCs are more often found in high density environments. In our work, we use deep imaging of the core regions of the Coma cluster down to an absolute magnitude of –8.2 and found that in this environment the nucleation fraction is higher than in the Virgo and Fornax clusters. We find nucleated galaxies in Coma as faint as –11.2 mag.

The chemical abundances of the gas-phase and stellar components of disc galaxies are relevant to understand their formation and evolution. It has been shown that an inside-out disc formation yields negative chemical profiles. However, a large spread in metallicity gradients, including positive ones, has been reported by recent and more precise observations, suggesting the action of other physics processes such as gas outflows and inflows, radial migration, and mergers and interactions. Cosmological simulations that includes chemical models provide a tools to tackle the origin of the metallicity profiles and the action of those processes that might affect them as a function of time. I present a summary of the current state-of-knowledge from a numerical point of view and discuss the main results from the analysis of the EAGLE simulations.

We present GASP, an ongoing ESO Large Program with MUSE aiming to study gas removal processes from galaxies at low redshift. GASP targets 100 galaxies with tails, tentacles and one-sided debris. MUSE data allows a detailed investigation of the ionized stripped gas, as well as of the gas and stars within the galaxy out to large distances from the galaxy center. We show the first results for two of the GASP galaxies that are striking cluster jellyfish galaxies of stellar masses ~ 1011M⊙.

We study fundamental properties of transonic galactic outflows in the gravitational potential of a cold dark matter halo (DMH) with a central super-massive black hole (SMBH) assuming a polytropic, steady and spherically symmetric state. We have classified the transonic solutions with respect to their topology in the phase space. As a result, we have found two types of transonic solutions characterized by a magnitude relationship between the gravity of DMH and that of SMBH. These two types of solutions have different loci of the transonic points; one transonic point is formed at a central region (< 0.01kpc) and another is at a distant region (> 100kpc). Also, mass fluxes and outflow velocities are different between the two solutions. These two transonic solutions may play different roles on the star formation history of galaxies and the metal contamination of intergalactic space. Furthermore, we have applied our model to the Sombrero galaxy. In this galaxy, the wide-spread hot gas is detected as an apparent trace of galactic outflows while the star-formation rate is disproportionately low, and the observed gas density distribution is quite similar to the hydrostatic state (Li et al. 2011). To solve this discrepancy, we propose a slowly accelerating outflow in which the transonic point forms in a distant region (~ 120 kpc) and the subsonic region spreads across the stellar distribution. In the subsonic region, the gas density distribution is similar to that of the hydrostatic state. Our model predicts the possibility of the slowly accelerating outflow in the Sombrero galaxy. Igarashi et al. 2014 used the isothermal model and well reproduced the observed gas density distribution, but the estimated mass flux (1.8M⊙/yr) is lager than the mass of the gas supplied by stars (0.3-0.4M⊙/yr). Then, we expect that the polytropic model may reproduce the observational mass of the supplied gas (Igarashi et al. 2015). Such slowly accelerating outflows should be distinguished from the conventional supersonic outflows frequently argued in star-forming galaxies.

Massive early-type galaxies undergo a significant process of evolution with redshift on the stellar mass vs size plane. Furthermore, this trend does not depend on the age of their stellar populations. Therefore, such an evolution should involve processes that do not include a significant amount of star formation, leaving (mostly) dry mergers as the main growth channel. By studying close pairs involving a massive galaxy, one can quantify the role of mergers on the growth of massive galaxies. A recent study based on the SHARDS dataset reveals that minor mergers cannot be the dominant mechanism to explain the bulk of size growth in these systems. Merging is found to provide a constant fractional growth rate of ~10% per Gyr from redshift z=1, corresponding to an overall stellar mass increase of 2× between z=1 and z=0.

By monitoring the arrival times from millisecond pulsars for years to decades, it is possible to search for, or place limits on, nanohertz frequency gravitational radiation. The most promising source of gravitational waves in this band is a stochastic background emitted from a population of supermassive black hole binaries. As these binaries are the direct product of of galaxy mergers and the properties of the SMBHs correlated strongly with their host galaxies, the gravitational wave emission of the binaries can be used to study how galaxies evolve. Here I discuss how pulsar timing can be used to search for gravitational waves, and how limits on the strength of the background are being used to challenge models of supermassive black hole formation and evolution.

The damped Lyman-α absorption-line systems (DLAs) that are observed in quasar spectra arise in neutral-gas-rich regions of intervening galaxies. With the highest neutral hydrogen column densities observed (N(HI) ⩾ 2 × 1020 atoms cm−2), they are known to trace the bulk of the neutral gas content of the Universe, and are thus powerful probes of galaxy formation and evolution. However, DLAs are extremely rare, and since the Lyman-α line falls in the UV for redshifts z < 1.65, not many are known at low redshift due to the limited availability of space data. Our HST surveys for DLAs in strong MgII absorbers have been successful at showing that MgII can be used as an unbiased tracer of DLAs. We present new results on their incidence, or redshift number density, dn/dz, and cosmological neutral gas mass density, ΩDLA, at redshifts 0.11 ⩽ z ⩽ 1.65, and incorporate results from higher and lower redshift studies in the literature to derive the evolution of neutral gas in the Universe.

I summarizes the main characteristics of a cosmological hydro-dynamical model which includes chemical evolution. In the last decade, galaxy formation models have made important progress which contribute to understand observations although there are still many open questions to be answered. Current chemo-dynamical cosmological simulations are able to reproduce global trends. They provide a suitable route to underpin physical events responsible of determining observed chemical patterns.

Massive galaxies with old stellar populations have been put forwards as a challenge to models in which cosmic structures grow hierarchically through gravitational instability. I will explain how the growth of massive galaxies is helped by features of hierarchical models. I give a brief outline of how the galaxy formation process is modelled in hierarchical cosmologies using semi-analytical models, and illustrate how these models can be refined as our understanding of processes such as star formation improves. I then present a brief survey of the current state of play in the modelling of massive galaxies and list some outstanding challenges.

Numerical simulations have shown that strong gravitational torque by non-axisymmetric components induce evolutionary processes such as redistribution of mass and angular momentum in the galactic disks and consequent change of chemical abundance profiles. If we hope to understand chemical evolution gradients and their evolution we must understand the secular processes and re-arrangement of material by non-axisymmetric components and vice-versa. The most obvious of these aforementioned non-axisymmetric components are bars - at least 2/3 of spiral galaxies host a bar, and possibly all disk galaxies have hosted a bar at some point in their evolution. While observationally it has been found that barred galaxies have shallower gas-phase metallicity gradients than non-barred galaxies, a complementary analysis of the stellar abundance profiles has not yet been undertaken. This is unfortunate because the study of both gas and stars is important in providing a complete picture, as the two components undergo (and suffer from) very different evolutionary process. We present here a pilot study of the gas and stellar metallicity and age distributions in a sample of barred and non-barred galaxies using 2D spectroscopic observations. We found that the majority of the stellar mass in our sample is composed of old (~10 Gyr) stars. This is true in the bulge and the disc region, even beyond two disc scalelengths. In the disc region, a larger fraction of young stars is present in the external parts of the disc compared with the inner disc. The disc growth is, therefore, compatible with a moderate inside-out formation scenario, where the luminosity-weighted age changes from ~10 Gyrs in the centre, to ~4 Gyrs at two disc scalelengths, depending upon the galaxy. However, the presence of substructure, like star forming rings, can produce stellar population trends that are not directly related with the growing of the disc but to the bar potential. In the disc region, the metallicity gradient always decrease with the radius. In the bulge region this is not always true and we find inverse metallicity gradients in several galaxies.

Galaxies are found to inhabit a variety of environments. They are often found in pairs, groups, or clusters. Cosmological simulations predict that these clusters are connected on a larger scale by filaments, but because these large scale structures are so vast, and because they are of intermediate density, observational constraints on their properties are difficult to achieve. We find a large-scale filament in the Abell 1763-Abell 1770 superstructure, determine that the star-formation therein is enhanced, and uncover a bent double lobe radio source midway across the filament. From the bend of this AGN'a jets, we probe the density of the surrounding intra-filament medium (IFM), constrained to be between 1-20x10−29 gm/cm3. This density is consistent with direct probes of the IFM as well as theoretical models.

The Arecibo Legacy Fast ALFA survey is in the process of yielding a complete HI dataset of the Virgo Cluster and its environs (Giovanelli et al. 2007, Kent et al., in preparation). Assuming a distance to Virgo of 16.7 Mpc, the minimum detectable HI mass by ALFALFA is of order 2 × 107 M⊙. A number of the HI detections appear to have interesting properties. Some appear associated with, but offset from, low surface brightness optical counterparts; others, at larger spatial offsets, may be tidally related to optical counterparts. Yet another class includes detections which are not identifiable with any optical counterparts. We present the ALFALFA results on these objects in the Virgo region, as well as followup aperture synthesis observations obtained with the VLA.

We present a new numerical model of star formation history and chemical evolution using the color-magnitude diagrams of resolved stars in galaxies. Our model differs from previous models in that it generates the simulated color-magnitude diagrams with the metal enrichment law calculated from the galactic chemical evolution model. We present a case of applying our model to a resolved galaxy in the Local Group: the Sextans dwarf spheroidal galaxy. It is found the the primary origin of the radial gradient of stellar population in this galaxy is star formation followed by metallicity.

The star-formation histories, masses and structural properties of galaxies are strongly dependent on their environment: massive, passively-evolving spheroids dominate cluster cores, while in field regions, galaxies are typically low-mass, star-forming and disk-dominated (e.g Blanton et al. 2005). Equally the global properties of galaxies have been found to be bimodally distributed around a stellar mass of ~3 × 1010 M⊙ (~M*+1) (e.g. Kauffmann et al. 2003). Large-scale spectroscopic surveys have shown that the evolution of massive galaxies is primarily driven by their merger history, rather than through direct interection with the cluster environment, as demonstrated by the finding of massive passively-evolving galaxies in all environments, mild observed environmental trends for their mean stellar ages, and the gradual SF-density relation in which star-formation is still seen to be suppressed in galaxies 3–4 virial radii from the nearest cluster. In contrast, in the dwarf regime (>M*+2) dramatic changes are seen in both the luminosity function and red galaxy fraction between those galaxies in the cores of clusters and those at the virial radius (Mercurio et al. 2006, Haines et al. 2006a).

We have examined the origins of the bimodality observed in the global properties of galaxies by comparing the environmental dependencies of star-formation for giant and dwarf galaxy populations. Using SDSS DR4 spectroscopic data to create a volume-limited sample complete to M*+3, we find that the environmental dependences of giant and dwarf galaxies are quite different, implying fundamental differences in their evolution (Haines et al. 2006b). Whereas the star-formation histories of giant galaxies are determined primarily by their merger history, this is not the case for dwarf galaxies. In particular, we find that old or passive dwarf galaxies are ONLY found as satellites within massive halos (clusters, groups or giant galaxies), with none in the lowest density regions. This implies that star-formation in dwarf galaxies must be much more resilient to the effects of mergers, and that the evolution of dwarf galaxies is primarily driven by the mass of their host halo, through effects such as suffocation, ram-pressure stripping or galaxy harassment.

Galaxy bimodality is caused by the bulge-disc nature of galaxies as opposed to two distinct galaxy classes. This is evident in the colour-structure plane which clearly shows that elliptical galaxies (bulge-only) lie in the red compact peak and late-type spiral galaxies (disc-dominated) lie in the blue diffuse peak. Early-type spirals (bulge plus disc systems) sprawl across both peaks. However after bulge-disc decomposition the bulges of early-type spirals lie exclusively in the red compact peak and their discs in the blue diffuse peak (exceptions exist but are rare, e.g., dust reddened edge-on discs and blue pseudo-bulges). Movement between these two peaks is not trivial because whilst switching off star-formation can transform colours from blue to red, modifying the orbits of ~1 billion stars from a planar diffuse structure to a triaxial compact structure is problematic (essentially requiring an equal mass merger). We propose that the most plausible explanation for the dual structure of galaxies is that galaxy formation proceeds in two stages. First an initial collapse phase (forming a centrally concentrated core and black hole), followed by splashback, infall and accretion (forming a planar rotating disc). Dwarf systems coule perhaps follow the same scenario but the lack of low luminosity bulge-disc systems would imply that the two components must rapidly blend to form a single flattened spheroidal system.

Using the Lyman break technique (e.g., Steidel et al. 1995), large samples of star-forming galaxies at $2 \lt z \lt 5$ have been identified and studied. These Lyman break galaxies (LBGs) are UV-luminous and thought to be similar to local starburst galaxies; they are relatively small (r$_{h}$=1-3 kpc), have relatively low mass (10$^{9.5-11}$M$_\odot$) and low extinction. However, their role in galaxy evolution is still debatable. Do all galaxies go through a Lyman Break phase? How biased is our view of galaxy evolution due to the Lyman break technique? Are LBGs the building blocks of larger systems or just small galaxies having their first burst of star-formation? Therefore, a census of the star-forming galaxy population as a function of time is needed in order to help us better understand how galaxies acquired their present morphology. In this contribution, I discuss the physical properties of a sample of UV-selected galaxies at intermediate redshifts. I conclude by showing that galaxies of all types, sizes and shapes are forming stars at intermediate-z. However, deep image and spectroscopy with large telescopes are needed in order to properly address their nature.