Disk galaxies at Z=0 and at High Redshift: an explanation of the observed evolution of damped Ly-alpha systems

The analysis of disk formation is based on the White & Rees (1978) picture, in which disk galaxies form by continuous cooling and accretion of gas within a merging hierarchy of dark matter halos. A simple Kennicutt law of star formation for disks, based on a single- fluid gravitational instability model is introduced. Since the gas supply in the disk is regulated by infall from the surrounding halo, the gas is always maintained at a critical threshold surface density. Chemical enrichment of the disks occurs when the surrounding hot halo gas is enriched with heavy elements ejected during surpernova explosions. This gas then cools onto the disk producing a new generation of metal-rich stars. I first show that models of this type can reproduce many of the observed properties of a typical spiral galaxy like the Milky Way, including its gas and stellar surface density profiles and the observed relationship between the ages and metallicities of solar neighbourhood stars. I then use the models to make inferences about the properties of disk galaxies at high redshift. The total neutral hydrogen density Omega(HI) increases at higher z. The predicted increase is mild, but is roughly consistent with the latest derivation of Omega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The models are also able to account for some of the other trends other trends seen in the high-redshift data, including the increase in the number of high column-density systems at high redshift, as well as the metallicity distribution of damped Lyman-alpha systems at redshifts 2-3.Comment: 14 pages with 8 figs included, uuencoded postscript file, to appear in the Proceedings of the Workshop on Coldgas at High Redshift, Kluwe

Large scale correlations in gas traced by MgII absorbers around low mass galaxies

The physical origin of the large-scale conformity in the colours and specific star formation rates of isolated low mass central galaxies and their neighbours on scales in excess of 1 Mpc is still under debate. One possible scenario is that gas is heated over large scales by feedback from active galactic nuclei (AGN), leading to coherent modulation of cooling and star formation between well-separated galaxies. In this Letter, the metal line absorption catalogue of Zhu & Menard (2013) is used to probe gas out to large projected radii around a sample of a million galaxies with stellar masses ~10^{10} M_{sun} and photometric redshifts in the range 0.4<z<0.8 selected from Sloan Digital Sky Survey imaging data. This galaxy sample covers an effective volume of 2.2 Gpc^3. A statistically significant excess of MgII absorbers is present around the red low mass galaxies compared to their blue counterparts out to projected radii of 10 Mpc. In addition, the equivalent width distribution function of MgII absorbers around low mass galaxies is shown to be strongly affected by the presence of a nearby (R_p<2 Mpc) radio-loud AGN out to projected radii of 5 Mpc.Comment: 5 pages, accepted in MNRAS Letter

Hierarchical Clustering and the Butcher-Oemler Effect

We show that the rapid evolution in the fraction of blue, star-forming galaxies seen in clusters as a function of redshift (the Butcher-Oemler effect) can be explained very simply if structure formation in the universe proceeeds hierarchically. We show that a rich cluster observed at high redshift has had a significantly different evolutionary history to a cluster of the same richness observed today. High redshift clusters take longer to assemble and thus under- go more merging at small lookback times. We have investigated two models of star formation in cluster galaxies: 1) a model in which star formation is induced by galaxy-galaxy mergers and interactions and 2) a model in which star formation is regulated by the infall of galaxies onto larger systems such as groups and clusters. Both models produce trends consistent with the Butcher- Oemler effect. Our models of cluster formation and evolution allow us to make predictions about trends in the observed properties of clusters with redshift. We find that there should be a correlation between the mass of the cluster or group and the strength of the observed Butcher-Oemler effect, with more massive systems exhibiting more evolution than less massive systems. We also predict that both the blue galaxy fraction and the incidence of interacting galaxies in rich clusters should rise continuously with redshift. Finally, we have explored the influence of cosmological parameters on our predictions for cluster evolution. We find that models in which structure formation occurs at very early epochs, such as low $\Omega$ models, predict rather little recent star formation and merging activity in clusters at redshifts of around 0.4.Comment: Latex file, 12 pages, postscript figures on reques

Physical origin of the large-scale conformity in the specific star formation rates of galaxies

Two explanations have been put forward to explain the observed conformity between the colours and specific star formation rates (SFR/$M_*$) of galaxies on large scales: 1) the formation times of their surrounding dark matter halos are correlated (commonly referred to as "assembly bias"), 2) gas is heated over large scales at early times, leading to coherent modulation of cooling and star formation between well-separated galaxies (commonly referred to as "pre-heating") . To distinguish between the pre-heating and assembly bias scenarios, we search for relics of energetic feedback events in the neighbourhood of central galaxies with different specific star formation rates. We find a significant excess of very high mass ($\log M_* > 11.3$) galaxies out to a distance of 2.5 Mpc around low SFR/$M_*$ central galaxies compared to control samples of higher SFR/$M_*$ central galaxies with the same stellar mass and redshift. We also find that very massive galaxies in the neighbourhood of low SFR/$M_*$ galaxies have much higher probability of hosting radio loud active galactic nuclei. The radio-loud AGN fraction in neighbours with $\log M_* > 11.3$ is four times higher around passive, non star-forming centrals at projected distances of 1 Mpc and two times higher at projected distances of 4 Mpc. Finally, we carry out an investigation of conformity effects in the recently publicly-released Illustris cosmological hydrodynamical simulation, which includes energetic input both from quasars and from radio mode accretion onto black holes. We do not find conformity effects of comparable amplitude on large scales in the simulations and we propose that gas needs to be pushed out of dark matter halos more efficiently at high redshifts.Comment: 8 pages, 8 figures, submitted to MNRA

Properties of AGN selected by their mid-IR colours: evidence for a physically distinct mode of black hole growth

We study the narrow emission line properties and stellar populations of a sample of 1385 AGN selected to have strong excess emission at mid-infrared wavelengths based on comparing Wide-field Infrared Survey Explorer W1-W2 band colours with optical stellar absorption line indicators. Our goal is to understand whether the physical conditions in the interstellar medium of these objects differ from those of local AGN selected by their optical emission line ratios. To enable this comparison, we construct a control sample of 50,000 optically-selected AGN with the same redshifts that do not have strong mid-IR excess emission. The mid-IR excess and control samples differ strongly in [OIII] line luminosity, ionized gas excitation mechanism, ionization state and electron density. We show that the radio-detected, mid-IR excess AGN constitute the most luminous and highly ionized AGN in the local Universe and they contribute primarily to the growth of black holes in the most massive galaxies. At least half of this black hole growth is occurring in galaxies with recent starbursts. The morphologies of these systems indicate that the starbursts have probably been triggered by galaxy-galaxy mergers and interactions. The most luminous AGN in our mid-IR excess sample have properties that are similar to the Type II quasars studied at higher redshifts. In contrast, the control sample constitute a class of lower ionization, less luminous AGN in more quiescent galaxies that contribute primarily to the growth of low mass black holes.Comment: 15 pages, 17 figures, accepted for publication in MNRA

The Outer Stellar Populations and Environments of Unusually HI-rich Galaxies

We investigate the nature of HI-rich galaxies from the ALFALFA and GASS surveys, which are defined as galaxies in the top 10th percentile in atomic gas fraction at a given stellar mass. We analyze outer (R>1.5 Re) stellar populations for a subset of face-on systems using optical g-r versus r-z colour/colour diagrams. The results are compared with those from control samples that are defined without regard to atomic gas content, but are matched in redshift, stellar mass and structural parameters. HI-rich early-type (C>2.6) and late-type (C<2.6) galaxies are studied separately. When compared to the control sample, the outer stellar populations of the majority of HI-rich early-type galaxies are shifted in the colour/colour plane along a locus consistent with younger stellar ages, but similar metallicities. The outer colours of HI-rich late-type galaxies are much bluer in r-z than the HI-rich early types, and we infer that they have outer disks which are both younger and more metal-poor. We then proceed to analyze the galaxy environments of HI-rich galaxies on scales of 500 kpc. HI-rich early-type galaxies with low (log M* < 10.5) stellar masses differ significantly from the control sample in that they are more likely to be central rather than satellite systems. Their satellites are also less massive and have younger stellar populations. Similar, but weaker effects are found for low mass HI-rich late-type galaxies. In addition, we find that the satellites of HI-rich late-types exhibit a greater tendency to align along the major axis of the primary. No environmental differences are found for massive (log M* > 10.5) HI-rich galaxies, regardless of type.Comment: 12 pages, 14 figures, submitted to MNRA

Transport Costs and the Size Distribution of a Linearly Arranged System of Cities

The question regarding the effects of changing transports costs on the size distribution of cities is an important topic of systems of cities research. The so-called New Economic Geography has already given some answers to this question. One central assumption in this kind of model is a very particular, simplified spatial structure. This contribution investigates the consequences of changing transport costs for a system of cities that are located equidistantly on a straight line. In the case of rising transport costs, the main outcome of this model is worker concentration in the central large cities, while the peripheral regions lose residents.transport costs, agglomeration, urban systems