The space density of spiral galaxies as function of their scale size, surface brightness and luminosity

The local space density of galaxies as a function of their basic structural parameters -luminosity, surface brightness and scale size- is still poorly known. Our poor knowledge is the result of strong selection biases against low surface brightness and small scale size galaxies in any optically selected sample. We derive bivariate space density distributions by correcting a sample of ~1000 local Sb-Sdm spiral galaxies for its selection effects. We present a parameterization of these bivariate distributions, based on a Schechter type luminosity function and a log-normal scale size distribution at a given luminosity. We next calculate the bivariate distributions as function of redshift using the Hubble Deep Field, and conclude that at higher redshift there is a decrease in space density of luminous, large scale size galaxies, but the density of smaller galaxies stays nearly the same.Comment: 8 pages, 2 figures. To appear in the conference proceedings of "Toward a New Millennium in Galaxy Morphology" edited by D.L. Block, I. Puerari, A. Stockton and D. Ferreira (Kluwer, Dordrecht

The Space Density of Spiral Galaxies as function of their Luminosity, Surface Brightness and Scalesize

The local space density of galaxies as a function of their basic structural parameters --like luminosity, surface brightness and scalesize-- is still poorly known. Our poor knowledge is mainly the result of strong selection biases against low surface brightness and small scalesize galaxies in any optically selected sample. We show that in order to correct for selection biases one has to obtain accurate surface photometry and distance estimates for a large (>~1000) sample of galaxies. We derive bivariate space density distributions in the (scalesize, surface brightness)-plane and the (luminosity, scalesize)-plane for a sample of ~1000 local Sb-Sdm spiral galaxies. We present a parameterization of these bivariate distributions, based on a Schechter type luminosity function and a log-normal scalesize distribution at a given luminosity. We show how surface brightness limits and (1+z)^4 cosmological redshift dimming can influence interpretation of luminosity function determinations and deep galaxy counts.Comment: 8 pages, 3 figures, to appear in the conference proceedings of IAU Colloquium 171, "The Low Surface Brightness Universe

The clustering of dark matter halos: scale-dependent bias on quasi-linear scales

We investigate the spatial clustering of dark matter halos, collapsing from $1-4 \sigma$ fluctuations, in the redshift range $0 - 5$ using N-body simulations. The halo bias of high redshift halos ($z \geq 2$) is found to be strongly non-linear and scale-dependent on quasi-linear scales that are larger than their virial radii ($0.5-10$ Mpc/h). However, at lower redshifts, the scale-dependence of non-linear bias is weaker and and is of the order of a few percent on quasi-linear scales at $z \sim 0$. We find that the redshift evolution of the scale dependent bias of dark matter halos can be expressed as a function of four physical parameters: the peak height of halos, the non-linear matter correlation function at the scale of interest, an effective power law index of the {\it rms} linear density fluctuations and the matter density of the universe at the given redshift. This suggests that the scale-dependence of halo bias is not a universal function of the dark matter power spectrum, which is commonly assumed. We provide a fitting function for the scale dependent halo bias as a function of these four parameters. Our fit reproduces the simulation results to an accuracy of better than 4 % over the redshift range $0\leq z \leq 5$. We also extend our model by expressing the non-linear bias as a function of the linear matter correlation function. It is important to incorporate our results into the clustering models of dark matter halos at any redshift, including those hosting early generations of stars and galaxies before reionization.Comment: Accepted for publication in MNRA

The evolution of the stellar mass versus halo mass relationship

We present an analysis of the predictions made by the Galform semi-analytic galaxy formation model for the evolution of the relationship between stellar mass and halo mass. We show that for the standard implementations of supernova feedback and gas reincorporation used in semi-analytic models, this relationship is predicted to evolve weakly over the redshift range 0<z<4. Modest evolution in the median stellar mass versus halo mass (SHM) relationship implicitly requires that, at fixed halo mass, the efficiency of stellar mass assembly must be almost constant with cosmic time. We show that in our model, this behaviour can be understood in simple terms as a result of a constant efficiency of gas reincorporation, and an efficiency of SNe feedback that is, on average, constant at fixed halo mass. We present a simple explanation of how feedback from active galactic nuclei (AGN) acts in our model to introduce a break in the SHM relation whose location is predicted to evolve only modestly. Finally, we show that if modifications are introduced into the model such that, for example, the gas reincorporation efficiency is no longer constant, the median SHM relation is predicted to evolve significantly over 0<z<4. Specifically, we consider modifications that allow the model to better reproduce either the evolution of the stellar mass function or the evolution of average star formation rates inferred from observations.Comment: Submitted to MNRAS after first round of referee comments, 26 pages, 16 figures, fixed a referenc

A dynamical model of supernova feedback : gas outflows from the interstellar medium.

We present a dynamical model of supernova feedback which follows the evolution of pressurized bubbles driven by supernovae in a multiphase interstellar medium (ISM). The bubbles are followed until the point of break-out into the halo, starting from an initial adiabatic phase to a radiative phase. We show that a key property which sets the fate of bubbles in the ISM is the gas surface density, through the work done by the expansion of bubbles and its role in setting the gas scaleheight. The multiphase description of the ISM is essential, and neglecting it leads to order-of-magnitude differences in the predicted outflow rates. We compare our predicted mass loading and outflow velocities to observations of local and high-redshift galaxies and find good agreement over a wide range of stellar masses and velocities. With the aim of analysing the dependence of the mass loading of the outflow, β (i.e. the ratio between the outflow and star formation rates), on galaxy properties, we embed our model in the galaxy formation simulation, GALFORM, set in the Λ cold dark matter framework. We find that a dependence of β solely on the circular velocity, as is widely assumed in the literature, is actually a poor description of the outflow rate, as large variations with redshift and galaxy properties are obtained. Moreover, we find that below a circular velocity of ≈80 km s−1, the mass loading saturates. A more fundamental relation is that between β and the gas scaleheight of the disc, hg, and the gas fraction, fgas, as β∝h1.1gf0.4gas, or the gas surface density, Σg, and the gas fraction, as β∝Σ−0.6gf0.8gas. We find that using the new mass loading model leads to a shallower faint-end slope in the predicted optical and near-IR galaxy luminosity functions

A dynamical model of supernova feedback: gas outflows from the interstellar medium

We present a dynamical model of supernova feedback which follows the evolution of pressurised bubbles driven by supernovae in a multi-phase interstellar medium (ISM). The bubbles are followed until the point of break-out into the halo, starting from an initial adiabatic phase to a radiative phase. We show that a key property which sets the fate of bubbles in the ISM is the gas surface density, through the work done by the expansion of bubbles and its role in setting the gas scaleheight. The multi-phase description of the ISM is essential, and neglecting it leads to order of magnitude differences in the predicted outflow rates. We compare our predicted mass loading and outflow velocities to observations of local and high-redshift galaxies and find good agreement over a wide range of stellar masses and velocities. With the aim of analysing the dependence of the mass loading of the outflow, beta (i.e. the ratio between the outflow and star formation rates), on galaxy properties, we embed our model in the galaxy formation simulation, GALFORM, set in the LCDM framework. We find that a dependence of beta solely on the circular velocity, as is widely assumed in the literature, is actually a poor description of the outflow rate, as large variations with redshift and galaxy properties are obtained. Moreover, we find that below a circular velocity of 80km/s the mass loading saturates. A more fundamental relation is that between beta and the gas scaleheight of the disk, hg, and the gas fraction, fgas, as beta hg^(1.1) fgas^(0.4), or the gas surface density, \Sigma_g, and the gas fraction, as beta \Sigma_g^(-0.6) fgas^(0.8). We find that using the new mass loading model leads to a shallower faint-end slope in the predicted optical and near-IR galaxy luminosity functions.Comment: 33 pages. Accepted for publication in MNRAS. Difference with previous version is a discussion of the effect of extreme interstellar medium conditions on beta and outflow velocit

Constraining SN feedback: a tug of war between reionization and the Milky Way satellites

Theoretical models of galaxy formation based on the cold dark matter cosmogony typically require strong feedback from supernova (SN) explosions in order to reproduce the Milky Way satellite galaxy luminosity function and the faint end of the field galaxy luminosity function. However, too strong a SN feedback also leads to the universe reionizing too late, and the metallicities of Milky Way satellites being too low. The combination of these four observations therefore places tight constraints on SN feedback. We investigate these constraints using the semi-analytical galaxy formation model galform. We find that these observations favour a SN feedback model in which the feedback strength evolves with redshift. We find that, for our best fit model, half of the ionizing photons are emitted by galaxies with rest-frame far-UV absolute magnitudes $M_{\rm AB}(1500{\rm \AA})<-17.5$, which implies that already observed galaxy populations contribute about half of the photons responsible for reionization. The $z=0$ descendants of these galaxies are mainly galaxies with stellar mass $M_*>10^{10}\,{\rm M}_{\odot}$ and preferentially inhabit halos with mass $M_{\rm halo}>10^{13}\,{\rm M}_{\odot}$.Comment: 17 pages, 12 figures, Accepted for publication on MNRA

Semi-analytical galaxy formation models and the high redshift universe

Semi-analytical models of galaxy formation based on hierarchical clustering now make a wide range of predictions for observable properties of galaxies at low and high redshift. This article concentrates on 2 aspects: (1) Self-consistent modelling of dust absorption predicts a mean UV extinction A_{UV} ~ 1 mag, depending only weakly on redshift, and similar to observational estimates. (2) The models predict that the Lyman-break galaxies found at z ~ 3 should be strongly clustered with a comoving correlation length r_0 = 4-7 Mpc/h (depending on the cosmology), in good agreement with subsequent observational determinations.Comment: 5 pages, 3 figures. To appear in Proceedings of the MPA/ESO Conference "Evolution of Large-scale Structure: from Recombination to Garching", ed. A.J. Banday et a

The Evolution of Galaxy Clustering in Hierarchical Models

The main ingredients of recent semi-analytic models of galaxy formation are summarised. We present predictions for the galaxy clustering properties of a well specified LCDM model whose parameters are constrained by observed local galaxy properties. We present preliminary predictions for evolution of clustering that can be probed with deep pencil beam surveys.Comment: To appear in the proceedings of the Marseille IGRAP99 conference "Clustering at High Redshift

Simulations and modelling of the ISM in galaxies

The latest observations of molecular gas and the atomic hydrogen content of local and high-redshift galaxies, coupled with how these correlate with star formation activity, have revolutionized our ideas about how to model star formation in a galactic context. A successful theory of galaxy formation has to explain some key facts: (i) high-redshift galaxies have higher molecular gas fractions and star formation rates than local galaxies, (ii) scaling relations show that the atomic-to-stellar mass ratio decreases with stellar mass in the local Universe, and (iii) the global abundance of atomic hydrogen evolves very weakly with time. We review how modern cosmological simulations of galaxy formation attempt to put these pieces together and highlight how approaches simultaneously solving dark matter and gas physics, and approaches first solving the dark matter N-body problem and then dealing with gas physics using semi-analytic models, differ and complement each other. We review the observable predictions, what we think we have learned so far and what still needs to be done in the simulations to allow robust testing by the new observations expected from telescopes such as ALMA, PdBI, LMT, JVLA, ASKAP, MeerKAT, SKA.Comment: To appear in the proceedings of the NRAO meeting: The Interstellar Medium in High Redshift galaxies Comes of Age, September 2012. 10 pages and 4 figure