Modelling star formation and feedback in simulations of interacting galaxies

We discuss a heuristic model to implement star formation and feedback in hydrodynamical simulations of galaxy formation and evolution. In this model, gas is allowed to cool radiatively and to form stars at a rate given by a simple Schmidt-type law. We assume that supernova feedback results in turbulent motions of gas below resolved scales, a process that can pressurize the diffuse gaseous medium effectively, even if it lacks substantial thermal support. Ignoring the complicated detailed physics of the feedback processes, we try to describe their net effect on the interstellar medium with a fiducial second reservoir of internal energy, which accounts for the kinetic energy content of the gas on unresolved scales. Applying the model to three-dimensional, fully self-consistent models of isolated disc galaxies, we show that the resulting feedback loop can be modelled with smoothed particle hydrodynamics such that converged results can be reached with moderate numerical resolution. With an appropriate choice of the free parameters, Kennicutt's phenomenological star formation law can be reproduced over many orders of magnitude in gas surface density. We also apply the model to mergers of equal-mass disc galaxies, typically resulting in strong nuclear starbursts. Confirming previous findings, the presence of a bulge can delay the onset of the starburst from the first encounter of the galaxies until their final coalescence. The final density profiles of the merger remnants are consistent with de Vaucouleurs profiles, except for the innermost region, where the newly created stars give rise to a luminous core with stellar densities that may be in excess of those observed in the cores of most elliptical galaxies. By comparing the isophotal shapes of collisionless and dissipative merger simulations we show that dissipation leads to isophotes that are more discy than those of corresponding collisionless simulations.     

The star formation histories of low surface brightness galaxies

We have performed deep imaging of a diverse sample of 26 low surface brightness galaxies (LSBGs) in the optical and the near-infrared. Using stellar population synthesis models, we find that it is possible to place constraints on the ratio of young to old stars (which we parametrize in terms of the average age of the galaxy), as well as the metallicity of the galaxy, using optical and near-infrared colours. LSBGs have a wide range of morphologies and stellar populations, ranging from older, high-metallicity earlier types to much younger and lower-metallicity late-type galaxies. Despite this wide range of star formation histories, we find that colour gradients are common in LSBGs. These are most naturally interpreted as gradients in mean stellar age, with the outer regions of LSBGs having lower ages than their inner regions. In an attempt to understand what drives the differences in LSBG stellar populations, we compare LSBG average ages and metallicities with their physical parameters. Strong correlations are seen between an LSBG's star formation history and its K -band surface brightness, K -band absolute magnitude and gas fraction. These correlations are consistent with a scenario in which the star formation history of an LSBG primarily correlates with its surface density and its metallicity correlates with both its mass and its surface density.     

The role of minor mergers in the recent star formation history of early-type galaxies

We demonstrate that the large scatter in the ultraviolet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation (RSF) in these objects can be reproduced by minor mergers in the standard Λ cold dark matter (ΛCDM) cosmology. Numerical simulations of mergers with mass ratios ≤1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement with the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions ≥20 per cent. Early-types that satisfy  ( NUV − r ) ≲ 3.8  are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ∼1.5 Gyr, while those that satisfy  3.8 < ( NUV − r ) < 5.5  are consistent with either recent mergers with mass ratios ≤1:6 or mergers with higher mass ratios that occurred more than ∼1.5 Gyr in the past. We demonstrate that the early-type colour–magnitude relations and colour distributions, in both the UV and optical spectral ranges, are consistent with the expected frequency of minor merging activity in the standard ΛCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level RSF observed in early-type galaxies in the nearby Universe.     

The evolution of star formation in quasar host galaxies

We have used far-infrared data from IRAS , Infrared Space Observatory ( ISO ), Spitzer Wide-Area Infrared Extragalactic (SWIRE), Submillimetre Common User Bolometer Array (SCUBA) and Max-Planck Millimetre Bolometer (MAMBO) to constrain statistically the mean far-infrared luminosities of quasars. Our quasar compilation at redshifts  0 < z < 6.5  and I -band luminosities  −20 < I _AB < −32  is the first to distinguish evolution from quasar luminosity dependence in such a study. We carefully cross-calibrate IRAS against Spitzer and ISO , finding evidence that IRAS 100-μm fluxes at <1 Jy are overestimated by ∼30 per cent. We find evidence for a correlation between star formation in quasar hosts and the quasar optical luminosities, varying as star formation rate (SFR)  ∝ L ^0.44±0.07_opt  at any fixed redshift below   z = 2  . We also find evidence for evolution of the mean SFR in quasar host galaxies, scaling as  (1 + z )^1.6±0.3  at   z < 2  for any fixed quasar I -band absolute magnitude fainter than −28. We find no evidence for any correlation between SFR and black hole mass at  0.5 < z < 4  . Our data are consistent with feedback from black hole accretion regulating stellar mass assembly at all redshifts.     

Accurate parameter estimation for star formation history in galaxies using SDSS spectra

To further our knowledge of the complex physical process of galaxy formation, it is essential that we characterize the formation and evolution of large data bases of galaxies. The spectral synthesis starlight code of Cid Fernandes et al. was designed for this purpose. Results of starlight are highly dependent on the choice of input basis of simple stellar population (SSP) spectra. Speed of the code, which uses random walks through the parameter space, scales as the square of the number of the basis spectra, making it computationally necessary to choose a small number of SSPs that are coarsely sampled in age and metallicity. In this paper, we develop methods based on a diffusion map that, for the first time, choose appropriate bases of prototype SSP spectra from a large set of SSP spectra designed to approximate the continuous grid of age and metallicity of SSPs of which galaxies are truly composed. We show that our techniques achieve better accuracy of physical parameter estimation for simulated galaxies. Specifically, we show that our methods significantly decrease the age–metallicity degeneracy that is common in galaxy population synthesis methods. We analyse a sample of 3046 galaxies in Sloan Digital Sky Survey Data Release 6 and compare the parameter estimates obtained from different basis choices.     

A submillimetre survey of the star formation history of radio galaxies

We present the results of the first major systematic submillimetre survey of radio galaxies spanning the redshift range 1< z <5. The primary aim of this work is to elucidate the star formation history of this sub class of elliptical galaxies by tracing the cosmological evolution of dust mass. Using SCUBA on the JCMT, we have obtained 850-μm photometry of 47 radio galaxies to a consistent rms depth of 1 mJy, and have detected dust emission in 14 cases. The radio galaxy targets have been selected from a series of low-frequency radio surveys of increasing depth (3CRR, 6CE, etc.), in order to allow us to separate the effects of increasing redshift and increasing radio power on submillimetre luminosity. Although the dynamic range of our study is inevitably small, we find clear evidence that the typical submillimetre luminosity (and hence dust mass) of a powerful radio galaxy is a strongly increasing function of redshift; the detection rate rises from ≃15 per cent at z <2.5 to ≳75 per cent at z >2.5, and the average submillimetre luminosity rises at a rate ∝(1+ z )³ out to z ≃4. Moreover, our extensive sample allows us to argue that this behaviour is not driven by underlying correlations with other radio galaxy properties such as radio power, radio spectral index, or radio source size/age. Although radio selection may introduce other more subtle biases, the redshift distribution of our detected objects is in fact consistent with the most recent estimates of the redshift distribution of comparably bright submillimetre sources discovered in blank field surveys. The evolution of submillimetre luminosity found here for radio galaxies may thus be representative of massive ellipticals in general.