Radially truncated galactic discs

We present the first results of a systematic analysis of radially truncated exponential discs for four galaxies of a complete sample of disc-dominated edge-on spiral galaxies.
The discs of our sample galaxies are truncated at similar radii on either side of their centres. With the possible exception of the disc of ESO 416-G25, it appears that the truncations in our sample galaxies are closely symmetric, in terms of both their sharpness and the truncation length. However, the truncations occur over a larger region and not as abruptly as found in previous studies.
We show that the truncated luminosity distributions of our sample galaxies, if also present in the mass distributions, comfortably meet the requirements for longevity. The formation and maintenance of disc truncations are probably closely related to stability requirements for galactic discs.     

Environmental effects on satellite galaxies: the link between concentration, size and colour profile

Using the Sloan Digital Sky Survey Data Release 4 group catalogue of Yang et al., we investigate sizes, concentrations, colour gradients and surface brightness profiles of central and satellite galaxies. We compare central and satellite galaxies at fixed stellar mass, in order to disentangle environmental from stellar mass dependencies. Early- and late-type galaxies are defined according to concentration. We find that at fixed stellar mass, late-type satellite galaxies have smaller radii and larger concentrations than late-type central galaxies. No such differences are found for early-type galaxies. We have also constructed surface brightness and colour profiles for the central and satellite galaxies in our sample. We find that late-type satellite galaxies have a lower surface brightness and redder colours than late-type central galaxies. We show that all observed differences between satellite and central galaxies can be explained by a simple fading model, in which the star formation in the disc decreases over time-scales of 2–3 Gyr after a galaxy becomes a satellite. Processes that induce strong morphological changes (e.g. harassment) and processes that strip the galaxy of its entire interstellar medium need not to be invoked in order to explain the environmental dependencies we find.     

The image separation distribution of strong lenses: halo versus subhalo populations

We present a halo model prediction of the image separation distribution of strong lenses. Our model takes into account the subhalo population, which has been ignored in previous studies, as well as the conventional halo population. Haloes and subhaloes are linked to central and satellite galaxies by adopting a universal scaling relation between masses of (sub)haloes and luminosities of galaxies. Our model predicts that 10–20 per cent of lenses should be caused by the subhalo population. The fraction of lensing by satellite galaxies (subhaloes) peaks at ∼1 arcsec and decreases rapidly with increasing image separations. We compute fractions of lenses which lie in groups and clusters and find them to be ∼14 and ∼4 per cent, respectively; nearly half of such lenses are expected to be produced by satellite galaxies, rather than central parts of haloes. We also study mass distributions of lensing haloes and find that, even at image separations of ∼3 arcsec, the deviation of lens mass distributions from isothermal profiles is large; at or beyond ∼3 arcsec, image separations are enhanced significantly by surrounding haloes. Our model prediction agrees reasonably well with observed image separation distributions from galaxy to cluster scales.     

Modelling the number counts of early-type galaxies by pure luminosity evolution

In this paper, we explore the plausible luminosity evolution of early-type galaxies in different cosmological models by constructing a set of pure luminosity evolution (PLE) models via the choices of the star-formation rate (SFR) parameters and formation redshift z _f of galaxies, with the observational constraints derived from the Hubble Space Telescope ( HST  ) morphological number counts for elliptical and S0 galaxies of the Medium Deep Survey (MDS) and the Hubble Deep Field (HDF). We find that the number counts of early-type galaxies can be explained by the pure luminosity evolution models, without invoking exotic scenarios such as merging or introducing an additional population, but the evolution should be nearly passive, with a high z _f assumed. The conclusion is valid in all of the three cosmological models we adopt in this paper. We also present the redshift distributions for three bins of observed magnitudes in the F814w passband, to show the redshift at which the objects that dominate the counts at a given magnitude may be found. The predictions of the redshift distribution of 22.5 <  b _j  < 24.0 are also presented for comparison with future data.     

Massive dark matter haloes around bright isolated galaxies in the 2dFGRS

We identify a large sample of isolated bright galaxies and their fainter satellites in the 2dF Galaxy Redshift Survey (2dFGRS). We analyse the dynamics of ensembles of these galaxies selected according to luminosity and morphological type by stacking the positions of their satellites and estimating the velocity dispersion of the combined set. We test our methodology using realistic mock catalogues constructed from cosmological simulations. The method returns an unbiased estimate of the velocity dispersion provided that the isolation criterion is strict enough to avoid contamination and that the scatter in halo mass at fixed primary luminosity is small. Using a maximum likelihood estimator that accounts for interlopers, we determine the satellite velocity dispersion within a projected radius of 175  h ⁻¹ kpc. The dispersion increases with the luminosity of the primary and is larger for elliptical galaxies than for spiral galaxies of similar b _J luminosity. Calibrating the mass–velocity dispersion relation using our mock catalogues, we find a dynamical mass within 175  h ⁻¹ kpc of     for elliptical galaxies and     for spiral galaxies. Finally, we compare our results with recent studies and investigate their limitations using our mock catalogues.     

The onset of warps in Spitzer observations of edge-on spiral galaxies

We analyse warps in the nearby edge-on spiral galaxies observed in the Spitzer /Infrared Array Camera (IRAC) 4.5-μm band. In our sample of 24 galaxies, we find evidence of warp in 14 galaxies. We estimate the observed onset radii for the warps in a subsample of 10 galaxies. The dark matter distribution in each of these galaxies are calculated using the mass distribution derived from the observed light distribution and the observed rotation curves. The theoretical predictions of the onset radii for the warps are then derived by applying a self-consistent linear response theory to the obtained mass models for six galaxies with rotation curves in the literature. By comparing the observed onset radii to the theoretical ones, we find that discs with constant thickness can not explain the observations; moderately flaring discs are needed. The required flaring is consistent with the observations. Our analysis shows that the onset of warp is not symmetric in our sample of galaxies. We define a new quantity called the onset-asymmetry index and study its dependence on galaxy properties. The onset asymmetries in warps tend to be larger in galaxies with smaller disc scalelengths. We also define and quantify the global asymmetry in the stellar light distribution, that we call the edge-on asymmetry in edge-on galaxies. It is shown that in most cases the onset asymmetry in warp is actually anticorrelated with the measured edge-on asymmetry in our sample of edge-on galaxies and this could plausibly indicate that the surrounding dark matter distribution is asymmetric.     

Relativistic beaming effects in the spectra of cores and hotspots in radio galaxies and quasars

We present the results of multiwavelength observations of cores and hotspots, at L , C , X and U bands with the Very Large Array, of a matched sample of radio galaxies and quasars selected from the Molonglo Reference Catalogue . We use these observations to determine the spectra of cores and hotspots, and test the unified scheme for radio galaxies and quasars. Radio cores have been detected at all wavelengths in all of the quasars in our sample, whereas only ∼50 per cent of the galaxies have cores detected in at least one of the wavelengths . The degree of core prominence in this sample is consistent with the unified scheme for radio galaxies and quasars. A comparison of the distributions of the two-point spectral index of the cores in our sample of lobe-dominated quasars, with the distributions in a matched sample of core-dominated quasars, shows that the distributions for these two classes are significantly different, and this is consistent with the expectations of the unified scheme. The difference in the spectral indices of the two hotspots on opposite sides is also significantly larger for quasars than for radio galaxies, as is expected in the unified scheme. We also investigate the relationship between the spectral index of the hotspots and the redshift or luminosity for our sample of sources.     

Discs of satellites: the new dwarf spheroidals

The spatial distributions of the most recently discovered ultra-faint dwarf satellites around the Milky Way and the Andromeda galaxy are compared to the previously reported discs-of-satellites (DoS) of their host galaxies. In our investigation, we pay special attention to the selection bias introduced due to the limited sky coverage of Sloan Digital Sky Survey (SDSS). We find that the new Milky Way satellite galaxies follow closely the DoS defined by the more luminous dwarfs, thereby further emphasizing the statistical significance of this feature in the Galactic halo. We also note a deficit of satellite galaxies with Galactocentric distances larger than  100 kpc  that are away from the DoS of the Milky Way. In the case of Andromeda, we obtain similar results, naturally complementing our previous finding and strengthening the notion that the DoS are optical manifestations of a phase-space correlation of satellite galaxies.     

Galaxy properties derived with spectral energy distribution fitting in the Hawaii-Hubble Deep Field-North

We compile multi-wavelength data from ultraviolet to infrared(IR) bands as well as redshift and source-type information, for a large sample of 178 341 sources in the Hawaii-Hubble Deep Field-North field. A total of 145 635 sources among the full sample are classified/treated as galaxies and have redshift information available. We derive physical properties for these sources utilizing the spectral energy distribution fitting code CIGALE that is based on Bayesian analysis. Through various consistency and robustness checks, we find that our stellar-mass and star-formation rate(SFR) estimates are reliable, which is mainly due to two facts. Firstly, we adopt the most up-to-date and accurate redshifts and point spread functionmatched photometry; and secondly, we make sensible parameter choices with the CIGALE code and take into account the influences of mid-IR/far-IR data, star-formation history models, and AGN contribution. We release our catalog of galaxy properties publicly(including, e.g., redshift, stellar mass, SFR, age, metallicity, dust attenuation). It is the largest of its kind in this field and should facilitate future relevant studies on the formation and evolution of galaxies.     

The local far-infrared galaxy colour–luminosity distribution: a reference for BLAST and Herschel/SPIRE submillimetre surveys

We measure the local galaxy far-infrared (FIR) 60 to 100 μm colour–luminosity distribution using an all-sky IRAS survey. This distribution is an important reference for the next generation of FIR–submillimetre surveys that have and will conduct deep extragalactic surveys at 250–500 μm. With the peak in dust-obscured star-forming activity leading to present-day giant ellipticals now believed to occur in submillimetre galaxies near   z ∼ 2.5  , these new FIR–submillimetre surveys will directly sample the spectral energy distributions of these distant objects at rest-frame FIR wavelengths similar to those at which local galaxies were observed by IRAS . We have taken care to correct for the temperature bias and the evolution effects in our IRAS 60-μm-selected sample. We verify that our colour–luminosity distribution is consistent with the measurements of the local FIR luminosity function, before applying it to the higher redshift Universe. We compare our colour–luminosity correlation with recent dust–temperature measurements of submillimetre galaxies and find evidence for pure luminosity evolution of the form  (1 + z )³  . This distribution will be useful for the development of evolutionary models for Balloon-borne Large Aperture Submillimeter Telescope (BLAST) and Spectral and Photometric Imaging Receiver (SPIRE) surveys as it provides a statistical distribution of the rest-frame dust temperatures for galaxies as a function of luminosity.     

The active nuclei of z < 1.0 3CRR radio sources

We combine Chandra and XMM–Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared (mid-IR) data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with   z < 1.0  from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations and the availability of more mid-IR data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type 2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss the recently discovered 'fundamental plane' of black hole activity, showing that care must be taken when placing radio-loud active galactic nucleus (AGN) on such diagnostic diagrams.     

The abundance and radial distribution of satellite galaxies

Using detailed mock galaxy redshift surveys (MGRSs) we investigate the abundance and radial distribution of satellite galaxies. The mock surveys are constructed using large numerical simulations and the conditional luminosity function (CLF), and are compared against data from the Two Degree Field Galaxy Redshift Survey (2dFGRS). We use Monte Carlo Markov chains to explore the full posterior distribution of the CLF parameter space, and show that the average relation between light and mass is tightly constrained and in excellent agreement with our previous models and with that of Vale & Ostriker. The radial number density distribution of satellite galaxies in the 2dFGRS reveals a pronounced absence of satellites at small projected separations from their host galaxies. This is (at least partly) owing to the overlap and merging of galaxy images in the 2dFGRS parent catalogue. Owing to the resulting close-pair incompleteness we are unfortunately unable to put meaningful constraints on the radial distribution of satellite galaxies; the data are consistent with a radial number density distribution that follows that of the dark matter particles, but we cannot rule out alternatives with a constant number density core. Marginalizing over the full CLF parameter space, we show that in a ΛCDM concordance cosmology the observed abundances of host and satellite galaxies in the 2dFGRS indicate a power spectrum normalization of  σ₈≃ 0.7  . The same cosmology but with  σ₈= 0.9  is unable to match simultaneously the abundances of host and satellite galaxies. This confirms our previous conclusions based on the pairwise peculiar velocity dispersions and the group multiplicity function.     

A search for the submillimetre counterparts to Lyman break galaxies

We have carried out targeted submillimetre observations as part of a programme to explore the connection between the rest-frame ultraviolet and far-infrared properties of star-forming galaxies at high redshift, which is currently poorly understood. On the one hand, the Lyman break technique is very effective at selecting     galaxies. On the other, 'blank-field' imaging in the submillimetre seems to turn up sources routinely, amongst which some are star-forming galaxies at similar redshifts. Already much work has been done searching for optical identifications of objects detected using the SCUBA instrument. Here we have taken the opposite approach, performing submillimetre photometry for a sample of Lyman break galaxies, the ultraviolet properties of which imply high star formation rates. The total signal from our Lyman break sample is undetected in the submillimetre, at an rms level of ∼0.5 mJy, which implies that the population of Lyman break galaxies does not constitute a large part of the recently detected blank-field submillimetre sources. However, our one detection suggests that with reasonable SCUBA integrations we might expect to detect those few Lyman break galaxies that are far-infrared brightest.     

Dust attenuation in starburst galaxies determined by measuring the dependence of the optical color indices on galaxy inclination

We use optical color indices (colors) from the SDSS database to study the effect of dust in starburst galaxies by mea‐suring the dependence of colors on galaxy inclination. Starburst galaxies with ongoing star formation, are rich with metals/dust and are, therefore, an excellent objects for studying the effect of dust in galaxies. They are selected using the [O III ]λ 5007/Hα vs. [N II ]λ 6584/Hβ diagram, that is, the BPT‐diagram. We use Kauffmann's empirical demarcation line in the BPT‐diagram to exclude galaxies with active galactic nuclei (AGN) from the sample because they have different physical and dust properties from normal galaxies. The sample is divided into bins according to galaxy stellar mass and 4000 Å break (which is a coarse measure of a galaxy star formation history; SFH) and the reddening with inclination is studied as a function of these two physical parameters. Assuming that the dust effect is negligible in the SDSS z ‐band, we derive the attenuation curves for these galaxies. We fit the attenuation curves with a simple power law and use power law index to interpret the relative distribution of dust and stars in the starburst galaxies (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The role of E+A and post-starburst galaxies – II. Spectral energy distributions and comparison with observations

In a previous paper, we have shown that the classical definition of E+A galaxies excludes a significant number of post-starburst galaxies. We suggested that analysing broad-band spectral energy distributions (SEDs) is a more comprehensive method to select and distinguish post-starburst galaxies than the classical definition of measuring equivalent widths of (Hδ) and [O  ii ] lines.
In this paper, we will carefully investigate this new method and evaluate it by comparing our model grid of post-starburst galaxies to observed E+A galaxies from the MORPHS catalogue.
In the first part, we investigate the UV-optical-NIR (near-infrared) SEDs of a large variety in terms of progenitor galaxies, burst strengths and time-scales of post-starburst models and compare them to undisturbed spiral, S0 and E galaxies as well as to galaxies in their starburst phase. In the second part, we compare our post-starburst models with the observed E+A galaxies in terms of Lick indices, luminosities and colours. We then use the new method of comparing the model SEDs with SEDs of the observed E+A galaxies.
We find that the post-starburst models can be distinguished from undisturbed spiral, S0 and E galaxies and galaxies in their starburst phase on the basis of their SEDs. It is even possible to distinguish most of the different post-starbursts by their SEDs. From the comparison with observations, we find that all observed E+A galaxies from the MORPHS catalogue can be matched by our models. However, only models with short decline time-scales for the star formation rate are possible scenarios for the observed E+A galaxies in agreement with our results from the first paper.     

Clustering Property of Wolf-Rayet Galaxies in the SDSS

We have analysed, for the first time, the clustering properties of Wolf-Rayet (W-R) galaxies, using a large sample of 846 W-R galaxies selected from the Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). We compute the cross-correlation function between W- R galaxies and a reference sample of galaxies drawn from the DR4. We compare the function to the results for control samples of non-W-R star-forming galaxies that are matched closely in redshift, luminosity, concentration, 4000-A break strength and specific star formation rate (SSFR). On scales larger than a few Mpc, W-R galaxies have almost the same clustering amplitude as the control samples, indicating that W-R galaxies and non-W-R control galax- ies populate dark matter haloes of similar masses. On scales between 0.1-1 h-1 Mpc, W-R galaxies are less clustered than the control samples, and the size of the difference depends on the SSFR. Based on both observational and theoretical considerations, we speculate that this negative bias can be interpreted by W-R galaxies residing preferentially at the centers of their dark matter haloes. We examine the distribution of W-R galaxies more closely using the SDSS galaxy group catalogue of Yang et al., and find that ～82% of our W-R galaxies are the central galaxies of groups, compared to ～74% for the corresponding control galaxies. We find that W-R galaxies are hosted, on average, by dark matter haloes of masses of 1012,3 M☉, compared to 1012,1 M? For centrally-located W-R galaxies and 1012,7 M☉ For satellite ones. We would like to point out that this finding, which provides a direct observational support to our conjecture, is really very crude due to the small number of W-R galaxies and the incom- pleteness of the group catalogue, and needs more work in future with larger samples.     

The growth of central and satellite galaxies in cosmological smoothed particle hydrodynamics simulations

We examine the accretion and merger histories of central and satellite galaxies in a smoothed particle hydrodynamics (SPH) cosmological simulation that resolves galaxies down to  7 × 10⁹ M_⊙  . Most friends-of-friends haloes in the simulation have a distinct central galaxy, typically 2–5 times more massive than the most massive satellite. As expected, satellites have systematically higher assembly redshifts than central galaxies of the same baryonic mass, and satellites in more massive haloes form earlier. However, contrary to the simplest expectations, satellite galaxies continue to accrete gas and convert it to stars; the gas accretion declines steadily over a period of 0.5–1 Gyr after the satellite halo merges with a larger parent halo. Satellites in a cluster mass halo eventually begin to lose baryonic mass. Typically, satellites in our simulation are 0.1–0.2 mag bluer than in models that assume no gas accretion on to satellites after a halo merger. Since   z = 1  , 27 per cent of central galaxies (above  3 × 10¹⁰ M_⊙  ) and 22 per cent of present-day satellite galaxies have merged with a smaller system above a 1:4 mass ratio; about half of the satellite mergers occurred after the galaxy became a satellite and half before. In effect, satellite galaxies can remain 'central' objects of halo substructures, with continuing accretion and mergers, making the transition in assembly histories and physical properties a gradual one. Implementing such a gradual transformation in semi-analytic models would improve their agreement with observed colour distributions of satellite galaxies in groups and with the observed colour dependence of galaxy clustering.     

Structure through colour: a pixel approach towards understanding galaxies

We present a study of pixel colour–magnitude diagrams (pCMDs) for a sample of 69 nearby galaxies chosen to span a wide range of Hubble types. Our goal is to determine how useful a pixel approach is for studying galaxies according to their stellar light distributions and content. The galaxy images were analysed on a pixel-by-pixel basis to reveal the structure of the individual pCMDs. We find that the average surface brightness (or projected mass density) in each pixel varies according to galaxy type. Early-type galaxies exhibit a clear 'prime sequence' and some pCMDs of face-on spirals reveal 'inverse-L' structures. We find that the colour dispersion at a given magnitude is found to be approximately constant in early-type galaxies but this quantity varies in the mid and late types. We investigate individual galaxies and find that the pCMDs can be used to pick out morphological features. We discuss the discovery of 'Red Hooks' in the pCMDs of six early-type galaxies and two spirals and postulate their origins. We develop quantitative methods to characterize the pCMDs, including measures of the blue-to-red light ratio and colour distributions of each galaxy and we organize these by morphological type. We compare the colours of the pixels in each galaxy with the stellar population models of Bruzual & Charlot to calculate star formation histories for each galaxy type and compare these to the stellar mass within each pixel. Maps of pixel stellar mass and mass-to-light ratio are compared to galaxy images. We apply the pCMD technique to three galaxies in the Hubble Ultra Deep Field to test the usefulness of the analysis at high redshift. We propose that these results can be used as part of a new system of automated classification of galaxies that can be applied at high redshift.     

Environment and mass dependencies of galactic λ spin parameter: cosmological simulations and observed galaxies compared

We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the λ spin parameter and the environment and mass of galaxies. In order to calculate the total value of λ for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies, which allows a rough estimate of the value of λ using only readily obtainable observables from the luminous galaxies. Use of a large volume-limited sample (upwards of 11 000) allows reliable inferences of mean values and dispersions of λ distributions. We find, in agreement with some N -body cosmological simulations, no significant dependence of λ on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with λ, in the sense that low-mass galaxies present both higher mean values of λ and associated dispersions, than high-mass galaxies. These results provide interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models.