On contamination and completeness in z≥ 5 Lyman-break galaxy surveys

A large population of z > 5 Lyman-break galaxies has been identified in recent years. However, the high-redshift galaxies selected by different surveys are subject to a variety of selection effects – some overt, others more subtle. We present an analysis of sample completeness and contamination issues in high-redshift surveys, focusing on surveys at z ≈ 5 and using a spectroscopically confirmed low-redshift sample from the DEEP2 survey in order to characterize contaminant galaxies. We find that most surveys underestimate their contamination from highly clustered galaxies at z ≈ 1 and stars. We consider the consequences of this for both the rest-frame ultraviolet luminosity function and the clustering signal from z ≈ 5 galaxies. We also find that sources with moderate strength Lyman α emission lines can be omitted from dropout surveys due to their blue colours, again effecting the derived luminosity functions. We discuss the points of comparison between different samples, and the applicability of survey-specific results to the population at z > 5 in general.     

A Study on the Photometric Redshifts of Faint Blue Galaxies in CDFS

From the COMBO-17 digital sky survey data, 1,231 faint blue galaxies with photometric redshifts of 0.1 < z < 0.3 in the sky region CDFS (Chandra Deep Field South) are selected as the sample. We have studied the distributions of the photometric redshifts of these sample galaxies, in the conditions that the photometric redshifts are obtained respectively by using only optical data and by using both optical and near-infrared data. The results indicate that there are 183 galaxies whose photometric redshifts derived from both optical and infrared data are greater than 1.2, that the rms error of the derived photometric redshifts is 0.046, and that to increase the photometric SNR is also helpful for discriminating those misjudged low-redshift galaxies by using only the optical data. We have studied as well the typical spectral energy distributions (SEDs) of these galaxies in the reference system at rest. It is found that for the high-redshift galaxies the observed near-infrared flux tends to be greater than the optical flux, while for the low-redshift galaxies the observed near-infrared flux tends to be less than the optical flux.     

Empirical population synthesis for 74 blue compact galaxies

We have observed the largest optical spectra sample of 97 blue compact galaxies. Stellar population properties of 74 star-forming BCGs of them were derived by comparing the equivalent widths of strong absorption features and continuum colors, using a method of empirical population synthesis based on star cluster sample. The results indicate that blue compact galaxies are typically age-composite stellar system, the continuum flux fractions at 5870Å due to old stellar components and young stellar components are both important for most of the galaxies. The stellar populations of blue compact galaxies present a variety of characteristics, and the contribution from different age and metallicity components is different. The star formation episodes are usually short, some galaxies maybe undergoing their first global episode of star formation, while for the most sample galaxies, older stars contribute to at most half the optical emission. Our results suggest that BCGs are old galaxies, in which star formation occurs in short intense burst separated by long quiescent phases.     

The evolution of 3CR radio galaxies from z1

We present the results of a comprehensive re-analysis of the images of a virtually complete sample of 28 powerful 3CR radio galaxies with redshifts 0.6< z <1.8 from the Hubble Space Telescope ( HST ) archive. Using a two-dimensional modelling technique we have derived scalelengths and absolute magnitudes for a total of 16 3CR galaxies with a median redshift of z =0.8. Our results confirm the basic conclusions of Best, Longair & Röttgering in that we also find z =1 3CR galaxies to be massive, well-evolved ellipticals, the infrared emission of which is dominated by starlight. However, we in fact find that the scalelength distribution of 3CR galaxies at z ≃1 is completely indistinguishable from that derived for their low-redshift counterparts from our own recently completed HST study of active galactic nuclei hosts at z ≃0.2. There is thus no evidence that 3CR radio galaxies at z ≃1 are dynamically different from 3CR galaxies at low redshift. Moreover, for a 10-object subsample we have determined the galaxy parameters with sufficient accuracy to demonstrate, for the first time, that the z ≃1 3CR galaxies follow a Kormendy relation that is indistinguishable from that displayed by low-redshift ellipticals if one allows for purely passive evolution. The implied rather modest level of passive evolution since z ≃1 is consistent with that predicted from spectrophotometric models provided one assumes a high formation redshift ( z ≥4) within a low-density universe. We conclude that there is no convincing evidence for significant dynamical evolution among 3CR galaxies in the redshift interval 0< z <1, and that simple passive evolution remains an acceptable interpretation of the K – z relation for powerful radio galaxies.     

The expected detection of dust emission from high-redshift Lyman α galaxies

Recent results have shown that a substantial fraction of high-redshift Lyman α (Lyα) galaxies contain considerable amounts of dust. This implies that Lyα galaxies are not primordial, as has been thought in the past. However, this dust has not been directly detected in emission; rather it has been inferred based on extinction estimates from rest-frame ultraviolet (UV) and optical observations. This can be tricky, as both dust and old stars redden galactic spectra at the wavelengths used to infer dust. Measuring dust emission directly from these galaxies is thus a more accurate way to estimate the total dust mass, giving us real physical information on the stellar populations and interstellar medium enrichment. New generation instruments, such as the Atacama Large Millimeter Array and Sub-Millimeter Array, should be able to detect dust emission from some of these galaxies in the submillimeter. Using measurements of the UV spectral slopes, we derive far-infrared flux predictions for of a sample of  23 z ≥ 4  Lyα galaxies. We find that in only a few hours, we can detect dust emission from 39 ± 22 per cent of our Lyα galaxies. Comparing these results to those found from a sample of 21 Lyman break galaxies (LBGs), we find that LBGs are on average 60 per cent more likely to be detected than Lyα galaxies, implying that they are more dusty, and thus indicating an evolutionary difference between these objects. These observations will provide better constraints on dust in these galaxies than those derived from their UV and optical fluxes alone. Undeniable proof of dust in these galaxies could explain the larger than expected Lyα equivalent widths seen in many Lyα galaxies today.     

Star-Forming Galaxies in Dense Environments

We report on preliminary results from an ongoing study of star-forming galaxies in the Virgo Cluster with the aim of understanding the role played by the environment on their evolution. First, the chemical evolution of nine spiral galaxies located in the inner parts of the cluster has been studied using chemical abundances recomputed for a sample of HII regions. Second, long-slit optical spectroscopy has been obtained for 22 blue dwarf galaxies selected across the Virgo central field. This sample is a subset of a larger list of dwarf galaxies for which deep Hα imaging has been collected. Chemical abundances have been derived for the sample using either a direct estimation of the electron temperature or empirical calibrations. Abundance results correlate with blue and near infrared magnitude, total HI mass and color. This revised version was published online in September 2006 with corrections to the Cover Date.     

The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum-likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and considering the importance of dust all compromise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy‐mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.     

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.     

X-ray luminosities of galaxies in groups

We have derived the X-ray luminosities of a sample of galaxies in groups, making careful allowance for contaminating intragroup emission. The L _X: L _B and L _X: L _FIR relations of spiral galaxies in groups appear to be indistinguishable from those in other environments, however the elliptical galaxies fall into two distinct classes. The first class is central-dominant group galaxies, which are very X-ray luminous and may be the focus of group cooling flows. All other early-type galaxies in groups belong to the second class, which populates an almost constant band of L _X/ L _B over the range 9.8< log  L _B<11.3 . The X-ray emission from these galaxies can be explained by a superposition of discrete galactic X-ray sources together with a contribution from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In the region where the optical luminosity of the non-central group galaxies overlaps with the dominant galaxies, the dominant galaxies are over an order of magnitude more luminous in X-rays.
We also compared these group galaxies with a sample of isolated early-type galaxies, and used previously published work to derive L _X: L _B relations as a function of environment. The non-dominant group galaxies have mean L _X/ L _B ratios very similar to those of isolated galaxies, and we see no significant correlation between L _X/ L _B and environment. We suggest that previous findings of a steep L _X: L _B relation for early-type galaxies result largely from the inclusion of group-dominant galaxies in samples.     

Model-independent measurements of bar pattern speeds

The pattern speed is one of the fundamental parameters that determines the structure of barred galaxies. This quantity is usually derived from indirect methods or by employing model assumptions. The number of bar pattern speeds derived using the model-independent Tremaine & Weinberg technique is still very limited. We present the results of model-independent measurements of the bar pattern speed in four galaxies ranging in Hubble type from SB0 to SBbc. Three of the four galaxies in our sample are consistent with bars being fast rotators. The lack of slow bars is consistent with previous observations and suggests that barred galaxies do not have centrally concentrated dark matter haloes. This contradicts simulations of cosmological structure formation and observations of the central mass concentration in nonbarred galaxies.     

Optical and infrared diagnostics of SDSS galaxies in the SWIRE survey

We present the rest-frame optical and infrared colours of a complete sample of  1114 z < 0.3  galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies.
We propose that the optical  ( g − r )  colour and infrared  log( L ₂₄/ L _3.6)  colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The  ( g − r )  colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the  log( L ₂₄/ L _3.6)  colour is more sensitive for bulge-dominated systems.
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red  ( g − r )  colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'.     

Multiwavelength study of 230 IRAS FSC galaxies

A sample of 230 galaxies has been compiled based on their IRAS FSC fluxes to study their multiwavelength properties and carry out comparative analyses with other similar samples. To understand the nature of these galaxies, a comparison with a sample of bright ULIRGs having fluxes at 60 μm fν > 1 Jy and 14 optically faint IRAS FSC galaxies is presented. This comparison shows that galaxies found by IRAS are not always strong infrared sources and that the objects from the sample of ULIRGs represent a sample of extreme galaxies, which are very powerful infrared sources. We have confirmed the consistency between Star Formation Rates (SFRs) calibrated based on luminosities of the PAH feature at 7.7 μm and radio 1.4 GHz flux densities. We have estimated the extinction for our 230 objects using SFRs calibrated from the PAH feature compared to ultraviolet flux, which shows that only 1% of the ultraviolet continuum typically escapes extinction by dust within a starburst.     

An ultraviolet-selected galaxy redshift survey: new estimates of the local star formation rate

We present the first results of an ongoing spectroscopic survey of galaxies selected in the rest frame ultraviolet (UV). The source catalogue has been constructed from a flux-limited sample of stars, galaxies and QSOs imaged at 2000 Å in Selected Area 57 with the FOCA balloon-borne imaging camera. Accurate positions for the UV sources have been obtained by matching with optical counterparts using APM scans of the Palomar Sky Survey limited at B   20.5. Here we present results derived from optical spectroscopy conducted with the WIYN telescope and the WHT for 142 faint sources. The redshift distribution for this UV-selected sample extends over 0 <  z  < 0.5, and a high fraction of the sources show intense nebular emission lines and UV–optical colours bluer than normal Hubble sequence galaxies. Such UV-selected surveys are thus a very efficient way to locate and study intermediate-redshift galaxies undergoing intense star formation. Although our sample is currently small, we derive a rest frame UV luminosity function with a steep faint-end slope consistent with that found for late-type galaxies in optical samples. However, the integrated luminosity density derived implies a volume-averaged star formation rate higher than other recent estimates, assuming a normal initial mass function. If representative of other UV fields, as suggested by UV number count studies, our data imply that the local abundance of star-forming galaxies may have been underestimated, and consequently claims for strong evolution in the global star formation rate in the range 0 <  z  < 1 overstated. An intensive study of a large UV-selected sample is likely to reveal important information on the declining population of star-forming galaxies of all types.     

The morphology,kinematics and metallicity of blue-core galaxies

We select 107 blue-core galaxies from the MaNGA survey, studying their morphology, kinematics as well as the gas-phase metallicity. Our results are as follows:(i) In our sample, 26% of blue-core galaxies have decoupled gas-star kinematics, indicating external gas accretion;15% have bar-like structure and 8% show post-merger features, such as tidal tails and irregular gas/star velocity field. All these processes/features, such as accreting external misaligned gas, interaction and bar, can trigger gas inflow. Thus the central star-forming activities lead to bluer colors in their centers(blue-core galaxies).(ii) By comparing with the SDSS DR7 star-forming galaxy sample, we find that the blue-core galaxies have higher central gas-phase metallicity than what is predicted by the local mass-metallicity relation. We explore the origin of the higher metallicity, finding that not only the blue-core galaxies, but also the flat-gradient and red-core galaxies all have higher metallicity. This can be explained by the combined effect of redshift and galaxy color.     

A ROSAT survey of Wolf–Rayet galaxies — II. The extended sample

We present results from an ongoing X-ray survey of Wolf–Rayet (WR) galaxies, a class of objects believed to be very young starbursts. This paper extends the first X-ray survey of WR galaxies by Stevens &38; Strickland by studying WR galaxies identified subsequent to the original WR galaxy catalogue of Conti.   Out of a sample of 40 new WR galaxies a total of 10 have been observed with the ROSAT PSPC, and of these seven have been detected (NGC 1365, NGC 1569, I Zw 18, NGC 3353, NGC 4449, NGC 5408 and a marginal detection of NGC 2366). Of these, all are dwarf starbursts except for NGC 1365, which is a barred spiral galaxy possibly with an active nucleus. We also report on observations of the related emission-line galaxy IRAS 0833+6517.   The X-ray properties of these galaxies are broadly in line with those found for the original sample; they are X-ray overluminous compared with their blue luminosity and have thermal spectra with typically kT  ∼ 0.4 − 1.0 keV. There are some oddities: NGC 5408 is very overluminous in X-rays, even compared with other WR galaxies; I Zw 18 has a harder X-ray spectrum; NGC 1365, although thought to contain an active nucleus, has X-ray properties that are broadly similar to other WR galaxies, and we suggest that the X-ray emission from NGC 1365 is due to starburst activity.   A good correlation between X-ray and blue luminosity is found for the WR galaxy sample as a whole. However, when just dwarf galaxies are considered there is little evidence of correlation. We discuss the implications of these results on our understanding of the X-ray emission from WR galaxies and suggest that the best explanation for the X-ray activity is starburst activity from a young starburst region.     

Star formation in nearby isolated galaxies

We use the FUV fluxes measured with the GALEX to study the star formation properties of galaxies collected in the “Local Orphan Galaxies” catalog (LOG). Among 517 LOG galaxies having radial velocities V _LG < 3500 km/s and Galactic latitudes |b| > 15°, 428 objects have been detected in FUV. We briefly discuss some scaling relations between the specific star formation rate (SSFR) and stellar mass, HI-mass, morphology, and surface brightness of galaxies situated in extremely low density regions of the Local Supercluster. Our sample is populated with predominantly late-type, gas-rich objects with the median morphological type of Sdm. Only 5% of LOG galaxies are classified as early types: E, S0, S0/a, however, they systematically differ from normal E and S0 galaxies by lower luminosity and presence of gas and dust. We find that almost all galaxies in our sample have their SSFR below 0.4 [Gyr^?1]. This limit is also true even for a sample of 270 active star-burst Markarian galaxies situated in the same volume. The existence of such a quasi-Eddington limit for galaxies seems to be a key factor which characterizes the transformation of gas into stars at the current epoch.     

Chemical abundances in a UV-selected sample of galaxies

We discuss the chemical properties of a sample of UV-selected intermediate-redshift  (0≲z≲0.4)  galaxies in the context of their physical nature and star-formation history. This work represents an extension of our previous studies of the rest-frame UV-luminosity function (Treyer et al.) and the star-formation properties of the same sample (Sullivan et al.) . We revisit the optical spectra of these galaxies and perform further emission-line measurements restricting the analysis to those spectra with the full set of emission lines required to derive chemical abundances. Our final sample consists of 68 galaxies with heavy-element abundance ratios and both UV and CCD B -band photometry. Diagnostics based on emission-line ratios show that all but one of the galaxies in our sample are powered by hot, young stars rather than by an AGN. Oxygen-to-hydrogen (O/H) and nitrogen-to-oxygen (N/O) abundance ratios are compared with those of various local and intermediate-redshift samples. Our UV-selected galaxies span a wide range of oxygen abundances, from ∼0.1 to 1 Z_⊙, intermediate between low-mass H  ii galaxies and massive starburst nuclei. For a given oxygen abundance, most have strikingly low N/O values. Moreover, UV-selected and H  ii galaxies systematically deviate from the usual metallicity–luminosity relation in the sense of being more luminous by  2–3 mag  . Adopting the 'delayed-release' chemical evolution model, we propose our UV-selected sources are observed at a special stage in their evolution, following a powerful starburst that enriched their ISM in oxygen and temporarily lowered their mass-to-light ratios. We discuss briefly the implications of our conclusions on the nature of similarly selected high-redshift galaxies.     

The dynamics of S0 galaxies and their Tully–Fisher relation

This paper investigates the detailed dynamical properties of a relatively homogeneous sample of disc-dominated S0 galaxies, with a view to understanding their formation, evolution and structure. By using high signal-to-noise ratio long-slit spectra of edge-on systems, we have been able to reconstruct the complete line-of-sight velocity distributions of stars along the major axes of the galaxies. From these data, we have derived both model distribution functions (the phase density of their stars) and the approximate form of their gravitational potentials.
The derived distribution functions are all consistent with these galaxies being simple disc systems, with no evidence for a complex formation history. Essentially no correlation is found between the characteristic mass scalelengths and the photometric scalelengths in these galaxies, suggesting that they are dark-matter dominated even in their inner parts. Similarly, no correlation is found between the mass scalelengths and asymptotic rotation speed, implying a wide range of dark matter halo properties.
By comparing their asymptotic rotation speeds with their absolute magnitudes, we find that these S0 galaxies are systematically offset from the Tully–Fisher relation for later-type galaxies. The offset in luminosity is what one would expect if star formation had been suddenly switched off a few Gyr ago, consistent with a simple picture in which these S0s were created from ordinary later-type spirals which were stripped of their star-forming interstellar medium when they encountered a dense cluster environment.     

RXTE observation of NGC 6240: a search for the obscured active nucleus

We present new data taken at 850 μm with SCUBA at the James Clerk Maxwell Telescope for a sample of 19 luminous infrared galaxies. Fourteen galaxies were detected. We have used these data, together with fluxes at 25, 60 and 100 μm from IRAS , to model the dust emission. We find that the emission from most galaxies can be described by an optically thin, single temperature dust model with an exponent of the dust extinction coefficient ( k _λ ∝ λ ^{− β} ) of β ≃1.4–2. A lower β ≃1 is required to model the dust emission from two of the galaxies, Arp 220 and NGC 4418. We discuss various possibilities for this difference and conclude that the most likely is a high dust opacity. In addition, we compare the molecular gas mass derived from the dust emission, M _850 μm, with the molecular gas mass derived from the CO emission, M _CO, and find that M _CO is on average a factor 2–3 higher than M _850 μm.