We present a power-spectrum analysis of the final 2dF Galaxy Redshift Survey (2dFGRS), employing a direct Fourier method. The sample used comprises 221 414 galaxies with measured redshifts. We ...investigate in detail the modelling of the sample selection, improving on previous treatments in a number of respects. A new angular mask is derived, based on revisions to the photometric calibration. The redshift selection function is determined by dividing the survey according to rest-frame colour, and deducing a self-consistent treatment of k-corrections and evolution for each population. The covariance matrix for the power-spectrum estimates is determined using two different approaches to the construction of mock surveys, which are used to demonstrate that the input cosmological model can be correctly recovered. We discuss in detail the possible differences between the galaxy and mass power spectra, and treat these using simulations, analytic models and a hybrid empirical approach. Based on these investigations, we are confident that the 2dFGRS power spectrum can be used to infer the matter content of the universe. On large scales, our estimated power spectrum shows evidence for the ‘baryon oscillations’ that are predicted in cold dark matter (CDM) models. Fitting to a CDM model, assuming a primordial ns= 1 spectrum, h= 0.72 and negligible neutrino mass, the preferred parameters are Ωmh= 0.168 ± 0.016 and a baryon fraction Ωb/Ωm= 0.185 ± 0.046 (1σ errors). The value of Ωmh is 1σ lower than the 0.20 ± 0.03 in our 2001 analysis of the partially complete 2dFGRS. This shift is largely due to the signal from the newly sampled regions of space, rather than the refinements in the treatment of observational selection. This analysis therefore implies a density significantly below the standard Ωm= 0.3: in combination with cosmic microwave background (CMB) data from the Wilkinson Microwave Anisotropy Probe (WMAP), we infer Ωm= 0.231 ± 0.021.
We investigate the stellar populations in a sample of 89 faint red galaxies in the Coma cluster, using high signal-to-noise ratio (S/N) spectroscopy from the 6.5-m MMT. Our sample is drawn from two ...1° fields, one centred on the cluster core and the other located 1° to the south-west of the cluster centre. The target galaxies are mostly 2–4 mag fainter than M*; galaxies with these luminosities have been previously studied only using small samples, or at low S/N. For a comparison sample we use published high-S/N data for red-sequence galaxies in the Shapley supercluster. We use state-of-the-art stellar population models (by R. Schiavon) to interpret the absorption-line indices and infer the single-burst-equivalent age and metallicity (Fe/H) for each galaxy, as well as the abundances of the light elements Mg, Ca, C and N. The ages of the Coma dwarfs span a wide range from <2 Gyr to >10 Gyr, with a strong environmental dependence. The oldest galaxies are found only in the core, while most of the galaxies in the outer south-west field have ages ∼3 Gyr. The galaxies have a metallicity range −1.0 ≲Fe/H≲ 0.0, and follow the same age–metallicity–mass plane as high-mass galaxies, but with increased intrinsic scatter. The Mg/Fe abundance ratios are on average slightly supersolar, and span a range −0.1 ≲Mg/Fe≲+0.4. The highest Mg enhancements are found only in the cluster core, while solar ratios predominate in the outskirts. We show that parametrized models with more complex star formation histories perform no better than single-burst models in reproducing the observed line indices. Assuming a star formation history dominated by a single burst, the number of dwarf galaxies on the red sequence in the Coma core has doubled since z≈ 0.7. Assuming instead an abruptly truncated constant star formation rate, the equivalent redshift is z≈ 0.4. These estimates bracket the red-sequence growth time-scales found by direct studies of distant clusters. In the south-west field, the red sequence was established only at z≈ 0.2 for a burst-dominated star formation history (z≈ 0.1 for the truncated case). Our observations confirm previous indications of very recently quenched star formation in this part of the cluster. Our results strongly support the scenario in which much of the cluster passive dwarf population (in this luminosity range) was generated by environment-driven transformation of infalling late-type galaxies.
The spectroscopic properties of galaxies in the Coma Cluster are compared with those of galaxies in rich clusters at z approx 0.5, to investigate the evolution of the star formation history in ...clusters. Luminous galaxies with M sub(V) <-20 and poststarburst/post-star-forming (k+a) spectra that constitute a significant fraction of galaxies in distant cluster samples are absent in Coma, where spectacular cases of k+a spectra are found instead at M sub(V) >-18.5 and represent a significant proportion of the cluster dwarf galaxy population. A simple inspection of their positions on the sky indicates that this type of galaxy does not show a preferential location within the cluster, but the bluest and strongest lined group of k+a galaxies lie in projection toward the central 1.4 Mpc of Coma and have radial velocities significantly higher than the cluster mean. We find a striking correlation between the positions of these young and strong poststarburst galaxies and substructure in the hot intracluster medium (ICM) identified from XMM-Newton data, with these galaxies lying close to the edges of two infalling substructures. This result strongly suggests that the interaction with the dense ICM could be responsible for the quenching of the star formation (thus creating the k+a spectrum) and, possibly, for any previous starburst. The evolution with redshift of the luminosity distribution of k+a galaxies can be explained by a "downsizing effect," with the maximum luminosity/mass of actively star-forming galaxies infalling onto clusters decreasing at lower redshift. We discuss the possible physical origin of this downsizing effect and the implications of our results for current scenarios of environmental effects on the star formation in galaxies.
We have performed a spectroscopic study of globular clusters (GCs) in the nearest giant elliptical NGC 5128 using the 2dF facility at the Anglo-Australian Telescope. We obtained integrated optical ...spectra for a total of 254 GCs, 79 of which are newly confirmed on the basis of their radial velocities and spectra. In addition, we obtained an integrated spectrum of the galaxy starlight along the southern major axis. We derive an empirical metallicity distribution function (MDF) for 207 GCs (∼14 per cent of the estimated total GC system) based upon Milky Way GCs. This MDF is multimodal at high statistical significance with peaks at Z/H∼−1.3 and −0.5. A comparison between the GC MDF and that of the stellar halo at 20 kpc (∼4Re) reveals close coincidence at the metal-rich ends of the distributions. However, an inner 8-kpc stellar MDF shows a clear excess of metal-rich stars when compared to the GCs. We compare a higher signal-to-noise ratio (S/N) subsample (147 GCs) with two stellar population models which include non-solar abundance ratio corrections. The vast majority of our sample (∼90 per cent) appears old, with ages similar to the Milky Way GC system. There is evidence for a population of intermediate-age (∼4–8 Gyr) GCs (≤15 per cent of the sample) which are on average more metal-rich than the old GCs. We also identify at least one younger cluster (∼1–2 Gyr) in the central regions of the galaxy. Our observations are consistent with a picture where NGC 5128 has undergone at least two mergers and/or interactions involving star formation and limited GC formation since z= 1, however the effect of non-canonical hot stellar populations on the integrated spectra of GCs remains an outstanding uncertainty in our GC age estimates.
ABSTRACT
We analyse the observed correlation between galaxy environment and Hα emission‐line strength, using volume‐limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn ...from the 2dF Galaxy Redshift Survey (MbJ< −19.5) and the Sloan Digital Sky Survey (Mr < −20.6). We characterize the environment by: (1) Σ5, the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) ρ1.1 and ρ5.5, three‐dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star‐forming and quiescent galaxies form two distinct populations, as characterized by their Hα equivalent width, W0(Hα). The relative numbers of star‐forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W0(Hα) amongst the star‐forming population is independent of environment. The fraction of star‐forming galaxies shows strong sensitivity to the density on large scales, ρ5.5, which is likely independent of the trend with local density, ρ1.1. We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for σ= 200–1000 km s‐1. Even in the lowest‐density environments, no more than ∼70 per cent of galaxies show significant Hα emission. Based on these results, we conclude that the present‐day correlation between star formation rate and environment is a result of short‐time‐scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy–galaxy interactions.
We compute the bispectrum of the 2dF Galaxy Redshift Survey (2dFGRS) and use it to measure the bias parameter of the galaxies. This parameter quantifies the strength of clustering of the galaxies ...relative to the mass in the Universe. By analysing 80 × 106 triangle configurations in the wavenumber range 0.1 < k < 0.5 h Mpc−1 (i.e. on scales roughly between 5 and 30 h−1 Mpc) we find that the linear bias parameter is consistent with unity: b1= 1.04 ± 0.11, and the quadratic (non-linear) bias is consistent with zero: b2=−0.054 ± 0.08. Thus, at least on large scales, optically selected galaxies do indeed trace the underlying mass distribution. The bias parameter can be combined with the 2dFGRS measurement of the redshift distortion parameter β≃Ωm0.6/b1, to yield Ωm= 0.27 ± 0.06 for the matter density of the Universe, a result that is determined entirely from this survey, independent of other data sets. Our measurement of the matter density of the Universe should be interpreted as Ωm at the effective redshift of the survey (z= 0.17).
We analyse the abundance ratios of the light elements Mg, Ca, C and N, relative to Fe, for 147 red-sequence galaxies in the Coma cluster and the Shapley Supercluster. The sample covers a 6 mag range ...in luminosity, from giant ellipticals to dwarfs at M★+ 4. We exploit the wide mass range to investigate systematic trends in the abundance ratios Mg/Fe, Ca/Fe, C/Fe and N/Fe. We find that each of these ratios can be well modelled using two-parameter relations of the form X/Fe=a0+a1 log σ+a2Fe/H, where σ is the velocity dispersion. Analysing these ‘X-planes’ reveals new structure in the abundance patterns, beyond the traditional one-parameter correlations (Mg/Fe–σ, etc.). The X-planes for the α elements, Mg and Ca, indicate a positive correlation with velocity dispersion, and simultaneously an anticorrelation with Fe/H (i.e. a1 > 0 and a2 < 0). Taking both effects into account dramatically reduces the scatter, compared to the traditional X/Fe−σ relations. For C and N, a similar correlation with velocity dispersion is recovered, but there is no additional dependence on Fe/H (i.e. a1 > 0 and a2≈ 0). The explicit dependence of X/Fe on two parameters is evidence that at least two physical processes are at work in setting the abundance patterns. The Fe/H dependence of Mg/Fe and Ca/Fe, at fixed σ, may result from different durations of star formation, from galaxy to galaxy, leading to a range in the degree of Type Ia supernova contribution to the chemical enrichment. The absence of corresponding Fe/H dependence for C and N is consistent with these elements being generated in intermediate- and low-mass stars, on time-scales similar to the Fe production. The σ dependence, at fixed Fe/H, is similar for elements Mg, C and N, despite their likely origin in stars of very different masses and lifetimes. Ca/Fe is positively correlated with σ, at fixed Fe/H, but its dependence is significantly less steep than that of Mg, C and N. We discuss possible origins for this pattern of trends and find no simple prescription that reproduces the observations. In particular, if C and N are produced on time-scales comparable to that of Fe, then the X/Fe–σ trends cannot be explained solely by a systematic variation of star formation time-scale with σ.
We have obtained radial velocity measurements for 51 new globular clusters around the Sombrero galaxy. These measurements were obtained using spectroscopic observations from the AAOmega spectrograph ...on the Anglo-Australian Telescope and the Hydra spectrograph at WIYN. Combining our own past measurements and velocity measurements obtained from the literature, we have constructed a large database of radial velocities that contains a total of 360 confirmed globular clusters. Previous studies' analyses of the kinematics and mass profile of the Sombrero globular cluster system have been constrained to the inner ~9' (~24 kpc or ~5R sub(e)), but our new measurements have increased the radial coverage of the data, allowing us to determine the kinematic properties of M104 out to ~15' (~41 kpc or ~9 sub(e)). We use our set of radial velocities to study the GC system kinematics and to determine the mass profile and V-band mass-to-light profile of the galaxy. We find that M/L sub(V) increases from 4.5 at the center to a value of 20.9 at 41 kpc (~9R sub(e) or 15'), which implies that the dark matter halo extends to the edge of our available data set. We compare our mass profile at 20 kpc (~4R sub(e) or ~7'.4) to the mass computed from X-ray data and find good agreement. We also use our data to look for rotation in the globular cluster system as a whole, as well as in the red and blue subpopulations. We find no evidence for significant rotation in any of these samples.
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