arXiv subjects

Steven Phillipps

Publications and source records attributed to Steven Phillipps.

59 records · Page 4Linked to original sources

arXiv1997

Luminosity Distributions within Rich Clusters - I: A Ubiquitous Dwarf-Rich Luminosity Function ?

From deep CCD observations of the cluster Abell 2554 we have recovered the cluster's luminosity distribution over a wide range of magnitude (-24 < M(R) < -16). We compare the derived A2554 cluster luminosity function (at redshift 0.1) with that of the local Coma Cluster (A1656) and the more distant (z = 0.2) cluster A963. The distribution is remarkably similar for these three clusters of comparable richness and morphology. All show a flat (α= -1.0) luminosity function for the giant galaxies (-24 < M(R) < -19.5) which exhibits a sharp upturn (α= -1.7) at some intermediate magnitude (M(R) = -19) and continues to rise to the limits of existing data. We suggest that such a luminosity function may be ubiquitous among rich clusters and that a similar form may apply for poorer clusters and possibly the field as well. The three cluster dwarf LFs are seen over a range of lookback times covering a quarter of the age of the universe. Therefore the similarity between the three measured LFs seems to rule out strong evolution of the dwarf populations in rich cluster environments, at least out to z = 0.2, unless richness effects conspire to conceal evolutionary changes.

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arXiv1996

The inferred redshift distribution of the faint blue excess

We infer the redshift distribution of the faint blue galaxy excess (FBE) at B=23.5 by subtracting the predicted distribution of giant/normal galaxies from the observed N(z) distribution for all types. This is possible because of the recent deep {\it Hubble Space Telescope} (HST) WFPC2 morphological number counts which have convincingly demonstrated that little evolution of the giant population is seen to B=26.0. The mean redshift of the FBE at B=23.5 is found to be _{FBE}=0.40 +/- 0.07 with upper and lower quartiles defined by z_{0.75}=0.58 +/- 0.05 and z_{0.25}=0.28 +/- 0.05, respectively. We compare this inferred FBE N(z) distribution to the predictions from three generic faint galaxy models: dwarf dominated (no evolution), pure luminosity evolution, and evolving dwarfs. The inferred FBE N(z) distribution strongly supports a hybrid evolving dwarf--rich model wherein a large population of dwarfs present at z=0.5 has subsequently faded to obscurity. The total integrated number density of dwarfs (down to M_{B}=-11) is estimated to be a factor of 20 times greater than that of E---Sc galaxies and the estimated fading to be 1.0 < Δm < 1.4 mags. Thus, the dwarf population is estimated to be responsible for ~30% of the luminosity density locally, rising to ~57% at z=0.5.

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arXiv1996

Is the luminosity distribution of field galaxies really flat ?

Recent observations of the galaxy population within rich clusters have found a characteristic luminosity distribution described by a flat (alpha = -1.0) Schechter function which exhibits an upturn at faint absolute magnitudes (B Mag = -18). Here we discuss whether such a form for the field luminosity distribution is ruled out by local and/or faint magnitude limited redshift surveys (MLRS). Our conclusions are that existing redshift surveys provide little constraints on the volume-density distribution of field galaxies faintwards of B Mag = -18. The local MLRS suffer from poor statistics over inhomogeneous volumes, while the faint MLRS are ambiguous because of the unknown nature of the ``faint blue excess'' and the ``normalization'' problem. Adopting a functional form similar to that seen in rich clusters we find that the maximum allowable faint end slope, based on the Mt Stromlo-APM redshift survey, is $α\approx -1.8$ faintwards of B Mag = -18.0 (Ho = 50 km/s/Mpc^{3}).

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arXiv1995

Hubble Space Telescope Counts of Elliptical Galaxies: Constraints on Cosmological Models ?

The interpretation of galaxy number counts in terms of cosmological models is fraught with difficulty due to uncertainties in the overall galaxy population (mix of morphological types, luminosity functions etc.) and in the observations (loss of low surface brightness images, image blending etc.). Many of these can be overcome if we use deep high resolution imaging of a single class of high surface brightness galaxies, whose evolution is thought to be fairly well understood. This is now possible by selecting elliptical and S0 galaxies using Hubble Space Telescope images from the Medium Deep Survey and other ultradeep WFPC2 images. In the present paper, we examine whether such data can be used to discriminate between open and closed universes, or between conventional cosmological models and those dominated by a cosmological constant. We find, based on the currently available data, that unless elliptical galaxies undergo very strong merging since $z \sim 1$ (and/or very large errors exist in the morphological classifications), then flat models dominated by a cosmological constant are ruled out. However, both an Einstein-de Sitter ($Ω_{0}=1$) model with standard passive stellar evolution and an open ($Ω_{0}=0.05$) model with no net evolution ({\it i.e.} cancelling stellar and dynamical evolution) predict virtually identical elliptical and S0 galaxy counts. Based on these findings and the recent reportings of $H_{o} \simeq 80$ km/s Mpc/s, we find that the maximum acceptable age of the universe is 13.3 Gyrs and a value of $\leq 9$ Gyrs favored. A flat---$Λ\neq 0$---universe is therefore {\it not} a viable solution to the $H_{o}$/globular cluster age problem.

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arXiv1995

Are disappearing dwarfs just lying low ?

Recent redshift surveys have shown that the excess galaxies seen in faint galaxy number counts (above those expected given the local galaxy luminosity function) are not evolved giants at high redshifts, but low to moderate luminosity objects at more modest redshifts. This has led to the suggestion that there was once an additional population of dwarf galaxies which has since disappeared, ie. there is non-conservation of galaxy number. Here we investigate the possibility that these disappearing dwarfs have actually evolved to become the population of very low surface brightness galaxies which is now being detected in nearby clusters.

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