Faint Galaxy Counts Research Articles

An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: high-resolution 870 μm source counts

We report the first counts of faint submillimetre galaxies (SMG) in the 870-um band derived from arcsecond resolution observations with the Atacama Large Millimeter Array (ALMA). We have used ALMA to map a sample of 122 870-um-selected submillimetre sources drawn from the (0.5x0.5)deg^2 LABOCA Extended Chandra Deep Field South Submillimetre Survey (LESS). These ALMA maps have an average depth of sigma(870um)~0.4mJy, some ~3x deeper than the original LABOCA survey and critically the angular resolution is more than an order of magnitude higher, FWHM of ~1.5" compared to ~19" for the LABOCA discovery map. This combination of sensitivity and resolution allows us to precisely pin-point the SMGs contributing to the submillimetre sources from the LABOCA map, free from the effects of confusion. We show that our ALMA-derived SMG counts broadly agree with the submillimetre source counts from previous, lower-resolution single-dish surveys, demonstrating that the bulk of the submillimetre sources are not caused by blending of unresolved SMGs. The difficulty which well-constrained theoretical models have in reproducing the high-surface densities of SMGs, thus remains. However, our observations do show that all of the very brightest sources in the LESS sample, S(870um)>12mJy, comprise emission from multiple, fainter SMGs, each with 870-um fluxes of <9mJy. This implies a natural limit to the star-formation rate in SMGs of <10^3 M_Sun/yr, which in turn suggests that the space densities of z>1 galaxies with gas masses in excess of ~5x10^10 M_Sun is <10^-5 Mpc^-3. We also discuss the influence of this blending on the identification and characterisation of the SMG counterparts to these bright submillimetre sources and suggest that it may be responsible for previous claims that they lie at higher redshifts than fainter SMGs.

The properties of submm galaxies in hierarchical models

We use the combined GALFORM semi-analytical model of galaxy formation and GRASIL spectrophotometric code to investigate the properties of galaxies selected via their sub-mm emission. Our fiducial model has previously been shown to fit the properties of local ULIRGs, as well as the number counts of faint sub-mm galaxies. Here, we test the model in detail by comparing the SEDs and stellar, dynamical, gas and halo masses of sub-mm galaxies against observational data. We precisely mimic the sub-mm and radio selection function of the observations and show that the predicted far-infrared properties of model galaxies with S_850>5mJy and S_1.4>30uJy are in good agreement with observations. Although the dust emission model does not assume a single dust temperature, the far-infrared SEDs are well described by single component modified black-body spectrum with characteristic temperature 32+/-5K. We also find evidence that the observations may have uncovered evolution in the far-infrared--radio relation in ULIRGs out to z~2. We show that the predicted redshift distribution of sub-mm galaxies provides a reasonable fit to the observational data with a median redshift z=2.0, with the radio-selected subset predicted to make up approximately 75% of the population. However, the predicted K-band and mid-infrared (3--8um) flux densities of the sub-mm galaxies (and LBGs) are up to a factor 10x fainter than observed. This discrepancy may indicate that the stellar masses of the sub-mm galaxies in the model are too low: M~10^10Mo, while observations suggest more massive systems, M~10^11Mo. Finally, we discuss the potential modifications to the models which may improve the fit to the observational data. [Abridged]

Galaxy evolution in the infrared: comparison of a hierarchical galaxy formation model with Spitzer data

We present predictions for the evolution of the galaxy luminosity function, number counts and redshift distributions in the IR based on the Lambda-CDM cosmological model. We use the combined GALFORM semi-analytical galaxy formation model and GRASIL spectrophotometric code to compute galaxy SEDs including the reprocessing of radiation by dust. The model, which is the same as that in Baugh et al (2005), assumes two different IMFs: a normal solar neighbourhood IMF for quiescent star formation in disks, and a very top-heavy IMF in starbursts triggered by galaxy mergers. We have shown previously that the top-heavy IMF seems to be necessary to explain the number counts of faint sub-mm galaxies. We compare the model with observational data from the SPITZER Space Telescope, with the model parameters fixed at values chosen before SPITZER data became available. We find that the model matches the observed evolution in the IR remarkably well over the whole range of wavelengths probed by SPITZER. In particular, the SPITZER data show that there is strong evolution in the mid-IR galaxy luminosity function over the redshift range z ~ 0-2, and this is reproduced by our model without requiring any adjustment of parameters. On the other hand, a model with a normal IMF in starbursts predicts far too little evolution in the mid-IR luminosity function, and is therefore excluded.

The 12 $\mathsf{\mu}$mISO-ESO-Sculptor and 24 $\mathsf{\mu}$mSpitzerfaint counts reveal a population of ULIRGs as dusty massive ellipticals

Context. Multi-wavelength galaxy number counts provide clues to the nature of galaxy evolution. The interpretation per galaxy type of the mid-IR faint counts obtained with ISO and Spitzer, consistent with the analysis of deep UV-optical-near IR galaxy counts, provide new constraints on the dust and stellar emission. Discovering the nature of new populations, such as high redshift ultra-luminous (≥10^(12) L_⊙) infrared galaxies (ULIRGs), is also crucial for understanding galaxy evolution at high redshifts. Aims. We first present the faint galaxy counts at 12 μm from the catalogue of the ISO-ESO-Sculptor Survey (ISO-ESS) published in a companion article (Seymour et al. 2007a, A&A, 475, 791). They go down to 0.31 mJy after corrections for incompleteness. We verify the consistency with the existing ISO number counts at 15 μm. Then we analyse the 12 μm (ISO-ESS) and the 24 μm (Spitzer) faint counts, to constrain the nature of ULIRGs, the cosmic star formation history and time scales for mass buildup. Methods. We show that the “normal” scenarios in our evolutionary code PEGASE, which had previously fitted the deep UV-opticalnear IR counts, are unsuccessful at 12 μm and 24 μm. We thus propose a new ULIRG scenario adjusted to the observed cumulative and differential 12 μm and 24 μm counts and based on observed 12 μm and 25 μm IRAS luminosity functions and evolutionary optical/mid-IR colours from PEGASE. Results. We succeed in simultaneously modelling the typical excess observed at 12 μm, 15 μm (ISO), and 24 μm (Spitzer) in the cumulative and differential counts by only changing 9% of normal galaxies (1/3 of the ellipticals) into ultra-bright dusty galaxies evolving as ellipticals, and interpreted as distant ULIRGs. These objects present similarities with the population of radio-galaxy hosts at high redshift. No number density evolution is included in our models even if minor starbursts due to galaxy interactions remain compatible with our results. Conclusions. Higher spectral and spatial resolution in the mid-IR, together with submillimeter observations using the future Herschel observatory, will be useful to confirm these results.

Faint blue galaxies revisited

If dwarf-elliptical galaxies formed their stars very rapdily (on timescales of less than 1 Gyr), they may in principle be detectable out to high redshift. Prior to the discovery of cosmic acceleration, it appeared that rapid and late formation dwarf elliptical galaxies might be required to explain the number counts of faint galaxies. A plausible hypothesis emerged: that photoionization by the UV background prevents gas cooling in low-mass halos until $z \lesssim 1.5$ . The discovery of cosmic acceleration eased the tension between predicted galaxy number counts and galaxy-evolution models. Nevertheless, there is some evidence for relatively late star formation in nearby dE's, and the photoionization delay mechanism still appears to have some merit. It is thus of interest to look back in time to see if we can find starbursting dwarf galaxies at moderate redshift. We review the connection between faint-blue galaxies and bursting-dwarf galaxies and discuss some attempts to identify progenitors to dE galaxies in the Hubble Ultra Deep Field (HUDF) observations. We find roughly 85 galaxies in the HUDF with redshifts $0.6 that appear to have formed most of their stars at $z . Of these, 70% have half-light radii less than 1.5 kpc. These are thus “smoking gun” candidates for dwarf galaxies that are either collapsing for the first time at moderate redshifts or have otherwise been unable to form stars for more than 1/3$ of a Hubble time.

The radio galaxy K-z relation: The $\mathsf{10^{12}}~$M\odot mass limit

The narrow K-z relation of powerful radio galaxies in the Hubble K-band diagram is often attributed to the stellar populations of massive elliptical galaxies. Because it extends over a large range of redshifts (), it is difficult to estimate masses at high redshifts by taking into account galaxy evolution. In the present paper, we propose to estimate the stellar masses of galaxies using the galaxy evolution model PÉGASE. We use star formation scenarios that successfully fit faint galaxy counts as well as galaxy templates. These scenarios also predict spectra at higher z, used to estimate valid photometric redshifts. The baryonic mass of the initial gas cloud is then derived. The K-z relation is remarkably reproduced by our evolutionary scenario for elliptical galaxies of baryonic mass , at all z up to . is also the maximum mass limit of all types of galaxies. Using another initial mass function (IMF), even a top-heavy one, does not alter our conclusions. The high value of observed at implies that massive clouds were already formed at early epochs. We also find that the limit is similar to the critical mass of a self-gravitating cloud regulated by cooling (Rees & Ostriker [CITE]; Silk [CITE]). Moreover, the critical size is remarkably close to the typical diameter of Lyα haloes surrounding distant radio galaxies. This confirms the validity of the method of baryonic mass determination based on the K-band luminosity. A puzzling question that remains to be answered is the short time-scale of mass-accumulation required to form such massive galaxies at . We discuss the dispersion of the K-z relation in terms of uncertainties on the mass limit. The link between the presence of the active nucleus and a large stellar mass is also discussed.

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000 Å. I. Results

We present the first detection of the mean flux of the optical extragalactic background light (EBL) at 3000, 5500, and 8000 A. Diffuse foreground flux at these wavelengths comes from terrestrial airglow, dust-scattered sunlight (zodiacal light), and dust-scattered Galactic starlight (diffuse Galactic light). We have avoided the brightest of these, terrestrial airglow, by measuring the absolute surface brightness of the night sky from above the Earth's atmosphere using the Wide Field Planetary Camera 2 (WFPC2) and Faint Object Spectrograph, both on board the Hubble Space Telescope (HST). On the ground we have used the du Pont 2.5 m Telescope at the Las Campanas Observatory (LCO) to obtain contemporaneous spectrophotometry of blank sky in the HST field of view to measure and then subtract foreground zodiacal light from the HST observations. We have minimized the diffuse Galactic light in advance by selecting the HST target field along a line of sight with low Galactic dust column density and then estimated the low-level Galactic foreground using a simple scattering model and the observed correlation between thermal, 100 μm emission, and optical scattered flux from the same dust. In this paper, we describe the coordinated LCO/HST program and the HST observations and data reduction and present the resulting measurements of the EBL. Galaxies brighter than V = 23 AB mag are not well sampled in an image the size of the WFPC2 field of view. We have therefore measured the EBL from unresolved and resolved galaxies fainter than V = 23 AB mag by masking out brighter galaxies. We write this as EBL23 to emphasize this bright magnitude cutoff. From absolute surface photometry using WFPC2 and ground-based spectroscopy, we find mean values for the EBL23 of 4.0 (±2.5), 2.7 (±1.4), and 2.2 (±1.0) in units of 10-9 ergs s-1 cm-2 sr-1 A-1 in the F300W, F555W, and F814W bandpasses, respectively. The errors quoted are 1 σ combined statistical and systematic uncertainties. The total flux measured in resolved galaxies with V > 23 AB mag by standard photometric methods is roughly 15% of the EBL23 flux in each band. We have also developed a new method of source photometry, uniquely suited to these data, with which we can measure the ensemble flux from detectable sources much more accurately than is possible with standard methods for faint galaxy photometry. Using this method, we have quantified systematic biases affecting standard galaxy photometry, which prevent light from being recovered in isophotes within a few percent of the sky level. These biases have a significant effect on faint galaxy counts. The flux from resolved sources as measured by our ensemble photometry method is 3.2 (±0.22), 0.89 (±0.01), and 0.76 (±0.01) in units of 10-9 ergs s-1 cm-2 sr-1 A-1 in the F300W, F555W, and F814W bandpasses, respectively, with 1 σ combined errors. These values, the total flux from resolved sources, represent absolute minima for the EBL23 in each band and are roughly 30% of the mean flux we measure for the total EBL23.

New Wide Field J- and K-band Galaxy Counts and the Extragalactic Background

We present new medium-deep wide field galaxy counts at J- and K-bands, performed in the ELAIS fields. The source detection limits are at J = 19.5 and K = 18.0 magnitudes. The sky coverage is 1 degree2, making our survey the largest to date at these wavelengths and magnitudes. Counts in this brightness range are critical for normalization of models when explaining the deepest galaxy counts. In our survey fields we find evidence for a higher than expected local galaxy density. If normalized to our counts, galaxy models do not need strong brightness evolution or exotic populations to be explained even at the faintest levels. We measure clearly sub-Euclidean slopes of d log N(m)/dm ~ 0.45 to 0.50 at 13 &lt; K &lt; 17.5 and 14.5 &lt; J &lt; 19. Normal, passively evolving stellar populations along with a high normalization of the local luminosity function and an open cosmological model best fit both the slope and amplitude of our number counts. We report a preliminary value, ~ 30 n Wm−2sr−1, for the K-band extragalactic background light using zodi-subtracted DIRBE-data and our observed star counts. The value is consistent with, e.g., Matsumoto (these Proceedings 2001) and Gorjian, V., Wright, E. L., &amp; Chary, R. R. 2000, ApJ, 536, 550, but is considerably higher than extrapolations from faint galaxy counts.

Identifying the Reionization Redshift from the Cosmic Star Formation Rate

We show that the cosmic star formation rate per comoving volume should exhibit a distinct drop around the reionization redshift, when the H II regions in the intergalactic medium around individual ionizing sources first overlapped. The drop results from the increase in the temperature of the intergalactic medium as it was photoionized, and the consequent suppression of the formation of low-mass galaxies. We show quantitatively that the detection of this drop, which is marked by a corresponding fall in the number counts of faint galaxies, should become feasible over the coming decade with the Next Generation Space Telescope.

Star Formation Histories From Faint Galaxy Counts

Multispectral faint galaxy counts, including the deepest Hubble Deep Field, are interpreted with the help of our evolution model PEGASE (Fioc and Rocca-Volmerange, 1997). The best fits correspond to galaxy formations at high redshifts, a pure luminosity evolution and classical luminosity functions. The adopted cosmology is a flat universe with the matter density parameter Ω M =0.3 and a cosmological constant ΩΔ = 0.7. A solution with Ω M = 0.01 (open universe) is also acceptable. But a flat universe with Ω M = 1 is clearly excluded. The star formation histories for galaxy types are derived from scenarios of evolution. The comparison with results already published in the litterature, arises puzzling problems needing a further analysis of star formation tracers, specifically for bright galaxies.

Young Stellar Populations at High Z: Where are They?

Our view on the deep universe is strongly biassed towards the starlight that directly escapes from high-redshift galaxies, since we know very little on the fraction of luminosity absorbed by dust. Attempts to correct for this effect directly from the slope of the UV spectra seem to suggest that a significant fraction of the UV flux is extinguished. New constraints are now set on the fraction of dust-enshrouded young stellar populations by the detection of the Cosmic Infrared Background, and faint galaxy counts at IR and submm wavelengths. We briefly review the observations and use a semi-analytic model of galaxy formation and evolution to predict number counts consistent with the background.

Low Surface Brightness Galaxies in Deep Surveys

AbstractWe examine the constraints that can be placed on the space density of low-surface-brightness galaxies from deep HST images. Such images, while covering only a small solid angle, provide enough depth and spatial resolution to detect LSB galaxies at moderate redshift and distinguish them from galaxies of higher surface brightness.We consider five simple models of the non-evolving or slowly-evolving population of LSB galaxies, motivated by various discussions in the recent literature. The basic results are (1) models with a large space-density of giant LSB galaxies at moderate redshift do not look like the real world and, (2) models with a large space-density of dwarf LSB galaxies are consistent with HST data (that is, they do not produce more faint LSB galaxies per unit solid angle than are detected at magnitudes I ≳ 23), but these LSB dwarf galaxies do not contribute much to faint galaxy counts unless they formed their stars in a rapid burst.

Deep optical imaging of the field of PC 1643+4631A&amp;B - I. Spatial distributions and the counts of faint galaxies

We present deep optical images of the PC 1643+4631 field, obtained at the William Herschel Telescope (WHT). This field contains two quasars at redshifts z=3.79 and 3.83 and a cosmic microwave background (CMB) decrement detected with the Ryle Telescope. The images are in U, G, V, R and I filters, and in two custom-built narrow-band filters. Using the object-finding algorithms focas and sextractor, we have constructed galaxy catalogues in each band. The catalogues created with the two detection algorithms are compared to determine the faint magnitude limits to which they are consistent. Using Monte Carlo simulations, we investigate the completeness of the catalogues at faint magnitudes. In particular, we compare the flux lost using isophotal apertures on a real image with that on a noise-only image: recovery of artificial galaxies from the noise-only image significantly overestimates the flux lost from the galaxies, and we find that the corrections made using this technique suffer a systematic error of some 0.4 mag. We determine that the catalogues are complete to RAB, GAB=25.0 and to UAB=25.5, and we investigate the number counts and spatial distributions of the galaxies in the field. The galaxy counts in each band are consistent with the results of other recent deep imaging surveys. We investigate the properties of several populations of objects in the field. Our narrow-band imaging does not reveal the presence of any bright emission-line galaxies, nor do we detect any obvious candidate for a further, possibly lensed, quasar image. The galaxies whose colours indicate that they lie at redshifts 1≲z≲2 are found to have similar properties as in other fields. However, we find that the number of high-redshift Lyman-break candidate galaxies in the field is significantly higher than would be expected from the mean surface density found in the surveys of Steidel et al. The Lyman-break candidates are strongly clustered, but they are not concentrated towards the CMB decrement; rather, they tend to lie towards the edges of the field. Finally, using the present data in combination with existing near-infrared imaging of the field, we investigate extremely red objects (EROs) in the field. We find a large surface density of EROs, and we find that their colours are consistent with those of slightly-reddened early-type galaxies at redshift z≈2.

Deep [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] Galaxy and Pair Counts as Tests of Merger History

We consider the use of counts of faint galaxy pairs as tests of the role of merging in galaxy evolution, using both simulations and Hubble Space Telescope (HST) data in comparison with models incorporating a range in the evolution of merger rate. Artificial data, including simulation of the surface-brightness selection effects implicit in HST imagery, indicate that simple pairing statistics are robust enough for this test. However, the differences between pair fractions for various plausible scenarios of merger-rate evolution are small enough to be easily hidden by statistical errors or environmental differences. With a Poisson-based algorithm, we identify pairs with physical distances between components 60 h-1 kpc. Deep HST WFPC2 images of the field around the radio galaxy 53W002 at z = 2.4 in F814W (I) and F606W (V) show that 12%?20% (~10 pairs) of galaxies at mI 24 or mV 24.6 are paired. This supports a mild enhancement of pair fraction toward high redshifts (1?2), as concluded in other recent studies of faint galaxy pairs. The magnitude limits eliminate most possible members of the grouping associated with 53W002 itself. We compare the counts of galaxies and galaxy pairs in the 53W002 field and other recent studies to a detailed model incorporating overall traditional galaxy spectral evolution and various merger rates, in conjunction with our previous evaluation of the local merger rate. The data support evolution of the merger rate, such that the comoving density of merging systems varies as (1 + z)?, with 0 ? ? ? 2. This is consistent with the constraint ? < 3, beyond which local elliptical galaxies (E/S0 types) are overproduced from merging, and includes the case ? ? 2 required by our model to support the merger hypothesis, whereby almost all elliptical galaxies are merger remnants. The ? ? 2 case also expects a large fraction of early galaxies undergoing merging. Therefore, we support the merger-rate evolution required to support the merger hypothesis and the expectation that early merging is important in the overall evolution of galaxies. Assuming merger remnants to be elliptical galaxies subsequently undergoing passive evolution, our model can reproduce at once counts of faint galaxies and galaxy pairs, as well as the present-day luminosity function of ellipticals, for the Hubble constant H0 ? 50 km s-1 Mpc-1 and a cosmological constant ? = 0. These lines of evidence do not appear to offer compelling arguments for more exotic cosmologies or star formation histories. A preliminary study shows the fraction of paired galaxies in the 53W002 field to be much larger than that in the Hubble Deep Field (5%?6% at most in the HDF), suggesting physical field-to-field fluctuations in pair statistics. Further progress to constrain merging history requires not only larger samples, preferably coupled to at least partial redshift information, but better indications as to which faint objects in multiple systems should be considered as independent galaxies and which are parts of single dynamical units.

The Star Formation History of the Carina Dwarf Galaxy

We have analyzed deep B and V photometry of the Carina dwarf spheroidal reaching below the old main-sequence turnoff to about V = 25. Using simulated color-magnitude diagrams to model a range of star formation scenarios, we have extracted a detailed, global star formation history. Carina experienced three significant episodes of star formation at about 15 Gyr, 7 Gyr, and 3 Gyr. Contrary to the generic picture of galaxy evolution, however, the bulk of star formation, at least 50%, occured during the episode 7 Gyr ago, which may have lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10-20% of its stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The remainder (~30%) formed relatively recently, only 3 Gyr ago. Interest in the local population of dwarf galaxies has increased lately due to their potential importance in the understanding of faint galaxy counts. We surmise that objects like Carina, which exhibits the most extreme episodic behavior of any of the dwarf spheroidal companions to the Galaxy, are capable of contributing to the observed excess of blue galaxies at B = 24 only if the star formation occurred instantaneously.

Semi-analytic modelling of galaxy evolution in the IR/submm range

This paper proposes a new semi-analytic modelling of galaxy properties in the IR/submm wavelength range, which is explicitly set in a cosmological framework. This type of approach has had some success in reproducing the optical properties of galaxies. We hereafter propose a simple extension to the IR/submm range. We estimate the IR/submm luminosities of ``luminous UV/IR galaxies'', and we explore how much star formation could be hidden in heavily--extinguished, ``ultraluminous IR galaxies'' by designing a family of evolutionary scenarios which are consistent with the current status of the ``cosmic constraints'', as well as with the IRAS luminosity function and faint counts, but with different high-z IR luminosity densities. However, these scenarios generate a Cosmic Infrared Background whose spectrum falls within the range of the isotropic IR component detected by Puget et al. (1996) and revisited by Guiderdoni et al. (1997). We give predictions for the faint galaxy counts and redshift distributions at IR and submm wavelengths. The submm range is very sensitive to the details of the evolutionary scenarios. As a result, the on-going and forthcoming observations with ISO and SCUBA (and later with SIRTF, SOFIA, FIRST and PLANCK) will put strong constraints on the evolution of galaxies at z=1 and beyond.

Cosmological Evolution of Dwarf Galaxies: The Influence of Star Formation and the Multiphase Interstellar Medium

A model is developed to explain the cosmological evolution of dwarf galaxies. The population of small galaxies is found to evolve rapidly for z < 1, which provides a natural explanation for the evolution observed in the galaxy luminosity function. A tail is found in the redshift distribution of the faint blue excess that can extend to a redshift of 2. The star formation history is followed in detail for these objects. Constraints on the metallicity are identified for which stars are formed with much higher efficiency in a multiphase interstellar medium than in massive galaxies. Blue dwarf galaxies at the current epoch are identified with this starburst mode. The collapse of 1 and 2 σ perturbations of the initial density fluctuation spectrum is followed using the extended standard hierarchical clustering formalism. The collapse of these perturbations is normally associated with the formation of dwarf galaxies. These objects have shallow gravitational potential wells, and their evolution strongly depends upon the cooling time of the gas. The latter is determined by the ionization and chemical equilibrium of the gas in the presence of the intergalactic and local stellar radiation fields. The latter generally dominates and creates a feedback mechanism that regulates the evolutionary timescale. To improve upon previous models, essential new astrophysical ingredients are incorporated, such as a more detailed description of the physical processes regulating the multiphase structure of the interstellar medium in dwarf galaxies and the effects of evolution in the galaxy's metallicity on the formation of stars in molecular clouds. It is found that for a low star formation rate of 0.1 M☉ yr-1, the cooling time of interstellar gas is longer than the local Hubble time until z ~ 1. At this epoch, a two-phase medium makes the dwarf interstellar medium less fragile against supernova explosions, and the volume filling factor of the hot phase (107 K) becomes of order unity. The resulting X-ray luminosity is consistent with observations of nearby dwarf galaxies if exchange processes between the hot and cold phases of the interstellar medium (i.e., evaporation and ablation of clouds due the supernova blast wave) are included. The tenuous halo gas has a temperature in excess of the escape velocity and exhibits very high ionization stages. The dark matter halo potential (M/L ~ 102) influences the loss rate of metals and the star formation history. It is found that if the metallicity is between 0.01 and 0.1 Z☉ then a period of rapid star formation, ~1-3 M☉ yr-1, can be excited for z ~ 1. The physical reason for this starburst mode in our model is that the free-fall time exceeds the ambipolar diffusion timescale for clouds. Consequently, molecular clouds are not magnetically supported and star formation can proceed much more efficiently compared with more massive and metal-rich galaxies. Our results are consistent with the redshift distribution of dwarf galaxies observed in the Hubble Deep Field. In particular, a rapidly evolving dwarf population can account for the excess number counts of faint galaxies: the blue excess. Predictions are made for the color evolution of the old stellar population in these systems, observable with the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on the Hubble Space Telescope in the near future.

Automated Morphological Classification in Deep [ITAL]Hubble Space Telescope UBVI[/ITAL] Fields: Rapidly and Passively Evolving Faint Galaxy Populations

We analyze deep Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images in U, B, V, I using artificial neural network (ANN) classifiers, which are based on galaxy surface brightness and light profile (but not on color nor on scale length, rhl). The ANN distinguishes quite well between E/S0, Sabc, and Sd/Irr+M galaxies (M for merging systems) for BJ 27 mag. We discuss effects from the cosmological surface brightness (SB) dimming and from the redshifted UV morphology on the classifications, and we correct for the latter. We present classifications in UBVI from (a) four independent human classifiers; (b) ANNs trained on V606 and I814 images; and (c) an ANN trained on images in the rest-frame UBV according to the expected redshift distribution as a function of BJ. For each of the three methods, we find that the fraction of galaxy types does not depend significantly on wavelength, and that they produce consistent counts as a function of type. The median scale length at BJ 27 mag is rhl 025-03 (1-2 kpc at z ≈ 1-2). Early- and late-type galaxies are fairly well separated in BVI color-magnitude diagrams for B 27 mag, with E/S0 galaxies being the reddest and Sd/Irr+M galaxies generally blue. We present the B-band galaxy counts for five WFPC2 fields as a function of morphological type for BJ 27 mag. E/S0 galaxies are only marginally above the no-evolution predictions, and Sabc galaxies are at most 0.5 dex above the nonevolving models for BJ 24 mag. The faint blue galaxy counts in the B band are dominated by Sd/Irr+M galaxies and can be explained by a moderately steep local luminosity function (LF) undergoing strong luminosity evolution. We suggest that these faint late-type objects (24 mag BJ 28 mag) are a combination of low-luminosity lower redshift dwarf galaxies, plus compact star-forming galaxies and merging systems at z 1-3, possibly the building blocks of the luminous early-type galaxies seen today.

Faint galaxy counts as a function of morphological type in a hierarchical merger model

The unprecedented resolution of the refurbished Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) has led to major advances in our understanding of galaxy formation. The high image quality in the Medium Deep Survey and Hubble Deep Field has made it possible, for the first time, to classify faint distant galaxies according to morphological type. These observations have revealed a large population of galaxies classed as irregulars or which show signs of recent merger activity. Their abundance rises steeply with apparent magnitude, providing a likely explanation for the large number of blue galaxies seen at faint magnitudes. We demonstrate that such a population arises naturally in a model in which structure forms hierarchically and which is dynamically dominated by cold dark matter. The number counts of irregular, spiral and elliptical galaxies as a function of magnitude seen in the HST data are well reproduced in this model.We present detailed predictions for the outcome of spectroscopic follow-up observations of the HST surveys. By measuring the redshift distributions of faint galaxies of different morphological types, these programmes will provide a test of the hierarchical galaxy formation paradigm and might distinguish between models with different cosmological parameters.

A BVRIJKHα Survey of 355 Nearby Galaxies

A multiband CCD-survey of 355 UGC galaxies in our local cosmological neighbourhood is undertaken in collaboration with S.D.M. White (MPI-fAstrophysik Garching), S. McGaugh (IoA, Cambridge), M. Dennefeld (IAP Paris), H. Ferguson (STScI), M. Rieke, A. Grauer (Steward Obs. Arizona). Optical observations are obtained on Calar Alto (MPIA Heidelberg) and at La Palma (RGO) – with data 40% complete at the moment. The selection criterion is diameter 1.5′ &lt; D25 &lt; 2.5′. A database of local galaxy properties is being established (including total luminosities, mean SB, diameters, colours, colour gradients, D/B-ratios, present SFRs). Using theoretical evolution models we can predict the bona fide appearance of the galaxy population at any given redshift and thus provide a secure reference point to interpreting galaxies observed at intermediate and high z (e.g. faint galaxy counts).