A Model for Star Formation, Gas Flows and Chemical Evolution in Galaxies at High Redshifts

Motivated by the increasing use of the Kennicutt-Schmidt (K-S) star formation law to interpret observations of high redshift galaxies, the importance of gas accretion to galaxy formation, and the recent observations of chemical abundances in galaxies at z~2-3, I use simple analytical models to assess the consistency of these processes of galaxy evolution with observations and with each other. I derive the time dependence of star formation implied by the K-S law, and show that the sustained high star formation rates observed in galaxies at z~2-3 require the accretion of additional gas. A model in which the gas accretion rate is approximately equal to the combined star formation and outflow rates broadly reproduces the observed trends of star formation rate with galaxy age. Using an analytical description of chemical evolution, I also show that this model, further constrained to have an outflow rate roughly equal to the star formation rate, reproduces the observed mass-metallicity relation at z~2.Comment: 7 pages, 3 figures. Accepted for publication in Ap

Gain control from beyond the classical receptive field in primate primary visual cortex

Gain control is a salient feature of information processing throughout the visual system. Heeger (1991, 1992) described a mechanism that could underpin gain control in primary visual cortex (VI). According to this model, a neuron's response is normalized by dividing its output by the sum of a population of neurons, which are selective for orientations covering a broad range. Gain control in this scheme is manifested as a change in the semisaturation constant (contrast gain) of a VI neuron. Here we examine how flanking and annular gratings of the same or orthogonal orientation to that preferred by a neuron presented beyond the receptive field modulate gain in V1 neurons in anesthetized marmosets (Callithrix jacchus). To characterize how gain was modulated by surround stimuli, the Michaelis-Menten equation was fitted to response versus contrast functions obtained under each stimulus condition. The modulation of gain by surround stimuli was modelled best as a divisive reduction in response gain. Response gain varied with the orientation of surround stimuli, but was reduced most when the orientation of a large annular grating beyond the classical receptive field matched the preferred orientation of neurons. The strength of surround suppression did not vary significantly with retinal eccentricity or laminar distribution. In the mannoset, as in macaques (Angelucci et al., 2002a,b), gain control over the sort of distances reported here (up to 10 deg) may be mediated by feedback from extrastriate areas

Muon decay in a linearly polarized laser field

In a previous paper, we showed that the decay rate of a muon is only slightly affected by the presence of a circularly polarized laser and we gave an analytic expression for the correction. In this paper, we present the analytical result for the case of a linearly polarized laser. Again the effect of the laser is small.Comment: 6 pages, no figure

On Iron Enrichment, Star Formation, and Type Ia Supernovae in Galaxy Clusters

The nature of star formation and Type Ia supernovae (SNIa) in galaxies in the field and in rich galaxy clusters are contrasted by juxtaposing the build-up of heavy metals in the universe inferred from observed star formation and supernovae rate histories with data on the evolution of Fe abundances in the intracluster medium (ICM). Models for the chemical evolution of Fe in these environments are constructed, subject to observational constraints, for this purpose. While models with a mean delay for SNIa of 3 Gyr and standard initial mass function (IMF) are consistent with observations in the field, cluster Fe enrichment immediately tracks a rapid, top-heavy phase of star formation -- although transport of Fe into the ICM may be more prolonged and star formation likely continues to redshifts <1. The source of this prompt enrichment is Type II supernovae (SNII) yielding at least 0.1 solar masses per explosion (if the SNIa rate normalization is scaled down from its value in the field according to the relative number of candidate progenitor stars in the 3-8 solar mass range) and/or SNIa explosions with short delay times associated with the rapid star formation mode. Star formation is >3 times more efficient in rich clusters than in the field, mitigating the overcooling problem in numerical cluster simulations. Both the fraction of baryons cycled through stars, and the fraction of the total present-day stellar mass in the form of stellar remnants, are substantially greater in clusters than in the field.Comment: 51 pages including 26 figures and 2 tables, accepted for publication in ApJ 5/4/0

Dust-to-Gas Ratio and Metallicity in Dwarf Galaxies

We examine the dust-to-gas ratio as a function of metallicity for dwarf galaxies [dwarf irregular galaxies (dIrrs) and blue compact dwarf galaxies (BCDGs)]. Using a one-zone model and adopting the instantaneous recycling approximation, we prepare a set of basic equations which describes processes of dust formation and destruction in a galaxy. Four terms are included for the processes: dust formation from heavy elements ejected by stellar mass loss, dust destruction in supernova remnants, dust destruction in star-forming regions, and accretion of heavy elements onto preexisting dust grains. Solving the equations, we compare the result with observational data of nearby dIrrs and BCDGs. The solution is consistent with the data within the reasonable ranges of model parameters constrained by the previous examinations. This means that the model is successful in understanding the dust amount of nearby galaxies. We also show that the accretion rate of heavy element onto preexisting dust grains is less effective than the condensation of heavy elements in dwarf galaxies.Comment: 14 pages LaTeX, 4 figures, to appear in Ap

Does the Number Density of Elliptical Galaxies Change at z<1?

We have performed a detailed V/Vmax test for a sample of the Canada-France Redshift Survey (CFRS) for the purpose of examining whether the comoving number density of field galaxies changes significantly at redshifts of z<1. Taking into account the luminosity evolution of galaxies which depends on their morphological type through different history of star formation, we obtain \sim 0.5 in the range of 0.3<z<0.8, where reliable redshifts were secured by spectroscopy of either absorption or emission lines for the CFRS sample. This indicates that a picture of mild evolution of field galaxies without significant mergers is consistent with the CFRS data. Early-type galaxies, selected by their (V-I)_{AB} color, become unnaturally deficient in number at z>0.8 due to the selection bias, thereby causing a fictitious decrease of . We therefore conclude that a reasonable choice of upper bound of redshift z \sim 0.8 in the V/Vmax test saves the picture of passive evolution for field ellipticals in the CFRS sample, which was rejected by Kauffman, Charlot, & White (1996) without confining the redshift range. However, about 10% of the CFRS sample consists of galaxies having colors much bluer than predicted for irregular galaxies, and their \avmax is significantly larger than 0.5. We discuss this population of extremely blue galaxies in terms of starburst that has just turned on at their observed redshifts.Comment: 11 pages including 3 figures, to appear in ApJ Letter

Age Dating of a High-Redshift QSO B1422+231 at Z=3.62 and its Cosmological Implications

The observed Fe II(UV+optical)/Mg II lambda lambda 2796,2804 flux ratio from a gravitationally lensed quasar B1422+231 at z=3.62 is interpreted in terms of detailed modeling of photoionization and chemical enrichment in the broad-line region (BLR) of the host galaxy. The delayed iron enrichment by Type Ia supernovae is used as a cosmic clock. Our standard model, which matches the Fe II/Mg II ratio, requires the age of 1.5 Gyr for B1422+231 with a lower bound of 1.3 Gyr, which exceeds the expansion age of the Einstein-de Sitter Omega_0=1 universe at a redshift of 3.62 for any value of the Hubble constant in the currently accepted range, H_0=60-80 km,s^{-1},Mpc^{-1}. This problem of an age discrepancy at z=3.62 can be unraveled in a low-density Omega_0<0.2 universe, either with or without a cosmological constant, depending on the allowable redshift range of galaxy formation. However, whether the cosmological constant is a required option in modern cosmology awaits a thorough understanding of line transfer processes in the BLRs.Comment: 7 pages including 3 figures, to appear in ApJ Letter

Metallicities of 0.3<z<1.0 Galaxies in the GOODS-North Field

We measure nebular oxygen abundances for 204 emission-line galaxies with redshifts 0.3<z<1.0 in the Great Observatories Origins Deep Survey North (GOODS-N) field using spectra from the Team Keck Redshift Survey (TKRS). We also provide an updated analytic prescription for estimating oxygen abundances using the traditional strong emission line ratio, R_{23}, based on the photoionization models of Kewley & Dopita (2003). We include an analytic formula for very crude metallicity estimates using the [NII]6584/Halpha ratio. Oxygen abundances for GOODS-N galaxies range from 8.2< 12+log(O/H)< 9.1 corresponding to metallicities between 0.3 and 2.5 times the solar value. This sample of galaxies exhibits a correlation between rest-frame blue luminosity and gas-phase metallicity (i.e., an L-Z relation), consistent with L-Z correlations of previously-studied intermediate-redshift samples. The zero point of the L-Z relation evolves with redshift in the sense that galaxies of a given luminosity become more metal poor at higher redshift. Galaxies in luminosity bins -18.5<M_B<-21.5 exhibit a decrease in average oxygen abundance by 0.14\pm0.05 dex from z=0 to z=1. This rate of metal enrichment means that 28\pm0.07% of metals in local galaxies have been synthesized since z=1, in reasonable agreement with the predictions based on published star formation rate densities which show that ~38% of stars in the universe have formed during the same interval. (Abridged)Comment: AASTeX, 49 pages, 16 figures, accepted for publication in The Astrophysical Journa

Testing the Relation Between the Local and Cosmic Star Formation Histories

Recently, there has been great progress toward observationally determining the mean star formation history of the universe. When accurately known, the cosmic star formation rate could provide much information about Galactic evolution, if the Milky Way's star formation rate is representative of the average cosmic star formation history. A simple hypothesis is that our local star formation rate is proportional to the cosmic mean. In addition, to specify a star formation history, one must also adopt an initial mass function (IMF); typically it is assumed that the IMF is a smooth function which is constant in time. We show how to test directly the compatibility of all these assumptions, by making use of the local (solar neighborhood) star formation record encoded in the present-day stellar mass function. Present data suggests that at least one of the following is false: (1) the local IMF is constant in time; (2) the local IMF is a smooth (unimodal) function; and/or (3) star formation in the Galactic disk was representative of the cosmic mean. We briefly discuss how to determine which of these assumptions fail, and improvements in observations which will sharpen this test.Comment: 14 pages in LaTeX (uses aaspp4.sty). 5 postscript figures. To appear in the Astrophysical Journa

Cluster Populations in A115 and A2283

This paper presents four color narrow-band photometry of clusters A115 ($z=0.191$) and A2283 ($z=0.182$) in order to follow the star formation history of various galaxy types. Although located at similar redshifts, the two clusters display very different fractions of blue galaxies (i.e. the Butcher-Oemler effect, $f_B = 0.13$ for A115, $f_B = 0.30$ for A2283). A system of photometric classification is applied to the cluster members that divides the cluster population into four classes based on their recent levels of star formation. It is shown that the blue population of each cluster is primarily composed of normal starforming (SFR < 1 M_{\sun} yrs$^{-1}$) galaxies at the high luminosity end, but with an increasing contribution from a dwarf starburst population below $M_{5500}= -20$. This dwarf starburst population appears to be the same population of low mass galaxies identified in recent HST imaging (Koo et al 1997), possible progenitors to present-day cluster dwarf ellipticals, irregulars and BCD's. Deviations in the color-magnitude relationship for the red galaxies in each cluster suggest that a population of blue S0's is evolving into present-day S0 colors at this epoch. The radial distribution of the blue population supports the prediction of galaxy harassment mechanisms for tidally induced star formation operating on an infalling set of gas-rich galaxies.Comment: 28 pages including 2 tables and 9 figures, AASTeX v4.0. Accepted by Ap.J. Data, referee report and response are avaliable from http://zebu.uoregon.edu/~j