Galaxies in SDSS and DEEP2: a quiet life on the blue sequence?

In the six billion years between redshifts z=1 and z=0.1, galaxies change due to the aging of their stellar populations, the formation of new stars, and mergers with other galaxies. Here I explore the relative importance of these various effects, finding that while mergers are likely to be important for the red galaxy sequence they are unlikely to affect more than 10% of the blue galaxy sequence. I compare the galaxy population at redshift z=0.1 from the Sloan Digital Sky Survey to that at z=1 from the Deep Extragalactic Evolutionary Probe 2. Galaxies are bluer at z=1: the blue sequence by about 0.3 mag and the red sequence by about 0.1 mag, in redshift z=0.1 (u-g) color. I evaluate the change in color and in the luminosity functions of the two sequences using some simplistic stellar population synthesis models. These models indicate that the luminous end of the red sequence fades less than passive evolution allows by about 0.2 mag. Due to a lack of luminous blue progenitors, ``dry'' mergers betweeen red galaxies then must create the luminous red population at z=0.1, if stellar population models are correct. The blue sequence colors and luminosity function are consistent with a reduction in the star-formation rate since redshift z=1 by a factor of about three, with no change in the number density to within 10%. These results restrict the number of blue galaxies that can fall onto the red sequence by any process, and in particular suggest that if mergers are catastrophic events they must be rare for blue galaxies.Comment: submitted to ApJ, summary and viewgraphs available at http://cosmo.nyu.edu/blanton/deep2sdss

Pickoff and spin-conversion quenchings of ortho-positronium in oxygen

The quenching processes of the thermalized ortho-positronium(o-Ps) on an oxygen molecule have been studied by the positron annihilation age-momentum correlation techinique(AMOC). The Doppler broadening spectrum of the 511 keV gamma-rays from the 2gamma annihilation of o-Ps in O_2 has been measured as a function of the o-Ps age. The rate of the quenching, consisting of the pickoff and the spin-conversion, is estimated from the positron lifetime spectrum. The ratio of the pickoff quenching rate to the spin-conversion rate is deduced from the Doppler broadening of the 511 keV gamma-rays from the annihilation of the o-Ps. The pickoff parameter ^1Z_eff, the effective number of the electrons per molecule which contribute to the pickoff quenching, for O_2 is determined to be 0.6 +- 0.4. The cross-section for the elastic spin-conversion quenching is determined to be (1.16 +- 0.01) * 10^{-19} cm^2.Comment: 4 pages with 5 eps figures, LaTeX2e(revtex4

On the Similarity between Cluster and Galactic Stellar Initial Mass Functions

The stellar initial mass functions (IMFs) for the Galactic bulge, the Milky Way, other galaxies, clusters of galaxies, and the integrated stars in the Universe are composites from countless individual IMFs in star clusters and associations where stars form. These galaxy-scale IMFs, reviewed in detail here, are not steeper than the cluster IMFs except in rare cases. This is true even though low mass clusters generally outnumber high mass clusters and the average maximum stellar mass in a cluster scales with the cluster mass. The implication is that the mass distribution function for clusters and associations is a power law with a slope of -2 or shallower. Steeper slopes, even by a few tenths, upset the observed equality between large and small scale IMFs. Such a cluster function is expected from the hierarchical nature of star formation, which also provides independent evidence for the IMF equality when it is applied on sub-cluster scales. We explain these results with analytical expressions and Monte Carlo simulations. Star clusters appear to be the relaxed inner parts of a widespread hierarchy of star formation and cloud structure. They are defined by their own dynamics rather than pre-existing cloud boundaries.Comment: 22 pages, 2 figures, ApJ, 648, in press, September 1, 200

Two Generations of Hexagonal CaAl_2Si_2O_8 (Dmisteinbergite) in the Type B2 FUN CAI STP-1

Dmisteinbergite (dmist) is a metastable hexag-onal form of CaAl_2Si_2O_8, with space group of P6_3/mcm, a = 5.10Å and c = 14.72Å [1]. First occurrence of meteoritic dmist has been reported in the Allende Type B2 FUN CAI STP-1 [2], where it appears to have crystallized from a ^(16)O-rich (Δ^(17)O ~ −25‰) silicate melt via rapid cooling [3]. Here we report on an-other textural occurrence of dmist in STP-1 - ^(16)O-poor (Δ^(17)O ~ −2‰) fine-grained crystals in alteration zone of the inclusion

The abundance of high-redshift objects as a probe of non-Gaussian initial conditions

The observed abundance of high-redshift galaxies and clusters contains precious information about the properties of the initial perturbations. We present a method to compute analytically the number density of objects as a function of mass and redshift for a range of physically motivated non-Gaussian models. In these models the non-Gaussianity can be dialed from zero and is assumed to be small. We compute the probability density function for the smoothed dark matter density field and we extend the Press and Schechter approach to mildly non-Gaussian density fields. The abundance of high-redshift objects can be directly related to the non-Gaussianity parameter and thus to the physical processes that generated deviations from the Gaussian behaviour. Even a skewness parameter of order 0.1 implies a dramatic change in the predicted abundance of z\gap 1 objects. Observations from NGST and X-ray satellites (XMM) can be used to accurately measure the amount of non-Gaussianity in the primordial density field.Comment: Minor changes to match the accepted ApJ version (ApJ, 539

Forsterite-Bearing Type B CAI with a Relict Eringaite-Bearing Ultra-Refractory CAI

Forsterite-bearing Type B (FoB) Ca,Al-rich inclusions (CAIs) are a rare type of coarse-grained igneous CAIs found almost exclusively in CV3 chondrites [1–5]. Here we describe the mineralogy, petrography, and oxygen-isotope compositions of a FoB CAI Al-2 from Allende containing a relict eringaite-bearing ultra-refractory (UR) inclusion. Eringaite is a Sc-rich garnet [Ca_3(Sc,Y,Ti)_2Si_3O_(12)] that has been recently identified in a cluster of UR inclusion fragments within an amoeboid olivine aggregate in Vigarano [6]

Damped Lyman alpha Absorbing Galaxies At Low Redshifts z<1 From Hierarchical Galaxy Formation Models

We investigate Damped Ly-alpha absorbing galaxies (DLA galaxies) at low redshifts z<1 in the hierarchical structure formation scenario to clarify the nature of DLA galaxies because observational data of such galaxies mainly at low redshifts are currently available. We find that our model well reproduces distributions of fundamental properties of DLA galaxies such as luminosities, column densities, impact parameters obtained by optical and near-infrared imagings. Our results suggest that DLA systems primarily consist of low luminosity galaxies with small impact parameters (typical radius about 3 kpc, surface brightness from 22 to 27 mag arcsec^{-2}) similar to low surface brightness (LSB) galaxies. In addition, we investigate selection biases arising from the faintness and from the masking effect which prevents us from identifying a DLA galaxy hidden or contaminated by a point spread function of a background quasar. We find that the latter affects the distributions of DLA properties more seriously rather than the former, and that the observational data are well reproduced only when taking into account the masking effect. The missing rate of DLA galaxies by the masking effect attains 60-90 % in the sample at redshift 0<z<1 when an angular size limit is as small as 1 arcsec. Furthermore we find a tight correlation between HI mass and cross section of DLA galaxies, and also find that HI-rich galaxies with M(HI) \sim 10^{9} M_sun dominate DLA systems. These features are entirely consistent with those from the Arecibo Dual-Beam Survey which is a blind 21 cm survey. Finally we discuss star formation rates, and find that they are typically about 10^{-2} M_sun yr^{-1} as low as those in LSB galaxies.Comment: 21 pages, 13 figures, Accepted for publication in Astrophsical Journa