2,910 research outputs found
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 = 5.10Ă
and c = 14.72Ă
[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 (Î^(17)O ~ â25â°) silicate melt via rapid cooling [3]. Here we report on an-other textural occurrence of dmist in STP-1 - ^(16)O-poor (Î^(17)O ~ â2â°) 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 [1â5]. 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
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