628 research outputs found
The Last Eight-Billion Years of Intergalactic SiIV Evolution
We identified 24 SiIV absorption systems with z <~ 1 from a blind survey of
49 low-redshift quasars with archival Hubble Space Telescope ultraviolet
spectra. We relied solely on the characteristic wavelength separation of the
doublet to automatically detect candidates. After visual inspection, we defined
a sample of 20 definite (group G = 1) and 4 "highly-likely" (G = 2) doublets
with rest equivalent widths W_r for both lines detected at > 3 sigma. The
absorber line density of the G = 1 doublets was dN_SiIV/dX = 1.4+0.4/-0.3 for
log N(Si+3) > 12.9. The best-fit power law to the G = 1 frequency distribution
of column densities f(N(Si+3)) had normalization k = (1.2+0.5/-0.4) x 10^-14
cm2 and slope alpha = -1.6+0.3/-0.3. Using the power-law model of f(N(Si+3)),
we measured the Si+3 mass density relative to the critical density: Omega(Si+3)
= (3.7+2.8/-1.7) x 10^-8 for 13 < log N(Si+3) < 15. From Monte Carlo sampling
of the distributions, we estimated our value to be a factor of 4.8+3.0/-1.9
higher than the 2 . From a simple linear fit to
Omega(Si+3) over the age of the Universe, we estimated a slow and steady
increase from z = 5.5 --> 0 with dOmega/dt_age = (0.61+/-0.23) x 10^-8 Gyr^-1.
We compared our ionic ratios N(Si+3)/N(C+3) to a 2 < z < 4.5 sample and
concluded, from survival analysis, that the two populations are similar, with
median = 0.16.Comment: 18 pages, 9 figures, 4 tables, added figures and new analysis,
results have changed, accepted to Ap
The spt-Crank for Ordinary Partitions
The spt-function was introduced by Andrews as the weighted counting
of partitions of with respect to the number of occurrences of the smallest
part. Andrews, Garvan and Liang defined the spt-crank of an -partition which
leads to combinatorial interpretations of the congruences of mod 5 and
7. Let denote the net number of -partitions of with spt-crank
. Andrews, Garvan and Liang showed that is nonnegative for all
integers and positive integers , and they asked the question of finding
a combinatorial interpretation of . In this paper, we introduce the
structure of doubly marked partitions and define the spt-crank of a doubly
marked partition. We show that can be interpreted as the number of
doubly marked partitions of with spt-crank . Moreover, we establish a
bijection between marked partitions of and doubly marked partitions of .
A marked partition is defined by Andrews, Dyson and Rhoades as a partition with
exactly one of the smallest parts marked. They consider it a challenge to find
a definition of the spt-crank of a marked partition so that the set of marked
partitions of and can be divided into five and seven equinumerous
classes. The definition of spt-crank for doubly marked partitions and the
bijection between the marked partitions and doubly marked partitions leads to a
solution to the problem of Andrews, Dyson and Rhoades.Comment: 22 pages, 6 figure
Characterizing the Low-Redshift Intergalactic Medium towards PKS1302-102
We present a detailed analysis of the intergalactic metal-line absorption
systems in the archival HST/STIS and FUSE ultraviolet spectra of the
low-redshift quasar PKS1302-102 (z_QSO = 0.2784). We supplement the archive
data with CLOUDY ionization models and a survey of galaxies in the quasar
field. There are 15 strong Lya absorbers with column densities logN_HI > 14. Of
these, six are associated with at least CIII 977 absorption (logN(C^++) > 13);
this implies a redshift density dN_CIII/dz = 36+13/-9 (68% confidence limits)
for the five detections with rest equivalent width W_r > 50 mA. Two systems
show OVI 1031,1037 absorption in addition to CIII (logN(O^+5) > 14). One is a
partial Lyman limit system (logN_HI = 17) with associated CIII, OVI, and SiIII
1206 absorption. There are three tentative OVI systems that do not have CIII
detected. For one OVI doublet with both lines detected at 3 sigma with W_r > 50
mA, dN_OVI/dz = 7+9/-4. We also search for OVI doublets without Lya absorption
but identify none. From CLOUDY modeling, these metal-line systems have
metallicities spanning the range -4 < [M/H] < -0.3. The two OVI systems with
associated CIII absorption cannot be single-phase, collisionally-ionized media
based on the relative abundances of the metals and kinematic arguments. From
the galaxy survey, we discover that the absorption systems are in a diverse set
of galactic environments. Each metal-line system has at least one galaxy within
500 km/s and 600 h^-1 kpc with L > 0.1 L_*.Comment: 21 pages in emulatepj form, 24 figures, 10 tables, accepted to Ap
Can Transition Metals and Group II Mono- and Dications Discriminate between Homo- and Heterochiral XYYX’ Dimers (X,X’=H,Me; Y=O,S,Se)?
We present a density functional theory and ab initio (MP2) study of stereoisomer discrimination between the homochiral and heterochiral dimers of the form M(XYYX’)2, where M is a cationic metal (Li+, Ca2+, Zn2+, Cu+, Cu2+) complexing chalcogen-chalcogen bridges (H2O2, H2S2, H2Se2, and their corresponding methyl and dimethyl derivatives). The heterochiral complexes examined were in general found to be more stable than the homochiral complexes, with the exception of several selenium-containing complexes. The large majority of the relative energy differences amounted to 1 kJ/mol or less, with the largest energy gap being 3.42 kJ/mol in the case of Ca2+(HSeSe(CH3))2 at B3LYP/aug-cc-pVTZ. Racemization mechanisms of these complexes and the description of their bonding using the Atoms in Molecules theory of Bader are also presented
The Gemini Deep Deep Survey: II. Metals in Star-Forming Galaxies at Redshift 1.3<z<2
The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample
of K<20.6 galaxies in the redshift range 0.8<z<2.0. Here we determine the
statistical properties of the heavy element enrichment in the interstellar
medium (ISM) of a subsample of 13 galaxies with 1.34<z<1.97 and UV absolute
magnitude M_2000 < -19.65. The sample contains 38% of the total number of
identified galaxies in the first two fields of the survey with z>1.3. The
selected objects have colors typical of irregular and Sbc galaxies. Strong
[OII] emission indicates high star formation activity in the HII regions
(SFR~13-106 M_sun/yr). The high S/N composite spectrum shows strong ISM MgII
and FeII absorption, together with weak MnII and MgI lines. The FeII column
density, derived using the curve of growth analysis, is logN_FeII =
15.54^{+0.23}_{-0.13}. This is considerably larger than typical values found in
damped Ly-alpha systems (DLAs) along QSO sight lines, where only 10 out of 87
(~11%) have logN_FeII > 15.2. High FeII column densities are observed in the
z=2.72 Lyman break galaxy cB58 (logN_FeII ~ 15.25) and in gamma-ray burst host
galaxies (logN_FeII ~ 14.8-15.9). Given our measured FeII column density and
assuming a moderate iron dust depletion (delta_Fe ~ 1 dex), we derive an
optical dust extinction A_V ~ 0.6. If the HI column density is log N(HI)<21.7
(as in 98% of DLAs), then the mean metallicity is Z/Z_sun > 0.2. The high
completeness of the GDDS sample implies that these results are typical of
star-forming galaxies in the 1<z<2 redshift range, an epoch which has
heretofore been particularly challenging for observational programs.Comment: ApJ in press, corrected HI column density estimat
Cosmic Star Formation History and its Dependence on Galaxy Stellar Mass
We examine the cosmic star formation rate (SFR) and its dependence on galaxy
stellar mass over the redshift range 0.8 < z < 2 using data from the Gemini
Deep Deep Survey (GDDS). The SFR in the most massive galaxies (M > 10^{10.8}
M_sun) was six times higher at z = 2 than it is today. It drops steeply from z
= 2, reaching the present day value at z ~ 1. In contrast, the SFR density of
intermediate mass galaxies (10^{10.2} < M < 10^{10.8} M_sun) declines more
slowly and may peak or plateau at z ~ 1.5. We use the characteristic growth
time t_SFR = rho_M / rho_SFR to provide evidence of an associated transition in
massive galaxies from a burst to a quiescent star formation mode at z ~ 2.
Intermediate mass systems transit from burst to quiescent mode at z ~ 1, while
the lowest mass objects undergo bursts throughout our redshift range. Our
results show unambiguously that the formation era for galaxies was extended and
proceeded from high to low mass systems. The most massive galaxies formed most
of their stars in the first ~3 Gyr of cosmic history. Intermediate mass objects
continued to form their dominant stellar mass for an additional ~2 Gyr, while
the lowest mass systems have been forming over the whole cosmic epoch spanned
by the GDDS. This view of galaxy formation clearly supports `downsizing' in the
SFR where the most massive galaxies form first and galaxy formation proceeds
from larger to smaller mass scales.Comment: Accepted for publication in ApJ
Evolved Galaxies at z > 1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems
We present spectroscopic evidence from the Gemini Deep Deep Survey (GDDS) for
a significant population of color-selected red galaxies at 1.3 < z < 2.2 whose
integrated light is dominated by evolved stars. Unlike radio-selected objects,
the z > 1.5 old galaxies have a sky density > 0.1 per sq. arcmin. Conservative
age estimates for 20 galaxies with z > 1.3; = 1.49, give a median age of
1.2 Gyr and = 2.4. One quarter of the galaxies have inferred z_f > 4.
Models restricted to abundances less than or equal to solar give median ages
and z_f of 2.3 Gyr and 3.3, respectively. These galaxies are among the most
massive and contribute approximately 50% of the stellar mass density at 1 < z <
2. The derived ages and most probable star formation histories suggest a high
star-formation-rate (300-500 solar masses per year) phase in the progenitor
population. We argue that most of the red galaxies are not descendants of the
typical z=3 Lyman break galaxies. Galaxies associated with luminous sub-mm
sources have the requisite star formation rates to be the progenitor
population. Our results point toward early and rapid formation for a
significant fraction of present day massive galaxies.Comment: 12 pages, 2 figures, 1 table, Accepted for publication, ApJ Letter
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