142 research outputs found
A Cross-Correlation Analysis of Mg II Absorption Line Systems and Luminous Red Galaxies from the SDSS DR5
We analyze the cross-correlation of 2,705 unambiguously intervening Mg II
(2796,2803A) quasar absorption line systems with 1,495,604 luminous red
galaxies (LRGs) from the Fifth Data Release of the Sloan Digital Sky Survey
within the redshift range 0.36<=z<=0.8. We confirm with high precision a
previously reported weak anti-correlation of equivalent width and dark matter
halo mass, measuring the average masses to be log M_h(M_[solar]h^-1)=11.29
[+0.36,-0.62] and log M_h(M_[solar]h^-1)=12.70 [+0.53,-1.16] for systems with
W[2796A]>=1.4A and 0.8A<=W[2796A]<1.4A, respectively. Additionally, we
investigate the significance of a number of potential sources of bias inherent
in absorber-LRG cross-correlation measurements, including absorber velocity
distributions and the weak lensing of background quasars, which we determine is
capable of producing a 20-30% bias in angular cross-correlation measurements on
scales less than 2'. We measure the Mg II - LRG cross-correlation for 719
absorption systems with v<60,000 km s^-1 in the quasar rest frame and find that
these associated absorbers typically reside in dark matter haloes that are
~10-100 times more massive than those hosting unambiguously intervening Mg II
absorbers. Furthermore, we find evidence for evolution of the redshift number
density, dN/dz, with 2-sigma significance for the strongest (W>2.0A) absorbers
in the DR5 sample. This width-dependent dN/dz evolution does not significantly
affect the recovered equivalent width-halo mass anti-correlation and adds to
existing evidence that the strongest Mg II absorption systems are correlated
with an evolving population of field galaxies at z<0.8, while the non-evolving
dN/dz of the weakest absorbers more closely resembles that of the LRG
population.Comment: 21 pages, 19 figures; Published in Astrophysical Journa
Low and High Ionization Absorption Properties of Mg II Absorption-Selected Galaxies at Intermediate Redshifts. II. Taxonomy, Kinematics, and Galaxies
[Abridged] We examine a sample of 45 Mg II absorption-selected systems over
the redshift range 0.4 to 1.4. Mg II and Fe II absorption profiles were
observed at a resolution of ~6 km/s with HIRES/Keck. Lyman-alpha and C IV data
were measured in archival FOS/HST spectra (~230 km/s). We perform a
multivariate analysis of W(MgII), W(FeII), W(CIV) and W(Lya) (rest-frame)
equivalent widths and the Mg II kinematic spread. We find five categories of Mg
II absorbers: "Classic", "C IV-deficient", "Single/Weak", "Double", and
"DLA/HI-Rich". There is a strong connection between low-ionization kinematics
and the location of an absorber on the W(CIV)-W(MgII) plane. In most absorbers
a significant fraction of the C IV arises in a phase separate from Mg II. Many
of the C IV profiles are resolved in the FOS spectra due to velocity
structure.. For 16 galaxies, we compare the available absorption-line
properties (taken from Churchill et al. 2000, Paper I) to the galaxy properties
but find no significant (greater than 3-sigma) correlations, although several
suggestive trends are apparent. We compare the locations of our intermediate
redshift absorbers on the W(CIV)-W(MgII) plane with those of lower and higher
redshift data taken from the literature and find evidence for evolution that is
connected with the Mg II kinematics. We discuss the potential of using the
above categorizations of absorbers to understand the evolution in the
underlying physical processes giving rise to the gas and governing its
ionization phases and kinematics.Comment: Accepted: The Astrophysical Journal; Work based upon data presented
in Paper I [astro-ph/0005585
Hubble Space Telescope Observations of Element Abundances in Low-redshift Damped Lyman-alpha Galaxies and Implications for the Global Metallicity-Redshift Relation
Most models of cosmic chemical evolution predict that the mass-weighted mean
interstellar metallicity of galaxies should rise with time from a low value
solar at to a nearly solar value at . In the
absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in
quasar spectra are expected to show such a rise in global metallicity. However,
it has been difficult to determine whether or not DLAs show this effect,
primarily because of the very small number of DLA metallicity measurements at
low redshifts. In an attempt to put tighter constraints on the low-redshift end
of the DLA metallicity-redshift relation, we have observed Zn II and Cr II
lines in four DLAs at , using the Space Telescope Imaging
Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These
observations have provided the first constraints on Zn abundances in DLAs with
. In all the three DLAs for which our observations offer meaningful
constraints on the metallicity, the data suggest that the metallicities are
much lower than the solar value. These results are consistent with recent
imaging studies indicating that these DLAs may be associated with dwarf or low
surface brightness galaxies. We combine our results with higher redshift data
from the literature to estimate the global mean metallicity-redshift relation
for DLAs. We find that the global mean metallicity shows at most a slow
increase with decreasing redshift. ...(Please see the paper for the complete
abstract).Comment: 56 pages, including 13 figures. Accepted for publication in the
Astrophysical Journa
A Lyman-alpha-only AGN from the Sloan Digital Sky Survey
The Sloan Digital Sky Survey has discovered a z=2.4917 radio-loud active
galactic nucleus (AGN) with a luminous, variable, low-polarization UV
continuum, H I two-photon emission, and a moderately broad Lyman-alpha line
(FWHM = 1430 km/s) but without obvious metal-line emission. SDSS
J113658.36+024220.1 does have associated metal-line absorption in three
distinct, narrow systems spanning a velocity range of 2710 km/s. Despite
certain spectral similarities, SDSS J1136+0242 is not a Lyman-break galaxy.
Instead, the Ly-alpha and two-photon emission can be attributed to an extended,
low-metallicity narrow-line region. The unpolarized continuum argues that we
see SDSS J1136+0242 very close to the axis of any ionization cone present. We
can conceive of two plausible explanations for why we see a strong UV continuum
but no broad-line emission in this `face-on radio galaxy' model for SDSS
J1136+0242: the continuum could be relativistically beamed synchrotron emission
which swamps the broad-line emission; or, more likely, SDSS J1136+0242 could be
similar to PG 1407+265, a quasar in which for some unknown reason the
high-ionization emission lines are very broad, very weak, and highly
blueshifted.Comment: AJ, in press, 10 pages emulateapj forma
A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the Sloan Digital Sky Survey
We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for
low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is
detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest
absorption. We have developed a novel Locally Calibrated Pixel (LCP) search
method that uses adjacent regions of the spectrum to calibrate interlopers and
spectral artifacts, which would otherwise limit the measurement of OVI
absorption. Despite the challenges presented by searching for weak OVI within
the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise,
we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2
(the null hypothesis has a chi^2=80 for 9 data points).
We interpret our results using synthetic spectra generated from a lognormal
density field assuming a mixed quasar-galaxy photoionizing background (Haardt &
Madau 2001) and that it dominates the ionization fraction of detected OVI. The
LCP search data can be fit by a constant metallicity model with [O/H] =
-2.15_(-0.09)^(+0.07), but also by models in which low-density regions are
unenriched and higher density regions have a higher metallicity. The
density-dependent enrichment model by Aguirre et al. (2008) is also an
acceptable fit. All our successful models have similar mass-weighted oxygen
abundance, corresponding to [_MW] = -2.45+-0.06. This result can be used
to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) =
1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the
mass-weighted mean oxygen abundance and the cosmic oxygen density in the
Lyman-alpha forest to date and indicates that it contains approximately 16% of
metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.Comment: 12 pages, 9 figures. Accepted by ApJ (minor changes
A search for non-pulsating, chemically normal stars in the Scuti instability strip using Kepler data
We identify stars in the δ Sct instability strip that do not pulsate in p modes at the 50-μmag limit, using Kepler data. Spectral classification and abundance analyses from high-resolution spectroscopy allow us to identify chemically peculiar stars, in which the absence of pulsations is not surprising. The remaining stars are chemically normal, yet they are not δ Sct stars. Their lack of observed p modes cannot be explained through any known mechanism. However, they are mostly distributed around the edges of the δ Sct instability strip, which allows for the possibility that they actually lie outside the strip once the uncertainties are taken into account.We investigated the possibility that the non-pulsators inside the instability strip could be unresolved binary systems, having components that both lie outside the instability strip.
If misinterpreted as single stars, we found that such binaries could generate temperature discrepancies of ∼300 K – larger than the spectroscopic uncertainties, and fully consistent with the observations. After these considerations, there remains one chemically normal nonpulsator that lies in the middle of the instability strip. This star is a challenge to pulsation theory. However, its existence as the only known star of its kind indicates that such stars are rare. We conclude that the δ Sct instability strip is pure, unless pulsation is shut down by diffusion or another mechanism, which could be interaction with a binary companion
Low and High Ionization Absorption Properties of Mg II Absorption-Selected Galaxies at Intermediate Redshifts. I. General Properties
We present extensive metal-line absorption properties for 45 absorption
systems that were selected by their Mg II absorption at redshifts between 0.4
and 1.4. For each system the properties of several chemical species are
determined, including a wide range of ionization conditions. In the optical,
the absorption systems have been observed at ~6 km/s resolution with
HIRES/Keck, which covered Mg II, several Fe II transitions, Mg I, and in some
cases (depending upon redshift), Ca II, Ti II, Mn II, and Al III. Ultraviolet,
lower resolution (~230 km/s) Faint Object Spectrograph data (1600 - 3275 Ang)
were obtained from the Hubble Space Telescope archive. These spectra covered Al
II, Al III, Si II, Si III, Si IV, C II, C III, C IV, N V, O VI, and several
Lyman series transitions, with coverage dependent upon the absorption system
redshift. From these data, we infer that Mg II absorbing galaxies at
intermediate redshifts have multiphase gaseous structures.Comment: Accepted: The Astrophysical Journa
A Population of X-ray Weak Quasars: PHL 1811 Analogs at High Redshift
We report the results from Chandra and XMM-Newton observations of a sample of
10 type 1 quasars selected to have unusual UV emission-line properties (weak
and blueshifted high-ionization lines; strong UV Fe emission) similar to those
of PHL 1811, a confirmed intrinsically X-ray weak quasar. These quasars were
identified by the Sloan Digital Sky Survey at high redshift (z~2.2); eight are
radio quiet while two are radio intermediate. All of the radio-quiet PHL 1811
analogs are notably X-ray weak by a mean factor of ~13. These sources lack
broad absorption lines and have blue UV/optical continua, suggesting they are
intrinsically X-ray weak. However, their average X-ray spectrum appears to be
harder than those of typical quasars, which may indicate the presence of heavy
intrinsic X-ray absorption. Our radio-quiet PHL 1811 analogs support a
connection between an X-ray weak spectral energy distribution and PHL 1811-like
UV emission lines; this connection provides an economical way to identify X-ray
weak type 1 quasars. The fraction of radio-quiet PHL 1811 analogs in the
radio-quiet quasar population is estimated to be < 1.2%. We have investigated
correlations between relative X-ray brightness and UV emission-line properties
for a sample combining radio-quiet PHL 1811 analogs, PHL 1811, and typical type
1 quasars. These correlation analyses suggest that PHL 1811 analogs may have
extreme wind-dominated broad emission-line regions. Observationally,
radio-quiet PHL 1811 analogs appear to be a subset (~30%) of radio-quiet
weak-line quasars. The existence of a subset of quasars in which
high-ionization "shielding gas" covers most of the BELR, but little more than
the BELR, could potentially unify the PHL 1811 analogs and WLQs. The two
radio-intermediate PHL 1811 analogs are X-ray bright. One of them appears to
have jet-dominated X-ray emission, while the nature of the other remains
unclear.Comment: ApJ accepted; 25 pages, 11 figures and 8 table
SDSS J0246-0825: A New Gravitationally Lensed Quasar from the Sloan Digital Sky Survey
We report the discovery of a new two-image gravitationally lensed quasar,
SDSS J024634.11-082536.2 (SDSS J0246-0825). This object was selected as a
lensed quasar candidate from the Sloan Digital Sky Survey (SDSS) by the same
algorithm that was used to discover other SDSS lensed quasars (e.g., SDSS
J0924+0219). Multicolor imaging with the Magellan Consortium's Walter Baade
6.5-m telescope and the spectroscopic observations using the W. M. Keck
Observatory's Keck II telescope confirm that SDSS J0246-0825 consists of two
lensed images (1\farcs04) of a source quasar at z=1.68.
Imaging observations with the Keck telescope and the Hubble Space Telescope
reveal an extended object between the two quasar components, which is likely to
be a lensing galaxy of this system. From the absorption lines in the spectra of
quasar components and the apparent magnitude of the galaxy, combined with the
expected absolute magnitude from the Faber-Jackson relation, we estimate the
redshift of the lensing galaxy to be z=0.724. A highly distorted ring is
visible in the Hubble Space Telescope images, which is likely to be the lensed
host galaxy of the source quasar. Simple mass modeling predicts the possibility
that there is a small (faint) lensing object near the primary lensing galaxy.Comment: 30 pages, 12 figures, The Astronomical Journal accepte
SDSS J0806+2006 and SDSS J1353+1138: Two New Gravitationally Lensed Quasars from the Sloan Digital Sky Survey
We report the discoveries of two, two-image gravitationally lensed quasars
selected from the Sloan Digital Sky Survey: SDSS J0806+2006 at z_s=1.540 and
SDSS J1353+1138 at z_s=1.629 with image separations of 1.40" and 1.41"
respectively. Spectroscopic and optical/near-infrared imaging follow-up
observations show that the quasar images have identical redshifts and possess
extended objects between the images that are likely to be lens galaxies at
z_l~0.6 in SDSS J0806+2006 and z_l~0.3 in SDSS J1353+1138. The field of SDSS
J0806+2006 contains several nearby galaxies that may significantly perturb the
system, and SDSS J1353+1138 has an extra component near its Einstein ring that
is probably a foreground star. Simple mass models with reasonable parameters
reproduce the quasar positions and fluxes of both systems.Comment: 27 pages, 7 figures, The Astronomical Journal accepte
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