24 research outputs found
A Quadruple-Phase Strong Mg II Absorber at z~0.9902 Toward PG 1634+706
The z=0.9902 system along the quasar PG 1634+706 line of sight is a strong
MgII absorber (W(2796)>0.3A) with only weak CIV absorption (it is
``CIV-deficient''). To study this system, we used high-resolution spectra from
both HST/STIS (R=30,000) and Keck/HIRES (R=45,000). These spectra cover key
transitions, such as MgI, MgII, FeII, SiII, CII, SiIII, CIII, SiIV, and CIV.
Assuming a Haardt and Madau extragalactic background spectrum, we modeled the
system with a combination of photoionization and collisional ionization. Based
on a comparison of synthetic spectra to the data profiles, we infer the
existence of the following four phases of gas: i) Seven MgII clouds have sizes
of 1-1000pc and densities of 0.002-0.1/cm^3, with a gradual decrease in density
from blue to red. The MgII phase gives rise to most of the CIV absorption and
resembles the warm, ionized inter-cloud medium of the Milky Way; ii) Instead of
arising in the same phase as MgII, MgI is produced in separate, narrow
components with b~0.75km/s. These small MgI pockets (~100AU) could represent a
denser phase (~200/cm^3) of the interstellar medium (ISM), analogous to the
small-scale structure observed in the Milky Way ISM; iii) A ``broad phase''
with a Doppler parameter, b~60km/s, is required to consistently fit Ly-alpha,
Ly-beta, and the higher-order Lyman-series lines. A low metallicity (log Z <=
-2) for this phase could explain why the system is ``CIV-deficient'', and also
why NV and OVI are not detected. This phase may be a galactic halo or it could
represent a diffuse medium in an early-type galaxy; iv) The strong absorption
in SiIV relative to CIV could be produced in an extra, collisionally ionized
phase with a temperature of T~60,000K. The collisional phase could exist in
cooling layers that are shock-heated by supernovae-related processes.Comment: 25 pages, 4 figures; to appear in ApJ, April 20, 200
A Catalog of Absorption Lines in Eight HST/STIS E230M 1.0 < z < 1.7 Quasar Spectra
We have produced a catalog of line identifications and equivalent width
measurements for all absorption features in eight ultraviolet echelle quasar
spectra. These spectra were selected as having the highest signal-to-noise
among the HST/STIS spectra obtained with the E230M grating. We identify 56
metal-line systems toward the eight quasars, and present plots of detected
transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the
features in the Lya forest region, redward of the incidence of the Lyb forest,
are metal lines. High ionization transitions are common. We see both O VI and C
IV in 88 - 90% of the metal-line systems for which the spectra cover the
expected wavelength. Si III is seen in 58%, while low ionization absorption in
C II, Si II, and/or Al II is detected in 50% of the systems for which they are
covered. This catalog will facilitate future studies of the Lya forest and of
metal-line systems of various types.Comment: 13 pages, 2 figures, submitted to Monthly Notices of the Royal
Astronomical Society, a complete version with the appendix and all figures is
available at http://www.astro.psu.edu/users/misawa/pub/Paper/qalcat.pdf.g
The Habitable Zone Planet Finder: A Proposed High Resolution NIR Spectrograph for the Hobby Eberly Telescope to Discover Low Mass Exoplanets around M Dwarfs
The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 10m
class Hobby Eberly telescope that will be capable of discovering low mass
planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution
R~ 50,000 and cover the wavelength range 0.9-1.65{\mu}m, the Y, J and H NIR
bands where most of the flux is emitted by mid-late type M stars, and where
most of the radial velocity information is concentrated. Enclosed in a chilled
vacuum vessel with active temperature control, fiber scrambling and mechanical
agitation, HZPF is designed to achieve a radial velocity precision < 3m/s, with
a desire to obtain <1m/s for the brightest targets. This instrument will enable
a study of the properties of low mass planets around M dwarfs; discover planets
in the habitable zones around these stars, as well serve as an essential radial
velocity confirmation tool for astrometric and transit detections around late M
dwarfs. Radial velocity observation in the near-infrared (NIR) will also enable
a search for close in planets around young active stars, complementing the
search space enabled by upcoming high-contrast imaging instruments like GPI,
SPHERE and PALM3K. Tests with a prototype Pathfinder instrument have already
demonstrated the ability to recover radial velocities at 7-10 m/s precision
from integrated sunlight and ~15-20 m/s precision on stellar observations at
the HET. These tests have also demonstrated the ability to work in the NIR Y
and J bands with an un-cooled instrument. We will also discuss lessons learned
about calibration and performance from our tests and how they impact the
overall design of the HZPF.Comment: 11 pages, 8 figures, to appear in Proc. SPIE 2010 Vol. 773
Evidence for Cold Accretion: Primitive Gas Flowing onto a Galaxy at z~0.274
We present UV and optical observations from the Cosmic Origins Spectrograph
on the Hubble Space Telescope and Keck of a z= 0.27395 Lyman limit system (LLS)
seen in absorption against the QSO PG1630+377. We detect H I absorption with
log N(HI)=17.06\pm0.05 as well as Mg II, C III, Si III, and O VI in this
system. The column densities are readily explained if this is a multi-phase
system, with the intermediate and low ions arising in a very low metallicity
([Mg/ H] =-1.71 \pm 0.06) photoionized gas. We identify via Keck spectroscopy
and Large Binocular Telescope imaging a 0.3 L_* star-forming galaxy projected
37 kpc from the QSO at nearly identical redshift (z=0.27406, \Delta v = -26
\kms) with near solar metallicity ([O/ H]=-0.20 \pm 0.15). The presence of very
low metallicity gas in the proximity of a near-solar metallicity, sub-L_*
galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A
search of the literature reveals that such low metallicity LLSs are not
uncommon. We found that 50% (4/8) of the well-studied z < 1 LLSs have
metallicities similar to the present system and show sub-L_* galaxies with rho
< 100 kpc in those fields where redshifts have been surveyed. We argue that the
properties of these primitive LLSs and their host galaxies are consistent with
those of cold mode accretion streams seen in galaxy simulations.Comment: Accepted for publication in the Astrophysical Journa
A Bare Molecular Cloud at z~0.45
Several neutral species (MgI, SiI, CaI, FeI) have been detected in a weak
MgII absorption line system (W_r(2796)~0.15 Angstroms) at z~0.45 along the
sightline toward HE0001-2340. These observations require extreme physical
conditions, as noted in D'Odorico (2007). We place further constraints on the
properties of this system by running a wide grid of photoionization models,
determining that the absorbing cloud that produces the neutral absorption is
extremely dense (~100-1000/cm^3), cold (<100 K), and has significant molecular
content (~72-94%). Structures of this size and temperature have been detected
in Milky Way CO surveys, and have been predicted in hydrodynamic simulations of
turbulent gas. In order to explain the observed line profiles in all neutral
and singly ionized chemical transitions, the lines must suffer from unresolved
saturation and/or the absorber must partially cover the broad emission line
region of the background quasar. In addition to this highly unusual cloud,
three other ordinary weak MgII clouds (within densities of ~0.005/cm^3 and
temperatures of ~10000K) lie within 500 km/s along the same sightline. We
suggest that the "bare molecular cloud", which appears to reside outside of a
galaxy disk, may have had in situ star formation and may evolve into an
ordinary weak MgII absorbing cloud.Comment: 15 pages, 4 figures, 4 tables, ApJ accepte
A Hubble Space Telescope Study of Lyman Limit Systems: Census and Evolution
We present a survey for optically thick Lyman limit absorbers at z<2.6 using
archival Hubble Space Telescope observations with the Faint Object Spectrograph
and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems
(LLSs) increasing the number of catalogued LLSs at z<2.6 by a factor of ~10. We
compile a statistical sample of 50 tau_LLS > 2 LLSs drawn from 249 QSO sight
lines that avoid known targeting biases. The incidence of such LLSs per unit
redshift, l(z)=dn/dz, at these redshifts is well described by a single power
law, l(z) = C1 (1+z)^gamma, with gamma=1.33 +/- 0.61 at z<2.6, or with
gamma=1.83 +/- 0.21 over the redshift range 0.2 < z < 4.9. The incidence of
LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6
Gyr from z=4.9 to 3.5; l(X) evolves much more slowly at low redshifts,
decreasing by a similar factor over the ~8 Gyr from z=2.6 to 0.25. We show that
the column density distribution function, f(N(HI)), at low redshift is not well
fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the
column density range 13 17.2. While low and high
redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is
some evidence that f(N(HI)) evolves with z for log N(HI) < 17.7, possibly due
to the evolution of the UV background and galactic feedback. Assuming LLSs are
associated with individual galaxies, we show that the physical cross section of
the optically thick envelopes of galaxies decreased by a factor of ~9 from z~5
to 2 and has remained relatively constant since that time. We argue that a
significant fraction of the observed population of LLSs arises in the
circumgalactic gas of sub-L* galaxies.Comment: Accepted by Ap