5,211 research outputs found
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
High Metallicity Mg II Absorbers in the z < 1 Lyman alpha Forest of PKS 0454+039: Giant LSB Galaxies?
We report the discovery of two iron-group enhanced high-metallicity Mg II
absorbers in a search through 28 Lyman Alpha forest clouds along the PKS
0454+039 sight line. Based upon our survey and the measured redshift number
densities of W_r(MgII) <= 0.3 A absorbers and Lyman Alpha absorbers at z ~ 1,
we suggest that roughly 5% of Lyman Alpha absorbers at z < 1 will exhibit
"weak" Mg II absorption to a 5-sigma W_r(2796) detection limit of 0.02 A. The
two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have W_r(Lya)
= 0.33 and 0.15 A, respectively. Based upon photoionization modeling, the H I
column densities are inferred to be in the range 15.8 <= log N(HI) <= 16.8
cm^-2. For the z = 0.6428 absorber, if the abundance pattern is solar, then the
cloud has [Fe/H] > -1; if its gas-phase abundance follows that of depleted
clouds in our Galaxy, then [Fe/H] > 0 is inferred. For the z = 0.9315 absorber,
the metallicity is [Fe/H] > 0, whether the abundance pattern is solar or
suffers depletion. Imaging and spectroscopic studies of the PKS 0454+039 field
reveal no candidate luminous objects at these redshifts. We discuss the
possibility that these Mg II absorbers may arise in the class of "giant" low
surface brightness galaxies, which have [Fe/H] >= -1, and even [Fe/H] >= 0, in
their extended disks. We tentatively suggest that a substantial fraction of
these "weak" Mg II absorbers may select low surface brightness galaxies out to
z ~ 1.Comment: Accepted The Astrophysical Journal; 25 pages; 6 encapsulated figure
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54
We present Far Ultraviolet Spectroscopic Explorer observations of the
star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not
detected above the HI absorption edge in our Galaxy. An upper limit is
evaluated by comparison with the background measured in regions of the detector
adjacent to the observed spectrum. A spectral window of 16 A, reasonably free
of additional HI Lyman series line absorption is used. No correction is needed
for molecular hydrogen absorption in our Galaxy but a foreground extinction of
0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is
obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison
with the number of ionizing photons derived from the H-alpha flux, this limit
translates into an upper limit of f_esc < 0.062 for the fraction of Lyman
continuum photons that escape the galaxy without being absorbed by interstellar
material. This limit compares with the limits obtained in three other nearby
galaxies and is compatible with the escape fractions predicted by models.
The upper limits obtained in nearby galaxies contrasts with the detection of
Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~
3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls
v5.0
Small scale structure in diffuse molecular gas from repeated FUSE and visible spectra of HD 34078
We present preliminary results from an ongoing program devoted to a study of
small scale structure in the spatial distribution of molecular gas. Our work is
based on multi-epoch FUSE and visible observations of HD34078. A detailed
comparison of H2, CH and CH+ absorption lines is performed. No short term
variations are seen (except for highly excited H2) but long-term changes in
N(CH) are clearly detected when comparing our data to spectra taken about 10
years ago.Comment: 4 pages, 2 figures, To appear in the Proceedings of the XVII IAP
Colloquium "Gaseous Matter in Galaxies and Intergalactic Space
Where are the absorbers towards Q2302+029?
We present images and spectroscopy of objects close to the sightline of
Q2302+029 in order to search for galaxies responsible for the remarkable z=0.7
high-ionization absorption line system found by Jannuzi et al (1996). This
system shows `normal' narrow OVI, NV, and CIV lines superimposed on broader
(3,000-5,000 km/s wide), unsaturated absorption troughs some 56,000 km/s away
from the QSO emission redshift (z=1.052). Despite reaching sensitivities
sufficient to detect 1/10 L* galaxies in the optical and 1/20 L* in the
infra-red, we are unable to detect any obvious bright galaxies which might be
responsible for the absorption beyond ~ 6 h-1 kpc of the sightline. This
supports the hypotheses that the absorption is either intrinsic to the QSO, or
arises in intracluster gas. Adopting either explanation is problematic: in the
first case, `associated' absorption at such high ejection velocities is hard to
understand, and challenges the conventional discrimination between intrinsic
and intervening absorbers; in the second case, the gas must reside in a ~ 40
h-1 Mpc long filament aligned along the line of sight in order to reproduce the
broad absorption. Spectroscopy of objects beyond the immediate vicinity of the
QSO sightline reveals a galaxy cluster at z=0.59, which coincides with strong
Lya and more narrow high ionization lines in the quasar spectrum. Here too, the
lack of galaxies at distances comparable to those found for, e.g.,
Lya-absorbing galaxies, suggests that the absorption may arise from
intracluster gas unassociated with any individual galaxies.Comment: 16 pages, accepted by Ap
Mg II Absorption Systems in SDSS QSO Spectra
We present the results of a MgII absorption-line survey using QSO spectra
from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than
0.3\AA and redshifts were identified and measured. We
find that the rest equivalent width ()
distribution is described very well by an exponential function , with
and \AA. Previously reported power law
fits drastically over-predict the number of strong lines. Extrapolating our
exponential fit under-predicts the number of \AA systems,
indicating a transition in near \AA. A combination of
two exponentials reproduces the observed distribution well, suggesting that
MgII absorbers are the superposition of at least two physically distinct
populations of absorbing clouds. We also derive a new redshift parameterization
for the number density of \AA lines:
and \AA. We find that the distribution steepens with decreasing redshift,
with decreasing from \AA at to \AA at
. The incidence of moderately strong MgII lines does not
show evidence for evolution with redshift. However, lines stronger than
\AA show a decrease relative to the no-evolution prediction with
decreasing redshift for . The evolution is stronger for
increasingly stronger lines. Since in saturated absorption lines is an
indicator of the velocity spread of the absorbing clouds, we interpret this as
an evolution in the kinematic properties of galaxies from moderate to low z.Comment: 50 pages, 26 figures, accepted for publication in Ap
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