50 research outputs found
The Low-Redshift Intergalactic Medium
The low-redshift Ly-alpha forest of absorption lines provides a probe of
large-scale baryonic structures in the intergalactic medium, some of which may
be remnants of physical conditions set up during the epoch of galaxy formation.
We discuss our recent Hubble Space Telescope (HST) observations and
interpretation of low-z Ly-alpha clouds toward nearby Seyferts and QSOs,
including their frequency, space density, estimated mass, association with
galaxies, and contribution to Omega-baryon. Our HST/GHRS detections of 70
Ly-alpha absorbers with N_HI > 10^12.6 cm-2 along 11 sightlines covering
pathlength Delta(cz) = 114,000 km/s show f(>N_HI) ~ N_HI^{-0.63 +- 0.04} and a
line frequency dN/dz = 200 +- 40 for N_HI > 10^12.6 cm-2 (one every 1500 km/s
of redshift). A group of strong absorbers toward PKS 2155-304 may be associated
with gas (400-800) h_75^-1 kpc from 4 large galaxies, with low metallicity (<
0.003 solar) and D/H < 2 x 10^-4. At low-z, we derive a metagalactic ionizing
radiation field from AGN of J_0 = 1.3^{+0.8 -0.5} x 10^-23 ergs/cm2/s/Hz/sr and
a Ly-alpha-forest baryon density Omega-baryon = (0.008 +- 0.004) h_75^-1 [J_-23
N_14 b_100]^{1/2} For clouds of characteristic size b = (100 kpc)b_100.Comment: 5 figure
CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
The relative abundances of atomic and molecular species in planet-forming
disks around young stars provide important constraints on photochemical disk
models and provide a baseline for calculating disk masses from measurements of
trace species. A knowledge of absolute abundances, those relative to molecular
hydrogen (H2), are challenging because of the weak rovibrational transition
ladder of H and the inability to spatially resolve different emission
components within the circumstellar environment. To address both of these
issues, we present new contemporaneous measurements of CO and H2 absorption
through the "warm molecular layer" of the protoplanetary disk around the
Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode
of the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2
absorption in this region for the first time. An analysis of the emission and
absorption spectrum of RW Aur shows components from the accretion region near
the stellar photosphere, the molecular disk, and several outflow components.
The warm H2 and CO absorption lines are consistent with a disk origin. We model
the 1092-1117A spectrum of RW Aur to derive log10
N(H2)~=~19.90 at T(H2) ~=~440~+/-~39 K. The CO
~--~ bands observed from 1410-1520A are best fit by log10
N(CO)~=~16.1~ at T(CO) ~=~200 K.
Combining direct measurements of the HI, H2, and CO column densities, we find a
molecular fraction in the warm disk surface of ~>=~0.47 and derive a
molecular abundance ratio of CO/H2~=~1.6~x~10, both
consistent with canonical interstellar dense cloud values.Comment: ApJ - accepted. 13 pages, 8 figure
The Local Ly-alpha Forest IV: STIS G140M Spectra and Results on the Distribution and Baryon Content of HI Absorbers
We present HST STIS/G140M spectra of 15 extragalactic targets, which we
combine with GHRS/G160M data to examine the statistical properties of the low-z
Ly-alpha forest. We evaluate the physical properties of these Ly-alpha
absorbers and compare them to their high-z counterparts. We determine that the
warm, photoionized IGM contains 29+/-4% of the total baryon inventory at z = 0.
We derive the distribution in column density, N_HI^(1.65+/-0.07) for 12.5 < log
[N_HI] 14.5. The slowing
of the number density evolution of high-W Ly-alpha clouds is not as great as
previously measured, and the break to slower evolution may occur later than
previously suggested (z~1.0 rather than 1.6). We find a 7.2sigma excess in the
two-point correlation function (TPCF) of Ly-alpha absorbers for velocity
separations less than 260 km/s, which is exclusively due to the higher column
density clouds. From our previous result that higher column density Ly-alpha
clouds cluster more strongly with galaxies, this TPCF suggests a physical
difference between the higher and lower column density clouds in our sample.Comment: 71 pages, 6 tables, 26 EPS figures, to appear in ApJ Supplemen
The Local Lyman-Alpha Forest: Absorbers in Galaxy Voids
We have conducted pointed redshift surveys for galaxies in the direction of
bright AGN whose HST far-UV spectra contain nearby (cz <~ 30,000 kms), low
column density (12.5 <= log N_{HI} (cm s^{-2}) <= 14.5) Ly-alpha forest
absorption systems. Here we present results for four lines-of-sight which
contain nearby (cz <~ 3000 kms) Ly-alpha absorbers in galaxy voids. Although
our data go quite deep (-13 <= M_{B}(limit) <= -14) out to impact parameters of
100-250 h_{70}^{-1} kpc, these absorbers remain isolated and thus appear to be
truly intergalactic, rather than part of galaxies or their halos. Since we and
others have discovered no galaxies in voids, the only baryons detected in the
voids are in the Ly-alpha ``clouds''. Using a photoionization model for these
clouds, the total baryonic content of the voids is 4.5% +/- 1.5% of the mean
baryon density.Comment: 5 pages, 1 figure, accepted for publication in Astrophysical Journal
Letter
The Metallicity of Intergalactic Gas in Cosmic Voids
We have used the Hubble/STIS and FUSE archives of ultraviolet spectra of
bright AGN to identify intergalactic Lya absorbers in nearby (z < 0.1) voids.
From a parent sample of 651 Lya absorbers, we identified 61 void absorbers
located more than 1.4/h_70 Mpc from the nearest L* or brighter galaxy.
Searching for metal absorption in high-quality (S/N > 10) spectra at the
location of three diagnostic metal lines (O VI 1032, C IV 1548, Si III 1206),
we detected no metal lines in any individual absorber, or in any group of
absorbers using pixel co-addition techniques. The best limits on metal-line
absorption in voids were set using four strong Lya absorbers with N(H I) >
10^{14} cm^-2, with 3-sigma equivalent-width limits ranging from 8 mA (O VI),
7-15 mA (C IV), and 4-10 mA (Si III). Photoionization modeling yields
metallicity limits Z < 10^{-1.8+/-0.4} Z_sun, from non-detections of C IV and O
VI, some 6 times lower than those seen in Lya and OVI absorbers at z < 0.1.
Although the void Lya absorbers could be pristine material, considerably deeper
spectra are required to rule out a universal metallicity floor produced by
bursts of early star formation, with no subsequent star formation in the voids.
The most consistent conclusion derived from these low-z results, and similar
searches at z = 3-5, is that galaxy filaments have increased their mean IGM
metallicity by factors of 30-100 since z = 3.Comment: Accepted for ApJ, 8 pages including Fig 1a,