1,091 research outputs found
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 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
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
Variations of Interstellar Gas-to-Dust Ratios at High Galactic Latitudes
Interstellar dust at high Galactic latitudes can influence astronomical
foreground subtraction, produce diffuse scattered light, and soften the
ultraviolet spectra of quasars. In a sample of 94 sight lines toward quasars at
high latitude and low extinction, we evaluate the interstellar "gas-to-dust
ratio" , using hydrogen column densities (H I and H) and
far-infrared estimates of dust reddening. In the Galactic plane, this ratio is
(in units of ). On average,
recent Planck estimates of in low-reddening sight lines are 12% higher
than those from Schlafly & Finkbeiner (2011), and exhibits
significant variations when measured at different radio telescopes. In a sample
of 51 quasars with measurements of both H I and H and , we find mean ratios (gas at all velocities) and
(low velocity only) using Planck data. High-latitude H
fractions are generally small (2-3% on average), although 9 of 39 sight lines
at have of 1-17%. Because FIR-inferred
is sensitive to modeled dust temperature and emissivity index
, gas-to-dust ratios have large, asymmetric errors at low . The
ratios are elevated in sight lines with high-velocity clouds, which contribute
but little reddening. In Complex C, the ratio decreases by 40% when
high velocity gas is excluded. Decreases in dust content are expected in
low-metallicity gas above the Galactic plane, resulting from grain destruction
in shocks, settling to the disk, and thermal sputtering in hot halo gas.Comment: Minor changes, accepted to Astrophysical Journal, 37 pages, 7 tables,
8 figure
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,
The Metagalactic Ionizing Radiation Field at Low Redshift
We compute the ionizing radiation field at low redshift, arising from
Seyferts, QSOs, and starburst galaxies. This calculation combines recent
Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN
database, and a new intergalactic opacity model based on Hubble Space Telescope
and Keck Ly-alpha absorber surveys. At z = 0 for AGN only, our best estimate
for the specific intensity at 1 Ryd is I_0 = 1.3 (+0.8/-0.5) x 10^-23
ergs/cm^2/s/Hz/sr, independent of H_0, Omega_0, and Lambda. The one-sided
ionizing photon flux is Phi_ion = 3400 (+2100/-1300) photons/cm^2/s, and the H
I photoionization rate is Gamma_HI = 3.2 (+2.0/-1.2) x 10^-14 s^-1 for alpha_s
= 1.8. We also derive Gamma_ HI for z = 0 - 4. These error ranges reflect
uncertainties in the spectral indexes for the ionizing EUV (alpha_s = 1.8 +/-
0.3) and the optical/UV (alpha_UV = 0.86 +/- 0.05), the IGM opacity model, the
range of Seyfert luminosities (0.001 - 100 L*) and the completeness of the
luminosity functions. Our estimate is a factor of three lower than the most
stringent upper limits on the ionizing background (Phi_ion < 10^4
photons/cm^2/s) obtained from H-alpha observations in external clouds, and it
lies within the range implied by other indirect measures. Starburst galaxies
with a sufficiently large Lyman continuum escape fraction, f_ esc > 0.05, may
provide a comparable background to AGN, I_0 (z=0) = 1.1 (+1.5/-0.7) x 10^{-23).
An additional component of the ionizing background of this magnitude would
violate neither upper limits from H-alpha observations nor the acceptable range
from other measurements.Comment: 30 pages, 9 figures, accepted for Astronomical J. (Oct. 1999
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