1,221 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 HI Environment of Nearby Lyman-alpha Absorbers
We present the results of a VLA and WSRT search for HI emission from the
vicinity of seven nearby clouds, which were observed in Lya absorption with HST
toward Mrk335, Mrk501 and PKS2155-304. We searched a volume of 40' x 40' x 1000
km/s. The HI mass sensitivity (5 sigma) varies from 5x10^6 to 5x10^8 Msun. We
detected HI emission in the vicinity of four out of seven absorbers. The
closest galaxy is a small dwarf galaxy at a projected distance of 68/h kpc from
the sight line toward Mrk335. It has the same velocity (V=1970 km/s) as one of
the absorbers, and has an HI mass of only 4x10^7 Msun. We found a more luminous
galaxy at the velocity (V=5100 km/s) of one of the absorbers toward
PKS2155-304, 230/h kpc from the sight line. Two other, stronger absorbers
toward PKS2155-304 at V=17,000 km/s are associated with a loose group of three
bright spiral galaxies, at projected distances of 300 to 600/h kpc. These
results support the conclusion that most nearby Lya forest clouds trace the
large-scale structures outlined by optically luminous galaxies. We do not find
any evidence for a physical association between an absorber and its closest
galaxy.Comment: 4 Tables, 11 Figures, to be published in Astron J. (Oct 1996) Vol 11
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
The Hot Intergalactic Medium - Galaxy Connection: Two Strong Absorbers in the Sightline Toward PG 1211+143
We present HST/STIS and FUSE spectra of the QSO PG 1211+143 (z_em = 0.081)
and a galaxy survey of the surrounding field. This sightline shows two strong
intergalactic absorption systems at cz ~ 15,300 and 19,300. This sightline
addresses the nature and origin of the OVI absorbers, and their connection to
galaxies. We explore the relationship of these absorbers to the nearby galaxies
and compare them to other OVI-bearing absorbers in diverse environments. At
15,300 km s^-1, we find four distinct HI components and associated CII, CIII,
CIV, SiII, SiIII, SiIV, NV, and OVI, lying near a spiral-dominated galaxy group
with a bright member galaxy 137 h_70^-1 kpc from the sightline. The observed
ions of C, Si, and N are likely to be photoionized, but the OVI is more
consistent with collisional ionization. The ion ratios in this absorber
resemble the highly-ionized Galactic HVCs; it may also trace the hot intragroup
medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km
s^-1, we find five HI components and associated CIII, SiIII, and
collisionally-ionized OVI lying 146 h_70^-1 kpc from an isolated galaxy. The
properties of the OVI-bearing gas are consistent with an origin in strong
shocks between low-metallicity gas (>= 2 - 6 % solar) and one or more of the
warm photoionized components. It is likely that these absorbers are related to
the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite
galaxies by interactions. However, we cannot reject completely the hypothesis
that they reside in the same large-scale structure in which the galaxies are
embedded but are otherwise not directly related.Comment: accepted by ApJ, to appear Feb 10 200
The Emergence of the Modern Universe: Tracing the Cosmic Web
This is the report of the Ultraviolet-Optical Working Group (UVOWG)
commissioned by NASA to study the scientific rationale for new missions in
ultraviolet/optical space astronomy approximately ten years from now, when the
Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific
theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to
0, occupying over 80% of cosmic time and beginning after the first galaxies,
quasars, and stars emerged into their present form. We considered
high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field
optical imaging (at least 10 arcmin square). The exciting science to be
addressed in the post-HST era includes studies of dark matter and baryons, the
origin and evolution of the elements, and the major construction phase of
galaxies and quasars. Key unanswered questions include: Where is the rest of
the unseen universe? What is the interplay of the dark and luminous universe?
How did the IGM collapse to form the galaxies and clusters? When were galaxies,
clusters, and stellar populations assembled into their current form? What is
the history of star formation and chemical evolution? Are massive black holes a
natural part of most galaxies? A large-aperture UV/O telescope in space
(ST-2010) will provide a major facility in the 21st century for solving these
scientific problems. The UVOWG recommends that the first mission be a 4m
aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field
imaging. In the coming decade, NASA should investigate the feasibility of an 8m
telescope, by 2010, with deployable optics similar to NGST. No high-throughput
UV/Optical mission will be possible without significant NASA investments in
technology, including UV detectors, gratings, mirrors, and imagers.Comment: Report of UV/O Working Group to NASA, 72 pages, 13 figures, Full
document with postscript figures available at
http://casa.colorado.edu/~uvconf/UVOWG.htm
Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy
While magnetoresistance (MR) has generally been found to be symmetric in
applied field in non-magnetic or magnetic metals, we have observed
antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and
transport measurements show that the antisymmetric MR is due to the appearance
of domain walls that run perpendicular to both the magnetization and the
current, a geometry existing only in materials with perpendicular magnetic
anisotropy. As a result, the extraordinary Hall effect (EHE) gives rise to
circulating currents in the vicinity of the domain walls that contributes to
the MR. The antisymmetric MR and EHE have been quantitatively accounted for by
a theoretical model.Comment: 17 pages, 4 figure
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