289 research outputs found
The Multiphase Intergalactic Medium towards PKS 2155-304
We study the cluster of H I and O VI absorption systems and the claimed
detection of O VIII absorption from the intergalactic medium at z ~ 0.0567,
associated with a group of galaxies toward the BL Lac object PKS 2155-304. As
measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet
Spectroscopic Explorer, and Chandra, this system appears to contain gas at a
variety of temperatures. We analyze this multi-phase gas in a clumpy-infall
model. From the absence of C IV and Si III absorption in the Ly-alpha clouds,
we infer metallicities less than 2.5-10% of solar values. The only metals are
detected in two O VI absorption components, offset by +/- 400 km/s from the
group barycenter (cz ~ 16,600 km/s). The O VI components may signify "nearside"
and "backside" infall into the group potential well, which coincides with the
claimed O VIII absorption. If the claimed O VIII detection is real, our
analysis suggests that clusters of strong Ly-alpha and O VI absorbers,
associated with groups of galaxies, may be the "signposts" of shock-heated,
metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI,
O VII, and O VIII, one may be able to clarify the heating mechanism of this
multiphase gas.Comment: Accepted for ApJL (2003), 11 pages, 1 color figur
Highly Ionized High-Velocity Clouds toward PKS 2155-304 and Markarian 509
To gain insight into four highly ionized high-velocity clouds (HVCs)
discovered by Sembach et al. (1999), we have analyzed data from the Hubble
Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) for the
PKS 2155-304 and Mrk 509 sight lines. We measure strong absorption in OVI and
column densities of multiple ionization stages of silicon (SiII/III/IV) and
carbon (CII/III/IV). We interpret this ionization pattern as a multiphase
medium that contains both collisionally ionized and photoionized gas. Toward
PKS 2155-304, for HVCs at -140 and -270 km/s, respectively, we measure
logN(OVI)=13.80+/-0.03 and log N(OVI)=13.56+/-0.06; from Lyman series
absorption, we find log N(HI)=16.37^(+0.22)_(-0.14) and 15.23^(+0.38)_(-0.22).
The presence of high-velocity OVI spread over a broad (100 km/s) profile,
together with large amounts of low-ionization species, is difficult to
reconcile with the low densities, n=5x10^(-6) cm^(-3), in the
collisional/photoionization models of Nicastro et al. (2002), although the HVCs
show a similar relation in N(SiIV)/N(CIV) versus N(CII)/N(CIV) as high-z
intergalactic clouds. Our results suggest that the high-velocity OVI in these
absorbers do not necessarily trace the WHIM, but instead may trace HVCs with
low total hydrogen column density. We propose that the broad high-velocity OVI
absorption arises from shock ionization, at bowshock interfaces produced from
infalling clumps of gas with velocity shear. The similar ratios of high ions
for HVC Complex C and these highly ionized HVCs suggest a common production
mechanism in the Galactic halo.Comment: 38 pages, including 10 figures. ApJ, 10 April, 2004. Replaced with
accepted versio
The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption towards HE 2347-4342
We provide an in-depth analysis of the He II and H I absorption in the
intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using
spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following
up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here
on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio
eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers
(soft radiation fields) with voids in the (H I) Ly-alpha distribution. These
effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc,
together with radiative transfer through a filamentary IGM whose opacity
variations control the penetration of 1-5 ryd radiation over 30-40 Mpc
distances. Owing to photon statistics and backgrounds, we can measure optical
depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and
reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to
460. Values of eta = 20-200 are consistent with models of photoionization by
quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may
require additional contributions from starburst galaxies, heavily filtered
quasar radiation, or density variations. Regions with eta < 30 may indicate the
presence of local hard sources. We find that eta is higher in "void" regions,
where H I is weak or undetected and 80% of the path length has eta > 100. These
voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation
softened by escape from the AGN cores or transfer through the "cosmic web". The
apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag
between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2.Comment: 27 pages, 7 figures, to appear in Ap
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,
Intergalactic Helium Absorption toward High-Redshift Quasars
The recent Hubble Space Telescope (HST) observations of the z(q) = 3.286 quasar Q0302-003 (Jakobsen et at. 1994) and the z(q) = 3.185 quasar Q1935-67 by Tytler (1995) show absorption edges at the redshifted wavelength of He II 304 A. A key goal is to distinguish between contributions from discrete Ly-alpha forest clouds and a smoothly distributed intergalactic medium (IGM). We model the contributions from each of these sources of He II absorption, including the distribution of line Doppler widths and column densities, the 'He II proximity effect' from the quasar, and a self-consistent derivation of the He II opacity of the universe as a function of the spectrum of ionizing sources, with the assumption that both the clouds and the IGM are photoionized. The He II edge can be fully accounted for by He II line blanketing for reasonable distributions of line widths and column densities in the Ly-alpha forest, provided that the ionizing sources have spectral index alpha(s) greater than 1.5, and any He II proximity effect is neglected. Even with some contribution from a diffuse IGM, it is difficult to account for the edge observed by Jakobsen et al. (1994) with a 'hard' source spectrum (alpha(s) less than 1.3). The proximity effect modifies the relative contributions of the clouds and IGM to tau(He II) near the quasar (z approx. less than z(q)) and markedly increases the amount of He II absorption required. This implies, for example, that to account for the He II edge with line blanketing alone, the minimum spectral index alpha(s) must be increased from 1.5 to 1.9. We demonstrate the need for higher resolution observations that characterize the change in transmission as z approaches z(q) and resolve line-free gaps in the continuum. We set limits on the density of the diffuse IGM and suggest that the IGM and Ly-alpha clouds are likely to be a significant repository for dark baryons
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
New HST Observations of the Halo Gas of NGC 3067: Limits on the Extragalactic Ionizing Background at Low Redshift and the Lyman Continuum Escape Fraction
We present UV spectroscopy from HST/GHRS and reanalyze existing H_alpha
images of the quasar/galaxy pair 3C 232/NGC 3067 and of the halo gas associated
with NGC 3067. The spectra permit measurement of, or limits on, the column
densities of Fe I, Fe II, Mg I, and Mg II in the absorbing cloud. Two distinct
models of the extragalactic radiation field are considered: (1) the ionizing
spectrum is dominated by a power-law extragalactic continuum, and (2) the
power-law spectrum contains a Lyman break, implying enhanced flux longward of
912 A relative to the hydrogen-ionizing flux. The H_alpha images constrain the
escape fraction of Lyman continuum photons from the galaxy to f_esc <= 0.02.
With the assumption that the cloud is shielded from all galactic contributions,
we can constrain the intensity and shape of the extragalactic continuum. For an
AGN-dominated power-law extragalactic spectrum, we derive a limit on the
extragalactic ionizing flux Phi_ion >= 2600 photons cm^-2 s^-1, or I_0 >=
10^-23 erg cm^-2 s^-1 Hz^-1 sr^-1 for an ionizing spectrum with power-law index
of 1.8 and a cloud of constant density. When combined with previous upper
limits from the absence of H_alpha recombination emission from intergalactic
clouds, our observations require 2600 <= Phi_ion <= 10000 photons cm^-2 s^-1.
We show that if galactic contributions to the incident radiation are important,
it is difficult to constrain Phi_ion. These results demonstrate that galactic
halo opacities and their wavelength dependence are crucial to understanding the
abundance of low-ionization metals in the IGM.Comment: 25 Pages LaTex, 8 PostScript Figures, accepted for publication in AJ,
Nov. 99 issu
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