399 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
HST/FOS spectra of PG 1351+64: An intrinsic absorber at low redshift
A 1 A resolution spectra of the nearby (z = 0.08797) Seyfert galaxy PG 1351+64 taken with the Faint Object Spectrograph (FOS) onboard the Hubble Space Telescope is presented. Spectral coverage runs from 1200-3200 A in the observed frame and includes emission and absorption features due to Ly-alpha, N 5, Si 4, C 4, and Mg 2. Three distinct intrinsic absorption systems in Ly-alpha, N 5, Si 4, and C 4, and tentatively in Mg 2, at velocities of 900 km/s, 1630 km/s, and 2900 km/s (plus or minus 100 km/s) relative to the emission-line redshift of the QSO were detected. The maximum relative velocity of these absorbers is less than 5000 km/s and therefore does not meet Weymann, Carswell, & Smith's criteria for Broad-Absorption-Line (BAL) QSO's at high-z. However, the absorptions are almost certainly intrinsic to the QSO given the low redshift of this object. In addition, PG 1351+64 is marginally radio-quiet, as are all BALQSO's, based on recent estimates of the radio-loud/radio-quiet dividing line. The narrow velocity width, less than 500 km/s, and low outflow velocities of the absorption systems are more similar to so called 'associated absorbers' seen at high-z in radio-loud quasars, but whose absorptions are thought to arise in clouds much farther from the nucleus (greater than 1 kpc) than are BAL clouds (1-10 pc). Despite the qualitative resemblance to the associated absorbers, the absorption systems in PG 1351+64 appear to be the low-luminosity analogs of BALQSO absorption troughs. The lower observed outflow velocities in PG 1351+64 are due to the much lower luminosity of the nuclear source in comparison to the high-z, high-luminosity BALQSO's. In addition, 'satellite' emission lines displaced 4000-5000 km/s blueward and redward of the Mg 2 emission were discovered
Radio Luminosities and Classificatory Criteria of BL Lacertae Objects
Using the sample of radio selected BL Lacertae objects (RBLs) and X-ray
selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we
calculated the luminosities of radio, optical and X-ray of each source and made
the statistical analysis among the luminosities at different wave-bands,
broad-band spectral indices from radio to X-ray () and peak
frequencies (). Our results are as follows: (i) there is a positive
correlation between radio luminosity and and a
negative correlation between and . High-energy peak BL Lacs
(HBLs) and low-energy peak BL Lacs (LBLs) can be distinguished very well, the
dividing lines are probably those of (erg/sec) and
(or )0.75 for - plot and
those of (erg/sec) and for the
- plot; (ii) there is a weak positive correlation between
optical luminosity and and a negatively weak
correlation between and ; (iii) there is no correlation
between X-ray luminosity and or between and
. From our analysis, we find that synchrotron radiation is the main
X-ray radiation mechanism for HBLs while inverse Compton scattering for LBLs.Comment: 9 pages, 3 figures. Submitted to A&
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
Radio line and continuum observations of quasar-galaxy pairs and the origin of low reshift quasar absorption line systems
There are a number of known quasars for which our line of sight to the high redshift quasar passes within a few Holmberg radii of a low redshift galaxy. In a few of these cases, spectra of the quasar reveal absorption by gas associated with the low redshift galaxy. A number of these pairs imply absorption by gas which lies well outside the optical disk of the associated galaxy, leading to models of galaxies with 'halos' or 'disks' of gas extending to large radii. The authors present observations of 4 such pairs. In three of the four cases, they find that the associated galaxy is highly disturbed, typically due to a gravitational interaction with a companion galaxy, while in the fourth case the absorption can be explained by clouds in the optical disk of the associated galaxy. They are led to an alternative hypothesis concerning the origin of the low redshift absorption line systems: the absorption is by gas clouds which have been gravitationally stripped from the associated galaxy. These galaxies are rapidly evolving, and should not be used as examples of absorption by clouds in halos of field spirals. The authors conclude by considering the role extended gas in interacting systems plays in the origin of higher redshift quasar absorption line systems
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