442 research outputs found
A Low Upper Limit to the Lyman Continuum Emission of two galaxies at z 3
Long exposure, long-slit spectra have been obtained in the UV/optical bands
for two galaxies at z=2.96 and z=3.32 to investigate the fraction of ionizing
UV photons escaping from high redshifts galaxies. The two targets are among the
brightest galaxies discovered by Steidel and collaborators and they have
different properties in terms of Lyman-alpha emission and dust reddening. No
significant Lyman continuum emission has been detected. The noise level in the
spectra implies an upper limit of f_{rel,esc}\equiv 3 f(900)/f(1500)< 16% for
the relative escape fraction of ionizing photons, after correction for
absorption by the intervening intergalactic medium. This upper limit is 4 times
lower than the previous detection derived from a composite spectrum of 29 Lyman
break galaxies at z 3.4. If this value is typical of the escape fraction of the
z 3 galaxies, and is added to the expected contribution of the QSO population,
the derived UV background is in good agreement with the one derived by the
proximity effect.Comment: 16 pages, 2 figures, ApJ Letters in pres
The Stellar UV Background at z<1.5 and the Baryon Density of Photoionized Gas
We use new studies of the cosmic evolution of star-forming galaxies to
estimate the production rate of ionizing photons from hot, massive stars at low
and intermediate redshifts. The luminosity function of blue galaxies in the
Canada-France Redshift Survey shows appreciable evolution in the redshift
interval z=0-1.3, and generates a background intensity at 1 ryd of J_L~ 1.3 x
10^{-21} f_{esc} ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1} at z~0.5, where f_esc is
the unknown fraction of stellar Lyman-continuum photons which can escape into
the intergalactic space, and we have assumed that the absorption is picket
fence-type. We argue that recent upper limits on the H-alpha surface brightness
of nearby intergalactic clouds constrain this fraction to be <~ 20%. The
background ionizing flux from galaxies can exceed the QSO contribution at z~
0.5 if f_{esc}>~ 6%. We show that, in the general framework of a diffuse
background dominated by QSOs and/or star-forming galaxies, the cosmological
baryon density associated with photoionized, optically thin gas decreases
rapidly with cosmic time. The results of a recent Hubble Space Telescope survey
of OVI absorption lines in QSO spectra suggest that most of this evolution may
be due to the bulk heating and collisional ionization of the intergalactic
medium by supernova events in young galaxy halos.Comment: 6 pages, Latex file, 2 figures, mn.sty, MNRAS in pres
A high space density of L* Active Galactic Nuclei at z~4 in the COSMOS field
Identifying the source population of ionizing radiation, responsible for the
reionization of the universe, is currently a hotly debated subject with
conflicting results. Studies of faint, high-redshift star-forming galaxies, in
most cases, fail to detect enough escaping ionizing radiation to sustain the
process. Recently, the capacity of bright quasi-stellar objects to ionize their
surrounding medium has been confirmed also for faint active galactic nuclei
(AGNs), which were found to display an escaping fraction of ~74% at z~4. Such
levels of escaping radiation could sustain the required UV background, given
the number density of faint AGNs is adequate. Thus, it is mandatory to
accurately measure the luminosity function of faint AGNs (L~L*) in the same
redshift range. For this reason we have conducted a spectroscopic survey, using
the wide field spectrograph IMACS at the 6.5m Baade Telescope, to determine the
nature of our sample of faint AGN candidates in the COSMOS field. This sample
was assembled using photometric redshifts, color, and X-ray information. We
ended up with 16 spectroscopically confirmed AGNs at 3.6<z<4.2 down to a
magnitude of i=23.0 for an area of 1.73 deg. This leads to an AGN
space density of ~1.6 (corrected) at z~4 for an
absolute magnitude of M=-23.5. This is higher than previous
measurements and seems to indicate that AGNs could make a substantial
contribution to the ionizing background at z~4. Assuming that AGN physical
parameters remain unchanged at higher redshifts and fainter luminosities, these
sources could be regarded as the main drivers of cosmic reionization.Comment: 10 pages, 3 figures, accepted for publication by Ap
The absorption spectrum of the QSO PKS 2126-158 (z_em = 3.27) at high resolution
Spectra of the z_em = 3.268 quasar PKS 2126-158 have been obtained in the
range lambda lambda 4300-6620 A with a resolution R = 27000 and an average
signal-to-noise ratio s/n = 25 per resolution element. The list of the
identified absorption lines is given together with their fitted column
densities and Doppler widths. The modal value of the Doppler parameter
distribution for the Ly-alpha lines is = 25 km/s. The column density
distribution can be described by a power-law dn/dN \propto N^{-beta} with beta
= 1.5. 12 metal systems have been identified, two of which were previously
unknown. In order to make the column densities of the intervening systems
compatible with realistic assumptions about the cloud sizes and the silicon to
carbon overabundance, it is necessary to assume a jump beyond the He II edge in
the spectrum of the UV ionizing background at z = 3 a factor 10 larger than the
standard predictions for the integrated quasar contribution. An enlarged sample
of C IV absorptions (71 doublets) has been used to analyze the statistical
properties of this class of absorbers strictly related to galaxies. The column
density distribution is well described by a single power-law, with beta=1.64
and the Doppler parameter distribution shows a modal value b = 14 km/s. The two
point correlation function has been computed in the velocity space for the
individual components of C IV features. A significant signal is obtained for
scales smaller than 200-300 km/s, xi(30< Delta v < 90 km/s) = 32.71 +- 2.89. A
trend of decreasing clustering amplitude with decreasing column density is
apparent, analogously to what has been observed for Ly-alpha lines.Comment: 16 pages, LaTeX (l-aa.sty), 13 ps figures, included in text with
epsf, Fig. 1 can be retrieved at http://www.sissa.it/~dodorico/paper.html,
accepted by Astronomy and Astrophysic
- âŠ