14 research outputs found
The Transverse Proximity Effect: A Probe to the Environment, Anisotropy, and Megayear Variability of QSOs
The transverse proximity effect is the expected decrease in the strength of
the Lya forest absorption in a QSO spectrum when another QSO lying close to the
line of sight enhances the photoionization rate above that due to the average
cosmic ionizing background. We select three QSOs from the Early Data Release of
the Sloan Digital Sky Survey that have nearby foreground QSOs, with proper line
of sight tangential separations of 0.50, 0.82, and 1.10 h^{-1} Mpc. We estimate
that the ionizing flux from the foreground QSO should increase the
photoionization rate by a factor (94, 13, 13) in these three cases, which would
be clearly detectable in the first QSO and marginally so in the other two. We
do not detect the transverse proximity effect. Three possible explanations are
provided: an increase of the gas density in the vicinity of QSOs, time
variability, and anisotropy of the QSO emission. We find that the increase of
gas density near QSOs can be important if they are located in the most massive
halos present at high redshift, but is not enough to fully explain the absence
of the transverse proximity effect. Anisotropy requires an unrealistically
small opening angle of the QSO emission. Variability demands that the
luminosity of the QSO with the largest predicted effect was much lower 10^6
years ago, whereas the transverse proximity effect observed in the HeII Lya
absorption in QSO 0302-003 by Jakobsen et al. (2003) implies a lifetime longer
than 10^7 years. A combination of all three effects may better explain the lack
of Lya absorption reduction. A larger sample of QSO pairs may be used to
diagnose the environment, anisotropy and lifetime distribution of QSOs.Comment: 27 pages, 13 figures, accepted by Ap
Faint AGN and the Ionizing Background
We determine the evolution of the faint, high-redshift, optical luminosity
function (LF) of AGN implied by several observationally-motivated models of the
ionizing background. Our results depend crucially on whether we use the total
ionizing rate measured by the proximity effect technique or the lower
determination from the flux decrement distribution of Ly alpha forest lines.
Assuming a faint-end LF slope of 1.58 and the SDSS estimates of the bright-end
slope and normalization, we find that the LF must break at M_B*=-24.2,-22.3,
-20.8 at z=3,4, 5 if we adopt the lower ionization rate and assume no stellar
contribution to the background. The break must occur at M_B*=-20.6,-18.7, -18.7
for the proximity effect background estimate. These values brighten by as much
as ~2 mag if high-z galaxies contribute to the background with an escape
fraction of ionizing photons consistent with recent estimates: f_e=0.16. By
comparing to faint AGN searches, we find that the typically-quoted proximity
effect estimates of the background imply an over-abundance of faint AGN (even
with f_e=1). Even adopting the lower bound on proximity effect measurements,
the stellar escape fraction must be high: f_e>0.2. Conversely, the lower flux-
decrement-derived background requires a limited stellar contribution: f_e<0.05.
Our derived LFs together with the locally-estimated black hole density suggest
that the efficiency of converting mass to light in optically-unobscured AGN is
somewhat lower than expected, <0.05. Comparison with similar estimates based on
X-ray counts suggests that more than half of all AGN are obscured in the
UV/optical. We also derive lower limits on typical AGN lifetimes and obtain
>10^7 yrs for favored cases.Comment: 19 pages, 16 figures. Accepted by Astrophysical Journa
Movies of Electrons in Atoms
Physicists have long been able to snap atomic-scale pictures by shining a beam of electrons at a target, but filming the electronic structure of an atom as it changes in time is the next goal. A rapid strobing of electron pulses less than a millionth of a billionth of a second long should do the trick, according to a theoretical analysis in the 24 December Physical Review Letters. The authors demonstrate with computer simulations that ultrafast electron pulses could track the breathing state of an excited atom or the hopping of electrons between atoms in a molecule. Such movies open up the possibility of controlling the electrons that drive chemical reactions