1,382 research outputs found
MACHOs, White Dwarfs, and the Age of the Universe
(Abridged Abstract) A favored interpretation of recent microlensing
measurements towards the Large Magellanic Cloud implies that a large fraction
(i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We
compare model white dwarf luminosity functions to the data from the
observational surveys in order to determine a lower bound on the age of any
substantial white dwarf halo population (and hence possibly on the age of the
Universe). We compare various theoretical white dwarf luminosity functions, in
which we vary hese three parameters, with the abovementioned survey results.
From this comparison, we conclude that if white dwarfs do indeed constitute
more than 10% of the local halo mass density, then the Universe must be at
least 10 Gyr old for our most extreme allowed values of the parameters. When we
use cooling curves that account for chemical fractionation and more likely
values of the IMF and the bolometric correction, we find tighter limits: a
white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5
Gyr). Our analysis also indicates that the halo white dwarfs almost certainly
have helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs,
22 page
A view of the narrow-line region in the infrared: active galactic nuclei with resolved fine-structure lines in the Spitzer archive
We queried the Spitzer archive for high-resolution observations with the
Infrared Spectrograph of optically selected active galactic nuclei (AGN) for
the purpose of identifying sources with resolved fine-structure lines that
would enable studies of the narrow-line region (NLR) at mid-infrared
wavelengths. By combining 298 Spitzer spectra with 6 Infrared Space Observatory
spectra, we present kinematic information of the NLR for 81 z<=0.3 AGN. We used
the [NeV], [OIV], [NeIII], and [SIV] lines, whose fluxes correlate well with
each other, to probe gas photoionized by the AGN. We found that the widths of
the lines are, on average, increasing with the ionization potential of the
species that emit them. No correlation of the line width with the critical
density of the corresponding transition was found. The velocity dispersion of
the gas, sigma, is systematically higher than that of the stars, sigma_*, in
the AGN host galaxy, and it scales with the mass of the central black hole,
M_BH. Further correlations between the line widths and luminosities L, and
between L and M_BH, are suggestive of a three dimensional plane connecting
log(M_BH) to a linear combination of log(sigma) and log(L). Such a plane can be
understood within the context of gas motions that are driven by AGN feedback
mechanisms, or virialized gas motions with a power-law dependence of the NLR
radius on the AGN luminosity. The M_BH estimates obtained for 35 type 2 AGN
from this plane are consistent with those obtained from the M_BH-sigma_*
relation.Comment: ApJ, revised to match the print versio
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