5,131 research outputs found
The extraordinary mid-infrared spectral properties of FeLoBAL Quasars
We present mid-infrared spectra of six FeLoBAL QSOs at 1<z<1.8, taken with
the Spitzer space telescope. The spectra span a range of shapes, from hot dust
dominated AGN with silicate emission at 9.7 microns, to moderately obscured
starbursts with strong Polycyclic Aromatic Hydrocarbon (PAH) emission. The
spectrum of one object, SDSS 1214-0001, shows the most prominent PAHs yet seen
in any QSO at any redshift, implying that the starburst dominates the mid-IR
emission with an associated star formation rate of order 2700 solar masses per
year. With the caveats that our sample is small and not robustly selected, we
combine our mid-IR spectral diagnostics with previous observations to propose
that FeLoBAL QSOs are at least largely comprised of systems in which (a) a
merger driven starburst is ending, (b) a luminous AGN is in the last stages of
burning through its surrounding dust, and (c) which we may be viewing over a
restricted line of sight range.Comment: ApJ, accepte
Spectral Templates from Multicolor Redshift Surveys
Understanding how the physical properties of galaxies (e.g. their spectral
type or age) evolve as a function of redshift relies on having an accurate
representation of galaxy spectral energy distributions. While it has been known
for some time that galaxy spectra can be reconstructed from a handful of
orthogonal basis templates, the underlying basis is poorly constrained. The
limiting factor has been the lack of large samples of galaxies (covering a wide
range in spectral type) with high signal-to-noise spectrophotometric
observations. To alleviate this problem we introduce here a new technique for
reconstructing galaxy spectral energy distributions directly from samples of
galaxies with broadband photometric data and spectroscopic redshifts.
Exploiting the statistical approach of the Karhunen-Loeve expansion, our
iterative training procedure increasingly improves the eigenbasis, so that it
provides better agreement with the photometry. We demonstrate the utility of
this approach by applying these improved spectral energy distributions to the
estimation of photometric redshifts for the HDF sample of galaxies. We find
that in a small number of iterations the dispersion in the photometric
redshifts estimator (a comparison between predicted and measured redshifts) can
decrease by up to a factor of 2.Comment: 25 pages, 9 figures, LaTeX AASTeX, accepted for publication in A
Hybrid expansions for local structural relaxations
A model is constructed in which pair potentials are combined with the cluster
expansion method in order to better describe the energetics of structurally
relaxed substitutional alloys. The effect of structural relaxations away from
the ideal crystal positions, and the effect of ordering is described by
interatomic-distance dependent pair potentials, while more subtle
configurational aspects associated with correlations of three- and more sites
are described purely within the cluster expansion formalism. Implementation of
such a hybrid expansion in the context of the cluster variation method or Monte
Carlo method gives improved ability to model phase stability in alloys from
first-principles.Comment: 8 pages, 1 figur
Stromgren Photometry from z=0 to z~1. The Method
We use rest-frame Stromgren photometry to observe clusters of galaxies in a
self-consistent manner from z=0 to z=0.8. Stromgren photometry of galaxies is
an efficient compromise between standard broad-band photometry and
spectroscopy, in the sense that it is more sensitive to subtle variations in
spectral energy distributions than the former, yet much less time-consuming
than the latter. Principal Component Analysis (PCA) is used to extract maximum
information from the Stromgren data. By calibrating the Principal Components
using well-studied galaxies (and stellar population models), we develop a
purely empirical method to detect, and subsequently classify, cluster galaxies
at all redshifts smaller than 0.8. Interlopers are discarded with unprecedented
efficiency (up to 100%). The first Principal Component essentially reproduces
the Hubble Sequence, and can thus be used to determine the global star
formation history of cluster members. The (PC2, PC3) plane allows us to
identify Seyfert galaxies (and distinguish them from starbursts) based on
photometric colors alone. In the case of E/S0 galaxies with known redshift, we
are able to resolve the age-dust- metallicity degeneracy, albeit at the
accuracy limit of our present observations. This technique will allow us to
probe galaxy clusters well beyond their cores and to fainter magnitudes than
spectroscopy can achieve. We are able to directly compare these data over the
entire redshift range without a priori assumptions because our observations do
not require k-corrections. The compilation of such data for different cluster
types over a wide redshift range is likely to set important constraints on the
evolution of galaxies and on the clustering process.Comment: 35 pages, 18 figures, accepted by ApJ
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