4,314 research outputs found
A Morphological and Multicolor Survey for Faint QSOs in the Groth-Westphal Strip
Quasars representative of the populous faint end of the luminosity function
are frustratingly dim with m~24 at intermediate redshift; moreover groundbased
surveys for such faint QSOs suffer substantial morphological contamination by
compact galaxies having similar colors. In order to establish a more reliable
ultrafaint QSO sample, we used the APO 3.5-m telescope to take deep groundbased
U-band CCD images in fields previously imaged in V,I with WFPC2/HST. Our
approach hence combines multicolor photometry with the 0.1" spatial resolution
of HST, to establish a morphological and multicolor survey for QSOs extending
about 2 magnitudes fainter than most extant groundbased surveys. We present
results for the "Groth-Westphal Strip", in which we identify 10 high likelihood
UV-excess candidates having stellar or stellar-nucleus+galaxy morphology in
WFPC2. For m(606)<24.0 (roughly B<24.5) the surface density of such QSO
candidates is 420 (+180,-130) per square degree, or a surface density of 290
(+160,-110) per square degree with an additional V-I cut that may further
exclude compact emission line galaxies. Even pending confirming spectroscopy,
the observed surface density of QSO candidates is already low enough to yield
interesting comparisons: our measures agree extremely well with the predictions
of several recent luminosity function models.Comment: 29 pages including 6 tables and 7 figures. As accepted for
publication in The Astronomical Journal (minor revisions
Galaxy Clustering Evolution in the UH8K Weak Lensing Fields
We present measurements of the two-point galaxy angular correlation function
as a function of apparent magnitude, color, and morphology. We present new
galaxy number counts to limiting magnitudes of I=24.0 and V=25.0. We find
to be well described by a power-law of slope -0.8. We find the
amplitude of the correlation function to decrease monotonically with
increasingly faint apparent magnitude. We compare with predictions utilizing
redshift distributions based on deep spectroscopic observations. We conclude
that simple redshift-dependent models which characterize evolution by means of
the epsilon parameter inadequately describe the observations. We find a strong
clustering dependence on V-I color because galaxies of extreme color lie at
similar redshifts and the angular correlation functions for these samples are
minimally diluted by chance projections.
We then present the first attempt to investigate the redshift evolution of
clustering, utilizing a population of galaxies of the same morphological type
and absolute luminosity. We study the dependence of on
redshift for Lstar early-type galaxies in the redshift range 0.2<z<0.9.
Although uncertainties are large, we find the evolution in the clustering of
these galaxies to be consistent with stable clustering [epsilon=0]. We find
Lstar early-type galaxies to cluster slightly more strongly (rnought =
5.25\pm0.28 \hMpc assuming epsilon=0) than the local full field population.
This is in good agreement with the 2dFGRS value for Lstar early-type galaxies
in the local universe (abridged).Comment: 41 pages, including 12 figs, 10 tables, to appear in Ap
Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei
The silicate cross section peak near 10um produces emission and absorption
features in the spectra of dusty galactic nuclei observed with the Spitzer
Space Telescope. Especially in ultraluminous infrared galaxies, the observed
absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A
foreground screen of obscuration cannot reproduce this observed feature, even
at large optical depth. Instead, the deep absorption requires a nuclear source
to be deeply embedded in a smooth distribution of material that is both
geometrically and optically thick. In contrast, a clumpy medium can produce
only shallow absorption or emission, which are characteristic of
optically-identified active galactic nuclei. In general, the geometry of the
dusty region and the total optical depth, rather than the grain composition or
heating spectrum, determine the silicate feature's observable properties. The
apparent optical depth calculated from the ratio of line to continuum emission
generally fails to accurately measure the true optical depth. The obscuring
geometry, not the nature of the embedded source, also determines the far-IR
spectral shape.Comment: To appear in ApJ
Isovector Giant Dipole Resonance from the 3D Time-Dependent Density Functional Theory for Superfluid Nuclei
A fully symmetry unrestricted Time-Dependent Density Functional Theory
extended to include pairing correlations is used to calculate properties of the
isovector giant dipole resonances of the deformed open-shell nuclei 172Yb
(axially deformed), 188Os (triaxially deformed), and 238U (axially deformed),
and to demonstrate good agreement with experimental data on nuclear
photo-absorption cross-sections for two different Skyrme force parametrizations
of the energy density functional: SkP and SLy4.Comment: 5 pages, 3 figures, published versio
Persistent currents in carbon nanotubes based rings
Persistent currents in rings constructed from carbon nanotubes are
investigated theoretically. After studying the contribution of finite
temperature or quenched disorder on covalent rings, the complexity due to the
bundle packing is addressed. The case of interacting nanotori and
self-interacting coiled nanotubes are analyzed in details in relation with
experiments.Comment: 7 sections, 9 figure
Monte Carlo simulation of virtual Compton scattering below pion threshold
This paper describes the Monte Carlo simulation developed specifically for
the VCS experiments below pion threshold that have been performed at MAMI and
JLab. This simulation generates events according to the (Bethe-Heitler + Born)
cross section behaviour and takes into account all relevant
resolution-deteriorating effects. It determines the `effective' solid angle for
the various experimental settings which are used for the precise determination
of photon electroproduction absolute cross section.Comment: 24 pages, 6 figures, to be published in Nuclear Instruments and
Methods in Physics Research, A One author adde
High resolution mid-infrared spectroscopy of ultraluminous infrared galaxies
(Abridged) We present R~600, 10-37um spectra of 53 ULIRGs at z<0.32, taken
using the IRS on board Spitzer. All of the spectra show fine structure emission
lines of Ne, O, S, Si and Ar, as well as molecular Hydrogen lines. Some ULIRGs
also show emission lines of Cl, Fe, P, and atomic Hydrogen, and/or absorption
features from C_2H_2, HCN, and OH. We employ diagnostics based on the
fine-structure lines, as well as the EWs and luminosities of PAH features and
the strength of the 9.7um silicate absorption feature (S_sil), to explore the
power source behind the infrared emission in ULIRGs. We show that the IR
emission from the majority of ULIRGs is powered mostly by star formation, with
only ~20% of ULIRGs hosting an AGN with a comparable or greater IR luminosity
than the starburst. The detection of the 14.32um [NeV] line in just under half
the sample however implies that an AGN contributes significantly to the mid-IR
flux in ~42% of ULIRGs. The emission line ratios, luminosities and PAH EWs are
consistent with the starbursts and AGN in ULIRGs being more extincted, and for
the starbursts more compact, versions of those in lower luminosity systems. The
excitations and electron densities in the NLRs of ULIRGs appear comparable to
those of lower luminosity starbursts, though there is evidence that the NLR gas
in ULIRGs is more dense. We show that the combined luminosity of the 12.81um
[NeII] and 15.56um [NeIII] lines correlates with both IR luminosity and the
luminosity of the 6.2 micron and 11.2 micron PAH features in ULIRGs, and use
this to derive a calibration between PAH luminosity and star formation rate.
Finally, we show that ULIRGs with 0.8 < S_sil < 2.4 are likely to be powered
mainly by star formation, but that ULIRGs with S_sil < 0.8, and possibly those
with S_sil > 2.4, contain an IR-luminous AGN.Comment: 62 pages in preprint format, 4 tables, 23 figures. ApJ accepte
The Nuclear Starburst in NGC 253
We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands,
broadband images in the J, H, and Ks bands, narrowband images centered at the
wavelengths of BrGamma and H2(1,0)S(1), and imaging spectroscopy centered on
[NeII](12.8um). We use these data and data from the literature in a
comprehensive re-assessment of the starburst in this galaxy. We derive the
supernova rate from the strength of the infrared [FeII] lines. We find that
most of the H2 infrared luminosity is excited by fluorescence in low density
gas. We derive a strong upper limit of ~37,000K for the stars exciting the
emission lines. We use velocity-resolved infrared spectra to determine the mass
in the starburst region. Most of this mass appears to be locked up in the old,
pre-existing stellar population. Using these constraints and others to build an
evolutionary synthesis model, we find that the IMF originally derived to fit
the starburst in M 82 (similar to a Salpeter IMF) also accounts for the
properties of NGC 253. The models indicate that rapid massive star formation
has been ongoing for 20-30 million years in NGC 253---that is, it is in a late
phase of its starburst. We model the optical emission line spectrum expected
from a late phase starburst and demonstrate that it reproduces the observed
HII/weak-[OI] LINER characteristics.Comment: 48 pages, 14 figures, uses AASTeX macros, to appear in Ap
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