144 research outputs found
On the Nature of Soft X-ray Weak Quasi-Stellar Objects
Recent studies of QSOs with ROSAT suggest the existence of a significant
population of Soft X-ray Weak QSOs (SXW QSOs) where the soft X-ray flux is ~
10-30 times smaller than in typical QSOs. As a first step in a systematic study
of these objects, we establish a well-defined sample of SXW QSOs which includes
all alpha_ox<=-2 QSOs from the Boroson & Green (1992) sample of 87 BQS QSOs.
SXW QSOs comprise about 11% of this optically selected QSO sample. From an
analysis of CIV absorption in the 55 BG92 QSOs with available CIV data, we find
a remarkably strong correlation between alpha_ox and the CIV absorption
equivalent width. This correlation suggests that absorption is the primary
cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption
properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs and
BAL QSOs. From a practical point of view, our correlation demonstrates that
selection by soft X-ray weakness is an effective (>=80% successful) and
observationally inexpensive way to find low-redshift QSOs with strong and
interesting ultraviolet absorption. We have also identified several notable
differences between the optical emission-line properties of SXW QSOs and those
of the other BG92 QSOs. SXW QSOs show systematically low [O III] luminosities
as well as distinctive H-beta profiles. They tend to lie toward the weak-[O
III] end of BG92 eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed
Seyferts and QSOs with similar values of eigenvector 1 have been suggested to
have extreme values of a primary physical parameter, perhaps mass accretion
rate relative to the Eddington rate (M-dot/M-dot_{Edd}). If these suggestions
are correct, it is likely that SXW QSOs also tend to have generally high values
of (M-dot/M-dot_{Edd}). (Abridged)Comment: 34 pages, ApJ accepted, also available from
http://www.astro.psu.edu/users/niel/papers/papers.htm
An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zw 1
We present an empirical template spectrum suitable for fitting/subtracting
and studying the FeII and FeIII line emission in the restframe UV spectra of
active galatic nuclei (AGNs), the first empirical UV iron template to cover the
full 1250 - 3090 A range. Iron emission is often a severe contaminant in
optical--UV spectra of AGNs. Its presence complicates and limits the accuracy
of measurements of both strong and weak emission lines and the continuum
emission, affecting studies of line and continuum interrelations, the
ionization structure, and elemental abundances in AGNs. Despite the wealth of
work on modeling the AGN FeII emission and the need to account for it in
observed AGN spectra, there is no UV template electronically available to aid
this process. The iron template we present is based on HST spectra of the
Narrow Line Seyfert 1, IZw1. Its intrinsic narrow lines (~900 km/s) and rich
iron spectrum make the template particularly suitable for use with most AGN
spectra. The iron emission spectrum and the line identifications and
measurements are presented and compared with the work of Laor et al. We
illustrate the application of the derived FeII and FeIII templates by fitting
and subtracting the iron emission from UV spectra of four high-z quasars and of
the nearby quasar, 3C273. We briefly discuss the small discrepancies between
this observed iron emission and the UV template, and compare the template with
previously published ones. We discuss the advantages and limitations of the
templates and of the template fitting method. We conclude that the templates
work sufficiently well to be a valuable and important tool for eliminating and
studying the iron emission in AGNs, at least until accurate theoretical iron
emission models are developed. (Abridged)Comment: 73 pages including 7 figures, 6 tables. To appear in ApJS. Preprint
is also available at
http://www.astronomy.ohio-state.edu/~vester/IronEmission
Optical spectroscopy of faint gigahertz peaked spectrum sources
We present spectroscopic observations of a sample of faint Gigahertz Peaked
Spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey
(WENSS). Redshifts have been determined for 19 (40%) of the objects. The
optical spectra of the GPS sources identified with low redshift galaxies show
deep stellar absorption features. This confirms previous suggestions that their
optical light is not significantly contaminated by AGN-related emission, but is
dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars,
justifying the use of these (probably) young radio sources as probes of galaxy
evolution. The optical spectra of GPS sources identified with quasars are
indistinguishable from those of flat spectrum quasars, and clearly different
from the spectra of Compact Steep Spectrum (CSS) quasars. The redshift
distribution of the GPS quasars in our radio-faint sample is comparable to that
of the bright samples presented in the literature, peaking at z ~ 2-3. It is
unlikely that a significant population of low redshift GPS quasars is missed
due to selection effects in our sample. We therefore claim that there is a
genuine difference between the redshift distributions of GPS galaxies and
quasars, which, because it is present in both the radio-faint and bright
samples, can not be due to a redshift-luminosity degeneracy. It is therefore
unlikely that the GPS quasars and galaxies are unified by orientation, unless
the quasar opening angle is a strong function of redshift. We suggest that the
GPS quasars and galaxies are unrelated populations and just happen to have
identical observed radio-spectral properties, and hypothesise that GPS quasars
are a sub-class of flat spectrum quasars.Comment: LaTeX, 13 pages. Accepted by MNRAS. For related papers see
http://www.ast.cam.ac.uk/~snelle
Long-Term Effects of Self-Control on Alcohol Use and Sexual Behavior among Urban Minority Young Women
High risk alcohol use and sexual behaviors peak in young adulthood and often occur in the same individuals. Alcohol use has been found to impair decision-making and contribute to high risk sexual activity. However, the association between alcohol use and risky sexual behavior may also reflect enduring individual differences in risk taking, sociability, self-control, and related variables. Both behaviors can serve similar functions related to recreation, interpersonal connection, and the pursuit of excitement or pleasure. The present study examined the extent to which high risk drinking and sexual behavior clustered together in a sample of urban minority young adult women, a demographic group at elevated risk for negative outcomes related to sexual health. We tested whether psychosocial functioning measured at the beginning of high school predicted classes of risk behaviors when girls were tracked longitudinally into young adulthood. Latent class analysis indicated three distinct profiles based on high risk drinking and sexual behavior (i.e., multiple sex partners) in young adulthood. The largest class (73% of the sample) reported low levels of risky drinking and sexual behavior. The next largest class (19%) reported high risk drinking and low risk sexual behavior, and the smallest class (8%) reported high levels of both behaviors. Compared to women from other racial/ethnic groups, black women were more likely to be categorized in the high risk drinking/low risk sex class. Multinomial logistic regression indicated that self-control in adolescence had a broad and enduring protective effect on risk behaviors eight years later and was associated with a greater probability of being in the low risk drinking/low risk sex class. Findings are discussed in terms of understanding the phenotypic expressions of risk behavior as they relate to early psychosocial development and the long-term protective function of self-control in reducing high risk drinking and sexual behaviors
Radio-optical scrutiny of compact AGN: Correlations between properties of pc-scale jets and optical nuclear emission
We study the correlations between the Very Long Baseline Array radio emission
at 15 GHz, extended emission at 151 MHz, and optical nuclear emission at 5100
AA for a complete sample of 135 compact jets. We use the partial Kendall's tau
correlation analysis to check the link between radio properties of parsec-scale
jets and optical luminosities of host AGN. We find a significant positive
correlation for 99 quasars between optical nuclear luminosities and total radio
(VLBA) luminosities of unresolved cores at 15 GHz originated at milliarcseconds
scales. For 18 BL Lacs, the optical continuum emission correlates with the
radio emission of the jet at 15 GHz. We suggest that the radio and optical
emission are beamed and originate in the innermost part of the
sub--parsec-scale jet in quasars. Analysis of the relation between the apparent
speed of the jet and the optical nuclear luminosity at 5100 AA supports the
relativistic beaming model for the optical emission generated in the jet, and
allows the peak values of the intrinsic optical luminosity of the jet and its
Lorentz factor to be estimated for the populations of quasars, BL Lacs, and
radio galaxies. The radio-loudness of quasars is found to increase at high
redshifts, which can be a result of lower efficiency of the accretion in AGN
having higher radio luminosities. A strong positive correlation is found
between the intrinsic kinetic power of the jet and the apparent luminosities of
the total and the unresolved core emission of the jet at 15 GHz. This
correlation is interpreted in terms of intrinsically more luminous parsec-scale
jet producing more luminous extended structure which is detectable at low radio
frequencies, 151 MHz. A possibility that the low frequency radio emission is
relativistically beamed in superluminal AGN and therefore correlates with radio
luminosity of the jet at 15 GHz can not be ruled out (abridged).Comment: 16 pages, 10 figuers; minor comments are added; accepted to A&
The Actin-Binding Protein Capulet Genetically Interacts with the Microtubule Motor Kinesin to Maintain Neuronal Dendrite Homeostasis
BACKGROUND: Neurons require precise cytoskeletal regulation within neurites, containing microtubule tracks for cargo transport in axons and dendrites or within synapses containing organized actin. Due to the unique architecture and specialized function of neurons, neurons are particularly susceptible to perturbation of the cytoskeleton. Numerous actin-binding proteins help maintain proper cytoskeletal regulation. METHODOLOGY/PRINCIPAL FINDINGS: From a Drosophila forward genetic screen, we identified a mutation in capulet--encoding a conserved actin-binding protein--that causes abnormal aggregates of actin within dendrites. Through interaction studies, we demonstrate that simultaneous genetic inactivation of capulet and kinesin heavy chain, a microtubule motor protein, produces elongate cofilin-actin rods within dendrites but not axons. These rods resemble actin-rich structures induced in both mammalian neurodegenerative and Drosophila Alzheimer's models, but have not previously been identified by loss of function mutations in vivo. We further demonstrate that mitochondria, which are transported by Kinesin, have impaired distribution along dendrites in a capulet mutant. While Capulet and Cofilin may biochemically cooperate in certain circumstances, in neuronal dendrites they genetically antagonize each other. CONCLUSIONS/SIGNIFICANCE: The present study is the first molecularly defined loss of function demonstration of actin-cofilin rods in vivo. This study suggests that simultaneous, seemingly minor perturbations in neuronal dendrites can synergize producing severe abnormalities affecting actin, microtubules and mitochondria/energy availability in dendrites. Additionally, as >90% of Alzheimer's and Parkinson's cases are sporadic this study suggests mechanisms by which multiple mutations together may contribute to neurodegeneration instead of reliance on single mutations to produce disease
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