4,390 research outputs found
Connecting Galaxy Evolution, Star Formation and the X-ray Background
As a result of deep hard X-ray observations by Chandra and XMM-Newton a
significant fraction of the cosmic X-ray background (CXRB) has been resolved
into individual sources. These objects are almost all active galactic nuclei
(AGN) and optical followup observations find that they are mostly obscured Type
2 AGN, have Seyfert-like X-ray luminosities (i.e., L_X ~ 10^{43-44} ergs
s^{-1}), and peak in redshift at z~0.7. Since this redshift is similar to the
peak in the cosmic star-formation rate, this paper proposes that the obscuring
material required for AGN unification is regulated by star-formation within the
host galaxy. We test this idea by computing CXRB synthesis models with a ratio
of Type 2/Type 1 AGN that is a function of both z and 2-10 keV X-ray
luminosity, L_X. The evolutionary models are constrained by parameterizing the
observed Type 1 AGN fractions from the recent work by Barger et al. The
parameterization which simultaneously best accounts for Barger's data, the CXRB
spectrum and the X-ray number counts has a local, low-L_X Type 2/Type 1 ratio
of 4, and predicts a Type 2 AGN fraction which evolves as (1+z)^{0.3}. Models
with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN
fractions. This particular evolution predicts a Type 2/Type 1 ratio of 1-2 for
log L_X > 44, and thus the deep X-ray surveys are missing about half the
obscured AGN with these luminosities. These objects are likely to be Compton
thick. Overall, these calculations show that the current data strongly supports
a change to the AGN unification scenario where the obscuration is connected
with star formation in the host galaxy rather than a molecular torus alone. The
evolution of the obscuration implies a close relationship between star
formation and AGN fueling, most likely due to minor mergers or interactions.Comment: 36 pages, 8 figures, ApJ in press. Minor changes to match published
versio
3-D Radiative Transfer Calculations of Radiation Feedback from Massive Black Holes: Outflow of Mass from the Dusty "Torus"
Observational and theoretical arguments suggest that the momentum carried in
mass outflows from AGN can reach several times L / c, corresponding to outflow
rates of hundreds of solar masses per year. Radiation pressure on lines alone
may not be sufficient to provide this momentum deposition, and the transfer of
reprocessed IR radiation in dusty nuclear gas has been postulated to provide
the extra enhancement. The efficacy of this mechanism, however, will be
sensitive to multi-dimensional effects such as the tendency for the reprocessed
radiation to preferentially escape along sight-lines of lower column density.
We use Monte Carlo radiative transfer calculations to determine the radiation
force on dusty gas residing within approximately 10 parsecs from an accreting
super-massive black hole. We calculate the net rate of momentum deposition in
the surrounding gas and estimate the mass-loss rate in the resulting outflow as
a function of solid angle for different black hole luminosities,
sightline-averaged column densities, clumping parameters, and opening angles of
the dusty gas. We find that these dust-driven winds carry momentum fluxes of
1-5 times L / c and correspond to mass-loss rates of 10-100 solar masses per
year for a 10^8 solar mass black hole radiating at or near its Eddington limit.
These results help to explain the origin of high velocity molecular and atomic
outflows in local ULIRGs, and can inform numerical simulations of galaxy
evolution including AGN feedback.Comment: 15 pages, 14 figures. Submitted to ApJ; v2 Corrected spelling and
other small typos; v3 Included additional details and references to match
accepted versio
Rapid N_H changes in NGC 4151
We have analyzed the two longest (elapsed time > 3 days) BeppoSAX
observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for
spectral variability on time-scales from a few tens of ksec to years. We found
in both cases highly significant spectral variability below ~ 6 keV down to the
shortest time-scales investigated. These variations can be naturally explained
in terms of variations in the low energy cut-off due to obscuring matter along
the line of sight. If the cut-off is modeled by two neutral absorption
components, one fully covering the source and the second covering only a
fraction of the source, the shortest time-scale of variability of a few days
constrains the location of the obscuring matter to within 3.4 X 10^4
Schwarzschild radii from the central X-ray source. This is consistent with the
distance of the Broad Emission Line Region, as inferred from reverberation
mapping, and difficult to reconcile with the parsec scale dusty molecular torus
of Krolik & Begelman (1988). We have also explored a more complex absorption
structure, namely the presence of an ionized absorber. Although the behaviour
of the ionization parameter is nicely consistent with the expectations, the
results are not completely satisfactory from the statistical point of view.
The overall absorption during the 2001 December observation is lower than in
all other historical observations with similar 2-10 keV flux. This suggests
that absorption variability plays a crucial role in the observed flux
variability of this source.Comment: Added references, corrected typos. 21 pages, 9 figures; accepted for
publication in Monthly Notices of the Royal Astronomical Society Main Journa
Gravitational bending of light by planetary multipoles and its measurement with microarcsecond astronomical interferometers
General relativistic deflection of light by mass, dipole, and quadrupole
moments of gravitational field of a moving massive planet in the Solar system
is derived. All terms of order 1 microarcsecond are taken into account,
parametrized, and classified in accordance with their physical origin. We
calculate the instantaneous patterns of the light-ray deflections caused by the
monopole, the dipole and the quadrupole moments, and derive equations
describing apparent motion of the deflected position of the star in the sky
plane as the impact parameter of the light ray with respect to the planet
changes due to its orbital motion. The present paper gives the physical
interpretation of the observed light-ray deflections and discusses the
observational capabilities of the near-future optical (SIM) and radio (SKA)
interferometers for detecting the Doppler modulation of the radial deflection,
and the dipolar and quadrupolar light-ray bendings by the Jupiter and the
Saturn.Comment: 33 pages, 10 figures, accepted to Phys. Rev.
The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme
Our goal is to characterize AGN populations by comparing their X-ray and
optical classifications. We present a sample of 99 spectroscopically identified
X-ray point sources in the XMM-LSS survey which are significantly detected in
the [2-10] keV band, and with more than 80 counts. We performed an X-ray
spectral analysis for all of these 99 X-ray sources. Introducing the fourfold
point correlation coefficient, we find only a mild correlation between the
X-ray and the optical classifications, as up to 30% of the sources have
differing X-ray and optical classifications: on one hand, 10% of the type 1
sources present broad emission lines in their optical spectra and strong
absorption in the X-rays. These objects are highly luminous AGN lying at high
redshift and thus dilution effects are totally ruled out, their discrepant
nature being an intrinsic property. Their X-ray luminosities and redshifts
distributions are consistent with those of the unabsorbed X-ray sources with
broad emission lines. On the other hand, 25/32 are moderate luminosity AGN,
which are both unabsorbed in the X-rays and only present narrow emission lines
in their optical spectra. The majority of them have an optical spectrum which
is representative of the host galaxy. We finally infer that dilution of the AGN
by the host galaxy seems to account for their nature. 5/25 have been defined as
Seyfert 2. In conclusion, most of these 32 discrepant cases can be accounted
for by the standard AGN unified scheme, as its predictions are not met for only
12% of the 99 X-ray sources. ABRIDGEDComment: 25 pages, 19 figures, Accepted for publication in A&
The Impact of HAART on the Respiratory Complications of HIV Infection: Longitudinal Trends in the MACS and WIHS Cohorts
Objective: To review the incidence of respiratory conditions and their effect on mortality in HIV-infected and uninfected individuals prior to and during the era of highly active antiretroviral therapy (HAART). Design: Two large observational cohorts of HIV-infected and HIV-uninfected men (Multicenter AIDS Cohort Study [MACS]) and women (Women's Interagency HIV Study [WIHS]), followed since 1984 and 1994, respectively. Methods: Adjusted odds or hazards ratios for incident respiratory infections or non-infectious respiratory diagnoses, respectively, in HIV-infected compared to HIV-uninfected individuals in both the pre-HAART (MACS only) and HAART eras; and adjusted Cox proportional hazard ratios for mortality in HIV-infected persons with lung disease during the HAART era. Results: Compared to HIV-uninfected participants, HIV-infected individuals had more incident respiratory infections both pre-HAART (MACS, odds ratio [adjusted-OR], 2.4; 95% confidence interval [CI], 2.2-2.7; p<0.001) and after HAART availability (MACS, adjusted-OR, 1.5; 95%CI 1.3-1.7; p<0.001; WIHS adjusted-OR, 2.2; 95%CI 1.8-2.7; p<0.001). Chronic obstructive pulmonary disease was more common in MACS HIV-infected vs. HIV-uninfected participants pre-HAART (hazard ratio [adjusted-HR] 2.9; 95%CI, 1.02-8.4; p = 0.046). After HAART availability, non-infectious lung diseases were not significantly more common in HIV-infected participants in either MACS or WIHS participants. HIV-infected participants in the HAART era with respiratory infections had an increased risk of death compared to those without infections (MACS adjusted-HR, 1.5; 95%CI, 1.3-1.7; p<0.001; WIHS adjusted-HR, 1.9; 95%CI, 1.5-2.4; p<0.001). Conclusion: HIV infection remained a significant risk for infectious respiratory diseases after the introduction of HAART, and infectious respiratory diseases were associated with an increased risk of mortality. © 2013 Gingo et al
A spectroscopic and proper motion search of Sloan Digital Sky Survey : red subdwarfs in binary systems
Red subdwarfs in binary systems are crucial for both model calibration and spectral classification. We search for red subdwarfs in binary systems from a sample of high proper motion objects with Sloan Digital Sky Survey spectroscopy. We present here discoveries from this search, as well as highlight several additional objects of interest. We find 30 red subdwarfs in wide binary systems including: two with spectral type of esdM5.5, 6 companions to white dwarfs and 3 carbon-enhanced red subdwarfs with normal red subdwarf companions. 15 red subdwarfs in our sample are partially resolved close binary systems. With this binary sample, we estimate the low limit of the red subdwarf binary fraction of similar to 10 per cent. We find that the binary fraction goes down with decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7 subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs have spectral type of >= M6 in our sample. We also derive relationships between spectral types and absolute magnitudes in the optical and near-infrared for M and L subdwarfs, and we present an M subdwarf sample with measured U, V, W space velocities.Peer reviewe
A large spectroscopic sample of L and T dwarfs from UKIDSS LAS : peculiar objects, binaries, and space density
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [F. Marocco, et al, 'A large spectroscopic sample of L and T dwarfs from UKIDSS LAS: peculiar objects, binaries, and space density', MNRAS, Vol. 449)4): 3651-3692, April 2015] is available online at: https://doi.org/10.1093/mnras/stv530.We present the spectroscopic analysis of a large sample of late-M, L, and T dwarfs from UKIDSS. Using the YJHK photometry from ULAS and the red-optical photometry from SDSS we selected a sample of 262 brown dwarf candidates and we followed-up 196 of them using X-shooter on the VLT. The large wavelength coverage (0.30-2.48 m) and moderate resolution (R~5000-9000) of X-shooter allowed us to identify peculiar objects including 22 blue L dwarfs, 2 blue T dwarfs, and 2 low gravity M dwarfs. Using a spectral indices-based technique we identified 27 unresolved binary candidates, for which we determined the spectral type of the potential components via spectral deconvolution. The spectra allowed us to measure the equivalent width of the prominent absorption features and to compare them to atmospheric models. Cross-correlating the spectra with a radial velocity standard, we measured the radial velocity for our targets, and we determined the distribution of the sample, which is centred at -1.71.2 km s with a dispersion of 31.5 km s. Using our results we estimated the space density of field brown dwarfs and compared it with the results of numerical simulations. Depending on the binary fraction, we found that there are (0.850.55) x 10 to (1.000.64) x 10 objects per cubic parsec in the L4-L6.5 range, (0.730.47) x 10 to (0.850.55) x 10 objects per cubic parsec in the L7-T0.5 range, and (0.740.48) x 10 to (0.880.56) x 10 objects per cubic parsec in the T1-T4.5 range. There seem to be an excess of objects in the L/T transition with respect to the late T dwarfs, a discrepancy that could be explained assuming a higher binary fraction than expected for the L/T transition, or that objects in the high-mass end and low-mass end of this regime form in different environments, i.e. following different IMFs.Peer reviewe
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