431 research outputs found
Bondi-Hoyle-Lyttleton accretion by binary stars
Binary stars often move through an ambient medium from which they accrete
material and angular momentum, as in triple-star systems, star-forming clouds,
young globular clusters and in the centres of galaxies. A binary form of
Bondi-Hoyle-Lyttleton accretion results whereby the accretion rate depends on
the binary properties: the stellar masses and separation, and the relative wind
speed. We present the results of simulations performed with the hydrodynamic
code GANDALF, to determine the mass accretion rates over a range of binary
separations, inclinations and mass ratios. When the binary separation is short,
the binary system accretes like a single star, while accretion onto stars in
wide binaries is barely affected by their companion. We investigate
intermediate-separation systems in some detail, finding that as the binary
separation is increased, accretion rates smoothly decrease from the rate equal
to that of a single star to the rate expected from two isolated stars. The form
of this decrease depends on the relative centre-of-mass velocity of the binary
and the gas, with faster-moving binaries showing a shallower decrease.
Accretion rates vary little with orbital inclination, except when the orbit is
side-on and the stars pass through each others' wakes. The specific angular
momentum accretion rate also depends on the inclination but is never sufficient
to prevent the binary orbit from contracting. Our results may be applied to
accretion onto protostars, pollution of stars in globular and nuclear clusters,
and wind mass-transfer in multiple stellar systems.Comment: 17 pages, 10 figures, accepted by MNRA
Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line Active Galactic Nuclei. I. Markers for Selection of Compelling Dual Active Galactic Nucleus Candidates
Merger-remnant galaxies with kpc-scale separation dual active galactic nuclei
(AGNs) should be widespread as a consequence of galaxy mergers and triggered
gas accretion onto supermassive black holes, yet very few dual AGNs have been
observed. Galaxies with double-peaked narrow AGN emission lines in the Sloan
Digital Sky Survey are plausible dual AGN candidates, but their double-peaked
profiles could also be the result of gas kinematics or AGN-driven outflows and
jets on small or large scales. To help distinguish between these scenarios, we
have obtained spatial profiles of the AGN emission via follow-up long-slit
spectroscopy of 81 double-peaked narrow-line AGNs in SDSS at 0.03 < z < 0.36
using Lick, Palomar, and MMT Observatories. We find that all 81 systems exhibit
double AGN emission components with ~kpc projected spatial separations on the
sky, which suggests that they are produced by kpc-scale dual AGNs or kpc-scale
outflows, jets, or rotating gaseous disks. In addition, we find that the
subsample (58%) of the objects with spatially compact emission components may
be preferentially produced by dual AGNs, while the subsample (42%) with
spatially extended emission components may be preferentially produced by AGN
outflows. We also find that for 32% of the sample the two AGN emission
components are preferentially aligned with the host galaxy major axis, as
expected for dual AGNs orbiting in the host galaxy potential. Our results both
narrow the list of possible physical mechanisms producing the double AGN
components, and suggest several observational criteria for selecting the most
promising dual AGN candidates from the full sample of double-peaked narrow-line
AGNs. Using these criteria, we determine the 17 most compelling dual AGN
candidates in our sample.Comment: 12 pages, 8 figures, published in ApJ. Modified from original version
to reflect referee's comment
Investigating the evolution of the dual AGN system ESO~509-IG066
We analyze the evolution of the dual AGN in ESO 509-IG066, a galaxy pair
located at z=0.034 whose nuclei are separated by 11 kpc. Previous observations
with XMM-Newton on this dual AGN found evidence for two moderately obscured
( cm) X-ray luminous ( erg/s) nuclear
sources. We present an analysis of subsequent Chandra, NuSTAR and Swift/XRT
observations that show one source has dropped in flux by a factor of 10 between
2004 and 2011, which could be explained by either an increase in the absorbing
column or an intrinsic fading of the central engine possibly due to a decrease
in mass accretion. Both of these scenarios are predicted by galaxy merger
simulations. The source which has dropped in flux is not detected by NuSTAR,
which argues against absorption, unless it is extreme. However, new Keck/LRIS
optical spectroscopy reveals a previously unreported broad H-alpha line which
is highly unlikely to be visible under the extreme absorption scenario. We
therefore conclude that the black hole in this nucleus has undergone a dramatic
drop in accretion rate. From AO-assisted near-infrared integral-field
spectroscopy of the other nucleus, we find evidence that the galaxy merger is
having a direct effect on the kinematics of the gas close to the nucleus of the
galaxy, providing a direct observational link between the galaxy merger and the
mass accretion rate on to the black hole.Comment: Accepted for publication in Ap
Search of sub-parsec massive binary black holes through line diagnosis II
Massive black hole binaries at sub-parsec separations may display in their
spectra anomalously small flux ratios between the MgII and CIV broad emission
lines, i.e. F_MgII/F_CIV <~ 0.1, due to the erosion of the broad line region
around the active, secondary black hole, by the tidal field of the primary. In
Paper I by Montuori et al. (2011), we focussed on broad lines emitted by gas
bound to the lighter accreting member of a binary when the binary is at the
center of a hollow density region (the gap) inside a circum-binary disc. The
main aim of this new study is at exploring the potential contribution to the
broad line emission by the circum-binary disc and by gaseous streams flowing
toward the black hole through the gap. We carry out a post-process analysis of
data extracted from a SPH simulation of a circum-binary disc around a black
hole binary. Our main result is that the MgII to CIV flux ratio can be reduced
to ~ 0.1 within an interval of sub-pc binary separations of the order of a ~
(0.01-0.2)(f_Edd/0.1)^(1/2) pc corresponding to orbital periods of ~ (20-200)
(f_Edd/0.1)^(3/4) years for a secondary BH mass in the range M_2 ~ 10^7-10^9
M_sun and a binary mass ratio of 0.3. At even closer separations this ratio
returns to increase to values that are indistinguishable from the case of a
single AGN (typically F_MgII/F_CIV ~ 0.3-0.4) because of the contribution to
the MgII line from gas in the circum-binary disc.Comment: 7 pages, 3 figure, accepted for publication in MNRA
HST Observations of the Double-Peaked Emission Lines in the Seyfert Galaxy Markarian 78: Mass Outflows from a Single AGN
Previous ground based observations of the Seyfert 2 galaxy Mrk 78 revealed a
double set of emission lines, similar to those seen in several AGN from recent
surveys. Are the double lines due to two AGN with different radial velocities
in the same galaxy, or are they due to mass outflows from a single AGN?We
present a study of the outflowing ionized gas in the resolved narrow-line
region (NLR) of Mrk 78 using observations from Space Telescope Imaging
Spectrograph (STIS) and Faint Object Camera (FOC) aboard the Hubble Space
Telescope(HST) as part of an ongoing project to determine the kinematics and
geometries of active galactic nuclei (AGN) outflows. From the spectroscopic
information, we deter- mined the fundamental geometry of the outflow via our
kinematics modeling program by recreating radial velocities to fit those seen
in four different STIS slit positions. We determined that the double emission
lines seen in ground-based spectra are due to an asymmetric distribution of
outflowing gas in the NLR. By successfully fitting a model for a single AGN to
Mrk 78, we show that it is possible to explain double emission lines with
radial velocity offsets seen in AGN similar to Mrk 78 without requiring dual
supermassive black holes.Comment: 22 pages, 7 figures (2 color), accepted for publication in The
Astrophysical Journa
PHIBSS: molecular gas content and scaling relations in z~1-3 normal star forming galaxies
We present PHIBSS, the IRAM Plateau de Bure high-z blue sequence CO 3-2
survey of the molecular gas properties in normal star forming galaxies (SFGs)
near the cosmic star formation peak. PHIBSS provides 52 CO detections in two
redshift slices at z~1.2 and 2.2, with log(M*(M_solar))>10.4 and
log(SFR(M_solar/yr))>1.5. Including a correction for the incomplete coverage of
the M*-SFR plane, we infer average gas fractions of ~0.33 at z~1.2 and ~0.47 at
z~2.2. Gas fractions drop with stellar mass, in agreement with cosmological
simulations including strong star formation feedback. Most of the z~1-3 SFGs
are rotationally supported turbulent disks. The sizes of CO and UV/optical
emission are comparable. The molecular gas - star formation relation for the
z=1-3 SFGs is near-linear, with a ~0.7 Gyrs gas depletion timescale; changes in
depletion time are only a secondary effect. Since this timescale is much less
than the Hubble time in all SFGs between z~0 and 2, fresh gas must be supplied
with a fairly high duty cycle over several billion years. At given z and M*,
gas fractions correlate strongly with the specific star formation rate. The
variation of specific star formation rate between z~0 and 3 is mainly
controlled by the fraction of baryonic mass that resides in cold gas.Comment: Submitted to the Astrophysical Journal; 14 figure
Radial Velocity Offsets Due to Mass Outflows and Extinction in Active Galactic Nuclei
We present a study of the radial velocity offsets between narrow emission
lines and host galaxy lines (stellar absorption and H I 21-cm emission) in
Seyfert galaxies with observed redshifts less than 0.043. We find that 35% of
the Seyferts in the sample show [O III] emission lines with blueshifts with
respect to their host galaxies exceeding 50 km/s, whereas only 6% show
redshifts this large, in qualitative agreement with most previous studies. We
also find that a greater percentage of Seyfert 1 galaxies show blueshifts than
Seyfert 2 galaxies. Using HST/STIS spatially-resolved spectra of the Seyfert 2
galaxy NGC 1068 and the Seyfert 1 galaxy NGC 4151, we generate geometric models
of their narrow-line regions (NLRs) and inner galactic disks, and show how
these models can explain the blueshifted [O III] emission lines in collapsed
STIS spectra of these two Seyferts. We conclude that the combination of mass
outflow of ionized gas in the NLR and extinction by dust in the inner disk
(primarily in the form of dust spirals) is primarily responsible for the
velocity offsets in Seyfert galaxies. More exotic explanations are not needed.
We discuss the implications of this result for the velocity offsets found in
higher redshift AGN.Comment: 25 pages, 7 figures, accepted for publication in the Astrophysical
Journa
A comparison of the strong lensing properties of the Sersic and the NFW profiles
We investigate the strong lensing properties of the Sersic profile as an
alternative to the NFW profile, focusing on applications to lens modelling of
clusters. Given an underlying Sersic dark matter profile, we study whether an
NFW profile can provide an acceptable fit to strong lensing constraints in the
form of single or multiple measured Einstein radii. We conclude that although
an NFW profile that fits the lensing constraints can be found in many cases,
the derived parameters may be biased. In particular, we find that for n~2,
which corresponds to massive clusters, the mass at r_200 of the best fit NFW is
overestimated (by a factor of ~2) and the concentration is very low (c~2). The
differences are important enough to warrant the inclusion of Sersic profile for
future analysis of strong lensing clusters.Comment: 19 pages (single column format), 11 figures. Accepted for publication
by JCA
Arc sensitivity to cluster ellipticity, asymmetries and substructures
We investigate how ellipticity, asymmetries and substructures separately
affect the ability of galaxy clusters to produce strong lensing events, i.e.
gravitational arcs, and how they influence the arc morphologies and fluxes.
This is important for those studies aiming, for example, at constraining
cosmological parameters from statistical lensing, or at determining the inner
structure of galaxy clusters through gravitational arcs. We do so by creating
two-dimensional gradually smoothed, differently elliptical and asymmetric
versions of some numerical models. On average, we find that the contributions
of ellipticity, asymmetries and substructures amount to ~40%, ~10% and ~30% of
the total strong lensing cross section, respectively. However, our analysis
shows that substructures play a more important role in less elliptical and
asymmetric clusters, even if located at large distances from the cluster
centers (~1Mpc/h). Conversely, their effect is less important in highly
asymmetric lenses. The morphology, position and flux of individual arcs are
strongly affected by the presence of substructures in the clusters. Removing
substructures on spatial scales <~50kpc/h, roughly corresponding to mass scales
<~5 10^{10}M_\odot/h, alters the image multiplicity of ~35% of the sources used
in the simulations and causes position shifts larger than 5'' for ~40% of the
arcs longer than 5''. We conclude that any model for cluster lens cannot
neglect the effects of ellipticity, asymmetries and substructures. On the other
hand, the high sensitivity of gravitational arcs to deviations from regular,
smooth and symmetric mass distributions suggests that strong gravitational
lensing is potentially a powerfull tool to measure the level of substructures
and asymmetries in clusters.Comment: 16 pages, 18 figures. Accepted version. Version with full resolution
images can be found at
http://www.ita.uni-heidelberg.de/~massimo/sub/publications.htm
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