845 research outputs found
The dynamical structure of isotropic spherical galaxies with a central black hole
We discuss the kinematical structure of a two-parameter family of isotropic
models with a central black hole. The family contains the slope of the central
density cusp and the relative black hole mass as parameters. Most of the basic
kinematical quantities of these models can be expressed analytically. This
family contains three distinct models where also the distribution function,
differential energy distribution and spatial LOSVDs can be expressed completely
analytically. Each of these models show a drastically different behaviour of
the distribution function. Although the effect of a black hole on the
distribution function is very strong, in particular for models with a shallow
density cusp, the differential energy distribution is only marginally affected.
We discuss in detail the effects of a central black hole on the LOSVDs. The
projected velocity dispersion increases with black hole mass at small projected
radii, but the effect of a black hole on the shape of the LOSVDs (characterized
by the h4 parameter) is less straightforward to interpret. Too much reliance on
the wings of the LOSVDs and the value of the h4 parameter to determine black
hole masses might hence be dangerous.Comment: 12 pages, 3 figures, accepted for publication in A&
Radial stability of a family of anisotropic Hernquist models with and without a supermassive black hole
We present a method to investigate the radial stability of a spherical
anisotropic system that hosts a central supermassive black hole (SBH). Such
systems have never been tested before for stability, although high anisotropies
have been considered in the dynamical models that were used to estimate the
masses of the central putative supermassive black holes. A family of analytical
anisotropic spherical Hernquist models with and without a black hole were
investigated by means of N-body simulations. A clear trend emerges that the
supermassive black hole has a significant effect on the overall stability of
the system, i.e. an SBH with a mass of a few percent of the total mass of the
galaxy can prevent or reduce the bar instabilities in anisotropic systems. Its
mass not only determines the strength of the instability reduction, but also
the time in which this occurs. These effects are most significant for models
with strong radial anisotropies. Furthermore, our analysis shows that unstable
systems with similar SBH but with different anisotropy radii evolve
differently: highly radial systems become oblate, while more isotropic models
tend to form into prolate structures. In addition to this study, we also
present a Monte-Carlo algorithm to generate particles in spherical anisotropic
systems.Comment: 16 pages, 12 figures, accepted for publication in MNRAS (some figures
have a lowered resolution
A low-frequency study of recently identified double-double radio galaxies
In order to understand the possible mechanisms of recurrent jet activity in
radio galaxies and quasars, which are still unclear, we have identified such
sources with a large range of linear sizes (220 917 kpc), and hence time
scales of episodic activity. Here we present high-sensitivity 607-MHz Giant
Metrewave Radio Telescope (GMRT) images of 21 possible double-double radio
galaxies (DDRGs) identified from the FIRST survey to confirm their episodic
nature. These GMRT observations show that none of the inner compact components
suspected to be hot-spots of the inner doubles are cores having a flat radio
spectrum, confirming the episodic nature of these radio sources. We have
indentified a new DDRG with a candidate quasar, and have estimated the upper
spectral age limits for eight sources which showed marginal evidence of
steepening at higher frequencies. The estimated age limits (11 52 Myr) are
smaller than those of the large-sized ( 1 Mpc) DDRGs.Comment: Accepted for publication in MNRAS. 14 pages, 7 figure
Discovery of a red quasar with recurrent activity
We report a new double-double radio quasar, DDRQ, J07464526 which exhibits
two cycles of episodic activity. From radio continuum observations at 607 MHz
using the GMRT and 1400 MHz from the FIRST survey we confirm its episodic
nature. We examine the SDSS optical spectrum and estimate the black hole mass
to be (8.20.3)10M from its observed MgII emission
line, and the Eddington ratio to be 0.03. The black hole mass is significantly
smaller than for the other reported DDRQ, J0935+0204, while the Eddington
ratios are comparable. The SDSS spectrum is significantly red continuum
dominated suggesting that it is highly obscured with
mag. This high obscuration further indicates the
existence of a large quantity of dust and gas along the line of sight, which
may have a key role in triggering the recurrent jet activity in such objects.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in Ap
FIR/submm spectroscopy with Herschel: first results from the VNGS and H-ATLAS surveys
The FIR/submm window is one of the least-studied regions of the
electromagnetic spectrum, yet this wavelength range is absolutely crucial for
understanding the physical processes and properties of the ISM in galaxies. The
advent of the Herschel Space Observatory has opened up the entire FIR/submm
window for spectroscopic studies. We present the first FIR/submm spectroscopic
results on both nearby and distant galaxies obtained in the frame of two
Herschel key programs: the Very Nearby Galaxies Survey and the Herschel ATLAS
Photo-centric variability of quasars caused by variations in their inner structure: Consequences on Gaia measurements
We study the photocenter position variability due to variations in the quasar
inner structure. We consider variability in the accretion disk emissivity and
torus structure variability due to different illumination by the central
source. We discuss possible detection of these effects by Gaia. Observations of
the photocenter variability in two AGNs, SDSS J121855+020002 and SDSS
J162011+1724327 have been reported and discussed. With investigation of the
variations in the quasar inner structure we explore how much this effect can
affect the position determination and whether it can be (or not) detected with
Gaia mission. We used (a) a model of a relativistic disk, including the
perturbation that can increase brightness of a part of the disk, and
consequently offset the photocenter position, and (b) a model of a dusty torus
which absorbs and re-emits the incoming radiation from accretion disk. We
estimated the value of the photocenter offset due to these two effects. We
found that perturbations in the inner structure can significantly offset the
photocenter. It depends on the characteristics of perturbation and accretion
disk and structure of the torus. In the case of two considered QSOs the
observed photocenter offsets cannot be explained by variations in the accretion
disk and other effects should be considered. We discussed possibility of
exploding stars very close to the AGN source, and also possibility that there
are two variable sources in the center of these two AGNs that may indicate a
binary super-massive black hole system on a kpc (pc) scale. The Gaia mission
seems to be very perspective, not only for astrometry, but also for exploring
the inner structure of AGNs. We conclude that variations in the quasar inner
structure can affect the observed photocenter (up to several mas). There is a
chance to observe such effect in the case of bright and low-redshifted QSOs.Comment: 12 pages, 8 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysics. Language improved, typos correcte
The Herschel exploitation of local galaxy Andromeda (HELGA) V: Strengthening the case for substantial interstellar grain growth
In this paper we consider the implications of the distributions of dust and
metals in the disc of M31. We derive mean radial dust distributions using a
dust map created from Herschel images of M31 sampling the entire far-infrared
(FIR) peak. Modified blackbodies are fit to approximately 4000 pixels with a
varying, as well as a fixed, dust emissivity index (beta). An overall metal
distribution is also derived using data collected from the literature. We use a
simple analytical model of the evolution of the dust in a galaxy with dust
contributed by stellar sources and interstellar grain growth, and fit this
model to the radial dust-to-metals distribution across the galaxy. Our analysis
shows that the dust-to-gas gradient in M31 is steeper than the metallicity
gradient, suggesting interstellar dust growth is (or has been) important in
M31. We argue that M31 helps build a case for cosmic dust in galaxies being the
result of substantial interstellar grain growth, while the net dust production
from stars may be limited. We note, however, that the efficiency of dust
production in stars, e.g., in supernovae (SNe) ejecta and/or stellar
atmospheres, and grain destruction in the interstellar medium (ISM) may be
degenerate in our simple model. We can conclude that interstellar grain growth
by accretion is likely at least as important as stellar dust production
channels in building the cosmic dust component in M31.Comment: 12 pages, 7 figures. Published in MNRAS 444, 797. This version is
updated to match the published versio
Efficiency of different selection strategies against boar taint in pigs
The breeding scheme of a Swiss sire line was modeled to compare different target traits and information sources for selection against boar taint. The impact of selection against boar taint on production traits was assessed for different economic weights of boar taint compounds. Genetic gain and breeding costs were evaluated using ZPlan+, a software based on selection index theory, gene flow method and economic modeling. Scenario I reflected the currently practiced breeding strategy as a reference scenario without selection against boar taint. Scenario II incorporated selection against the chemical compounds of boar taint, androstenone (AND), skatole (SKA) and indole (IND) with economic weights of −2.74, −1.69 and −0.99 Euro per unit of the log transformed trait, respectively. As information sources, biopsy-based performance testing of live boars (BPT) was compared with genomic selection (GS) and a combination of both. Scenario III included selection against the subjectively assessed human nose score (HNS) of boar taint. Information sources were either station testing of full and half sibs of the selection candidate or GS against HNS of boar taint compounds. In scenario I, annual genetic gain of log-transformed AND (SKA; IND) was 0.06 (0.09; 0.02) Euro, which was because of favorable genetic correlations with lean meat percentage and meat surface. In scenario II, genetic gain increased to 0.28 (0.20; 0.09) Euro per year when conducting BPT. Compared with BPT, genetic gain was smaller with GS. A combination of BPT and GS only marginally increased annual genetic gain, whereas variable costs per selection candidate augmented from 230 Euro (BPT) to 330 Euro (GS) or 380 Euro (both). The potential of GS was found to be higher when selecting against HNS, which has a low heritability. Annual genetic gain from GS was higher than from station testing of 4 full sibs and 76 half sibs with one or two measurements. The most effective strategy to reduce HNS was selecting against chemical compounds by conducting BPT. Because of heritabilities higher than 0.45 for AND, SKA and IND and high genetic correlations to HNS, the (correlated) response in units of the trait could be increased by 62% compared with scenario III with GS and even by 79% compared with scenario III, with station testing of siblings with two measurements. Increasing the economic weights of boar taint compounds amplified negative effects on average daily gain, drip loss and intramuscular fat percentag
Modelling high resolution ALMA observations of strongly lensed highly star forming galaxies detected by <i>Herschel</i>
We have modelled ∼ 0.1 arcsec resolution ALMA imaging of six strong gravitationally lensed galaxies detected by the Herschel Space Observatory. Our modelling recovers mass properties of the lensing galaxies and, by determining magnification factors, intrinsic properties of the lensed sub-millimetre sources. We find that the lensed galaxies all have high ratios of star formation rate to dust mass, consistent with or higher than the mean ratio for high redshift sub-millimetre galaxies and low redshift ultra-luminous infra-red galaxies. Source reconstruction reveals that most galaxies exhibit disturbed morphologies. Both the cleaned image plane data and the directly observed interferometric visibilities have been modelled, enabling comparison of both approaches. In the majority of cases, the recovered lens models are consistent between methods, all six having mass density profiles that are close to isothermal. However, one system with poor signal to noise shows mildly significant differences
The physical characteristics of the gas in the disk of Centaurus A using the Herschel Space Observatory
We search for variations in the disk of Centaurus A of the emission from
atomic fine structure lines using Herschel PACS and SPIRE spectroscopy. In
particular we observe the [C II](158 m), [N II](122 and 205 m), [O
I](63 and 145 m) and [O III](88 m) lines, which all play an important
role in cooling the gas in photo-ionized and photodissociation regions. We
determine that the ([C II]+[O I])/ line ratio, a proxy for the
heating efficiency of the gas, shows no significant radial trend across the
observed region, in contrast to observations of other nearby galaxies. We
determine that 10 - 20% of the observed [C II] emission originates in ionized
gas. Comparison between our observations and a PDR model shows that the
strength of the far-ultraviolet radiation field, , varies between
and and the hydrogen nucleus density varies between
and cm, with no significant radial trend in
either property. In the context of the emission line properties of the
grand-design spiral galaxy M51 and the elliptical galaxy NGC 4125, the gas in
Cen A appears more characteristic of that in typical disk galaxies rather than
elliptical galaxies.Comment: Accepted for publication in the Astrophysical Journal. 22 pages, 10
figures, 5 table
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