122 research outputs found
A systematic search for massive black hole binaries in SDSS spectroscopic sample
We present the results of a systematic search for massive black hole binaries
in the Sloan Digital Sky Survey spectroscopic database. We focus on bound
binaries, under the assumption that one of the black holes is active. In this
framework, the broad lines associated to the accreting black hole are expected
to show systematic velocity shifts with respect to the narrow lines, which
trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929
galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture
of two quasars at two different redshifts. The spectral model is a data-driven
dimensionality reduction of the SDSS quasar spectra based on a matrix
factorization. We identified 32 objects with peculiar spectra. Nine of them can
be interpreted as black hole binaries. This doubles the number of known black
hole binary candidates. We also report on the discovery of a new class of
extreme double-peaked emitters with exceptionally broad and faint Balmer lines.
For all the interesting sources, we present detailed analysis of the spectra,
and discuss possible interpretations.Comment: 10 pages, 2 figures, accepted for publication in Ap
Photometric identification of blue horizontal branch stars
We investigate the performance of some common machine learning techniques in
identifying BHB stars from photometric data. To train the machine learning
algorithms, we use previously published spectroscopic identifications of BHB
stars from SDSS data. We investigate the performance of three different
techniques, namely k nearest neighbour classification, kernel density
estimation and a support vector machine (SVM). We discuss the performance of
the methods in terms of both completeness and contamination. We discuss the
prospect of trading off these values, achieving lower contamination at the
expense of lower completeness, by adjusting probability thresholds for the
classification. We also discuss the role of prior probabilities in the
classification performance, and we assess via simulations the reliability of
the dataset used for training. Overall it seems that no-prior gives the best
completeness, but adopting a prior lowers the contamination. We find that the
SVM generally delivers the lowest contamination for a given level of
completeness, and so is our method of choice. Finally, we classify a large
sample of SDSS DR7 photometry using the SVM trained on the spectroscopic
sample. We identify 27,074 probable BHB stars out of a sample of 294,652 stars.
We derive photometric parallaxes and demonstrate that our results are
reasonable by comparing to known distances for a selection of globular
clusters. We attach our classifications, including probabilities, as an
electronic table, so that they can be used either directly as a BHB star
catalogue, or as priors to a spectroscopic or other classification method. We
also provide our final models so that they can be directly applied to new data.Comment: To appear in A&A. 19 pages, 22 figures. Tables 7, A3 and A4 available
electronically onlin
Finding rare objects and building pure samples: Probabilistic quasar classification from low resolution Gaia spectra
We develop and demonstrate a probabilistic method for classifying rare
objects in surveys with the particular goal of building very pure samples. It
works by modifying the output probabilities from a classifier so as to
accommodate our expectation (priors) concerning the relative frequencies of
different classes of objects. We demonstrate our method using the Discrete
Source Classifier, a supervised classifier currently based on Support Vector
Machines, which we are developing in preparation for the Gaia data analysis.
DSC classifies objects using their very low resolution optical spectra. We look
in detail at the problem of quasar classification, because identification of a
pure quasar sample is necessary to define the Gaia astrometric reference frame.
By varying a posterior probability threshold in DSC we can trade off sample
completeness and contamination. We show, using our simulated data, that it is
possible to achieve a pure sample of quasars (upper limit on contamination of 1
in 40,000) with a completeness of 65% at magnitudes of G=18.5, and 50% at
G=20.0, even when quasars have a frequency of only 1 in every 2000 objects. The
star sample completeness is simultaneously 99% with a contamination of 0.7%.
Including parallax and proper motion in the classifier barely changes the
results. We further show that not accounting for class priors in the target
population leads to serious misclassifications and poor predictions for sample
completeness and contamination. (Truncated)Comment: MNRAS accepte
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
Structure of the SMC - Stellar component distribution from 2MASS data
The spatial distribution of the SMC stellar component is investigated from
2MASS data. The morphology of the different age populations is presented. The
center of the distribution is calculated and compared with previous
estimations. The rotation of the stellar content and possible consequence of
dark matter presence are discussed. The different stellar populations are
identified through a CMD diagram of the 2MASS data. Isopleth contour maps are
produced in every case, to reveal the spatial distribution. The derived density
profiles are discussed. The older stellar population follows an exponential
profile at projected diameters of about 5 kpc (~5 deg) for the major axis and
~4 kpc for the minor axis, centred at RA: 0h:51min, Dec: -73deg 7' (J2000.0).
The centre coordinates are found the same for all the different age population
maps and are in good accordance with the kinematical centre of the SMC. However
they are found considerably different from the coordinates of the centre of the
gas distribution. The fact that the older population found on an exponential
disk, gives evidence that the stellar content is rotating, with a possible
consequence of dark matter presence. The strong interactions between the MCs
and the MilkyWay might explain the difference in the distributions of the
stellar and gas components. The lack in the observed velocity element, that
implies absence of rotation, and contradicts with the consequences of
exponential profile of the stellar component, may also be a result of the
gravitational interactions.Comment: 7 Pages, 6 figures, accepted for publication in A&
The nature of massive black hole binary candidates - II. Spectral energy distribution atlas
Recoiling supermassive black holes (SMBHs) are considered one plausible physical mechanism to explain high velocity shifts between narrow and broad emission lines sometimes observed in quasar spectra. If the sphere of influence of the recoiling SMBH is such that only the accretion disc is bound, the dusty torus would be left behind, hence the SED should then present distinctive features (i.e. a mid-infrared deficit). Here, we present results from fitting the spectral energy distributions (SEDs) of 32 type-1 AGN with high velocity shifts between broad and narrow lines. The aim is to find peculiar properties in the multiwavelength SEDs of such objects by comparing their physical parameters (torus and disc luminosity, intrinsic reddening, and size of the 12 μm emitter) with those estimated from a control sample of ∼1000 typical quasars selected from the Sloan Digital Sky Survey in the same redshift range. We find that all sources, with the possible exception of J1154+0134, analysed here present a significant amount of 12 μm emission. This is in contrast with a scenario of an SMBH displaced from the centre of the galaxy, as expected for an undergoing recoil event
Structure and evolution of circumbinary disks around supermassive black hole (SMBH) binaries
It is generally believed that gaseous disks around supermassive black hole
(SMBH) binaries in centers of galaxies can facilitate binary merger and give
rise to observational signatures both in electromagnetic and gravitational wave
domains. We explore general properties of circumbinary disks by reformulating
standard equations for the viscous disk evolution in terms of the viscous
angular momentum flux F_J. In steady state F_J is a linear function of the
specific angular momentum, which is a generalization of (but is not equivalent
to) the standard constant \dot M disk solution. If the torque produced by the
central binary is effective at stopping gas inflow and opening a gap (or
cavity) in the disk, then the inner part of the circumbinary disk can be
approximated as a constant F_J disk. We compute properties of such disks in
different physical regimes relevant for SMBH binaries and use these results to
understand the gas-assisted evolution of SMBH pairs starting at separations
10^{-4}-10^{-2} pc. We find the following. (1) Pile-up of matter at the inner
edge of the disk leads to continuous growth of the torque acting on the binary
and can considerably accelerate its orbital evolution compared to the
gravitational wave-driven decay. (2) Torque on the binary is determined
non-locally and does not in general reflect the disk properties in the vicinity
of the binary. (3) Binary evolution depends on the past history of the disk
evolution. (4) Eddington limit can be important in circumbinary disks even if
they accrete at sub-Eddington rates at late stages of binary evolution. (5)
Circumbinary disk self-consistently evolved under the action of the binary
torque emits more power and has spectrum different from the spectrum of
constant \dot M disk - it is steeper (\nu F_\nu\propto \nu^{12/7}) and extends
to shorter wavelength, facilitating its detection.Comment: Discussion of overflow across the orbit of the secondary added in
section 5.2.6. 23 pages, 10 figures, submitted to Ap
Luminous AGB stars in nearby galaxies. A study using Virtual Observatory tools
Aims. This study focuses on very luminous Mbol<-6.0 mag AGB stars with
J-Ks>1.5 mag and H-Ks>0.4 mag in the LMC, SMC, M31, and M33 from 2MASS data.
Methods.The data were taken from the 2MASS All-Sky Point Source catalogue
archive. We used Virtual Observatory tools and took advantage of its
capabilities at various stages in the analysis.
Results. It is well known that stars with the colors we selected correspond
mainly to carbon stars. Although the most luminous AGBs detected here contain a
large number of carbon stars,they are not included in existing catalogues
produced from data in the optical domain, where they are not visible since they
are dust-enshrouded. A comparison of the AGB stars detected with combined near
and mid-infrared data from MSX and 2MASS in the LMC shows that 10% of the
bright AGB stars are bright carbon stars never detected before and that the
other 50% are OH/IR oxygen rich stars, whereas the 40% that remain were not
cross-matched.
Conclusions. The catalogues of the most luminous AGB stars compiled here are
an important complement to existing data. In the LMC, these bright AGB stars
are centrally located, whereas they are concentrated in an active
star-formation ring in M31. In the SMC and M33, there are not enough of them to
draw definite conclusions, although they tend to be centrally located. Their
luminosity functions are similar for the four galaxies we studied.Comment: 16 pages, 12 figures, 4 tables (Appendix A), accepted in A&
The expected performance of stellar parametrization with Gaia spectrophotometry
Gaia will obtain astrometry and spectrophotometry for essentially all sources
in the sky down to a broad band magnitude limit of G=20, an expected yield of
10^9 stars. Its main scientific objective is to reveal the formation and
evolution of our Galaxy through chemo-dynamical analysis. In addition to
inferring positions, parallaxes and proper motions from the astrometry, we must
also infer the astrophysical parameters of the stars from the
spectrophotometry, the BP/RP spectrum. Here we investigate the performance of
three different algorithms (SVM, ILIUM, Aeneas) for estimating the effective
temperature, line-of-sight interstellar extinction, metallicity and surface
gravity of A-M stars over a wide range of these parameters and over the full
magnitude range Gaia will observe (G=6-20mag). One of the algorithms, Aeneas,
infers the posterior probability density function over all parameters, and can
optionally take into account the parallax and the Hertzsprung-Russell diagram
to improve the estimates. For all algorithms the accuracy of estimation depends
on G and on the value of the parameters themselves, so a broad summary of
performance is only approximate. For stars at G=15 with less than two
magnitudes extinction, we expect to be able to estimate Teff to within 1%, logg
to 0.1-0.2dex, and [Fe/H] (for FGKM stars) to 0.1-0.2dex, just using the BP/RP
spectrum (mean absolute error statistics are quoted). Performance degrades at
larger extinctions, but not always by a large amount. Extinction can be
estimated to an accuracy of 0.05-0.2mag for stars across the full parameter
range with a priori unknown extinction between 0 and 10mag. Performance
degrades at fainter magnitudes, but even at G=19 we can estimate logg to better
than 0.2dex for all spectral types, and [Fe/H] to within 0.35dex for FGKM
stars, for extinctions below 1mag.Comment: MNRAS, in press. Minor corrections made in v
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