9,410 research outputs found
Selection bias in the M_BH-sigma and M_BH-L correlations and its consequences
It is common to estimate black hole abundances by using a measured
correlation between black hole mass and another more easily measured observable
such as the velocity dispersion or luminosity of the surrounding bulge. The
correlation is used to transform the distribution of the observable into an
estimate of the distribution of black hole masses. However, different
observables provide different estimates: the Mbh-sigma relation predicts fewer
massive black holes than does the Mbh-L relation. This is because the sigma-L
relation in black hole samples currently available is inconsistent with that in
the SDSS sample, from which the distributions of L or sigma are based: the
black hole samples have smaller L for a given sigma or have larger sigma for a
given L. This is true whether L is estimated in the optical or in the NIR. If
this is a selection rather than physical effect, then the Mbh-sigma and Mbh-L
relations currently in the literature are also biased from their true values.
We provide a framework for describing the effect of this bias. We then combine
it with a model of the bias to make an estimate of the true intrinsic
relations. While we do not claim to have understood the source of the bias, our
simple model is able to reproduce the observed trends. If we have correctly
modeled the selection effect, then our analysis suggests that the bias in the
relation is likely to be small, whereas the relation is
biased towards predicting more massive black holes for a given luminosity. In
addition, it is likely that the Mbh-L relation is entirely a consequence of
more fundamental relations between Mbh and sigma, and between sigma and L. The
intrinsic relation we find suggests that at fixed luminosity, older galaxies
tend to host more massive black holes.Comment: 12 pages, 7 figures. Accepted by ApJ. We have added a figure showing
that a similar bias is also seen in the K-band. A new appendix describes the
BH samples as well as the fits used in the main tex
The Stellar Mass Fundamental Plane: The virial relation and a very thin plane for slow-rotators
Early-type galaxies -- slow and fast rotating ellipticals (E-SRs and E-FRs)
and S0s/lenticulars -- define a Fundamental Plane (FP) in the space of
half-light radius , enclosed surface brightness and velocity
dispersion . Since and are distance-independent
measurements, the thickness of the FP is often expressed in terms of the
accuracy with which and can be used to estimate sizes .
We show that: 1) The thickness of the FP depends strongly on morphology. If the
sample only includes E-SRs, then the observed scatter in is ,
of which only is intrinsic. Removing galaxies with
further reduces the observed scatter to ( intrinsic). The observed scatter increases to the usually
quoted in the literature if E-FRs and S0s are added. If the FP is defined using
the eigenvectors of the covariance matrix of the observables, then the E-SRs
again define an exceptionally thin FP, with intrinsic scatter of only
orthogonal to the plane. 2) The structure within the FP is most easily
understood as arising from the fact that and are nearly
independent, whereas the and correlations are nearly
equal and opposite. 3) If the coefficients of the FP differ from those
associated with the virial theorem the plane is said to be `tilted'. If we
multiply by the global stellar mass-to-light ratio and we account
for non-homology across the population by using S\'ersic photometry, then the
resulting stellar mass FP is less tilted. Accounting self-consistently for
gradients will change the tilt. The tilt we currently see suggests that
the efficiency of turning baryons into stars increases and/or the dark matter
fraction decreases as stellar surface brightness increases.Comment: 13 pages, 9 figures, 3 tables, accepted for publication in MNRA
A Search for the Most Massive Galaxies. II. Structure, Environment and Formation
We study a sample of 43 early-type galaxies, selected from the SDSS because
they appeared to have velocity dispersion > 350 km/s. High-resolution
photometry in the SDSS i passband using HRC-ACS on board the HST shows that
just less than half of the sample is made up of superpositions of two or three
galaxies, so the reported velocity dispersion is incorrect. The other half of
the sample is made up of single objects with genuinely large velocity
dispersions. None of these objects has sigma larger than 426 +- 30 km/s. These
objects define rather different relations than the bulk of the early-type
galaxy population: for their luminosities, they are the smallest, most massive
and densest galaxies in the Universe. Although the slopes of the scaling
relations they define are rather different from those of the bulk of the
population, they lie approximately parallel to those of the bulk "at fixed
sigma". These objects appear to be of two distinct types: the less luminous
(M_r>-23) objects are rather flattened and extremely dense for their
luminosities -- their properties suggest some amount of rotational support and
merger histories with abnormally large amounts of gaseous dissipation. The more
luminous objects (M_r<-23) tend to be round and to lie in or at the centers of
clusters. Their properties are consistent with the hypothesis that they are
BCGs. Models in which BCGs form from predominantly radial mergers having little
angular momentum predict that they should be prolate. If viewed along the major
axis, such objects would appear to have abnormally large sigma for their sizes,
and to be abnormally round for their luminosities. This is true of the objects
in our sample once we account for the fact that the most luminous galaxies
(M_r<-23.5), and BCGs, become slightly less round with increasing luminosity.Comment: 21 pages, 19 figures, accepted for publication in MNRA
Direction-Dependent Polarised Primary Beams in Wide-Field Synthesis Imaging
The process of wide-field synthesis imaging is explored, with the aim of
understanding the implications of variable, polarised primary beams for
forthcoming Epoch of Reionisation experiments. These experiments seek to detect
weak signatures from redshifted 21cm emission in deep residual datasets, after
suppression and subtraction of foreground emission. Many subtraction algorithms
benefit from low side-lobes and polarisation leakage at the outset, and both of
these are intimately linked to how the polarised primary beams are handled.
Building on previous contributions from a number of authors, in which
direction-dependent corrections are incorporated into visibility gridding
kernels, we consider the special characteristics of arrays of fixed dipole
antennas operating around 100-200 MHz, looking towards instruments such as the
Square Kilometre Array (SKA) and the Hydrogen Epoch of Reionization Arrays
(HERA). We show that integrating snapshots in the image domain can help to
produce compact gridding kernels, and also reduce the need to make complicated
polarised leakage corrections during gridding. We also investigate an
alternative form for the gridding kernel that can suppress variations in the
direction-dependent weighting of gridded visibilities by 10s of dB, while
maintaining compact support.Comment: 15 pages, 4 figures. Accepted for publication in JA
The luminosity and stellar mass Fundamental Plane of early-type galaxies
From a sample of ~50000 early-type galaxies from the SDSS, we measured the
traditional Fundamental Plane in four bands. We then replaced luminosity with
stellar mass, and measured the "stellar mass" FP. The FP steepens slightly as
one moves from shorter to longer wavelengths: the orthogonal fit has slope 1.40
in g and 1.47 in z. The FP is thinner at longer wavelengths: scatter is 0.062
dex in g, 0.054 dex in z. The scatter is larger at small galaxy sizes/masses;
at large masses measurement errors account for essentially all of the observed
scatter. The FP steepens further when luminosity is replaced with stellar mass,
to slope ~ 1.6. The intrinsic scatter also reduces further, to 0.048 dex. Since
color and stellar mass-to-light ratio are closely related, this explains why
color can be thought of as the fourth FP parameter. However, the slope of the
stellar mass FP remains shallower than the value of 2 associated with the
virial theorem. This is because the ratio of dynamical to stellar mass
increases at large masses as M_d^0.17. The face-on view of the stellar mass
kappa-space suggests that there is an upper limit to the stellar density for a
given dynamical mass, and this decreases at large masses: M_*/R_e^3 ~ M_d^-4/3.
We also study how the estimated coefficients a and b of the FP are affected by
other selection effects (e.g. excluding small sigma biases a high; excluding
fainter L biases a low). These biases are seen in FPs which have no intrinsic
curvature, so the observation that a and b scale with L and sigma is not, by
itself, evidence that the Plane is warped. We show that the FP appears to curve
sharply downwards at the small mass end, and more gradually downwards towards
larger masses. Whereas the drop at small sizes is real, most of the latter
effect is due to correlated errors.Comment: 17 pages, 15 figures, MNRAS in press. Added appendix on possible
sample contamination by disk
Long-term impact risk for (101955) 1999 RQ36
The potentially hazardous asteroid (101955) 1999 RQ36 has the possibility of
collision with the Earth in the latter half of the 22nd century, well beyond
the traditional 100-year time horizon for routine impact monitoring. The
probabilities accumulate to a total impact probability of approximately 10E-3,
with a pair of closely related routes to impact in 2182 comprising more than
half of the total. The analysis of impact possibilities so far in the future is
strongly dependent on the action of the Yarkovsky effect, which raises new
challenges in the careful assessment of longer term impact hazards.
Even for asteroids with very precisely determined orbits, a future close
approach to Earth can scatter the possible trajectories to the point that the
problem becomes like that of a newly discovered asteroid with a weakly
determined orbit. If the scattering takes place late enough so that the target
plane uncertainty is dominated by Yarkovsky accelerations then the thermal
properties of the asteroid,which are typically unknown, play a major role in
the impact assessment. In contrast, if the strong planetary interaction takes
place sooner, while the Yarkovsky dispersion is still relatively small compared
to that derived from the measurements, then precise modeling of the
nongravitational acceleration may be unnecessary.Comment: Reviewed figures and some text change
Does environment affect the star formation histories of early-type galaxies?
Differences in the stellar populations of galaxies can be used to quantify
the effect of environment on the star formation history. We target a sample of
early-type galaxies from the Sloan Digital Sky Survey in two different
environmental regimes: close pairs and a general sample where environment is
measured by the mass of their host dark matter halo. We apply a blind source
separation technique based on principal component analysis, from which we
define two parameters that correlate, respectively, with the average stellar
age (eta) and with the presence of recent star formation (zeta) from the
spectral energy distribution of the galaxy. We find that environment leaves a
second order imprint on the spectra, whereas local properties - such as
internal velocity dispersion - obey a much stronger correlation with the
stellar age distribution.Comment: 5 pages, 2 figures. Proceedings of JENAM 2010, Symposium 2:
"Environment and the formation of galaxies: 30 years later
A Search for the Most Massive Galaxies. III. Global and Central Structure
We used the Advanced Camera for Surveys on board the Hubble Space Telescope
to obtain high resolution i-band images of the centers of 23 single galaxies,
which were selected because they have SDSS velocity dispersions larger than 350
km/s. The surface brightness profiles of the most luminous of these objects
(M_i<-24) have well-resolved `cores' on scales of 150-1000 pc, and share
similar properties to BCGs. The total luminosity of the galaxy is a better
predictor of the core size than is the velocity dispersion. The correlations of
luminosity and velocity dispersion with core size agree with those seen in
previous studies of galaxy cores. Because of high velocity dispersions, our
sample of galaxies can be expected to harbor the most massive black holes, and
thus have large cores with large amounts of mass ejection. The mass-deficits
inferred from core-Sersic fits to the surface-brightness profiles are
approximately double the black-hole masses inferred from the M_bh-sigma
relation and the same as those inferred from the M_bh-L relation. The less
luminous galaxies (M_i>-23) tend to have steeper `power-law' inner profiles,
higher-ellipticity, diskier isophotes, and bulge-to-total ratios of order 0.5
-- all of which suggest that they are `fast-rotators' and rotational motions
could have contaminated the velocity dispersion estimate. There are obvious
dust features within about 300 pc of the center in about 35% of the sample,
predominantly in power-law rather than core galaxies.Comment: 27 Pages, 22 Figures, 2 Tables, Accepted for Publication in MNRA
Curvature in the scaling relations of early-type galaxies
We select a sample of about 50,000 early-type galaxies from the Sloan Digital
Sky Survey (SDSS), calibrate fitting formulae which correct for known problems
with photometric reductions of extended objects, apply these corrections, and
then measure a number of pairwise scaling relations in the corrected sample. We
show that, because they are not seeing corrected, the use of Petrosian-based
quantities in magnitude limited surveys leads to biases, and suggest that this
is one reason why Petrosian-based analyses of BCGs have failed to find
significant differences from the bulk of the early-type population. These
biases are not present when seeing-corrected parameters derived from
deVaucouleur fits are used. Most of the scaling relations we study show
evidence for curvature: the most luminous galaxies have smaller velocity
dispersions, larger sizes, and fainter surface brightnesses than expected if
there were no curva-ture. These statements remain true if we replace
luminosities with stellar masses; they suggest that dissipation is less
important at the massive end. There is curvature in the dynamical to stellar
mass relation as well: the ratio of dynamical to stellar mass increases as
stellar mass increases, but it curves upwards from this scaling both at small
and large stellar masses. In all cases, the curvature at low masses becomes
apparent when the sample becomes dominated by objects with stellar masses
smaller than 3 x 10^10 M_Sun. We quantify all these trends using second order
polynomials; these generally provide significantly better description of the
data than linear fits, except at the least luminous end.Comment: 15 pages, 17 figures, Accepted by MNRA
- …