119 research outputs found
Galaxies at z=4 and the Formation of Population II
We report the discovery of four high-redshift objects (3.3 < z < 4) observed
behind the rich cluster CL0939+4713 (Abell 851). One object (DG 433) has a
redshift of z=3.3453; the other three objects have redshifts of z\approx 4: A0
at z=3.9819, DG 353 and P1/P2 at z=3.9822. It is possible that all four objects
are being lensed in some way by the cluster, DG 433 being weakly sheared, A0
being strongly sheared, and DG 353 and P1/P2 being an image pair of a common
source object; detailed modelling of the cluster potential will be necessary to
confirm this hypothesis. The weakness of common stellar wind features like N V
and especially C IV in the spectra of these objects argues for sub-solar
metallicities, at least as low as the SMC. DG 353 and DG 433, which have
ground-based colors, are moderately dusty [E_{int}(B-V) < 0.15], similar to
other z>3 galaxies. Star formation rates range from 2.5 (7.8) h^{-2} to 22.
(78.) h^{-2} M_{\odot}/yr, for q_0=0.5 (0.05), depending on assumptions about
gravitational lensing and extinction, also typical of other z>3 galaxies. These
objects are tenatively identified as the low-metallicity proto-spheroid clumps
that will merge to form the Population II components of today's spheroids.Comment: 16 pages, including 2 PostScript figures. Needs aaspp4.sty
(included). Accepted for publication in the Astrophysical Journa
A Quintet Of Black Hole Mass Determinations
We report five new measurements of central black hole masses based on Space Telescope Imaging Spectrograph and Wide Field Planetary Camera 2 observations with the Hubble Space Telescope (HST) and on axisymmetric, three-integral, Schwarzschild orbit-library kinematic models. We selected a sample of galaxies within a narrow range in velocity dispersion that cover a range of galaxy parameters (including Hubble type and core/power-law surface density profile) where we expected to be able to resolve the galaxy's sphere of influence based on the predicted value of the black hole mass from the M-sigma relation. We find masses for the following galaxies: NGC3585, M(BH) = 3.4(-0.6)(+1.5) x 10(8) M(circle dot;) NGC 3607, M(BH) = 1.2(-0.4)(+0.4) x 10(8) M(circle dot); NGC 4026, M(BH) = 2.1(-0.4)(+0.7) x 10(8) M(circle dot); and NGC 5576, M(BH) = 1.8(-0.4)(+0.3) x 10(8) M(circle dot), all significantly excluding M(BH) = 0. For NGC 3945, M(BH) = 9(-21)(+17) x 10(6) M(circle dot), which is significantly below predictions from M-sigma and M-L relations and consistent with MBH = 0, though the presence of a double bar in this galaxy may present problems for our axisymmetric code.NASA/HST GO-5999, GO-6587, GO-6633, GO-7468, GO-9107NASA NAS 5-26555Astronom
The M-sigma and M-L Relations in Galactic Bulges and Determinations of their Intrinsic Scatter
We derive improved versions of the relations between supermassive black hole
mass (M_BH) and host-galaxy bulge velocity dispersion (sigma) and luminosity
(L) (the M-sigma and M-L relations), based on 49 M_BH measurements and 19 upper
limits. Particular attention is paid to recovery of the intrinsic scatter
(epsilon_0) in both relations. We find log(M_BH / M_sun) = alpha + beta *
log(sigma / 200 km/s) with (alpha, beta, epsilon_0) = (8.12 +/- 0.08, 4.24 +/-
0.41, 0.44 +/- 0.06) for all galaxies and (alpha, beta, epsilon_0) = (8.23 +/-
0.08, 3.96 +/- 0.42, 0.31 +/- 0.06) for ellipticals. The results for
ellipticals are consistent with previous studies, but the intrinsic scatter
recovered for spirals is significantly larger. The scatter inferred reinforces
the need for its consideration when calculating local black hole mass function
based on the M-sigma relation, and further implies that there may be
substantial selection bias in studies of the evolution of the M-sigma relation.
We estimate the M-L relationship as log(M_BH / M_sun) = alpha + beta * log(L_V
/ 10^11 L_sun,V) of (alpha, beta, epsilon_0) = (8.95 +/- 0.11, 1.11 +/- 0.18,
0.38 +/- 0.09); using only early-type galaxies. These results appear to be
insensitive to a wide range of assumptions about the measurement errors and the
distribution of intrinsic scatter. We show that culling the sample according to
the resolution of the black hole's sphere of influence biases the relations to
larger mean masses, larger slopes, and incorrect intrinsic residuals.Comment: 27 pages, 18 figures, 7 tables, ApJ accepte
The stellar population histories of early-type galaxies. III. The Coma Cluster
We present stellar population parameters of twelve early-type galaxies (ETGs)
in the Coma Cluster based on spectra obtained using the Low Resolution Imaging
Spectrograph on the Keck II Telescope. Our data allow us to examine in detail
the zero-point and scatter in their stellar population properties. Our ETGs
have SSP-equivalent ages of on average 5-8 Gyr with the models used here, with
the oldest galaxies having ages of ~10 Gyr old. This average age is identical
to the mean age of field ETGs. Our ETGs span a large range in velocity
dispersion but are consistent with being drawn from a population with a single
age. Specifically, ten of the twelve ETGs are consistent within their formal
errors of having the same age, 5.2+/-0.2 Gyr, over a factor of more than 750 in
mass. We therefore find no evidence for downsizing of the stellar populations
of ETGs in the core of the Coma Cluster. We suggest that Coma Cluster ETGs may
have formed the majority of their mass at high redshifts but suffered small but
detectable star formation events at z~0.1-0.3. Previous detections of
'downsizing' from stellar populations of local ETGs may not reflect the same
downsizing seen in lookback studies of RSGs, as the young ages of the local
ETGs represent only a small fraction of their total masses. (abridged)Comment: 49 pages, 20 figures (19 EPS, 1 JPEG). MNRAS, in press. For version
with full resolution of Fig. 1 see
http://www.astro.rug.nl/~sctrager/coma.pdf; for Table 2, see
http://www.astro.rug.nl/~sctrager/coma_table2.pdf; for Table B3, see
http://www.astro.rug.nl/~sctrager/coma_tableB3.pd
The Centers of Early-Type Galaxies with HST. IV. Central Parameter Relations
We analyze Hubble Space Telescope surface-brightness profiles of 61
elliptical galaxies and spiral bulges (hot galaxies). Luminous hot galaxies
have cuspy cores with steep outer power-law profiles that break at r ~ r_b to
shallow inner profiles with logslope less than 0.3. Faint hot galaxies show
steep, largely featureless power-law profiles at all radii and lack cores. The
centers of power-law galaxies are up to 1000 times denser in mass and
luminosity than the cores of large galaxies at a limiting radius of 10 pc. At
intermediate magnitudes (-22.0 < M_V < -20.5), core and power-law galaxies
coexist, and there is a range in r_b at a given luminosity of at least two
orders of magnitude. Central properties correlate with global rotation and
shape: core galaxies tend to be boxy and slowly rotating, whereas power-law
galaxies tend to be disky and rapidly rotating. The dense power-law centers of
disky, rotating galaxies are consistent with their formation in gas-rich
mergers. The parallel proposition that cores are simply the by-products of
gas-free stellar mergers is less compelling. For example, core galaxies accrete
small, dense, gas-free galaxies at a rate sufficient to fill in low-density
cores if the satellites survived and sank to the center. An alternative model
for core formation involves the orbital decay of massive black holes (BHs): the
BH may heat and eject stars from the center, eroding a power law if any exists
and scouring out a core. An average BH mass per spheroid of 0.002 times the
stellar mass yields reasonably good agreement with the masses and radii of
observed cores and in addition is consistent with the energetics of AGNs and
kinematic detections of BHs in nearby galaxies.Comment: 40 pages (Tex) with 10 figures and 4 tables (Postscript). To appear
in the November 1997 Astronomical Journal. The discussion section is
significantly revised from the original submission to Astro-ph, dated October
1996. One figure is slightly altered, and the data tables are the sam
The slope of the black-hole mass versus velocity dispersion correlation
Observations of nearby galaxies reveal a strong correlation between the mass
of the central dark object M and the velocity dispersion sigma of the host
galaxy, of the form log(M/M_sun) = a + b*log(sigma/sigma_0); however, published
estimates of the slope b span a wide range (3.75 to 5.3). Merritt & Ferrarese
have argued that low slopes (<4) arise because of neglect of random measurement
errors in the dispersions and an incorrect choice for the dispersion of the
Milky Way Galaxy. We show that these explanations account for at most a small
part of the slope range. Instead, the range of slopes arises mostly because of
systematic differences in the velocity dispersions used by different groups for
the same galaxies. The origin of these differences remains unclear, but we
suggest that one significant component of the difference results from Ferrarese
& Merritt's extrapolation of central velocity dispersions to r_e/8 (r_e is the
effective radius) using an empirical formula. Another component may arise from
dispersion-dependent systematic errors in the measurements. A new determination
of the slope using 31 galaxies yields b=4.02 +/- 0.32, a=8.13 +/- 0.06, for
sigma_0=200 km/s. The M-sigma relation has an intrinsic dispersion in log M
that is no larger than 0.3 dex. In an Appendix, we present a simple model for
the velocity-dispersion profile of the Galactic bulge.Comment: 37 pages, 9 figure
The Demography of Massive Dark Objects in Galaxy Centres
We construct dynamical models for a sample of 36 nearby galaxies with Hubble
Space Telescope photometry and ground-based kinematics. The models assume that
each galaxy is axisymmetric, with a two-integral distribution function,
arbitrary inclination angle, a position-independent stellar mass-to-light ratio
Upsilon, and a central massive dark object (MDO) of arbitrary mass M_bh. They
provide acceptable fits to 32 of the galaxies for some value of M_bh and
Upsilon; the four galaxies that cannot be fit have kinematically decoupled
cores. The mass-to-light ratios inferred for the 32 well-fit galaxies are
consistent with the fundamental plane correlation Upsilon \propto L^0.2, where
L is galaxy luminosity. In all but six galaxies the models require at the 95%
confidence level an MDO of mass M_bh ~ 0.006 M_bulge = 0.006 Upsilon L. Five of
the six galaxies consistent with M_bh=0 are also consistent with this
correlation. The other (NGC 7332) has a much stronger upper limit on M_bh. We
consider various parameterizations for the probability distribution describing
the correlation of the masses of these MDOs with other galaxy properties. One
of the best models can be summarized thus: a fraction f ~0.97 of galaxies have
MDOs, whose masses are well described by a Gaussian distribution in log
(M_bh/M_bulge) of mean -2.27 and width ~0.07.Comment: 28 pages including 13 figures and 4 tables. Submitted to A
A Stellar Dynamical Measurement of the Black Hole Mass in the Maser Galaxy NGC 4258
We determine the mass of the black hole at the center of the spiral galaxy
NGC 4258 by constructing axisymmetric dynamical models of the galaxy. These
models are constrained by high spatial resolution imaging and long-slit
spectroscopy of the nuclear region obtained with the {\em Hubble Space
Telescope}, complemented by ground-based observations extending to larger
radii. Our best mass estimate is \MBH = (3.3 \pm 0.2) \times 10^7 \MSun for
a distance of 7.28 Mpc (statistical errors only). This is within 15% of
(3.82\pm 0.01) \times 10^7 \MSun, the mass determined from the kinematics of
water masers (rescaled to the same distance) assuming they are in Keplerian
rotation in a warped disk. The construction of accurate dynamical models of NGC
4258 is somewhat compromised by an unresolved active nucleus and color
gradients, the latter caused by variations in the stellar population and/or
obscuring dust. These problems are not present in the other black
hole mass determinations from stellar dynamics that have been published by us
and other groups; thus, the relatively close agreement between the stellar
dynamical mass and the maser mass in NGC 4258 enhances our confidence in the
black hole masses determined in other galaxies from stellar dynamics using
similar methods and data of comparable quality.Comment: 58 pages, submitted to ApJ. Some figures excluded due to size. The
entire paper is at http://www.noao.edu/noao/staff/lauer/nuker_papers.htm
The Masses of Nuclear Black Holes in Luminous Elliptical Galaxies and Implications for the Space Density of the Most Massive Black Holes
Black hole masses predicted from the Mbh-sigma relationship conflict with
those predicted from the Mbh-L relationship for the most luminous galaxies,
such as brightest cluster galaxies (BCGs). This is because stellar velocity
dispersion, sigma, increases only weakly with L for BCGs and other giant
ellipticals. The Mbh-L relationship predicts that the most luminous BCGs may
have Mbh approaching 10^{10}M_sol, while the M-sigma relationship always
predicts Mbh<3X10^9M_sol. We argue that the Mbh-L relationship is a plausible
or even preferred description for BCGs and other galaxies of similar
luminosity. If cores in central stellar density are formed by binary BHs, then
the inner-core cusp radius, r_gamma, may be an independent witness of Mbh.
Using structural parameters derived from a large sample of early-type galaxies
observed by HST, we argue that L is superior to sigma as an indicator of
r_gamma in luminous galaxies. The observed r_gamma-Mbh relationship for 11 core
galaxies with measured Mbh appears to be consistent with the Mbh-L relationship
for BCGs. BCGs have large cores appropriate for their large luminosities that
may be difficult to generate with the modest BH masses inferred from the
Mbh-sigma relationship. Mbh~L may be expected to hold for BCGs, if they were
formed in dissipationless mergers, which should preserve ratio of BH to stellar
mass. This picture appears to be consistent with the slow increase in sigma
with L and the more rapid increase in effective radii with L seen in BCGs. If
BCGs have large BHs commensurate with their luminosities, then the local BH
mass function for Mbh>3X10^9M_sol may be nearly an order of magnitude richer
than that inferred from the Mbh-sigma relationship. The volume density of QSOs
at earlier epochs may favor the predictions from the Mbh-L relationship.Comment: 62 pages, 18 figures, submitted to the Astrophysical Journal; revised
after initial revie
Axisymmetric Dynamical Models of the Central Regions of Galaxies
We present axisymmetric, orbit superposition models for 12 galaxies using
data taken with the Hubble Space Telescope (HST) and ground-based
observatories. In each galaxy, we detect a central black hole (BH) and measure
its mass to accuracies ranging from 10% to 70%. We demonstrate that in most
cases the BH detection requires both the HST and ground-based data. Using the
ground-based data alone does provide an unbiased measure of the BH mass
(provided they are fit with fully general models), but at a greatly reduced
significance. The most significant correlation with host galaxy properties is
the relation between the BH mass and the velocity dispersion of the host
galaxy; we find no other equally strong correlation, and no second parameter
that improves the quality of the mass-dispersion relation. We are also able to
measure the stellar orbital properties from these general models. The most
massive galaxies are strongly biased to tangential orbits near the BH,
consistent with binary BH models, while lower-mass galaxies have a range of
anisotropies, consistent with an adiabatic growth of the BH.Comment: 25 pages, accepted for publication in The Astrophysical Journa
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