1,958 research outputs found
Spectroscopy of the globular clusters in M87
With a velocity dispersion of 370 + or - 50 km/sec the globular cluster system of M87 is kinematically hotter than the stars in the giant elliptical itself. This is consistent with the clusters' shallower density distribution for isotropic orbits. The mean metallicity of the 27 clusters in the sample analyzed here is no more than a factor of 2 more metal rich than the cluster system of the Milky Way, but considerably more metal poowr than the integrated starlight in the field at a radius of 1' from the center of M87. There is no evidence for the existence of young clusters in the system. The mass-radius relation between 1' and 5' required to contain the globular clusters joins on to that required to contain the hot gas around M87
The Hubble constant and dark energy
The Hubble Constant measured from the anisotropy in the cosmic microwave
background (CMB) is shown to be independent of small changes from the standard
model of the redshift dependence of dark energy. Modifications of the Friedmann
equation to include phantom power (w < -1), textures (w = -2/3) and curvature
are considered, and constraints on these dark energy contributors from
supernova observations are derived. Modified values for the density of matter
inferred from cosmic density perturbations and from the CMB under these
circumstances are also estimated, as exemplified by 2df and SDSS.Comment: PASP accepted; PASP September 201
Maximum likelihood method for fitting the Fundamental Plane of the 6dF Galaxy Survey
We have used over 10,000 early-type galaxies from the 6dF Galaxy Survey
(6dFGS) to construct the Fundamental Plane across the optical and near-infrared
passbands. We demonstrate that a maximum likelihood fit to a multivariate
Gaussian model for the distribution of galaxies in size, surface brightness and
velocity dispersion can properly account for selection effects, censoring and
observational errors, leading to precise and unbiased parameters for the
Fundamental Plane and its intrinsic scatter. This method allows an accurate and
robust determination of the dependencies of the Fundamental Plane on variations
in the stellar populations and environment of early-type galaxies.Comment: 3 pages, 1 figure, to appear in the proceedings of the IAU Symposium
262 "Stellar Populations: Planning for the Next Decade", Charlot and Bruzual
ed
The composition of HB stars : RR Lyrae variables
We used moderately high-resolution, high S/N spectra to study the chemical
composition of 10 field ab-type RR Lyrae stars. A new temperature scale was
determined from literature Infrared Flux Method measures of subdwarfs and the
Kurucz (1992) model atmospheres, and used to calibrate colors for both dwarfs
and RR Lyraes. The applicability of Kurucz (1992) model atmospheres in the
analysis of RR Lyraes at minimum light was analyzed: we found that they are
able to reproduce colors, excitation and ionization equilibria as well as the
wings of Halpha. We derived abundances for 21 species. The metal abundances of
the program stars span the range -2.50<[Fe/H]<+0.17$. Lines of most elements
are found to form in LTE conditions. Fe lines satisfy very well the excitation
and ionization equilibria. RR Lyraes share the typical abundance pattern of
other stars of similar [Fe/H]: alpha-elements are overabundant by about 0.4dex
and Mn is underabundant by about 0.6dex in stars with [Fe/H]<-1. Significant
departures from LTE are found only for a few species. We used our new [Fe/H]
abundances, as well as values from Butler and coworkers (corrected to our
system), and from high resolution spectroscopy of globular clusters giants, to
obtain a new calibration of the DeltaS index: [Fe/H]= -0.194(\pm 0.011)DeltaS
-0.08(\pm 0.18) and to update the metallicity calibration of the Ca II K line
index: [Fe/H]= 0.65(\pm 0.17)W'(K) -3.49(\pm 0.39). Finally, our new
metallicity scale was used to revise the [Fe/H] dependence of the absolute
magnitude of RR Lyrae stars, Mv: Mv = 0.20(\pm 0.03)[Fe/H] + 1.06(\pm 0.04).Comment: 59 pages, Latex using aaspp.sty, ps-files of text, tables (21) and
figures (23) available from ftp://boas3.bo.astro.it/pub/gisella To appear in
October 1995 Astronomical Journa
Radiative transfer modelling of parsec-scale dusty warped discs
Warped discs have been found on (sub-)parsec scale in some nearby Seyfert
nuclei, identified by their maser emission. Using dust radiative transfer
simulations we explore their observational signatures in the infrared in order
to find out whether they can partly replace the molecular torus. Strong
variations of the brightness distributions are found, depending on the
orientation of the warp with respect to the line of sight. Whereas images at
short wavelengths typically show a disc-like and a point source component, the
warp itself only becomes visible at far-infrared wavelengths. A similar variety
is visible in the shapes of the spectral energy distributions. Especially for
close to edge-on views, the models show silicate feature strengths ranging from
deep absorption to strong emission for variations of the lines of sight towards
the warp. To test the applicability of our model, we use the case of the
Circinus galaxy, where infrared interferometry has revealed a highly elongated
emission component matching a warped maser disc in orientation and size. Our
model is for the first time able to present a physical explanation for the
observed dust morphology as coming from the AGN heated dust. As opposed to
available torus models, a warped disc morphology produces a variety of silicate
feature shapes for grazing lines of sight, close to an edge-on view. This could
be an attractive alternative to a claimed change of the dust composition for
the case of the nearby Seyfert 2 galaxy NGC 1068, which harbours a warped maser
disc as well.Comment: accepted by MNRA
Measuring the Hubble Constant with the Hubble Space Telescope
Ten years ago our team completed the Hubble Space Telescope Key Project on
the extragalactic distance scale. Cepheids were detected in some 25 galaxies
and used to calibrate four secondary distance indicators that reach out into
the expansion field beyond the noise of galaxy peculiar velocities. The result
was H_0 = 72 +/- 8 km/sec/Mpc and put an end to galaxy distances uncertain by a
factor of two. This work has been awarded the Gruber Prize in Cosmology for
2009.Comment: Gruber Prize Lecture to be published in Transactions of the IA
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