788 research outputs found
The morphology-density relation for dwarf galaxies
The morphology-density relation is examined for dwarf galaxies with absolute magnitudes -18 less than or equal to M sub B sub T less than or equal to -12.5, based on a deep photographic survey of nearby groups and clusters of galaxies. Results are given. Compared to dwarf ellipticals, dwarf irregulars form a more extended population in nearby clusters, and may in fact be entirely absent from the cluster cores. The spatial distribution of dwarf ellipticals in clusters depends on luminosity and the presence or absence of nucleation. Nucleated dE's and non-nucleated dE's fainter than M sub B sub T approx. -13.5 are concentrated toward the centers of clusters like the giant E and S0 galaxies. In contrast, non-nucleated dE's brighter than M sub B sub T approx. -14.5 are distributed like the spirals and irregulars. The intrinsic shapes of the bright non-nucleated dE's are similar to those of the dwarf irregulars, suggesting a possible evolutionary connection between these two classes of galaxies
The Tolman Surface Brightness Test for the Reality of the Expansion. II. The Effect of the Point-Spread Function and Galaxy Ellipticity on the Derived Photometric Parameters
To complete the Tolman surface brightness test on the reality of the
expansion of the Universe, we need to measure accurately the surface brightness
profiles of the high-redshift galaxy sample. We, therefore, investigate the
effects of various sizes of point-spread-functions composed of telescope
diffraction, CCD pixel resolutions, and ground-based seeing on the measurements
of mean surface brightness. We have done the calculations using two synthetic
galaxies of effective radii of 0.70" and 0.25" with point-spread functions of
0.1, 0.3, and 0.9 arcseconds. We have also compared actual observations of
three high-redshift galaxies in the cluster Cl 1324 + 3011 (z = 0.76) made both
with the Keck telescopes in seeing of about 0.9" and with HST which has a PSF
that is approximately ten times smaller. The conclusion is that HST data can be
used as far into the galaxy image as a Petrosian metric radius of eta = 1.3
magnitudes, whereas the ground-based data will have systematic errors of up to
2.9 magnitudes in the mean surface brightness at eta values of less than 2.2
magnitudes. In the final section, we compare the differences in derived average
surface brightness for nearly circular galaxy images compared with highly
flattened images. The comparison is made by using the two reduction procedures
of (1) integrating the profile curves using circular apertures, and (2)
approximating an ``equivalent circular'' galaxy that is highly elongated by
using an ``effective'' radius of sqrt{ab}, where a and b are the semi-major and
semi-minor axis, respectively, of the best-fitting ellipse. The conclusion is
that the two methods of reduction give nearly identical results and that either
method can be used to analyze the low and high-redshift galaxy samples used in
the Tolman test.Comment: 15 pages, 9 figures; accepted for publication in Astronomical Journa
The Schizophrenic Spectrum of LSR 1610-0040: a Peculiar M Dwarf/Subdwarf
We present a moderate resolution (R=2000), 0.8-4.1 micron spectrum of LSR
1610-0040, a high proper motion star classified as an early-type L subdwarf by
Lepine and collaborators based on its red-optical spectrum. The near-infrared
spectrum of LSR 1610-0040 does not fit into the (tentative) M/L subdwarf
sequence but rather exhibits a mix of characteristics found in the spectra of
both M dwarfs and M subdwarfs. In particular, the near-infrared spectrum
exhibits a Na I doublet and CO overtone bandheads in the K band, and Al I and K
I lines and an FeH bandhead in the H band, all of which have strengths more
typical of field M dwarfs. Furthermore the spectrum of Gl 406 (M6 V) provides a
reasonably good match to the 0.6-4.1 micron spectral energy distribution of LSR
1610. Nevertheless the near-infrared spectrum of LSR 1610 also exhibits
features common to the spectra of M subdwarfs including a strong Ti I multiplet
centered at ~0.97 microns, a weak VO band at ~1.06 microns, and possible
collision-induced H_2 absorption in the H and K bands. We discuss a number of
possible explanations for the appearance of the red-optical and near-infrared
spectrum of LSR 1610-0040. Although we are unable to definitively classify LSR
1610-0040, the preponderance of evidence suggests that it is a mildly
metal-poor M dwarf. Finally, we tentatively identify a new band of TiO at ~0.93
microns in the spectra of M dwarfs.Comment: Accepted for publication in the Astronomical Journa
Large Magellanic Cloud Distance from Cepheid Variables using Least Squares Solutions
Distance to the Large Magellanic Cloud (LMC) is determined using the Cepheid
variables in the LMC. We combine the individual LMC Cepheid distances obtained
from the infrared surface brightness method and a dataset with a large number
of LMC Cepheids. Using the standard least squares method, the LMC distance
modulus can be found from the ZP offsets of these two samples. We have adopted
both a linear P-L relation and a ``broken'' P-L relation in our calculations.
The resulting LMC distance moduli are 18.48+-0.03 mag and 18.49+-0.04 mag
(random error only), respectively, which are consistent to the adopted 18.50
mag in the literature.Comment: 2 pages and 1 figure, to appear in proceeding of "Galaxies in the
Local Volume" Sydney 8-13 July 200
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
The Detailed Forms of the LMC Cepheid PL and PLC Relations
Possible deviations from linearity of the LMC Cepheid PL and PLC relations
are investigated. Two datasets are studied, respectively from the OGLE and
MACHO projects. A nonparametric test, based on linear regression residuals,
suggests that neither PL relation is linear. If colour dependence is allowed
for then the MACHO PL relation is found to deviate more significantly from the
linear, while the OGLE PL relation is consistent with linearity. These finding
are confirmed by fitting "Generalised Additive Models" (nonparametric
regression functions) to the two datasets. Colour dependence is shown to be
nonlinear in both datasets, distinctly so in the case of the MACHO Cepheids. It
is also shown that there is interaction between the period and colour functions
in the MACHO data.Comment: 20 pages, 20 figures, MNRAS accepte
Temperature Differences in the Cepheid Instability Strip Require Differences in the Period-Luminosity Relation in Slope and Zero Point
A graphical and an algebraic demonstration is made to show why the slope and
zero point of the Cepheid period-luminosity (P-L) relation is rigidly coupled
with the slope and zero point of the Cepheid instability strip in the HR
diagram. The graphical demonstration uses an arbitrary (toy) ridge line in the
instability strip, while the algebraic demonstration uses the pulsation
equation into which the observed P-L relations for the Galaxy and the LMC are
put to predict the temperature zero points and slopes of the instability
strips. Agreement between the predicted and measured instability strip slopes
argue that the observed P-L differences between the Galaxy and LMC are real. In
another proof, the direct evidence for different P-L slopes in different
galaxies is shown by comparing the Cepheid data in the Galaxy, the combined
data in NGC 3351 and NGC 4321, in M31, LMC, SMC, IC 1613, NGC 3109, and in
Sextans A+B. The P-L slopes for the Galaxy, NGC 3351, NGC 4321, and M31 are
nearly identical and are the steepest in the sample. The P-L slopes decrease
monotonically with metallicity in the order listed, showing that the P-L
relation is not the same in different galaxies, complicating their use in
calibrating the extragalactic distance scale.Comment: 15 pages, 1 table, 4 figures, submitted to The Ap
Environmental Dependence of the Structure of Brightest Cluster Galaxies
We measure the Petrosian structural properties of 33 brightest cluster
galaxies (BCGs) at redshifts z<0.1 in X-ray selected clusters with a wide range
of X-ray luminosities. We find that some BCGs show distinct signatures in their
Petrosian profiles, likely to be due to cD haloes. We also find that BCGs in
high X-ray luminosity clusters have shallower surface brightness profiles than
those in low X-ray luminosity clusters. This suggests that the BCGs in high
X-ray luminosity clusters have undergone up to twice as many equal-mass mergers
in their past as those in low X-ray luminosity clusters. This is qualitatively
consistent with the predictions of hierarchical structure formation.Comment: 11 pages, accepted for publication in MNRA
Mapping the galactic gravitational potential with peculiar acceleration
It has been suggested recently that the change in cosmological redshift (the
Sandage test of expansion) could be observed in the next generation of large
telescopes and ultra-stable spectrographs. In a recent paper we estimated the
change of peculiar velocity, i.e. the peculiar acceleration, in nearby galaxies
and clusters and shown it to be of the same order of magnitude as the typical
cosmological signal. Mapping the acceleration field allows for a reconstruction
of the galactic gravitational potential without assuming virialization. In this
paper we focus on the peculiar acceleration in our own Galaxy, modeled as a
Kuzmin disc and a dark matter spherical halo. We estimate the peculiar
acceleration for all known Galactic globular clusters and find some cases with
an expected velocity shift in excess of 20 cm/sec for observations fifteen
years apart, well above the typical cosmological acceleration. We then compare
the predicted signal for a MOND (modified Newtonian dynamics) model in which
the spherical dark matter halo is absent. We find that the signal pattern is
qualitatively different, showing that the peculiar acceleration field could be
employed to test competing theories of gravity. However the difference seems
too small to be detectable in the near future.Comment: 11 pages, 10 figures, 3 tables, minor changes, accepted for
publication by MNRA
Neutrino mass constraint from CMB and its degeneracy with other cosmological parameters
We show that the cosmic microwave background (CMB) data of WMAP can give
subelectronvolt limit on the neutrino mass: m_nu < 0.63 eV (95% CL). We also
investigate its degeneracy with other cosmological parameters. In particular,
we show the Hubble constant derived from the WMAP data decreases considerably
when the neutrino mass is a few times 0.1 eV.Comment: 3 pages, 2 figures, prepared for the TAUP2007 Proceeding
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