924 research outputs found
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
Dark Energy and the quietness of the Local Hubble Flow
The linearity and quietness of the Local () Hubble Flow (LHF) in
view of the very clumpy local universe is a long standing puzzle in standard
and in open CDM cosmogony. The question addressed in this paper is whether the
antigravity component of the recently discovered dark energy can cool the
velocity flow enough to provide a solution to this puzzle. We calculate the
growth of matter fluctuations in a flat universe containing a fraction
of dark energy obeying the time independent equation of state
. We find that dark energy can indeed cool the LHF. However the
dark energy parameter values required to make the predicted velocity dispersion
consistent with the observed value have been ruled out
by other observational tests constraining the dark energy parameters and
. Therefore despite the claims of recent qualitative studies dark
energy with time independent equation of state can not by itself explain the
quietness and linearity of the Local Hubble Flow.Comment: 4 pages, 3 figures, accepted in Phys. Rev. D. Minor corrections, one
figure adde
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
Allan Sandage and the Cosmic Expansion
This is an account of Allan Sandage's work on (1) The character of the
expansion field. For many years he has been the strongest defender of an
expanding Universe. He later explained the CMB dipole by a local velocity of
220 +/- 50 km/s toward the Virgo cluster and by a bulk motion of the Local
supercluster (extending out to ~3500 km/s) of 450-500 km/s toward an apex at
l=275, b=12. Allowing for these streaming velocities he found linear expansion
to hold down to local scales (~300 km/s). (2) The calibration of the Hubble
constant. Probing different methods he finally adopted - from
Cepheid-calibrated SNe Ia and from independent RR Lyr-calibrated TRGBs - H_0 =
62.3 +/- 1.3 +/- 5.0 km/s/Mpc.Comment: 12 pages, 11 figures, 1 table, Submitted to Astrophysics and Space
Science, Special Issue on the Fundamental Cosmic Distance Scale in the Gaia
Er
Application of the Limit Cycle Model to Star Formation Histories in Spiral Galaxies: Variation among Morphological Types
We propose a limit-cycle scenario of star formation history for any
morphological type of spiral galaxies. It is known observationally that the
early-type spiral sample has a wider range of the present star formation rate
(SFR) than the late-type sample. This tendency is understood in the framework
of the limit-cycle model of the interstellar medium (ISM), in which the SFR
cyclically changes in accordance with the temporal variation of the mass
fraction of the three ISM components. When the limit-cycle model of the ISM is
applied, the amplitude of variation of the SFR is expected to change with the
supernova (SN) rate. Observational evidence indicates that the early-type
spiral galaxies show smaller rates of present SN than late-type ones. Combining
this evidence with the limit-cycle model of the ISM, we predict that the
early-type spiral galaxies show larger amplitudes in their SFR variation than
the late-types. Indeed, this prediction is consistent with the observed wider
range of the SFR in the early-type sample than in the late-type sample. Thus,
in the framework of the limit-cycle model of the ISM, we are able to interpret
the difference in the amplitude of SFR variation among the morphological
classes of spiral galaxies.Comment: 12 pages LaTeX, to appear in A
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
Distances from the Correlation between Galaxy Luminosities and Rotation Rates
A large luminosity--linewidth template sample is now available, improved
absorption corrections have been derived, and there are a statistically
significant number of galaxies with well determined distances to supply the
zero point. A revised estimate of the Hubble Constant is H_0=77 +-4 km/s/Mpc
where the error is the 95% probability statistical error. Systematic
uncertainties are potentially twice as large.Comment: 21 pages, 9 figures. Invited chapter for the book `Post-Hipparcos
Cosmic Candles', Eds. F. Caputo and A. Heck (Kluwer Academic Publishers,
Dordrecht
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
The Age of the Oldest Stars in the Local Galactic Disk From Hipparcos Parallaxes of G and K Subgiants
We review the history of the discovery of field subgiant stars and their
importance in the age dating of the Galactic disk. We use the cataloged data
from the Hipparcos satellite in this latter capacity. Based on Hipparcos
parallaxes accurate to 10% or better, the absolute magnitude of the lower
envelope of the nearly horizontal subgiant sequence for field stars in the H-R
diagram for B-V colors between 0.85 and 1.05 is measured to be M_V = 4.03 +/-
0.06. The age of the field stars in the solar neighborhood is found to be 7.9
+/- 0.7 Gyr by fitting the theoretical isochrones for [Fe/H] = +0.37 to the
lower envelope of the Hipparcos subgiants. The same grid of isochrones yields
ages, in turn, of 4.0 +/- 0.2 Gyr, 6.2 +/- 0.5 Gyr, and 7.5 to 10 Gyr for the
old Galactic clusters M67, NGC188, and NGC6791. The ages of both the Galactic
disk in the solar neighborhood and of NGC6791 are, nevertheless, likely between
3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of
13.5 Gyr. The most significant results are (1) the supermetallicity of the
oldest local disk stars, and (2) the large age difference between the most
metal-poor component of the halo and the thick and thin disk in the solar
neighborhood. These facts are undoubtedly related and pose again the problem of
the proper scenario for the timing of events in the formation of the halo and
the Galactic disk in the solar neighborhood. [Abstract Abridged]Comment: 44 pages, 12 Figures; accepted for publication in PASP; high
resolution versions of Figures 1, 2, 6 and 9 available at
http://bubba.ucdavis.edu/~lubin/Sandage
Dim galaxies and outer halos of galaxies missed by 2MASS ? The near-infrared luminosity function and density
By using high-resolution and deep Ks band observations of early-type galaxies
of the nearby Universe and of a cluster at z=0.3 we show that the two
luminosity functions (LFs) of the local universe derived from 2MASS data miss a
fair fraction of the flux of the galaxies (more than 20 to 30%) and a whole
population of galaxies of central brightness fainter than the isophote used for
detection, but bright enough to be included in the published LFs. In
particular, the fraction of lost flux increases as the galaxy surface
brightness become fainter. Therefore, the so far derived LF slopes and
characteristic luminosity as well as luminosity density are underestimated.
Other published near-infrared LFs miss flux in general, including the LF of the
distant field computed in a 3 arcsec aperture.Comment: A&A in pres
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