5,715 research outputs found
The stellar populations of the central region of M31
We continue the analysis of the dataset of our spectroscopic observation
campaign of M31, by deriving simple stellar population properties (age
metallicity and alpha-elements overabundance) from the measurement of Lick/IDS
absorption line indices. We describe their two-dimensional maps taking into
account the dust distribution in M31. 80\% of the values of our age
measurements are larger than 10 Gyr. The central 100 arcsec of M31 are
dominated by the stars of the classical bulge of M31. They are old (11-13 Gyr),
metal-rich (as high as [Z/H]~0.35 dex) at the center with a negative gradient
outwards and enhanced in alpha-elements ([alpha/Fe]~ 0.28+- 0.01 dex). The bar
stands out in the metallicity map, where an almost solar value of [Z/H]
(~0.02+-0.01 dex) with no gradient is observed along the bar position angle
(55.7 deg) out to 600 arcsec from the center. In contrast, no signature of the
bar is seen in the age and [alpha/Fe] maps, that are approximately
axisymmetric, delivering a mean age and overabundance for the bar and the
boxy-peanut bulge of 10-13 Gyr and 0.25-0.27 dex, respectively. The
boxy/peanut-bulge has almost solar metallicity (-0.04+- 0.01 dex). The
mass-to-light ratio of the three components is approximately constant at M/LV ~
4.4-4.7 Msol/Lsol. The disk component at larger distances is made of a mixture
of stars, as young as 3-4 Gyr, with solar metallicity and smaller M/LV (~3+-0.1
Msol/Lsol). We propose a two-phase formation scenario for the inner region of
M31, where most of the stars of the classical bulge come into place together
with a proto-disk, where a bar develops and quickly transforms it into a
boxy-peanut bulge. Star formation continues in the bulge region, producing
stars younger than 10 Gyr, in particular along the bar, enhancing its
metallicity. The disk component appears to build up on longer time-scales.Comment: Language-edited version, Accepted for publication in A&
Simultaneous brachial diplegia and rotational vertigo due to combined spinal anterior and vertebrobasilar embolism
The stellar dynamics and mass of NGC 1316 using the radial velocities of planetary nebulae
We present a study of the kinematics of the outer regions of the early-type
galaxy NGC 1316, based on radial velocity measurements of 43 planetary nebulae
as well as deep integrated-light absorption line spectra. The smoothed velocity
field of NGC 1316 indicates fast rotation at a distance of 16 kpc, possibly
associated with an elongated feature orthogonal to the inner dust lanes. The
mean square stellar velocity is approximately independent of radius, and the
estimated total mass of the system is 2.6 x 10^11 M_sun within a radius of 16
kpc, implying an integrated mass-to-light ratio of M/L_B = 8.Comment: 39 pages, 14 figures, in press on The Astrophysical Journal n. 50
The old and heavy bulge of M31 I. Kinematics and stellar populations
We present new optical long-slit data along 6 position angles of the bulge
region of M31. We derive accurate stellar and gas kinematics reaching 5 arcmin
from the center, where the disk light contribution is always less than 30%, and
out to 8 arcmin along the major axis, where the disk makes 55% of the total
light. We show that the velocity dispersions of McElroy (1983) are severely
underestimated (by up to 50 km/s) and previous dynamical models have
underestimated the stellar mass of M31's bulge by a factor 2. Moreover, the
light-weighted velocity dispersion of the galaxy grows to 166 km/s, thus
reducing the discrepancy between the predicted and measured mass of the black
hole at the center of M31. The kinematic position angle varies with distance,
pointing to triaxiality. We detect gas counterrotation near the bulge minor
axis. We measure eight emission-corrected Lick indices. They are approximately
constant on circles. We derive the age, metallicity and alpha-element
overabundance profiles. Except for the region in the inner arcsecs of the
galaxy, the bulge of M31 is uniformly old (>12 Gyr, with many best-fit ages at
the model grid limit of 15 Gyr), slightly alpha-elements overabundant
([alpha/Fe]~0.2) and at solar metallicity, in agreement with studies of the
resolved stellar components. The predicted u-g, g-r and r-i Sloan color
profiles match reasonably well the dust-corrected observations. The stellar
populations have approximately radially constant mass-to-light ratios (M/L_R ~
4-4.5 for a Kroupa IMF), in agreement with stellar dynamical estimates based on
our new velocity dispersions. In the inner arcsecs the luminosity-weighted age
drops to 4-8 Gyr, while the metallicity increases to above 3 times the solar
value.Comment: Accepted for publication in A&
Stellar kinematics for the central spheroid in the Polar Disk Galaxy NGC4650A
We have obtained high angular resolution, high signal-to-noise spectra of the
Calcium triplet absorption lines on the photometric axes of the stellar
spheroid in the polar disk galaxy NGC4650A. Along the major axis, the observed
rotation and velocity dispersion measurements show the presence of a
kinematically decoupled nucleus, and a flat velocity dispersion profile. The
minor axis kinematics is determined for the first time: along this direction
some rotation is measured, and the velocity dispersion is nearly constant and
slightly increases at larger distances from the center. The new high resolution
kinematic data suggest that the stellar component in NGC4650A resembles a
nearly-exponential oblate spheroid supported by rotation. The main implications
of these results on the previous mass models for NGC4650A are discussed.
Moreover, the new kinematic data set constraints on current models for the
formation scenarios of Polar Ring Galaxies (PRGs), supporting a slow accretion
rather then a secondary strong dissipative event.Comment: 25 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Constraints on H_0 from the Central Velocity Dispersions of Lens Galaxies
We employ Schwarzschild's method of orbit modeling to constrain the mass
profiles of the central lens galaxies in Q0957+561 and PG 1115+080. We combine
the measured central projected stellar velocity dispersions of these galaxies
with the self-similar radial profiles of the rms velocity and of the
Gauss-Hermite moment h_4 observed in nearby galaxies for 0 < R < 2 R_eff. For
Q0957+561, we find a 16% uncertainty in the galaxy mass, and formal 2-sigma
limits on the Hubble constant of H_0 = (61 +13/-15) km/s/Mpc. For PG 1115+080,
we find that none of the viable lens models can be ruled out, so that H_0 is
not yet strongly constrained by this system.Comment: Revised version accepted by ApJ: slightly modified results for both
lens sytems. 18 pages, with 7 inline Postscript figures, LaTeX, aaspp4.sty;
postscript paper w/figs (490 kb) also available at
http://cfa-www.harvard.edu/~romanow/orbit.post.v2.ps.g
Kinetics and mechanism of proton transport across membrane nanopores
We use computer simulations to study the kinetics and mechanism of proton
passage through a narrow-pore carbon-nanotube membrane separating reservoirs of
liquid water. Free energy and rate constant calculations show that protons move
across the membrane diffusively in single-file chains of hydrogen-bonded water
molecules. Proton passage through the membrane is opposed by a high barrier
along the effective potential, reflecting the large electrostatic penalty for
desolvation and reminiscent of charge exclusion in biological water channels.
At neutral pH, we estimate a translocation rate of about 1 proton per hour and
tube.Comment: 4 pages, 4 figure
Is Galactic Structure Compatible with Microlensing Data?
We generalize to elliptical models the argument of Kuijken (1997), which
connects the microlensing optical depth towards the Galactic bulge to the
Galactic rotation curve. When applied to the latest value from the MACHO
collaboration for the optical depth for microlensing of bulge sources, the
argument implies that the Galactic bar cannot plausibly reconcile the measured
values of the optical depth, the rotation curve and the local mass density.
Either there is a problem with the interpretation of the microlensing data, or
our line of sight to the Galactic centre is highly atypical in that it passes
through a massive structure that wraps only a small distance around the
Galactic centre.Comment: Submitted to ApJ Letters. 8 pages LaTeX, 3 figures. Corrected error
in description of microlensing observation
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