518 research outputs found
A New Spiral Arm of the Galaxy: The Far 3-Kpc Arm
We report the detection in CO of the far-side counterpart of the well-known
expanding 3-Kpc Arm in the central region of the Galaxy. In a CO
longitude-velocity map at b = 0 deg the Far 3-Kpc Arm can be followed over at
least 20 deg of Galactic longitude as a faint lane at positive velocities
running parallel to the Near Arm. The Far Arm crosses l = 0 deg at +56 km/s,
quite symmetric with the -53 km/s expansion velocity of the Near Arm. In
addition to their symmetry in longitude and velocity, we find that the two arms
have linewidths (~21 km/s), linear scale heights (~103 pc FWHM), and H2 masses
per unit length (~4.3 x 10^6 Mo/kpc) that agree to 26% or better. Guided by the
CO, we have also identified the Far Arm in high-resolution 21 cm data and find,
subject to the poorly known CO-to-H2 ratio in these objects, that both arms are
predominately molecular by a factor of 3-4. The detection of these symmetric
expanding arms provides strong support for the existence of a bar at the center
of our Galaxy and should allow better determination of the bar's physical
properties.Comment: ApJ Letters, accepted 9 July 0
Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy
Non-axisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the outer Lindblad resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bar's pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Way's bar. The combined likelihood function of the bar's pattern speed and angle has its maximum for a pattern speed of Omega(b) = (1.89 +/- 0.08) x Omega(0), where Omega(0) is the local circular frequency. Assuming a solar radius of 8.05 kpc and a local circular velocity of 238 km s(-1), this corresponds to Omega(b) = 56 +/- 2km s(-1) kpc(-1). On the other hand, the bar's orientation phi(b) cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Omega(b)/Omega(0) = 1.91+0.0044 (phi(b)(deg) - 48), with standard deviation of 0.02. For example, for an angle of phi(b) = 30 deg the pattern speed is 54.0 +/- 0.5 km s(-1) kpc(-1). These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance
Bar Diagnostics in Edge-On Spiral Galaxies. I. The Periodic Orbits Approach
We develop diagnostics to detect the presence and orientation of a bar in an
edge-on disk, using its kinematical signature in the position-velocity diagram
(PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral
galaxy mass model, we briefly review the orbital properties of two-dimensional
non-axisymmetric disks and identify the main families of periodic orbits. We
use those families as building blocks to model real galaxies and calculate the
PVDs obtained for various realistic combinations of periodic orbit families and
for a number of viewing angles with respect to the bar. We show that the global
structure of the PVD is a reliable bar diagnostic in edge-on disks.
Specifically, the presence of a gap between the signatures of the families of
periodic orbits in the PVD follows directly from the non-homogeneous
distribution of the orbits in a barred galaxy. Similarly, material in the two
so-called forbidden quadrants of the PVD results from the elongated shape of
the orbits. We show how the shape of the signatures of the dominant x1 and x2
families of periodic orbits in the PVD can be used efficiently to determine the
viewing angle with respect to the bar and, to a lesser extent, to constrain the
mass distribution of an observed galaxy. We also address the limitations of the
models when interpreting observational data.Comment: 22 pages, 9 figures (AASTeX, aaspp4.sty). Accepted for publication in
The Astrophysical Journa
Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
HIRES on the Keck I telescope has been used to measure the first radial
velocities for stars belonging to eleven, heavily-reddened globular clusters in
the direction of the inner Galaxy. The question of kinematic substructuring
among the Galactic globular cluster system is investigated using an updated
catalog of globular cluster distances, metallicities and velocities. It is
found that the population of metal-rich globular clusters shows significant
rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc
of the Galactic center, the measured rotation velocity and line-of-sight
velocity dispersion are similar to those of bulge field stars. We investigate
claims that the metal-rich clusters are associated with the central Galactic
bar by comparing the kinematics of the innermost clusters to that of the atomic
hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich
and metal-poor clusters bears a remarkable similarity to that of the gas,
including the same non-circular motions which have traditionally been
interpreted as evidence for a Galactic bar, or, alternatively, a
non-axisymmetric bulge. However, uncertainties in the existing
three-dimensional Galactocentric positions for most of the clusters do not yet
allow an unambiguous discrimination between the competing scenarios of
membership in a rigidly rotating bar, or in a bulge which is an oblate
isotropic rotator. We conclude that the majority of metal-rich clusters within
the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and
not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available
at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical
Journa
The properties of the Galactic bar implied by gas kinematics in the inner Milky Way
Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way
have long been known to be inconsistent with circular motion in an axisymmetric
potential. Several lines of evidence suggest that the Galaxy is barred, and gas
flow in a barred potential could be consistent with the observed ``forbidden''
velocities and other features in the data. We compare the H I observations to
l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a
family of barred potentials. The gas flow pattern is very sensitive to the
parameters of the assumed potential, which allows us to discriminate among
models. We present a model that reproduces the outer contour of the H I l-V
diagram reasonably well; this model has a strong bar with a semimajor axis of
3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR),
and a pattern speed of 42 km/s/kpc, and matches the data best when viewed from
34\deg to the bar major axis. The behavior of the models, combined with the
constraint that the shocks in the Milky Way bar should resemble those in
external barred galaxies, leads us to conclude that wide ranges of parameter
space are incompatible with the observations. In particular we suggest that the
bar must be fairly strong, must have an ILR, and cannot be too end-on, with the
bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit
larger forbidden velocities over a wider longitude range than are seen in
molecular gas; this important difference is the reason our favored model
differs so significantly from other recently proposed models.Comment: 23 pages, 14 figures, 1 table, uses emulateapj and psfig, 640 kb.
Submitted to Ap
The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies
[Abridged]We compare both the Milky Way and M31 galaxies to local external
disk galaxies within the same mass range, using their relative locations in the
planes formed by V_flat versus M_K, j_disk, and the average Fe abundance of
stars in the galaxy outskirts. We find, for all relationships, that the MW is
systematically offset by ~ 1 sigma, showing a significant deficiency in stellar
mass, in angular momentum, in disk radius and [Fe/H] in the stars in its
outskirts at a given V_flat. On the basis of their location in the M_K, V_flat,
and R_d volume, the fraction of spirals like the MW is 7+/-1%, while M31
appears to be a "typical'' spiral. Our Galaxy appears to have escaped any
significant merger over the last ~10 Gyrs which may explain why it is deficient
by a factor 2 to 3 in stellar mass, angular momentum and outskirts metallicity
and then, unrepresentative of the typical spiral. As with M31, most local
spirals show evidence for a history shaped mainly by relatively recent merging.
We conclude that the standard scenario of secular evolution is generally unable
to reproduce the properties of most (if not all) spiral galaxies. However, the
so-called "spiral rebuilding'' scenario proposed by Hammer et al. 2005 is
consistent with the properties of both distant galaxies and of their
descendants - the local spirals.Comment: 14 pages, 6 figures, to appear in Ap
Relationship between the Velocity Ellipsoids of Galactic-Disk Stars and their Ages and Metallicities
The dependences of the velocity ellipsoids of F-G stars of the thin disk of
the Galaxy on their ages and metallicities are analyzed based on the new
version of the Geneva-Copenhagen Catalog. The age dependences of the major,
middle, and minor axes of the ellipsoids, and also of the dispersion of the
total residual veltocity, obey power laws with indices 0.25,0.29,0.32, and 0.27
(with uncertainties \pm 0.02). Due to the presence of thick-disk objects, the
analogous indices for all nearby stars are about a factor of 1.5 larger.
Attempts to explain such values are usually based on modeling relaxation
processes in the Galactic disk. With increasing age, the velocity ellipsoid
increases in size and becomes appreciably more spherical, turns toward the
direction of the Galactic center, and loses angular momentum. The shape of the
velocity ellipsoid remains far from equilibrium. With increasing metallicity,
the velocity ellipsoid for stars of mixed age increases in size, displays a
weak tendency to become more spherical, and turns toward the direction of the
Galactic center (with these changes occurring substantially more rapidly in the
transition through the metallicity [Fe/H]= -0.25). Thus, the ellipsoid changes
similarly to the way it does with age; however, with decreasing metallicity,
the rotational velocity about the Galactic center monotonically increases,
rather than decreases(!). Moreover, the power-law indices for the age
dependences of the axes depend on the metallicity, and display a maximum near
[Fe/H]=-0.1. The age dependences of all the velocity-ellipsoid parameters for
stars with equal metallicity are roughly the same. It is proposed that the
appearance of a metallicity dependence of the velocity ellipsoids for thin-disk
stars is most likely due to the radial migration of stars.Comment: 15 pages, 6 figures, accepted 2009, Astronomy Reports, Vol. 53 No. 9,
P.785-80
Coupling the dynamics and the molecular chemistry in the Galactic center
The physical conditions of the Galactic center (GC) clouds moving with
non-circular velocities are not well-known. We have studied the physical
conditions of these clouds with the aim of better understanding the origin of
the outstanding physical conditions of the GC molecular gas and the possible
effect of the large scale dynamics on these physical conditions.Using published
CO(1-0) data, we have selected a set of clouds belonging to all the kinematical
components seen in the longitude-velocity diagram of the GC. We have done a
survey of dense gas in all the components using the J=2-1 lines of CS and SiO
as tracers of high density gas and shock chemistry. We have detected CS and SiO
emission in all the kinematical components. The gas density and the SiO
abundance of the clouds in non-circular orbits are similar those in the nuclear
ring (GCR). Therefore, in all the kinematical components there are dense clouds
that can withstand the tidal shear. However, there is no evidence of star
formation outside the GCR. The high relative velocity and shear expected in the
dust-lanes along the bar major axis could inhibit the star formation process,
as observed in other galaxies. The high SiO abundances derived in the
non-circular velocity clouds are likely due to the large-scale shocks that
created the dust lanesComment: One figure as an independent PDF file. Accepted by A&
Solid phase extraction for removal of matrix effects in lipophilic marine toxin analysis by liquid chromatography-tandem mass spectrometry
The potential of solid phase extraction (SPE) clean-up has been assessed to reduce matrix effects (signal suppression or enhancement) in the liquid chromatography-tandem mass spectrometry (LC¿MS/MS) analysis of lipophilic marine toxins. A large array of ion-exchange, silica-based, and mixed-function SPE sorbents was tested. Polymeric sorbents were found to retain most of the toxins. Optimization experiments were carried out to maximize recoveries and the effectiveness of the clean-up. In LC¿MS/MS analysis, the observed matrix effects can depend on the chromatographic conditions used, therefore, two different HPLC methods were tested, using either an acidic or an alkaline mobile phase. The recovery of the optimized SPE protocol was around 90% for all toxins studied and no break-through was observed. The matrix effects were determined by comparing signal response from toxins spiked in crude and SPE-cleaned extracts with those derived from toxins prepared in methanol. In crude extracts, all toxins suffered from matrix effects, although in varying amounts. The most serious effects were observed for okadaic acid (OA) and pectenotoxin-2 (PTX2) in the positive electrospray ionization mode (ESI+). SPE clean-up on polymeric sorbents in combination with the alkaline LC method resulted in a substantial reduction of matrix effects to less than 15% (apparent recovery between 85 and 115%) for OA, yessotoxin (YTX) in ESI¿ and azaspiracid-1 (AZA1), PTX2, 13-desmethyl spirolides C (SPX1), and gymnodimine (GYM) in ESI+. In combination with the acidic LC method, the matrix effects after SPE were also reduced but nevertheless approximately 30% of the matrix effects remained for PTX2, SPX1, and GYM in ESI+. It was concluded that SPE of methanolic shellfish extracts can be very useful for reduction of matrix effects. However, the type of LC and MS methods used is also of great importance. SPE on polymeric sorbents in combination with LC under alkaline conditions was found the most effective method
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