281 research outputs found
Detecting Galactic MACHOs with VERA through Astrometric Microlensing of Distant Radio Sources
In this paper we investigate the properties of astrometric microlensing of
distant radio sources (QSOs and radio galaxies) due to MACHOs, and discuss
their implications for VERA (VLBI Exploration of Radio Astrometry). First we
show that in case of astrometric microlensing of distant sources, the event
duration is only a function of the lens mass and tangential velocity, but
independent of the lens distance, in contrast to the well-known three-fold
degeneracy for photometric microlensing. Moreover, the lens mass is scaled
by the tangential velocity as , rather than
which is the case for photometric microlensing. Thus, in
astrometric microlensing the dependence of the lens mass on the unknown
parameter is weaker, indicating that the duration of astrometric
microlensing event is a better quantity to study the mass of lensing objects
than that of photometric microlensing. We also calculate the optical depth and
event rate, and show that within 20 of the galactic center a typical
event rate for 10 as-level shift is larger than event
per year, assuming that a quarter of the halo is made up with MACHOs. This
indicates that if one monitors a few hundred sources for 20 years, such
an astrometric microlensing event is detectable. Since a typical event duration
is found to be fairly long (5 to 15 years), the frequency of the monitoring
observation can be relatively low, i.e., once per six months, which is rather
reasonable for practical observations. We discuss practical strategy for
observing astrometric microlensing with VERA, and argue that an astrometric
microlensing event due to MACHOs can be detected by VERA within a few decades.Comment: 10 pages, 6 figures included, accepted for publication in PAS
Mass of the Galaxy Inferred from Outer Rotation Curve
Using an outer rotation curve of the Galaxy, we explore the galactic
constants and the mass of the Galaxy. We show that \Theta_0 of 200 km/s is more
favorable than the IAU standard value of 220 km/s, and also show that if
\Theta_0 is smaller than 207 km/s the rotation curve beyond 2R_0 is declining
in Keplerian fashion. In the case of \Theta_0= 200 km/s and R_0= 7.6 kpc, the
total mass and the extent of the Galaxy inferred from the rotation curve are
2.0+/-0.3x 10^{11} M_\odot and 15 kpc, respectively. These results may
significantly change the previous view of the Galaxy, that its outer region is
dominated by a massive dark halo extending out to several tens of kpc.Comment: Latex, 4 pages and 3 figures, to appear in PASJ Letter Vol.48 No.
Nuclear Rotation Curves of Galaxies in the CO Line Emission
We have obtained high-resolution position-velocity (PV) diagrams along the
major axes of the central regions of nearby galaxies in the CO-line emission
using the Nobeyama 45-m telescope and the Millimeter Array. Nuclear rotation
curves for 14 galaxies have been derived based on the PV diagrams using the
envelope-tracing method. The nuclear rotation curves for most of the galaxies
show a steep rise within a few hundred pc, which indicates a high-density
concentration of mass.
Keywords: Galaxies: general - Galaxies: structure - ISM: molecular lineComment: To appear in AJ, Plain TeX, Figures on reques
On the Keplerian Rotation Curves of Galaxies
We propose a criterion for examining whether or not the uncertainty of the
outer rotation curves is sufficiently small to distinguish a Keplerian rotation
curve from flat ones. We have applied this criterion to both Keplerian and
non-Keplerian rotation curves so far obtained, and investigated their relative
fraction. We also studied the minimum extent of the dark halos indicated by
non-Keplerian rotation curves using the criterion. We have found that one
cannot rule out the possibility that a significant fraction of rotation curves
become Keplerian within 10-times the disk scale length. If the Keplerian
rotation curves so far observed trace the mass truncation, several galaxies may
have rather small halos, the extent of which is not larger than twice that of
the optical disk.Comment: Latex, 6 pages and 3 figures, to appear in PASJ Vol.49 No.
Declining Rotation Curve and Brown Dwarf MACHOs
If the Galactic rotation speed at the Solar circle is km s
or smaller, which is supported by several recent studies, the rotation curve of
the Galaxy could be declining in the outermost region. Motivated by this, we
investigate the effect of such declining rotation curve on the estimate of the
MACHO mass and the fractional contribution of the MACHOs to the Galactic dark
halo. Using Hernquist and Plummer halo models instead of the standard halo
model, we find that the MACHO mass could be significantly smaller than that for
the standard halo case. In particular, there exists a certain set of halo
parameters for which the MACHO mass is 0.1 or less and at the same
time the MACHO contribution to the total mass of the halo is almost 100 %. This
result indicates that a halo which consists solely of brown dwarfs can be
consistent with both of the observed microlensing properties and the
constraints from the rotation curve, provided the outer rotation curve is
indeed declining.Comment: 8 pages and 4 figures, accepted for publication in ApJ Letter
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