509 research outputs found
A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*
The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
, of the nucleus at the Galactic Center. This is interesting in view of
the predicted circularized flow within , based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. In addition, the
self-Comptonization of the sub-mm bump appears to produce an X-ray flux
exceeding that due to bremsstrahlung from this region, which may account for
the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However,
the required accretion rate in the Keplerian flow is orders of magnitude
smaller than that predicted by the Bondi-Hoyle simulations. We speculate that
rapid evaporation, in the form of a wind, may ensue from the heating associated
with turbulent mixing of gas elements with large eccentricity as they settle
down into a more or less circular (i.e., low eccentricity) trajectory. The
spectrum of Sgr A* longward of mm may be generated outside of the
Keplerian flow, where the gas is making a transition from a quasi-spherical
infall into a circularized pattern.Comment: 40 pages, 9 figure
Polarized mm And sub-mm Emission From Sgr A* At The Galactic Center
The recent detection of significant linear polarization at mm and sub-mm
wavelengths in the spectrum of Sgr A* (if confirmed) will be a useful probe of
the conditions within several Schwarzschild radii () of the event horizon
at the Galactic Center. Hydrodynamic simulations of gas flowing in the vicinity
of this object suggest that the infalling gas circularizes when it approaches
within of the black hole. We suggest that the sub-mm ``excess'' of
emission seen in the spectrum of Sgr A* may be associated with radiation
produced within the inner Keplerian region and that the observed polarization
characteristics provide direct evidence for this phenomenon. The overall
spectrum from this region, including the high-energy component due to
bremsstrahlung and inverse Compton scattering processes, is at or below the
recent {\it Chandra} measurement, and may account for the X-ray source if it
turns out to be the actual counterpart to Sgr A*.Comment: 12 pages, 2 figures. published in APJ Letter
Measuring the Black Hole Spin in Sgr A*
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a
Keplerian structure whose peak emission occurs within several Schwarzschild
radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if
confirmed, is presumably the self-Comptonized component from this region. In
this paper, we suggest that sub-mm timing observations could yield a signal
corresponding to the period P_0 of the marginally stable orbit, and therefore
point directly to the black hole's spin a. Sgr A*'s mass is now known to be
(2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive
black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of
a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild
black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital
frequency with the innermost stable circular orbit is made feasible by the
transition from optically thick to thin emission at sub-mm wavelengths. With
stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum
is thus produced at the smallest radius. We caution, however, that theoretical
uncertainties in the structure of the emission region may still produce some
ambiguity in the timing signal. Given that Sgr A*'s flux at mm is
several Jy, these periods should lie within the temporal-resolving capability
of sub-mm telescopes using bolometric detectors. A determination of P_0 should
provide not only a value of a, but it should also define the angular momentum
vector of the orbiting gas in relation to the black hole's spin axis. In
addition, since the X-ray flux detected by Chandra appears to be the
self-Comptonized mm to sub-mm component, these temporal fluctuations may also
be evident in the X-ray signal.Comment: 15 pages, 1 figures. Accepted for publication in ApJ Letter
High resolution observations of compact H II regions at 230 GHz
Based on the idea that star formation goes on progressively in molecular clouds, a search was conducted for protostars by mapping compact H II regions at a frequency of 250 GHz. The IRAM 30 m radio telescope was used with a (3)He cooled bolometer. Twenty compact H II regions usually obtaining twice the expected free-free flux density, positionally coincident with the H II region, were observed. Even fine structure within the H II regions can be traced in the maps as in the case of G75.84+0.40 near ON-2. The high degree of coincidence between the 250 and 5 GHz map of Harris shows that the excess flux density observed must come from dust mixed with the ionized gas. Part of the dust must however be accumulated in the outer part of the H II region, since in some cases the contours are shifted outwards relative to the radio maps. This is consistent with the fact that in those cases where enough information is available to make a model fit, temperatures were derived of 80 + or - 30 K
The simultaneous spectrum of Sgr A* from 20cm to 1mm and the nature of the mm-excess
We report results from a multiwavelength campaign to measure the simultaneous
spectrum of the super-massive black hole candidate Sgr A* in the Galactic
Center from cm to mm-wavelengths using the VLA, BIMA, the Nobeyama 45m, and the
IRAM 30m telescopes. The observations confirm that the previously detected
mm-excess is an intrinsic feature of the spectrum of Sgr A*. The excess can be
interpreted as due to the presence of an ultra-compact component of
relativistic plasma with a size of a few Schwarzschild radii near the black
hole. If so, Sgr A* might offer the unique possibility to image the putative
black hole against the background of this component with future mm-VLBI
experiments.Comment: Apj, in press, (AAS, emulateapj) LaTex, 6 pages, preprint also
available at
http://www.mpifr-bonn.mpg.de/mpivlb/falcke/publications.html#campaig
Goodyera pubescens (Willd.) R. Br.
https://thekeep.eiu.edu/herbarium_specimens_byname/4044/thumbnail.jp
Infalling Gas Towards the Galactic Center
VLA maps of ammonia emission were made for the Galactic Center region. The
NH3(1,1) and NH3(2,2) transitions were observed in three 2' x 2' fields
covering Sgr A* and the region 3' immediately south of it. In the central 3
parsecs surrounding Sgr A* we find emission which appears to be associated with
the circumnuclear disk (CND), both morphologically and kinematically. This
central emission is connected to a long, narrow 2 pc x 10 pc streamer of clumpy
molecular gas located towards the south, which appears to be carrying gas from
the nearby 20 km/s giant molecular cloud (GMC) to the circumnuclear region. We
find a velocity gradient along the streamer, with progressively higher
velocities as the gas approaches Sgr A*. The streamer stops at the location of
the CND, where the line width of the NH3 emission increases dramatically. This
may be the kinematic signature of accretion onto the CND. The ratio of the
NH3(2,2)/NH3(1,1) emission indicates that the gas is heated at the northern tip
of the streamer, located inside the eastern edge of the CND. The morphology,
kinematics and temperature gradients of the gas all indicate that the southern
streamer is located at the Galactic Center and is interacting with the
circumnuclear region.Comment: 11 pages, 10 figures, accepted by The Astrophysical Journal (figure 1
contours have been corrected
Fraxinus nigra Pott
https://thekeep.eiu.edu/herbarium_specimens_byname/2922/thumbnail.jp
The line-of-sight distribution of the gas in the inner 60 pc of the Galaxy
2MASS K_S band data of the inner 60 pc of the Galaxy are used to reconstruct
the line-of-sight distances of the giant molecular clouds located in this
region. Using the 2MASS H band image of the same region, two different
populations of point sources are identified according to their flux ratio in
the two bands. The population of blue point sources forms a homogeneous
foreground that has to be subtracted before analyzing the K_S band image. The
reconstruction is made using two basic assumptions: (i) an axis-symmetric
stellar distribution in the region of interest and
(ii) optically thick clouds with an area filling factor of ~1 that block all
light of stars located behind them. Due to the reconstruction method, the
relative distance between the different cloud complexes is a robust result,
whereas it is not excluded that the absolute distance with respect to Sgr A* of
structures located more than 10 pc in front of Sgr A* are understimated by up
to a factor of 2. It is shown that all structures observed in the 1.2 mm
continuum and in the CS(2-1) line are present in absorption. We place the 50 km
s^-1 cloud complex close to, but in front of, Sgr A*. The 20 km s^-1 cloud
complex is located in front of the 50 km s^-1 cloud complex and has a large LOS
distance gradient along the direction of the galactic longitude. The bulk of
the Circumnuclear Disk is not seen in absorption. This leads to an upper limit
of the cloud sizes within the Circumnuclear Disk of ~0.06 pc.Comment: 12 pages with 17 figures. Accepted for publication in A&
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