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, $r_S$, of the nucleus at the Galactic Center. This is interesting in view of the predicted circularized flow within $\sim 5-10 r_S$, 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 $\sim 1-2$ 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 ($r_S$) 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 $5-25 r_S$ 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 $\nu\sim 1$ 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

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

Goodyera pubescens (Willd.) R. Br.

https://thekeep.eiu.edu/herbarium_specimens_byname/4044/thumbnail.jp

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&