Nature 438, 62-64 (3 November 2005) | doi: 10.1038/nature04205

 A size of 1 AU for the radio source Sgr A* at the centre of the Milky Way

    Zhi-Qiang Shen1, K. Y. Lo2, M.-C. Liang3, Paul T. P. Ho4,5 and J.-H. Zhao4

Although it is widely accepted that most galaxies have supermassive black holes
at their centres1, 2, 3, concrete proof has proved elusive. Sagittarius A* (Sgr
A*)4, an extremely compact radio source at the centre of our Galaxy, is the best
candidate for proof5, 6, 7, because it is the closest. Previous very-long-baseline 
interferometry observations (at 7 mm wavelength) 
reported that Sgr A* is approx2 astronomical units (au) in size8, but this is still
larger than the 'shadow' (a remarkably dim inner region encircled by a bright
ring) that should arise from general relativistic effects near the event horizon
of the black hole9. Moreover, the measured size is wavelength dependent10. Here
we report a radio image of Sgr A* at a wavelength of 3.5 mm, demonstrating that
its size is approx1 au. When combined with the lower limit on its mass11, the
lower limit on the mass density is 6.5 times 1021Mcircle dot pc-3 (where Mcircle
dot is the solar mass), which provides strong evidence that Sgr A* is a
supermassive black hole. The power-law relationship between wavelength and
intrinsic size (sizewavelength1.09) explicitly rules out explanations other than
those emission models with stratified structure, which predict a smaller
emitting region observed at a shorter radio wavelength.

   1. Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 
200030, China
   2. National Radio Astronomy Observatory, 520 Edgemont Road, 
Charlottesville,
Virginia 22903, USA
   3. Division of Geological and Planetary Sciences, California 
Institute of
Technology, Pasadena, California 91125, USA
   4. Harvard-Smithsonian CfA, 60 Garden Street, Cambridge, 
Massachusetts 02138, USA
   5. Institute of Astronomy & Astrophysics, Academia Sinica, PO Box 
23-141,
Taipei 106, Taiwan, China 

Correspondence to: Zhi-Qiang Shen1 Correspondence and requests for 
materials
should be addressed to Z.-Q.S. (Email: zshen@shao.ac.cn).

Received 1 July 2005; Accepted 31 August 2005

 Sent by odyssey(Jianfeng Zhou, Tsinghua Center for Astrophysics, 
zhoujf@mail.tsinghua.edu.cn)

See paper of :

A CCD Photometric and Morphological Study of the Extended Halo and
Filaments of ESO 383-45: A Galaxy Undergoing Ram Pressure Stripping, or a
Tidal Merger Remnant?
S. N. Kemp, Eduardo de la Fuente, A. Franco-Balderas, and J. Meaburn
Page 680  [ http://www.journals.uchicago.edu/cgi-bin/resolve?ApJ61687 ]

Title: Chandra Observations of the Interacting Galaxies NGC 3395/3396 (Arp 270)
Authors: N. J. Brassington (1), A. M. Read (2), T. J. Ponman (1) ((1)
 University of Birmingham, (2) University of Leicester)
Comments: 18 pages, 16 figures, accepted for publication in MNRAS
\\
 In this paper we present the results of a 20 ks high resolution Chandra X-ray observation of the peculiar galaxy pair NGC 3395/3396, a system at a very early stage of merging, and less evolved than the famous Antennae and Mice merging systems. Previously unpublished ROSAT HRI data are also presented. The point source population and the hot diffuse gas in this system are investigated, and compared with other merging galaxy pairs. 16 X-ray point sources are detected in Arp 270, 7 of which are classified as ULXs (Lx > 10^39 erg/s). From spectral fits and the age of the system it seems likely that these are predominantly high mass X-ray binaries. The diffuse gas emits at a global temperature of ~0.5 keV, consistent with temperatures observed in other interacting systems, and we see no evidence of the starburst-driven hot gaseous outflows seen in more evolved systems such as The Mice and The Antennae. It is likely that these features are absent from Arp 270 as the gas has had insufficient time to break out of the galaxy disks. 32% of the luminosity of Arp 270 arises from the diffuse gas in the system, this is low when compared to later stage merging systems and gives further credence that this is an early stage merger. Comparing the ULX population of Arp 270 to other merging systems, we derive a relationship between the star formation rate of the system, indicated by Lfir, and the number (N(ULX)) and luminosity (Lulx) of its ULX population. We find Nulx proportional to Lfir^0.18 and Lulx proportional to Lfir^0.54. These relationships, coupled with the relation of the point source X-ray luminosity (Lxp) to Lk and Lfir+uv (Colbert et al. 2003), indicate that the ULX sources in an interacting system have contributions from both the old and young stellar populations.
\\ ( http://arXiv.org/abs/astro-ph/0504284 ,  649kb)

Decomposition of the Visible and Dark Matter in the Einstein Ring 0047-2808 by Semilinear Inversion

S. Dye and S. J. Warren
Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BW, UK

Received 2004 July 13; accepted 2004 December 17

ABSTRACT

We measure the mass density profile of the lens galaxy in the Einstein ring system 0047-2808 using our semilinear inversion method developed in an earlier paper. By introducing an adaptively gridded source plane, we are able to eliminate the need for regularization of the inversion. This removes the problem of a poorly defined number of degrees of freedom, encountered by inversion methods that employ regularization, and so allows a proper statistical comparison between models. We confirm previous results indicating that the source is double and that a power-law model gives a significantly better fit than the singular isothermal ellipsoid model. We measure a slope = 2.11 ± 0.04. We find, furthermore, that a dual-component constant M/L baryonic+dark halo model gives a significantly better fit than the power-law model, at the 99.7% confidence level. The inner logarithmic slope of the dark halo profile is found to be 0.87 (95% CL), consistent with the predictions of cold dark matter simulations of structure formation. We determine an unevolved B-band mass-to-light ratio for the baryons (only) of 3.05 h65 M/LB (95% CL). This is the first measurement of the baryonic M/L of a single galaxy by purely gravitational lens methods. The baryons account for 65% (95% CL) of the total projected mass, or, assuming spherical symmetry, 84% (95% CL) of the total three-dimensional mass within the mean radius of 116 (7.5 h kpc) traced by the ring. Finally, at the level of >3 , we find that the halo mass is rounder than the baryonic distribution and that the two components are offset in orientation from one another.

Subject headings: dark mattergravitational lensing

\\
Paper: astro-ph/0504086
Date: Mon, 4 Apr 2005 21:33:42 GMT   (776kb)

Title: The Vega Debris Disk -- A Surprise from Spitzer
Authors: K. Y. L. Su (1), G. H. Rieke (1), K. A. Misselt (1), J. A. Stansberry
 (1), A. Moro-Martin (1,3), K. R. Stapelfeldt (2), M. W. Werner (2), D. E.
 Trilling (1), G. J. Bendo (1), K. D. Gordon (1), D. C. Hines (4), M. C. Wyatt
 (5), W. S. Holland (5), M. Marengo (6), S. T. Megeath (6), and G. G. Fazio
 (6) ((1) Steward Observatory, (2) JPL/Caltech, (3) Max-Planck-Institut fur
 Astronomie, (4) Space Science Institute, (5) Royal Observatory, (6) CfA)
Comments: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2,
 3a, 3b and 4 have been degraded to lower resolutions.)
\\
 We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved and its angular size is much larger than found previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and 105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are circular, smooth and without clumpiness at all three wavelengths. The radial surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an r^-1 power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger than 180 um and at radii of 86-200 AU from the star, can reproduce the observed 850 um flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters amongthese large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances where we detect its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS results. This rate would require a very massive asteroidal reservoir for the
dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and subsequent collisional cascade.
\\ ( http://arXiv.org/abs/astro-ph/0504086 ,  776kb)

Paper: astro-ph/0504036
Date: Fri, 1 Apr 2005 20:17:09 GMT   (997kb)

Title: Gemini and Chandra observations of Abell 586, a relaxed strong-lensing cluster
Authors: E. S. Cypriano, G. B. Lima Neto, L.Sodr\'e Jr, Jean Paul Kneib and  Luis E. Campusano
Comments: ApJ accepted, 20 pages, 11 figures

 We analyze the mass content of the massive strong-lensing cluster Abell 586 (z = 0.17). We use optical data (imaging and spectroscopy) obtained with the Gemini Multi-Object Spectrograph (GMOS) mounted on the 8-m Gemini-North telescope, together with publicly available X-ray data taken with the Chandra space telescope. Employing different techniques -- velocity distribution of galaxies, weak gravitational lensing, and X-ray spatially resolved spectroscopy -- we derive mass and velocity dispersion estimates from each of them. All estimates agree well with each other, within a 68% confidence level, indicating a velocity dispersion of 1000 -- 1250 km/s. The projected mass distributions obtained through weak-lensing and X-ray emission are strikingly similar, having nearly circular geometry. We suggest that Abell 586 is probably a truly relaxed cluster, whose last major merger occurred more than ~4 Gyr ago.
\\ ( http://arXiv.org/abs/astro-ph/0504036 ,  997kb)