85 research outputs found
M31* and its circumnuclear environment
We present a multiwavelength investigation of the circumnuclear environment
of M31. Based on Chandra/ACIS data, we tightly constrain the X-ray luminosity
of M31*, the central supermassive black hole of the galaxy, to be L (0.3-7
keV)<= 1.2x10^{36}erg/s, approximately 10^{-10} of the Eddington luminosity.
From the diffuse X-ray emission, we characterize the circumnuclear hot gas
with a temperature of ~0.3 keV and a density of ~0.1 cm^{-3}. In the absence of
an active SMBH and recent star formation, the most likely heating source for
the hot gas is Type Ia SNe. The presence of cooler, dusty gas residing in a
nuclear spiral has long been known in terms of optical line emission and
extinction. We further reveal the infrared emission of the nuclear spiral and
evaluate the relative importance of various possible ionizing sources. We show
evidence for interaction between the nuclear spiral and the hot gas, probably
via thermal evaporation. This mechanism lends natural understandings to 1) the
inactivity of M31*, in spite of a probably continuous supply of gas from outer
disk regions, and 2) the launch of a bulge outflow of hot gas, primarily
mass-loaded from the circumnuclear regions. One particular prediction of such a
scenario is the presence of gas with intermediate temperatures arising from the
conductive interfaces. The FUSE observations do show strong OVI1032
and 1038 absorption lines against the bulge starlight, but the effective OVI
column density (~4x10^{14} cm^{-2}), may be attributed to foreground gas
located in the bulge and/or the highly inclined disk of M31. Our study strongly
argues that stellar feedback, particularly in the form of energy release from
SNe Ia, may play an important role in regulating the evolution of SMBHs and the
interstellar medium in galactic bulges.Comment: Submitted to MNRAS, 33 pages, 9 figures. Comments welcom
A confirmed location in the Galactic halo for the high-velocity cloud 'chain A'
The high-velocity clouds of atomic hydrogen, discovered about 35 years ago,
have velocities inconsistent with simple Galactic rotation models that
generally fit the stars and gas in the Milky Way disk. Their origins and role
in Galactic evolution remain poorly understood, largely for lack of information
on their distances. The high-velocity clouds might result from gas blown from
the Milky Way disk into the halo by supernovae, in which case they would enrich
the Galaxy with heavy elements as they fall back onto the disk. Alternatively,
they may consist of metal-poor gas -- remnants of the era of galaxy formation,
accreted by the Galaxy and reducing its metal abundance. Or they might be truly
extragalactic objects in the Local Group of galaxies. Here we report a firm
distance bracket for a large high-velocity cloud, Chain A, which places it in
the Milky Way halo (2.5 to 7 kiloparsecs above the Galactic plane), rather than
at an extragalactic distance, and constrains its gas mass to between 10^5 and 2
times 10^6 solar masses.Comment: 8 pages, including 4 postscript figures. Letter to Nature, 8 July
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