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به دنبال فرمولی برای سیاه چاله ها در NGC 922
#1
[تصویر:  13548752961.jpg]







In this holiday season of home cooking and carefully-honed recipes, some
astronomers are asking: what is the best mix of ingredients for stars
to make the largest number of plump black holes?




They are tackling this problem by studying the number of black holes in
galaxies with different compositions. One of these galaxies, the ring
galaxy NGC 922, is seen in this composite image containing X-rays from
NASA's Chandra X-ray Observatory (red) and optical data from the Hubble
Space Telescope (pink, yellow and blue).




NGC 922 was formed by the collision between two galaxies – one seen in
this image and another located outside the field of view. This
collision triggered the formation of new stars in the shape of a ring.
Some of these were massive stars that evolved and collapsed to form
black holes.




Most of the bright X-ray sources in Chandra's image of NGC 922 are black
holes pulling material in from the winds of massive companion stars.
Seven of these are what astronomers classify as "ultraluminous X-ray
sources" (ULXs). These are thought to contain stellar-mass black holes
that are at least ten times more massive than the sun, which places them
in the upper range for this class of black hole. They are a different
class from the supermassive black holes found at the centers of
galaxies, which are millions to billions of times the mass of the sun.




Theoretical work suggests that the most massive stellar-mass black holes
should form in environments containing a relatively small fraction of
elements heavier than hydrogen and helium, called “metals” by
astronomers. In massive stars, the processes that drive matter away
from the stars in stellar winds work less efficiently if the fraction of
metals is smaller. Thus, stars with fewer of these metals among their
ingredients should lose less of their mass through winds as they evolve.
A consequence of this reduced mass loss is that a larger proportion of
massive stars will collapse to form black holes when their nuclear fuel
is exhausted. This theory appeared to be supported by the detection of a
large number (12) of ULXs in the Cartwheel galaxy, where stars
typically contain only about 30% of the metals found in the sun.




To test this theory, scientists studied NGC 922, which contains about
the same fraction of metals as the sun, meaning that this galaxy is
about three times richer in metals than the Cartwheel galaxy. Perhaps
surprisingly, the number of ULXs found in NGC 922 is comparable to the
number seen in the Cartwheel galaxy. Rather, the ULX tally appears to
depend only on the rate at which stars are forming in the two galaxies,
not on the fraction of metals they contain.




One explanation for these results is that the theory predicting the most
massive stellar-mass black holes should form in metal poor conditions
is incorrect. Another explanation is that the metal fraction in the
Cartwheel galaxy is not low enough to have a clear effect on the
production of unusually massive stellar-mass black holes, and therefore
will not cause an enhancement in the number of ULXs. Recent models
incorporating the evolution of stars suggest that a clear enhancement in
the number of ULXs might only be seen when the metal fraction falls
below about 15% of the Sun's value. Astronomers are investigating this
possibility by observing galaxies with extremely low metal fractions
using Chandra. The number of ULXs is being compared with the number
found in galaxies with higher metal content. The results of this work
will be published in a future paper.




A paper describing the results for NGC 922 was published in the March 10, 2012 issue of the Astrophysical Journal.
The authors were Andrea Prestwich and Jose Luis Galache of the
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA; Tim
Linden from University of Santa Cruz in Santa Cruz, CA; Vicky Kalogera
from Northwestern University in Evanston, IL; Andreas Zezas from CfA and
University of Crete in Crete, Greece; Tim Roberts from University of
Durham in Durham, UK; Roy Kilgard from Wesleyan University in
Middletown, CT; Anna Wolter and Ginevra Trinchieri from INAF in Milano,
Italy.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science and
flight operations from Cambridge, Mass.




Credits: X-ray: NASA/CXC/SAO/A. Prestwich et al; Optical: NASA/STScI




› Read more/access all images


› Chandra's Flickr photoset




J.D. Harrington, 202-358-0321
Headquarters, Washington
j.d.harrington@nasa.gov

Janet Anderson, 256-544-0034
Marshall Space Flight Center, Huntsville, Ala.
janet.l.anderson@nasa.gov

Megan Watzke 617-496-7998
Chandra X-ray Center, Cambridge, Mass.
mwatzke@cfa.harvard.edu
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  سیاه چاله ای پرجرم در ngc4178 amir astronomer 0 273 26-10-2012، 12:38 AM
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  کهکشان یا سیاه چاله؛ کدام یک اول بوجود آمد؟ M@h$A 0 337 11-01-2012، 06:52 PM
آخرین ارسال: M@h$A

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