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Mystery in the Heart of Andromeda

A strange blue light near the core of the Andromeda Galaxy promises to tell us much about black holes and the behavior of objects near them. First spotted in 1995 by the Hubble Space Telescope, the blue light was thought to emmanate from a single, massive star, or possibly an exotic source of energy that was little understood. But new spectroscopic observations show that the light is actually made up of 400 stars packed into a disk only one light year across.

Now this is a very strange finding, for these young stars — thought to be on the order of 200 million years old — are revolving around the black hole at the center of Andromeda so closely that they should be torn apart. How could gas and dust coalesce to form stars in such an environment? Mysteries, of course, are just what astronomers like to find; they often lead to enough new data to revise earlier theories and produce more complete explanations.

Image: This artist’s concept shows a view across a mysterious disk of young, blue stars encircling a supermassive black hole at the core of the neighboring Andromeda Galaxy (M31). The region around the black hole is barely visible at the center of the disk. The background stars are the typical older, redder population of stars that inhabit the cores of most galaxies. Under the black hole’s gravitational grip, the stars are traveling very fast: 2.2 million miles an hour (3.6 million kilometers an hour, or 1,000 kilometers a second). Credit: NASA, ESA and A. Schaller (for STScI).

And that black hole at Andromeda’s center seems to be confirmed. The Hubble data are precise enough to rule out other exotic alternatives. The black hole that emerges, so to speak, has a mass equivalent to 140 million Suns. The blue stars that surround it are, in turn, nestled inside an elliptical ring of older, redder stars. And the fact that the blue stars are as young as they are implies they are only the most recent manifestation of a recurring phenomenon.

“The blue stars in the disk are so short-lived that it is unlikely in the long 12-billion-year history of Andromeda that such a short-lived disk would appear now,” said Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona. “That’s why we think that the mechanism that formed this disk of stars probably formed other stellar disks in the past and will trigger them again in the future. We still don’t know, however, how such a disk could form in the first place. It still remains an enigma.”

Centauri Dreams‘ note: Forty detections of black holes have been accomplished, but in almost all cases the evidence is not conclusive. Until now, the only two unambiguous detections, according to astronomer John Kormendy of the University of Texas in Austin, were in the galaxy NGC 4258 and in our own Milky Way. Andromeda gives us three confirmed black holes, and opens up new speculation on star formation in circumstances where it was once thought impossible.

More in this European Space Agency news release. The team’s results will appear in the September 20 issue of The Astrophysical Journal.

Comments on this entry are closed.

  • ljk January 8, 2007, 13:52

    Andromeda Galaxy Five Time Bigger Than Thought

    The discovery of several large, metal-poor stars located
    far from the center of the Andromeda galaxy suggests our
    nearest galactic neighbor might be up to five times larger
    than previously thought.


  • ljk May 9, 2007, 21:37

    The Collision Between The Milky Way And Andromeda

    Authors: T.J. Cox, Abraham Loeb (Harvard/CfA)

    (Submitted on 8 May 2007)

    Abstract: We use a N-body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years – within the Sun’s lifetime. During the first close encounter of the two galaxies, there is a 12% chance that the Sun will be pulled from its present position and reside in the extended tidal material. After the second close encounter, there is a 30% chance that the Sun will reside in the extended tidal material, and a 2.7% chance that our Sun will be more tightly bound to Andromeda than to the Milky Way. Eventually, after the merger has completed, the Sun is likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late–forming elliptical galaxies.


    submitted to MNRAS


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0705.1170v1 [astro-ph]

    Submission history

    From: Thomas Cox J [view email]

    [v1] Tue, 8 May 2007 20:40:59 GMT (1132kb)


  • ljk May 28, 2007, 21:49

    An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters

    Authors: S. Galleti, M. Bellazzini, L. Federici, A. Buzzoni, F. Fusi Pecci

    (Submitted on 28 May 2007)

    Abstract: We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M31 galaxy. We obtained low resolution spectra of 48 targets selected from the XSC of 2MASS, as in Galleti et al. (2005). The observed candidates have been robustly classified according to their radial velocity and by verifying their extended/point-source nature from ground-based optical images. Among the 48 observed candidates clusters we found 5 genuine remote globular clusters. One of them has been already identified independently by Mackey et al. (2007), their GC1; the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40 kpc 40 kpc. At odds with the Milky Way, M31 appears to have a significant population of very bright globular clusters in its extreme outskirts.


    16 pages including 6 pages published only in the electronic edition of the Journal. Accepted for publication in A&A


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0705.4037v1 [astro-ph]

    Submission history

    From: Silvia Galleti [view email]

    [v1] Mon, 28 May 2007 12:30:01 GMT (326kb)


  • ljk March 19, 2008, 18:44

    The Nature and Origin of Substructure in the Outskirts of M31. I. Surveying the Stellar Content with HST/ACS

    Authors: J. C. Richardson, A. M. N. Ferguson, R. A. Johnson, M. J. Irwin, N. R. Tanvir, D. C. Faria, R. A. Ibata, K. V. Johnston, G. F. Lewis

    (Submitted on 18 Mar 2008)

    Abstract: We present the largest and most detailed survey to date of the stellar populations in the outskirts of M31 based on the analysis of 14 deep HST/ACS pointings spanning the range 11.5-45.0 kpc. We conduct a quantitative comparison of the resolved stellar populations in these fields and identify several striking trends. The color-magnitude diagrams (CMDs), which reach ~3 magnitudes below the red clump, can be classified into two main categories based on their morphologies. `Stream-like’ fields, so named for their similarity to the CMD of the giant stellar stream, are characterized by a red clump that slants bluewards at fainter magnitudes and an extended horizontal branch. On the other hand, `disk-like’ fields exhibit rounder red clumps with significant luminosity width, lack an obvious horizontal branch and show evidence for recent star formation (~0.25 – 2.0 Gyr ago).

    We compare the spatial and line-of-sight distribution of stream-like fields with a recent simulation of the giant stream progenitor orbit and find an excellent agreement. These fields, found across the face of M31, attest to the high degree of pollution caused by this event. Disk-like material resides in the extended disk structure of M31 and is detected out to 44 kpc (projected); the uniform populations in these fields, including the ubiquitous presence of young populations, and the strong rotation reported elsewhere are most consistent with a scenario in which this structure has formed through heating and disruption of the existing thin disk, perhaps due to the impact of the giant stream progenitor. Our comparative analysis sheds new light on the likely composition of two of the ultra-deep pointings formerly presented as pure outer disk and pure halo in the literature.

    Comments: Accepted to AJ. this ftp URL contains high resolution figures. 16 pages, 10 figures; best seen in color on above link

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.2614v1 [astro-ph]

    Submission history

    From: Jenny Richardson [view email]

    [v1] Tue, 18 Mar 2008 13:11:05 GMT (415kb)