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Omega Centauri: When Galaxies Collide

By Larry Klaes

An alternative title for Larry’s new story might be “Toward a Science of Galactic Archaeology.” For the vast cities of stars we see in the night sky are in a constant, if extremely long-term, process of re-shaping themselves through encounters with other galaxies, an activity whose traces in the distant past may still be detectable. In fact, astronomers hoping to learn more about such collisions may have a interesting remnant close at hand. As Larry writes, Omega Centauri offers some characteristics that set it apart from the average globular cluster, and point to a much different origin.

Just days ago, the team that operates the Hubble Space Telescope (HST) released a large collection of images on the eighteenth anniversary of the astronomical instrument’s deployment into Earth orbit that show dozens of galaxies doing what the team called “interactions” with each other, but which can just as easily be described as collisions.

The new Hubble images show massive islands of hundreds of billions of stars being pulled into each other by their immense collective gravities. These collisions – while causing few actual impacts between stars – reshape the merging galaxies over millions of years in a cosmic ballet that seems unbelievably slow to us short-lived humans. Billions of stars and any planets they possess are whipped about into new regions of space; many are even flung away into the intergalactic realm forever.

Our Milky Way galaxy is no stranger to such mergers. Astronomers have found the faded remains of smaller galaxies that were “consumed” by the Milky Way, their stars melting into the larger stellar population and their origins nearly obliterated by time. The Milky Way’s two major satellite galaxies, called the Large and Small Magellanic Clouds, appear to have been distorted by an ancient close passage with their giant neighbor.

In the next few billion years, as our Sun is reaching the end of its existence, our heavens will be filled with the suns of our neighboring spiral galaxy Messier 31, better known as the Andromeda galaxy, as the two stellar islands come together in their own cosmic ballet. Astronomers who recently ran computer simulations on that future merger say our Solar System – whatever is left of it by then – will be pulled even further from the Milky Way’s center, where it currently resides at a distance of 26,000 light years. As our galaxy and Andromeda take on new shapes as they move through each other (and eventually come back to form one massive elliptical galaxy) Earth’s night skies will have numerous new stars, forming constellations totally unfamiliar to the ones we know now.

Omage Centauri wider view

As the Hubble astrophotos and many others taken over the last century show, these galactic “interactions” have been happening since the days when galaxies first formed after the Big Bang 13.7 billion years ago. They will continue to do so for eons to come, until the last suns burn out across the Universe, leaving only clusters of dark stellar remnants, immense black holes, and dust. And one of them may be closer than we think, with new evidence that a long-recognized star cluster residing inside the Milky Way may actually have been an independent galaxy in the distant past.

Image: This image shows the southern Milky Way patch. At the centre is the constellation Centaurus, where the globular cluster Omega Centauri is located. Credits: A. Fujii.

The stellar group in question is known as Omega Centauri, a massive collection of over ten million suns roughly 18,000 light years from our planet, visible to unaided vision from Earth’s Southern Hemisphere. Ancient astronomers of the pre-telescopic era recorded Omega Centauri as a star. English astronomer John Herschel was the first to label it as a globular star cluster in the 1830s, the category it has since retained.

Despite the designation, Omega Centauri has never quite fit in with the rest of the 150 or so known globular star clusters that encircle the Milky Way like moths around a flame. In addition to having ten times more stars than the average globular cluster, these same suns are also of diverse ages and types, whereas most stars in a typical globular cluster are all roughly the same. These facts, along with Omega Centauri’s chemical makeup and its path through the Milky Way, have been making astronomers suspect the cluster was once something even larger and more magnificent long ago, until its fateful meeting with our galaxy.

A paper released this month by scientists from HST and the Gemini South Observatory in Chile has added an important discovery to this theory: A rare intermediate black hole dwelling at the heart of the star cluster.

By monitoring the motions of suns near the core of Omega Centauri, the astronomers determined that a very heavy object had to exist at the celestial object’s center: The remains of a massive star that collapsed upon itself to form a gravitational “pit” in space 40,000 times more massive than our Sun. While this is much smaller than the black hole at the center of our Milky Way, which weighs in at four million solar masses, team member Karl Gebhardt noted there is “only one [other] example of an intermediate-mass black hole – in the globular cluster G1, in the nearby Andromeda galaxy.”

Omega Centauri

Omega Centauri may indeed have once been what astronomers call a dwarf galaxy, until the star island was essentially consumed by the Milky Way as it drifted through intergalactic space. The outer suns of Omega Centauri were likely stripped away by our galaxy as the two bodies merged, which made the dwarf galaxy end up appearing like a globular cluster. However, Omega Centauri was still massive enough to retain many of its stars closer to its center, thanks in no small part to its black hole, thus remaining much larger than the average globular cluster.

Image: A new discovery has resolved some of the mystery surrounding Omega Centauri, the largest and brightest globular cluster in the sky. Results obtained by Hubble and the Gemini Observatory reveal that the globular cluster may have a rare intermediate-mass black hole hidden in its centre, implying that is likely not a globular cluster at all, but a dwarf galaxy stripped of its outer stars. Credits: NASA/ ESA/ STScI/ AURA (The Hubble Heritage Team).

Having an actual galaxy so relatively close for astronomers to study will provide new information on the nature of the 100 billion stellar islands that populate the known Universe. This data should reveal how galaxies form and the role that the massive black holes which reside in most galactic cores play in their births.

The paper is Noyola et al., “Gemini and Hubble Space Telescope Evidence for an Intermediate Mass Black Hole in Omega Centauri,” accepted by the Astrophysical Journal and available online.

Comments on this entry are closed.

  • Ron S May 20, 2008, 11:54

    “…Omega Centauri was still massive enough to retain many of its stars closer to its center, thanks in no small part to its black hole…”

    I think this is backwards. A galaxy is naturally denser in its center, and is therefore the breeding ground for massive black holes. If Omega Centauri is a dwarf galaxy remnant, it is not too surprising that the gravitationally dense core, including its black hole, would survive through long interaction with the mainly less dense large body of our galaxy. In other words, the black hole is the result of the pre-collision dwarf galaxy core formation, and survives with that core since the (now) cluster’s interior binding is stronger than external interaction. The dwarf galaxy’s “suburbs” would have faired less well.

  • dad2059 May 20, 2008, 12:39

    Larry: Isn’t this the cluster that was estimated to be close to 12 billion years old, almost the age of the Universe?

    That would make it one of the oldest galactic artifacts in the Universe, and one relatively close by in stellar terms.

    Great post!

  • ljk May 28, 2008, 14:13

    On the Fundamental Plane of the Galactic Globular Cluster System

    Authors: M. Pasquato, G. Bertin

    (Submitted on 24 May 2008)

    Abstract: The globular clusters (GC) of our Galaxy have been found to lie close to a plane in the log(R_e), log(sigma), SB_e space, on the continuation of the Fundamental Plane (FP) known to characterize the properties of early-type galaxies. We reexamine the issue on a sample of 48 GCs selected in terms of homogeneity criteria for the photometric data available from the literature and perform a model-independent analysis of surface brightness profiles (SBP) and distance moduli, estimating error bars and studying selection effects with non-parametric statistical tests. We determine the coefficients of the FP and their error bars. The scatter from the FP relation is likely to be intrinsic, i.e. not due to measurement errors only.

    We find that in the standard FP coordinates our sample occupies a slim, axisymmetric region of parameter space, suggesting that the scaling relation might be around a Fundamental Line, rather than a plane, confirming a result noted earlier. This is likely to be the origin of the difficulties in the fit by a plane mentioned in previous investigations. Such FL relation would imply a pure photometric scaling law, which might be tested on wider samples and on extra-galactic GC systems. We find a correlation of the residuals from the FP relation with the central slope of the SBP. No other correlations are found. Finally, we reconstruct the distribution of the values of the quantity log(K_V/(M/L)) (virial coefficient divided by the mass-to-light ratio) through kernel density estimation and find evidence for bimodality, which suggests that the galactic GC system may be composed of at least two dynamically different populations. Yet, these populations do not reflect the standard dichotomy between disk and halo clusters. (abridged).

    Comments: 11 pages, 4 figures, accepted for publication in A&A, no boldface text

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.3766v1 [astro-ph]

    Submission history

    From: Mario Pasquato [view email]

    [v1] Sat, 24 May 2008 16:07:12 GMT (34kb)


  • ljk June 13, 2008, 15:11

    How Do Disks Survive Mergers?

    Authors: Philip F. Hopkins, Thomas J. Cox, Joshua D. Younger, Lars Hernquist (CfA)

    (Submitted on 10 Jun 2008)

    Abstract: We develop a physical model for how galactic disks survive and/or are destroyed in interactions. Based on dynamical arguments, we show gas primarily loses angular momentum to internal torques in a merger. Gas within some characteristic radius (a function of the orbital parameters, mass ratio, and gas fraction of the merging galaxies), will quickly lose angular momentum to the stars sharing the perturbed disk, fall to the center and be consumed in a starburst. A similar analysis predicts where violent relaxation of the stellar disks is efficient.

    Our model allows us to predict the stellar and gas content that will survive to re-form a disk in the remnant, versus being violently relaxed or contributing to a starburst. We test this in hydrodynamic simulations and find good agreement as a function of mass ratio, orbital parameters, and gas fraction, in simulations spanning a wide range in these properties and others, including different prescriptions for gas physics and feedback. In an immediate sense, the amount of disk that re-forms can be understood in terms of well-understood gravitational physics, independent of details of ISM gas physics or feedback. This allows us to explicitly quantify the requirements for such feedback to (indirectly) enable disk survival, by changing the pre-merger gas content and distribution.

    The efficiency of disk destruction is a strong function of gas content: we show how and why sufficiently gas-rich major mergers can, under general conditions, yield systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of our results in semi-analytic models.

    Comments: 29 pages, 16 figures, submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.1739v1 [astro-ph]

    Submission history

    From: Philip Hopkins [view email]

    [v1] Tue, 10 Jun 2008 20:15:53 GMT (2042kb)


  • ljk August 7, 2008, 1:45

    Globular Clusters Like to Be Near the Center of the Action

    Written by Nancy Atkinson

    August 5, 2008

    Globular clusters are gravitationally bound, dense concentrations of stars. There can be hundreds of thousands of stars in a cluster, and they are so close together that it’s hard to distinguish globular clusters outside of our galaxy from stars within our own galaxy just using ground-based telescopes: in other words, these big bunches of far away stars can look like a single, nearby star.

    But astronomers recently used the Hubble Space Telescope’s sharp eyes to identify, incredibly, over 11,000 globular clusters in the Virgo cluster of galaxies. And in doing so, they also noticed something interesting about where the globulars are located.

    Globular clusters don’t seem to form uniformly from galaxy to galaxy; instead they like to be where the action is near the center of galaxy clusters. The globulars are also more prevalent in dwarf galaxies near the center of the cluster of galaxies.

    Hubble’s Advanced Camera for Surveys resolved the star clusters in 100 galaxies of various sizes, shapes, and brightnesses, even in faint, dwarf galaxies. Comprised of over 2,000 galaxies, the Virgo cluster is the nearest large galaxy cluster to Earth, located about 54 million light-years away.

    Astronomers have long known that the giant elliptical galaxy at the cluster’s center, M87, hosts a larger-than-predicted population of globular star clusters. The origin of so many globulars has been a long-standing mystery.

    “Our study shows that the efficiency of star cluster formation depends on the environment,” said Patrick Cote of the Herzberg Institute of Astrophysics in Victoria, British Columbia. “Dwarf galaxies closest to Virgo’s crowded center contained more globular clusters than those farther away.”

    Full article here:


  • ljk August 14, 2008, 23:16

    Obscured clusters. II. GLIMPSE-C02 – A new metal rich globular cluster in the Milky Way

    Authors: R. Kurtev (1), V.D. Ivanov (2), J. Borissova (1), S. Ortolani (3) ((1) Valparaiso University, Chile (2) ESO, Chile (3)Padova University, Italy)

    (Submitted on 11 Aug 2008)

    Abstract: The estimated total number of Milky Way globulars is 160+-20. The question of whether there are any more undiscovered globular clusters in the Milky Way is particularly relevant with advances in near and mid-IR instrumentation.

    This investigation is a part of a long-term project to search the inner Milky Way for hidden star clusters and to study them in detail. GLIMPSE-C02 (G02) is one of these objects, situated near the Galactic plane (l=14.129deg, b=-0.644deg). Our analysis is based on SOFI/NTT JHKs imaging and low resolution (R~1400) spectroscopy of three bright cluster red giants in the K atmospheric window. We derived the metal abundance by analysis of these spectra and from the slope of the RGB.

    The cluster is deeply embedded in dust and undergoes a mean reddening of Av~24.8+-3 mag. The distance to the object is D=4.6+-0.7kpc. The metal abundance of G02 is [Fe/H](H96)=-0.33+-0.14 and [Fe/H](CG)=-0.16+-0.12 using different scales. The best fit to the radial surface brightness profile with a single-mass King’s model yields a core radius rc=0.70 arcmin (0.9pc), tidal radius rt=15 arcmin (20pc), and central oncentration c=1.33.

    We demonstrate that G02 is new Milky Way globular cluster, among the most metal rich globular clusters in the Galaxy. The object is physically located at the inner edge of the thin disk and the transition region with the bulge, and also falls in the zone of the “missing” globulars toward the central region of the Milky Way.

    Comments: 5 pages, 6 figures, accepted for publication in Astronomy and Astrophysics

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0808.1565v1 [astro-ph]

    Submission history

    From: Radostin Georgiev Kurtev [view email]

    [v1] Mon, 11 Aug 2008 19:39:50 GMT (241kb)


  • ljk December 5, 2008, 1:26


    December 2, 2008

    Holiday Glitter With Omega Centauri

    Written by Nancy Atkinson

    Omega Centauri. Credit: ESO

    A new image of Omega Centauri shows the globular cluster glittering away as one of the finest jewels of the southern hemisphere night sky. It contains millions of stars and is located about 17,000 light-years from Earth in the constellation of Centaurus, and sparkles at magnitude 3.7, appearing nearly as large as the full moon on the southern night sky.

    Visible with the unaided eye from a clear, dark observing site, when seen through even a modest amateur telescope, the Omega Centauri can be seen as incredible, densely packed sphere of glittering stars. But when astronomers use a professional telescopes, they are able to uncover amazing secrets of this beautiful globular cluster.

    This new image is based on data collected with the Wide Field Imager (WFI), mounted on the 2.2-metre diameter Max-Planck/ESO telescope, located at ESO’s La Silla observatory, high up in the arid mountains of the southern Atacama Desert in Chile.

    Omega Centauri is about 150 light-years across and is the most massive of all the Milky Way’s globular clusters. It is thought to contain some ten million stars!

    Recent research into this intriguing celestial giant suggests that there is a medium sized black hole sitting at its center. Observations made with the Hubble Space Telescope and the Gemini Observatory showed that stars at the cluster’s center were moving around at an unusual rate — the cause, astronomers concluded, was the gravitational effect of a massive black hole with a mass of roughly 40,000 times that of the Sun.

    The presence of this black hole is just one of the reasons why some astronomers suspect Omega Centauri to be an imposter. Some believe that it is in fact the heart of a dwarf galaxy that was largely destroyed in an encounter with the Milky Way.

    Other evidence (see here and here) points to the several generations of stars present in the cluster — something unexpected in a typical globular cluster, which is thought to contain only stars formed at one time. Whatever the truth, this dazzling celestial object provides professional and amateur astronomers alike with an incredible view on clear dark nights.

    Source: ESO


  • ljk February 16, 2009, 22:40

    A Chandra Study of the Galactic Globular Cluster Omega Centauri

    Authors: Daryl Haggard (University of Washington), Adrienne M. Cool (San Francisco State University), Melvyn B. Davies (Lund Observatory)

    (Submitted on 13 Feb 2009)

    Abstract: We analyze a ~70 ksec Chandra ACIS-I exposure of the globular cluster Omega Centauri (NGC 5139). The ~17 amin x 17 amin field of view fully encompasses three core radii and almost twice the half-mass radius. We detect 180 sources to a limiting flux of ~4.3×10^-16 erg/cm^2/s (Lx = 1.2×10^30 erg/s at 4.9 kpc). After accounting for the number of active galactic nuclei and possible foreground stars, we estimate that 45-70 of the sources are cluster members. Four of the X-ray sources have previously been identified as compact accreting binaries in the cluster–three cataclysmic variables (CVs) and one quiescent neutron star.

    Correlating the Chandra positions with known variable stars yields eight matches, of which five are probable cluster members that are likely to be binary stars with active coronae. Extrapolating these optical identifications to the remaining unidentified X-ray source population, we estimate that 20-35 of the sources are CVs and a similar number are active binaries. This likely represents most of the CVs in the cluster, but only a small fraction of all the active binaries.

    We place a 2-sigma upper limit of Lx < 3×10^30 erg/s on the integrated luminosity of any additional faint, unresolved population of sources in the core. We explore the significance of these findings in the context of primordial vs. dynamical channels for CV formation. The number of CVs per unit mass in Omega Cen is at least 2-3 times lower than in the field, suggesting that primordial binaries that would otherwise lead to CVs are being destroyed in the cluster environment.

    Comments: Accepted for publication in ApJ, 14 pages (emulateapj), 6 figures, 2 tables

    Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0902.2397v1 [astro-ph.HE]

    Submission history

    From: Daryl Haggard [view email]

    [v1] Fri, 13 Feb 2009 21:20:22 GMT (221kb)


  • ljk April 6, 2009, 14:01

    The non-peculiar velocity dispersion profile of the stellar system omega Centauri

    Authors: A. Sollima, M. Bellazzini, R. L. Smart, M. Correnti, E. Pancino, F. R. Ferraro, D. Romano

    (Submitted on 3 Apr 2009)

    Abstract: We present the results of a survey of radial velocities over a wide region extending from r~10 arcmin out to r~80 arcmin (~1.5 tidal radii) within the massive star cluster omega Centauri. The survey was performed with FLAMES@VLT, to study the velocity dispersion profile in the outer regions of this stellar system. We derived accurate radial velocities for a sample of 2557 newly observed stars, identifying 318 bona-fide cluster red giants.

    Merging our data with those provided by Pancino et al. (2007), we assembled a final homogeneous sample of 946 cluster members that allowed us to trace the velocity dispersion profile from the center out to r~32 arcmin. The velocity dispersion appears to decrease monotonically over this range, from a central value of sigma_{v}~17.2 Km/s down to a minimum value of sigma_{v}~5.2 Km/s.

    The observed surface brightness profile, rotation curve, velocity dispersion profile and ellipticity profile are simultaneously well reproduced by a simple dynamical model in which mass follows light, within the classical Newtonian theory of gravitation.

    The comparison with an N-body model of the evolution of a system mimicking omega Cen during the last 10 orbits into the Galactic potential suggests that (a) the rotation of stars lying in the inner ~20 arcmin of the clusters is not due to the effects of the tidal field of the Milky Way, as hypothized by other authors, and (b) the overall observational scenario is still compatible with the possibility that the outer regions of the cluster are subject to some tidal stirring.

    Comments: 12 pages, 12 figures, accepted for publication by MNRAS

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0904.0571v1 [astro-ph.SR]

    Submission history

    From: Antonio Sollima [view email]

    [v1] Fri, 3 Apr 2009 13:13:24 GMT (567kb)


  • ljk May 6, 2009, 13:01

    New Limits on an Intermediate Mass Black Hole in Omega Centauri: I. Hubble Space Telescope Photometry and Proper Motions

    Authors: Jay Anderson, Roeland P. van der Marel

    (Submitted on 5 May 2009)

    Abstract: We analyze data from the Hubble Space Telescope’s Advanced Camera for Surveys of the globular cluster Omega Cen. We construct a photometric and proper-motion catalog using the GO-9442, GO-10252, and GO-10775 data sets. The 2.5- to 4-year baseline between observations yields a catalog of some $10^5$ proper motions, with 53,382 high-quality measurements in a central field. We determine the cluster center to ~1-arcecond accuracy using two different star-count methods. We also determine the kinematical center of the proper motions, which agrees with the star-count center to within its ~4.6-arcsecond uncertainty.

    The proper-motion dispersion of the cluster increases gradually inwards, but there is no variation in kinematics with position within the central ~15 arcsec: there is no dispersion cusp and no stars with unusually high velocities. We measure for the first time in any globular cluster the variation in proper-motion dispersion with mass along the main sequence, and find the cluster not yet to be in equipartition.

    Our proper-motion results do not confirm the arguments put forward by Noyola, Gebhardt & Bergmann to suspect an intermediate-mass black hole (IMBH) in Omega Cen. In Paper II we present new dynamical models for the high-quality data presented here, with the aim of putting quantitative contraints on the mass of any possible IMBH.

    Comments: 28 pages, 24 figures, Submitted to ApJ

    Subjects: Galaxy Astrophysics (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0905.0627v1 [astro-ph.GA]

    Submission history

    From: Jay Anderson [view email]

    [v1] Tue, 5 May 2009 16:15:27 GMT (3787kb)


  • ljk May 6, 2009, 13:12

    New Limits on an Intermediate Mass Black Hole in Omega Centauri: II. Dynamical Models

    Authors: Roeland P. van der Marel, Jay Anderson

    (Submitted on 5 May 2009)

    Abstract: We present a detailed dynamical analysis of the projected density and kinematical data available for the globular cluster Omega Cen. We solve the spherical anisotropic Jeans equation to predict the projected profiles of the RMS velocity in each of the three orthogonal coordinate directions (line of sight, proper motion radial, and proper motion tangential).

    We fit the models to new HST star count and proper motion data near the cluster center presented in Paper I, combined with existing ground-based measurements. We also derive and model the Gauss-Hermite moments of the observed proper motion distributions.

    The projected density profile is consistent with being flat near the center, with an upper limit gamma=0.07 on the central logarithmic slope. The RMS proper motion profile is also consistent with being flat near the center, and there are no unusually fast-moving stars. The models provide a good fit and yield a 1-sigma upper limit MBH < 1.2E4 solar masses on the mass of a possible intermediate-mass black hole (IMBH). The inferred upper limit corresponds to MBH/Mtot < 0.43%. We combine this with results for other clusters and discuss the implications for globular cluster IMBH demographics.

    Tighter limits will be needed to rule out or establish whether globular clusters follow the same black hole demographics correlations as galaxies. The arguments put forward by Noyola et al. (2008) to suspect an IMBH in Omega Cen are not confirmed by our study; the IMBH mass they suggested is firmly ruled out.

    Comments: 27 pages, 9 figures, submitted to ApJ

    Subjects: Galaxy Astrophysics (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0905.0638v1 [astro-ph.GA]

    Submission history

    From: Jay Anderson [view email]

    [v1] Tue, 5 May 2009 16:15:51 GMT (174kb)


  • ljk July 7, 2009, 13:18

    MAGIC observation of Globular Cluster M13 and its millisecond pulsars

    Authors: T. Jogler, C. Delgado Mendez, M. T. Costado, W. Bednarek, J. Sitarek, for the MAGIC Collaboration

    (Submitted on 6 Jul 2009)

    Abstract: Based on MAGIC observations from June and July 2007, we present upper limits to the E>140 GeV emission from the globular cluster M13. Those limits allow us to constrain the population of millisecond pulsars within M13 and to test models for acceleration of leptons inside their magnetospheres and/or surrounding.

    We conclude that in M13 either millisecond pulsars are fewer than expected or they accelerate leptons less efficiently than predicted.

    Comments: 4 pages, 3 figures, Contribution to the 31st ICRC 2009 Lodz

    Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

    Cite as: arXiv:0907.0987v1 [astro-ph.HE]

    Submission history

    From: Tobias Jogler [view email]

    [v1] Mon, 6 Jul 2009 13:57:18 GMT (301kb)


  • ljk July 9, 2009, 13:11

    Pfleiderer2: identification of a new globular cluster in the Galaxy

    Authors: Sergio Ortolani, Charles Bonatto, Eduardo Bica, Beatriz Barbuy

    (Submitted on 7 Jul 2009)

    Abstract: We provide evidence that indicate the star cluster Pfleiderer 2, which is projected in a rich field, as a newly identified Galactic globular cluster. Since it is located in a crowded field, core extraction and decontamination tools were applied to reveal the cluster sequences in B, V and I Color-Magnitude Diagrams (CMDs).

    The main CMD features of Pfleiderer 2 are a tilted Red Giant Branch, and a red Horizontal Branch, indicating a high metallicity around solar. The reddening is E(B-V)=1.01.

    The globular cluster is located at a distance from the Sun d$_{\odot}$ = 16$\pm$2 kpc.

    The cluster is located at 2.7 kpc above the Galactic plane and at a distance from the Galactic center of R$_{\rm GC}$=9.7 kpc, which is unusual for a metal-rich globular cluster.

    Comments: Accepted by The Astronomical Journal

    Subjects: Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0907.1225v1 [astro-ph.GA]

    Submission history

    From: Charles Bonatto [view email]

    [v1] Tue, 7 Jul 2009 14:04:20 GMT (423kb)


  • ljk September 9, 2009, 11:44

    Deep Gemini/GMOS imaging of an extremely isolated globular cluster in the Local Group

    Authors: A. D. Mackey, A. M. N. Ferguson, M. J. Irwin, N. F. Martin, A. P. Huxor, N. R. Tanvir, S. C. Chapman, R. A. Ibata, G. F. Lewis, A. W. McConnachie

    (Submitted on 8 Sep 2009)

    Abstract: We report on deep imaging of a remote M31 globular cluster, MGC1, obtained with Gemini/GMOS. Our colour-magnitude diagram for this object extends ~5 magnitudes below the tip of the red giant branch and exhibits features consistent with an ancient metal-poor stellar population, including a long, well-populated horizontal branch. The red giant branch locus suggests MGC1 has a metal abundance [M/H] ~ -2.3.

    We measure the distance to MGC1 and find that it lies ~160 kpc in front of M31 with a distance modulus of 23.95 +/- 0.06. Combined with its large projected separation of 117 kpc from M31 this implies a deprojected radius of Rgc = 200 +/- 20 kpc, rendering it the most isolated known globular cluster in the Local Group by some considerable margin.

    We construct a radial brightness profile for MGC1 and show that it is both centrally compact and rather luminous, with Mv = -9.2. Remarkably, the cluster profile shows no evidence for a tidal limit and we are able to trace it to a radius of at least 450 pc, and possibly as far as ~900 pc. The profile exhibits a power-law fall-off with exponent -2.5, breaking to -3.5 in its outermost parts.

    This core-halo structure is broadly consistent with expectations derived from numerical models, and suggests that MGC1 has spent many gigayears in isolation.

    Comments: Accepted for publication in MNRAS

    Subjects: Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0909.1456v1 [astro-ph.GA]

    Submission history

    From: Dougal Mackey [view email]

    [v1] Tue, 8 Sep 2009 11:17:45 GMT (1408kb)


  • ljk September 10, 2009, 10:16

    Comparing the image of Omega Centauri taken by HST in 2002 (embedded in
    this article above) and one just released after the fourth and probably last
    servicing mission:


  • ljk September 14, 2009, 9:12

    The Center of Globular Cluster Omega Centauri

    Credit: NASA, ESA, and the Hubble SM4 ERO Team

    Explanation: What is left over after stars collide? To help answer this question, astronomers have been studying the center of the most massive ball of stars in our Milky Way Galaxy. In the center of globular cluster Omega Centauri, stars are packed in 10,000 times more densely than near our Sun.

    Pictured above, the newly upgraded Hubble Space Telescope has resolved the very center of Omega Centauri into individual stars. Visible are many faint yellow-white stars that are smaller than our Sun, several yellow-orange stars that are Red Giants, and an occasional blue star.

    When two stars collide they likely either combine to form one more massive star, or they stick, forming a new binary star system. Close binary stars interact, sometimes emitting ultraviolet or X-ray light when gas falls from one star onto the surface of a compact companion such as a white dwarf or neutron star. Two such binaries have now been located in Omega Centauri’s center.

    The star cluster lies about 15,000 light-years away and is visible toward the constellation of Centaurus.


  • ljk September 30, 2009, 0:06

    Radial distribution of the multiple stellar populations in omega Centauri

    Authors: A. Bellini (1,2), G. Piotto (1), L. R. Bedin (2), I. R. King (3), J. Anderson (2), A. P. Milone (1), Y. Momany (4). ((1) Univ.PD, (2) STScI, (3) Univ.of Washington Seattle, (4) INAF-OAPD)

    (Submitted on 25 Sep 2009)

    Abstract: We present a detailed study of the radial distribution of the multiple populations identified in the Galactic globular cluster omega Cen.

    We used both space-based images (ACS/WFC and WFPC2) and ground-based images (FORS1@VLT and WFI@2.2m ESO telescopes) to map the cluster from the inner core to the outskirts (~20 arcmin). These data sets have been used to extract high-accuracy photometry for the construction of color-magnitude diagrams and astrometric positions of ~900 000 stars.

    We find that in the inner ~2 core radii the blue main sequence (bMS) stars slightly dominate the red main sequence (rMS) in number. At greater distances from the cluster center, the relative numbers of bMS stars with respect to rMS drop steeply, out to ~8 arcmin, and then remain constant out to the limit of our observations.

    We also find that the dispersion of the Gaussian that best fits the color distribution within the bMS is significantly greater than the dispersion of the Gaussian that best fits the color distribution within the rMS.

    In addition, the relative number of intermediate-metallicity red-giant-branch stars which includes the progeny of the bMS) with respect to the metal-poor component (the progeny of the rMS) follows a trend similar to that of the main-sequence star-count ratio N_bMS/N_rMS.

    The most metal-rich component of the red-giant branch follows the same distribution as the intermediate-metallicity component. We briefly discuss the possible implications of the observed radial distribution of the different stellar components in omega Cen.

    Comments: 16 pages, 14 figures (6 in low resolution), 3 tables. Accepted for publication in Astronomy and Astrophysics on 23 September 2009

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0909.4785v1 [astro-ph.SR]

    Submission history

    From: Andrea Bellini [view email]

    [v1] Fri, 25 Sep 2009 20:12:49 GMT (1736kb)


  • ljk September 30, 2009, 0:11

    Stromgren photometry of Galactic Globular Clusters. II Metallicity distribution of red giants in Omega Centauri

    Authors: A. Calamida (1), G. Bono (2), P.B. Stetson (3), L.M. Freyhammer (4), A.M. Piersimoni (5), R. Buonanno (2), F. Caputo (6), S. Cassisi (5), M. Castellani (6), C.E. Corsi (6), M. Dall’Ora (7), S. Degl’Innocenti (8), I. Ferraro (6), F. Grundahl (9), M. Hilker (1), M. Monelli (10), M. Nonino (11), N. Patat (1), A. Pietrinferni (5), P.G. Prada Moroni (8), F. Primas (1), L. Pulone (6), T. Richtler (12), M. Romaniello (1), J. Storm (13), A.R. Walker (14) ((1) ESO (2) Univ. Roma Tor Vergata (3) HIA-NRC (4) Univ. Lancashire (5) INAF/OATe (6) INAF/OAR (7) INAF/OAC (8) Univ. Pisa (9) Aarhus Univ. (10) IAC (11) OAT (12) Univ. Concepcion (13) Astrophys. In. Potsdam (14) CTIO)

    (Submitted on 25 Sep 2009)

    Abstract: We present new intermediate-band Stroemgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster Omega Cen.

    Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J,H,K_s images). The final optical-NIR catalog covers a region of more than 20*20 arcmin squared across the cluster center.

    We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant (RG) stars. By adopting different Stroemgren metallicity indices we estimate the photometric metallicity for ~4,000 RGs, the largest sample ever collected.

    The metallicity distributions show multiple peaks ([Fe/H]_phot=-1.73+/-0.08,-1.29+/-0.03,-1.05+/-0.02,-0.80+/-0.04,-0.42+/-0.12 and -0.07+/-0.08 dex) and a sharp cut-off in the metal-poor tail ([Fe/H]_phot<=-2 dex) that agree quite well with spectroscopic measurements.

    We identify four distinct sub-populations,namely metal-poor (MP,[Fe/H]<=-1.49), metal-intermediate (MI,-1.49<[Fe/H]<=-0.93), metal-rich (MR,-0.95<[Fe/H]-1.7 dex).

    Comments: 28 pages, 22 figures, accepted on ApJ

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0909.4661v1 [astro-ph.SR]

    Submission history

    From: Annalisa Calamida Dr. [view email]

    [v1] Fri, 25 Sep 2009 13:37:35 GMT (985kb)


  • ljk October 9, 2009, 22:03

    On the origin of the helium-rich population in Omega Centauri

    Authors: D. Romano, M. Tosi, M. Cignoni, F. Matteucci, E. Pancino, M. Bellazzini

    (Submitted on 7 Oct 2009)

    Abstract: To study the possible origin of the huge helium enrichment attributed to the stars on the blue main sequence of Omega Centauri, we make use of a chemical evolution model that has proven able to reproduce other major observed properties of the cluster, namely, its stellar metallicity distribution function, age-metallicity relation and trends of several abundance ratios with metallicity.

    In this framework, the key condition to satisfy all the available observational constraints is that a galactic-scale outflow develops in a much more massive parent system, as a consequence of multiple supernova explosions in a shallow potential well. This galactic wind must carry out preferentially the metals produced by explosive nucleosynthesis in supernovae, whereas elements restored to the interstellar medium through low-energy stellar winds by both asymptotic giant branch (AGB) stars and massive stars must be mostly retained.

    Assuming that helium is ejected through slow winds by both AGB stars and fast rotating massive stars (FRMSs), the interstellar medium of Omega Centauri’s parent galaxy gets naturally enriched in helium in the course of its evolution.

    Comments: 10 pages, 5 figures, accepted for publication in MNRAS

    Subjects: Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0910.1299v1 [astro-ph.GA]

    Submission history

    From: Donatella Romano [view email]

    [v1] Wed, 7 Oct 2009 15:50:48 GMT (76kb)


  • ljk October 31, 2010, 12:48

    October 26, 2010 09:00 AM (EDT)

    News Release Number: STScI-2010-28

    Hubble Data Used to Look 10,000 Years into the Future

    October 26, 2010: The globular star cluster Omega Centauri has caught the attention of sky watchers ever since the ancient astronomer Ptolemy first catalogued it 2,000 years ago. Ptolemy, however, thought Omega Centauri was a single star. He didn’t know that the “star” was actually a beehive swarm of nearly 10 million stars, all orbiting a common center of gravity. The stars are so tightly crammed together that astronomers had to wait for the powerful vision of NASA’s Hubble Space Telescope to peer deep into the core of the “beehive” and resolve individual stars. Hubble’s vision is so sharp it can even measure the motion of many of these stars, and over a relatively short span of time.

    Full article here:


  • ljk February 17, 2012, 22:07

    Starbursts May Actually Destroy Globular Clusters

    by Nancy Atkinson on February 14, 2012

    It seems logical to assume that long ago, the amount of globular clusters increased in our galaxy during star-making frenzies called ‘starbursts.’ But a new computer simulation shows just the opposite: 13 billion years ago, starbursts may have actually destroyed many of the globular clusters that they helped to create.

    “It is ironic to see that starbursts may produce many young stellar clusters, but at the same time also destroy the majority of them,” said Dr. Diederik Kruijssen of the Max Planck Institute for Astrophysics. “This occurs not only in galaxy collisions, but should be expected in any starburst environment”

    Full article here: