Archive for globular clusters

VLT/MUSE spectroscopy suggests a central intermediate mass black hole in globular cluster NGC 6397

Posted in astronomy with tags , on February 5, 2016 by Tim Kendall

NGC6397-NRGBhiTwo new papers appearing today on the astro-ph preprint server have highlighted the capabilities of a new instrument at ESO/VLT, the Multi Unit Spectroscopic Explorer, MUSE. Studying the velocity dispersion of stars in the globular cluster NGC 6397, astronomers infer a central black hole of  some six hundred solar masses. They have also been able to construct the first complete spectroscopic HR diagram for a globular cluster, using nearly 19000 stellar spectra. Image: Antilhue/Chile;, 14.5″ mirror, prime focus f/9. From the abstracts: (Paper I/Paper II)

We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12,000 individual stars in the globular cluster NGC 6397. The stars are deblended with a PSF (point spread function) fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis using a full-spectrum fitting method based on a large grid of PHOENIX [theoretical model] spectra. With 18,932 spectra from 12,307 stars in NGC 6397 we have analysed the largest sample so far available for a single globular cluster. We derived a mean radial velocity of 17.84 ± 0.07 km/s and a mean metallicity of [Fe/H]= −2.120 ± 0.002, with the latter seemingly varying with temperature for stars on the RGB. We determine effective temperature and [Fe/H] from the spectra, and surface gravity from HST photometry. This is the first very comprehensive HRD for a globular cluster based on the analysis of several thousands of stellar spectra. Furthermore, two interesting objects were identified with one being a post-AGB star and the other a possible millisecond-pulsar companion.

We present a detailed analysis of the kinematics of the galactic globular cluster NGC 6397 based on more than ~18,000 spectra obtained with the novel integral field spectrograph MUSE. While NGC 6397 is often considered a core collapse cluster, our analysis suggests a flattening of the surface brightness profile at the smallest radii. Although it is among the nearest globular clusters, the low velocity dispersion of NGC 6397 of <5 km/s imposes heavy demands on the quality of the kinematical data. We show that despite its limited spectral resolution, MUSE reaches an accuracy of 1 km/s in the analysis of stellar spectra. We find slight evidence for a rotational component in the cluster and the velocity dispersion profile that we obtain shows a mild central cusp. To investigate the nature of this feature, we calculate spherical Jeans models and compare these models to our kinematical data. This comparison shows that if a constant mass-to-light ratio is assumed, the addition of an intermediate-mass black hole with a mass of 600 M_sun brings the model predictions into agreement with our data, and therefore could be at the origin of the velocity dispersion profile. We further investigate cases with varying mass-to-light ratios and find that a compact dark stellar component can also explain our observations. However, such a component would closely resemble the black hole from the constant mass-to-light ratio models as this component must be confined to the central ~5 arcsec of the cluster and must have a similar mass. Independent constraints on the distribution of stellar remnants in the cluster or kinematic measurements at the highest possible spatial resolution should be able to distinguish the two alternatives.

IDL TIFF fileHubble image of the central regions of NGC 6397 (Wikipedia). In 2006, a study using such data was published that showed a clear lower limit in the intrinsic brightness of the cluster population of faint stars at around visual magnitude 26. The authors therefore were able to deduce observationally the lower limit for the mass necessary for stars to develop a core capable of fusion: roughly 0.083 times the mass of the Sun.


A new class of “dark” globular clusters around Centaurus A

Posted in astronomy with tags , on May 14, 2015 by Tim Kendall

Centaurus A haloImage and text: ESO,ESA/Hubble, NASA. Digitized Sky Survey. Acknowledgement: Davide de Martin

Observations with ESO’s Very Large Telescope in Chile have discovered a new class of “dark” globular star clusters around the giant galaxy Centaurus A. These mysterious objects look similar to normal clusters, but contain much more mass and may either harbour unexpected amounts of dark matter, or contain massive black holes — neither of which was expected nor is understood. Globular star clusters are huge balls of thousands of stars that orbit most galaxies. They are among the oldest known stellar systems in the Universe and have survived through almost the entire span of galaxy growth and evolution.

Matt Taylor, a PhD student at the Pontificia Universidad Catolica de Chile, Santiago, Chile, and holder of an ESO Studentship, is lead author of the new study. He sets the scene: “Globular clusters and their constituent stars are keys to understanding the formation and evolution of galaxies. For decades, astronomers thought that the stars that made up a given globular cluster all shared the same ages and chemical compositions — but we now know that they are stranger and more complicated creatures.” The elliptical galaxy Centaurus A (also known as NGC 5128) is the closest giant galaxy to the Milky Way and is suspected to harbour as many as 2000 globular clusters. Many of these globulars are brighter and more massive than the 150 or so orbiting the Milky Way. Matt Taylor and his team have now made the most detailed studies so far of a sample of 125 globular star clusters around Centaurus A using the FLAMES instrument on ESO’s Very Large Telescope at the Paranal Observatory in northern Chile.

The full science paper is available via ESO (link under above image). While I ponder that with the benefit of hindsight it appears natural that massive galaxies like Cen A, totally different from our own Milky Way, should harbour some very odd (to our eyes) globular clusters, I can’t resist instead here to note some other, more uncertain and maybe even stranger dark-related news.  Atmospheric scientists may have discovered clouds of positrons associated with thunderstorms here on Earth. Why they have not annihilated is a mystery, and no excess gamma ray emission appears to be observed. Story and quote:

A terrifying few moments flying into the top of an active thunderstorm in a research aircraft has led to an unexpected discovery that could help explain the longstanding mystery of how lightning gets initiated inside a thunderstorm. University of New Hampshire physicist Joseph Dwyer and lightning science colleagues from the University of California at Santa Cruz and Florida Tech describe the turbulent encounter and discovery in a paper to be published in the Journal of Plasma Physics. In August 2009, Dwyer and colleagues were aboard a National Center for Atmospheric Research Gulfstream V when it inadvertently flew into the extremely violent thunderstorm—and, it turned out, through a large cloud of positrons, the antimatter opposite of electrons, that should not have been there. To encounter a cloud of positrons without other associated physical phenomena such as energetic gamma-ray emissions was completely unexpected, thoroughly perplexing and contrary to currently understood physics. “The fact that, apparently out of nowhere, the number of positrons around us suddenly increased by more than a factor of 10 and formed a cloud around the aircraft is very hard to understand. We really have no good explanation for it,” says Dwyer, a lightning expert and the UNH Peter T. Paul Chair in Space Sciences at the Institute for the Study of Earth, Oceans, and Space. It is known that thunderstorms can sometimes make flashes of energetic gamma rays, which may produce pairs of electrons and positrons when they interact with air. But the appearance of positrons should then coincide with a large increase in the number of gamma rays. “We should have seen bright gamma-ray emissions along with the positrons,” Dwyer says. “But in our observations, we first saw a positron cloud, then another positron cloud about seven kilometers away and then we saw a bright gamma-ray glow afterwards. So it’s all not making a whole lot of sense.”


One possible explanation for the sudden appearance of positrons is that the aircraft itself dramatically influenced the electrical environment of the thunderstorm but that, Dwyer says, would be very surprising. It’s also possible the researchers were detecting a kind of exotic electrical discharge inside the thunderstorm that involves positrons. “This is the idea of ‘dark lightning,’ which makes a lot of positrons,” says Dwyer. “In detecting the positrons, it’s possible we were seeing sort of the fingerprint of dark lightning. It’s possible, but none of the explanations are totally satisfying.”

Light in dark red sky

Globular clusters and stellar archaeology: Hubble views IC 4499

Posted in astronomy with tags , , on August 20, 2014 by Tim Kendall

IC 4499: A globular cluster’s age revisited
Stellar archaeology is a new term which has come into use to describe the observational recovery of multiple generations of stars using modern computer-generated models. These take into account the complex underlying physics of stellar atmospheres. Globular clusters are excellent laboratories for such studies. Image credit: ESA/Hubble & NASA

IC 4499 is a somewhat special case. Its mass lies somewhere between low-mass globulars, which show a single generation build-up, and the more complex and massive globulars which can contain more than one generation of stars. By studying objects like IC 4499 astronomers can therefore explore how mass affects a cluster’s contents. Astronomers found no sign of multiple generations of stars in IC 4499 — supporting the idea that less massive clusters in general only consist of a single stellar generation.

While IC 4499 is a special case, the abstract of the new paper by Chantereau et al. outlines the general idea that multiple populations of stars are likely to exist in globular clusters, and these multiple generations may be discerned by their differing chemical and dynamical characteristics:

Globular clusters are among the oldest structures in the Universe and they host today low-mass stars and no gas. However, there has been a time when they formed as gaseous objects hosting a large number of short-lived, massive stars. Many details on this early epoch have been depicted recently through unprecedented dissection of low-mass globular cluster stars via spectroscopy and photometry. In particular, multiple populations have been identified, which bear the nucleosynthetic fingerprints of the massive hot stars long disappeared. Here we discuss how massive star archeology can been done through the lens of these multiple populations.

Abell 1689 – a massive galaxy cluster 700 megaparsecs distant

Posted in astronomy with tags , , on September 15, 2013 by Tim Kendall

Image and full story credits are here and the image is now at APOD.

The astronomers used Hubble’s Advanced Camera for Surveys to peer deep inside the heart of Abell 1689, detecting the visible-light glow of 10,000 globular clusters, some as dim as 29th magnitude. Based on that number, Blakeslee’s team estimated that more than 160,000 globular clusters are huddled within a diameter of 2.4 million light-years. “Even though we are looking deep into the cluster, we’re only seeing the brightest globular clusters, and only near the center of Abell 1689 where Hubble was pointed,” he said. The brightness of most of the globular clusters is estimated to be 31st magnitude. This is out of reach for Hubble, but not for NASA’s James Webb Space Telescope, an infrared observatory scheduled to launch later this decade. By going fainter, Webb should be able to see many more of the globular clusters.

This great image of Abell 1689 is a fantastic demonstration of gravitational lensing and I urge readers to follow the link and find out more about how these new observations of extragalactic globular clusters can shed light on the dark matter problem.