Where there once was 158, there is now more… Globular clusters, that is. Thanks to ESO’s VISTA survey telescope at the Paranal Observatory in Chile, the Via Lactea (VVV) survey has cut through the gas and dust of the Milky Way to reveal the first star cluster that is far beyond our center. But keep your eyes on the prize, because as dazzling as the cluster called UKS 1 is on the right is, the one named VVV CL001 on the left isn’t as easy to spot.
Need more? Then keep on looking, because VVV CL001 isn’t alone. The next victory for VISTA is VVV CL002, which is shown in the image below. What makes it special? It’s quite possible that VVV CL002 is the closest of its type to the center of our galaxy. While you might think discoveries of this type are commonplace, they are actually out of the ordinary. The last was documented in 2010 and it’s only through systematically studying the central parts of the Milky Way in infrared light that new ones turn up. To add even more excitement to the discovery, there is a possibility that VVV CL001 is gravitationally bound to UKS 1, making it a binary pair! However, without further study, this remains unverified.
Thanks to the hard work of the VVV team led by Dante Minniti (Pontificia Universidad Catolica de Chile) and Philip Lucas (Centre for Astrophysics Research, University of Hertfordshire, UK) we’re able to feast our eyes on even more. About 15,000 light years away on the other side of the Milky Way, they’ve turned up VVV CL003 – an open cluster. Due the intristic faintness of these new objects, it’s a wonder we can see them at all… In any light!
Globular clusters are generally some of the oldest structures in our galaxy. Many of the most famous ones formed around the same time as our galaxy, some 13 billion years ago. However, some are distinctly younger. While many classification schemes are used, one breaks globular clusters into three groups: an old halo group which includes the oldest of the clusters, those in the disk and bulge of the galaxy which tend to have higher metallicity, and a younger population of halo clusters. The latter of these provides a bit of a problem since the galaxy should have settled into a disk by the time they formed, depriving them of the necessary materials to form in the first place. But a new study suggests a solution that’s not of this galaxy.
The new study looked at the distribution of these younger clusters around our Milky Way. Of the three classifications for globular clusters discussed, the young halo clusters are scattered well beyond the range of the other populations. The young halo extends to as much as 120 kiloparsecs (400 thousand light years) while the old halo clusters tend to lie within 30 kiloparsecs (100 thousand light years). Additionally, the young clusters don’t appear to be rotating with the disk of the galaxy whereas the old halo slowly orbits in the same direction as the disk.
In looking more carefully at the positions of these satellites, the team, led by Stefan Keller at the Australian National University, found that the younger population tends to lie in a wide plane that is tilted from the rotational axis of our galaxy by a mere 8°.
This plane is strikingly similar to another recognized grouping of objects: Many of the known dwarf galaxies lie in a nearly identical plane, known as the Plane of Satellites (PoS). This finding suggests that this population of globular clusters is a relic of cannibalized galaxies. Even more interesting is that, while these objects are younger than the distinctly “old” population, there is still a large variation in their ages. This implies that this plane wasn’t created by the accretion of one, or even a few minor galaxies, but a consistent feeding of small galaxies onto the Milky Way for much of the history of the universe, and all from the same direction. Studies of the distribution of satellites around our nearest major neighbor, M31, the Andromeda galaxy, has turned up a similar preferred plane, tilted some 59° from its disk.
One explanation for this is that this is a preferred direction that traces invisible filaments of dark matter. While dark matter distributions are difficult to predict, models haven’t accounted for such strong filamentary structure on such small scales. Rather, in the neighborhood of our galaxy, the overall distribution is described as an oblate spheroid. One of the reasons astronomers believe our own dark matter halo is so nicely shaped is the way it is affecting the Sagittarius dwarf galaxy which is slowly being accreted onto our own. If the dark matter were more wispy, it should be stretched out in different manners.
Another possibility the authors consider is that the objects were created in a preferred plane “from the break up of a large progenitor at early times”. In other words, the filament could be a fossil of larger structure before our galaxy formed along which these dwarf galaxies formed and from which these galaxies could have been slowly accreting over the history of the galaxy.
Globular Clusters have a story to tell. These dense clumps of thousands of stars are relics of the early history of our galaxy, preserving information of the galaxy’s properties from their formation. Knowing this, astronomers have used globular clusters for nearly 30 years to probe how our galaxy has evolved. New observations from Hubble, add surprising new insight to this picture.
One of the advantages to studying clusters, is that the large number of stars allows astronomers to accurately determine some properties of the constituent stars far better than they could if the stars were isolated. In particular, since clusters all form in a short span of time, all stars will have the same age. More massive stars will die off first, peeling away from the main sequence before their lower mass brothers. How far this point, where stars leave the main sequence, has progressed is indicative of the age of the cluster. Since globular clusters have such a rich population of stars, their H-R diagrams are well detailed and the turn-off becomes readily apparent.
Using ages found in this manner, astronomers can use these clusters to get a snapshot of what the conditions of the galaxy were like when it formed. In particular, astronomers have studied the amount of elements heavier than helium, called “metals”, as the galaxy has aged. One of the first findings using globular clusters to probe this age-metallicity relationship was that there was a notable difference in the way the inner portion and the outer portion of the galaxy has evolved. Globular clusters revealed that the inner 15 kpc evolved heavier elements faster than the outer portions. Such findings allow for astronomers to test models of galactic formation and evolution and have helped to support models involving halos of dark matter.
While these results have been confirmed by numerous follow-up studies, the sampling of globular clusters is still somewhat skewed. Many of the globular clusters studied were part of the Galactic Globular Cluster Treasury project conducted using the Hubble Space Telescope’s Advanced Camera for Surveys (HST/ACS). In order to minimize the time spent using the much demanded telescope, the team was only able to target relatively nearby globular clusters. As such, the most distant cluster they could observe was NGC 4147 which is ~21 kpc from the galactic center. Other studies have made use of Hubble’s Wide Field Planetary Camera 2 and pushed the radius back to over 50 kpc from the galactic center. However, currently only 6 globular clusters with distances over 50 kpc have been included in this larger study. Interestingly, there has been a notable absence of clusters between 15 and 50 kpc, leaving a gap in the fuller knowledge.
This gap is the target of a recent study by a team of astronomers led by Aaron Dotter from the Space Telescope Science Institute in Maryland. In the new study, the team examines 6 globular clusters. Three of them (IC 4499, NGC 6426, and Ruprecht 106) are towards the inner edge of this range, lying between 15 and 20 kpc from the galactic center while the other three (NGC 7006, Palomar 15, and Pyxis) each lie around 40 kpc.
Again making use of the HST/ACS, the team found that all of the clusters were younger than globular clusters from the inner portions of the galaxy with similar metalicities. But three of the clusters, IC 4499, Ruprecht 106, and Pyxis were significantly younger to the tune of 1-2 billion years younger again supporting the picture that inner clusters had evolved faster. Additionally, this finding of a sharp difference helps to support the picture that the outer clusters underwent a different evolutionary process, aside from the rapid enrichment in the inner halo. One suggestion is that many of the outer halo clusters were originally formed in dwarf galaxies and later accreted into the Milky Way due to the timescales on which clusters in such smaller galaxies are thought to evolve.
Globular clusters form during the earliest stages in a galaxy’s development, so clusters like this one, M 107, or NGC 6171 are some of the oldest objects in the Universe. Typically, globular clusters formed about 10 billion years ago, and astronomers say that studying these objects can provide significant insights into how galaxies, and their component stars evolve. While M 107 has been observed many times, this new look from ESO shows a stunning view of this swarm of stars.
Located about 21,000 light years away, M107 is not visible to the naked eye. But, with an apparent magnitude of about eight, it can easily be observed from a dark site with binoculars or a small telescope.
The globular cluster is about 13 arcminutes across and is found in the constellation of Ophiuchus, north of the pincers of Scorpius. Roughly half of the Milky Way’s known globular clusters are actually found in the constellations of Sagittarius, Scorpius and Ophiuchus, in the general direction of the centre of the Milky Way. This is because they are all in elongated orbits around the central region and are on average most likely to be seen in this direction.
This image is composed from exposures taken through the blue, green and near-infrared filters by the Wide Field Camera (WFI) on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile.
Using four years of data from the Hubble Space Telescope’s Advanced Camera for Surveys, astronomers have made the most accurate measurements of the movement of stars in the globular cluster Omega Centauri, and now can predict their movements for the next 10,000 years. This “beehive” of stars is tightly crammed together, so resolving the individual stars was a job that perhaps only Hubble could do. “It takes high-speed, sophisticated computer programs to measure the tiny shifts in the positions of the stars that occur in only four years’ time,” says astronomer Jay Anderson of the Space Telescope Science Institute in Baltimore, Md., who conducted the study with fellow Institute astronomer Roeland van der Marel. “Ultimately, though, it is Hubble’s razor-sharp vision that is the key to our ability to measure stellar motions in this cluster.”
Astronomers say that the precise measurement of star motions in giant clusters can yield insights into how stellar groupings formed in the early universe, and whether an “intermediate mass” black hole, one roughly 10,000 times as massive as our Sun, might be lurking among the stars.
Analyzing archived images taken over a four-year period by Hubble’s astronomers have made the most accurate measurements yet of the motions of more than 100,000 cluster inhabitants, the largest survey to date to study the movement of stars in any cluster.
The astronomers used the Hubble images, which were taken in 2002 and 2006, to make a movie simulation of the frenzied motion of the cluster’s stars. The movie shows the stars’ projected migration over the next 10,000 years.
Omega Centauri is the biggest and brightest globular cluster in the Milky Way, and one of the few that can be seen by the unaided eye. It is located in the constellation Centaurus, Omega Centauri, so is viewable in the southern skies, and is one of about 150 such clusters in our Milky Way Galaxy.
In this video below, astronomers Jay Anderson and Roeland van der Marel discuss their in-depth study of the giant cluster Omega Centauri.
Often in astronomy, one observable property traces another property which may be more difficult to observe directly; X-ray activity on stars can be used to trace turbulent heating of the photosphere. CO is used to trace cold H2. Sometimes these correlations make sense. Activities in stars produce the X-ray emissions. Other times, the tracer seems distantly related at best.
This is the case of a newly discovered correlation between the mass of the central black hole of galaxies and the number of globular clusters they contain. What can this relationship teach astronomers? Why does it hold for some types of galaxies better than others? And where does it come from in the first place.
The mass of a galaxy’s super massive black hole (SMBH) is known to have a strong relationship between many features of their host galaxies. It has identified to follow the range of velocities of stars in the galaxy, the mass and luminosity of the bulge of spiral galaxies, and the total amount of dark matter in galaxies. Because dark matter in the halo of galaxies and the luminosity have also been known to correspond to the number of globular clusters, Andreas Burkert of the Max-Planck-Institute for Extraterrestrial Physics in Germany, and Scott Tremaine at Princeton wondered if they could cut out the middlemen of dark matter and luminosity and still maintain a strong correlation between the central SMBH and the number of globular clusters.
Their initial investigation involved only 13 galaxies, but a follow-up study by Gretchen and William Harris and submitted to the Monthly Notices of the Royal Astronomical Society, increased the number of galaxies included in the survey to 33. The results of these studies indicated that for elliptical galaxies, the SMBH-GC relationship is evident. However, for lenticular galaxies there was no clear correlation. While there appeared to be a trend for classical spirals, the small number of data points (4) would not provide a strong statistical case independently, but did appear to follow the trend established by the elliptical galaxies.
Although the correlation appeared strong in most cases, about 10% of the galaxies included in the larger surveys were clear outliers. This included the Milky Way which has a SMBH mass that falls significantly short of the expectation from cluster number. One source of error the authors of the original study suspect is that it is possible that, in some cases, objects identified as globular clusters may have been misidentified and in actuality, be the cores of tidally stripped dwarf galaxies. Regardless, the relationship as it stands presently, seems to be quite strong and is even more tightly defined than that of the correlation between that of the SMBH mass and velocity dispersion that implied the potential relationship in the first place. The reason for the discordance in lenticular galaxies has not yet been explained and no reasons have yet been postulated.
NGC 1399, an elliptical galaxy about 65 million light years from Earth. Credit: NASA, Chandra
A dense stellar remnant has been ripped apart by a black hole a thousand times as massive as the Sun. If confirmed, this discovery would be a cosmic double play: it would be strong evidence for an intermediate mass black hole — which has been a hotly debated topic — and would mark the first time such a black hole has been caught tearing a star apart. Scientists believe a mysterious intense X-ray emission, called an “ultraluminous X-ray source” or ULX is responsible for the destruction. “Astronomers have made cases for stars being torn apart by supermassive black holes in the centers of galaxies before, but this is the first good evidence for such an event in a globular cluster,” said Jimmy Irwin of the University of Alabama, who led the study.
The new results come from the Chandra X-ray Observatory and the Magellan telescope, and were announced at the 215th American Astronomical Society meeting today.
The scenario is based on Chandra observations, which revealed the ULX in a dense cluster of old stars, and optical observations that showed a peculiar mix of elements associated with the X-ray emission. Taken together, a case can be made that the X-ray emission is produced by debris from a disrupted white dwarf star that is heated as it falls towards a massive black hole. The optical emission comes from debris further out that is illuminated by these X-rays.
The intensity of the X-ray emission places the source in the category, meaning that it is more luminous than any known stellar X-ray source, but less luminous than the bright X-ray sources (active galactic nuclei) associated with supermassive black holes in the nuclei of galaxies. The nature of ULXs is a mystery, but one suggestion is that some ULXs are black holes with masses between about a hundred and several thousand times that of the Sun, a range intermediate between stellar-mass black holes and supermassive black holes located in the nuclei of galaxies.
This ULX is in a globular cluster, NGC 1399, an elliptical galaxy about 65 million light-years from Earth that is a very old and crowded conglomeration of stars. Astronomers have suspected that globular clusters could contain intermediate-mass black holes, but conclusive evidence for this has been elusive.
Irwin and his colleagues obtained optical spectra of the object using the Magellan I and II telescopes in Las Campanas, Chile. These data reveal emission from gas rich in oxygen and nitrogen but no hydrogen, a rare set of signals from globular clusters. The physical conditions deduced from the spectra suggest that the gas is orbiting a black hole of at least 1,000 solar masses. The abundant amount of oxygen and absence of hydrogen indicate that the destroyed star was a white dwarf, the end phase of a solar-type star that has burned its hydrogen leaving a high concentration of oxygen. The nitrogen seen in the optical spectrum remains an enigma.
“We think these unusual signatures can be explained by a white dwarf that strayed too close to a black hole and was torn apart by the extreme tidal forces,” said coauthor Joel Bregman of the University of Michigan.
Theoretical work suggests that the tidal disruption-induced X-ray emission could stay bright for more than a century, but it should fade with time. So far, the team has observed there has been a 35% decline in X-ray emission from 2000 to 2008.
Irwin said at today’s press conference that a new survey just getting started will look for more globular clusters with x-ray sources.