SAO: The Densest Galaxy

[bystander]

The Densest Galaxy
Smithsonian Astrophysical Observatory
Weekly Science Update | 2013 Sep 13

A Hubble image of the central region of the galaxy M60, showing at its outskirts the small dwarf galaxy M60-UDC1 (inside the red circle). This dwarf is found to be the galaxy with the highest known density of stars (a more typical dwarf galaxy is shown inside the dotted red circle. The diagonal white streak is an artifact). (Credit: NASA/HST; J. Strader)

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Our nearest neighbor star, Proxima Centauri, is about 4.2 light-years away. This distance is typical (on average) between stars (or gravitationally bound stellar systems) in the Milky Way and similar galaxies. In terms of density, there are about 0.02 stars per cubic light-year in the general neighborhood of the Sun. At the other extreme of stellar density, a globular cluster - a spherical collection of as many as a million stars orbiting a galaxy - can have an average of over ten stars per cubic light-year, and even higher densities near its center. Astronomers are trying to understand what limits the density of stellar neighborhoods, if and how the average densities differ in different sized galaxies, and the relationship of stellar densities to the sizes of the supermassive black holes at galaxy centers.

A dwarf galaxy is small in both size and mass, with 100-1000 times fewer stars than the Milky Way. Astronomers interested in stellar densities are naturally curious about the densities in such small systems. CfA astronomers Pepi Fabbiano and Nelson Caldwell and their colleagues spotted a strange dwarf galaxy on the outskirts of a much larger galaxy, Messier 60, about sixty million light-years away (that is, in our cosmic vicinity). This dwarf is only about 80 light-years in radius (compared with our Milky Way's radius of tens of thousands of light-years), but the scientists determined that it has a stellar mass of density of over one hundred stars per cubic light-year, making it the densest known galaxy.

The astronomers used the Chandra X-ray Observatory to reveal an X-ray source at the dwarf galaxy's center. The source could be either a nuclear supermassive black hole or perhaps a pair of binary stars emitting X-rays - additional observations are needed to resolve the issue. The authors point out that if the central object is a nuclear black hole, it is likely that this dwarf galaxy was once a much larger galaxy that collided with another galaxy. It was stripped of most of its matter in the interaction but its nucleus, left relatively unaffected, remained behind to consolidate the remnants into the dense system seen today. While it is always fascinating to discover new extrema in the cosmic zoo, these results also shed light on processes that affect galaxy evolution and the properties of dense stellar systems.

The Densest Galaxy - Jay Strader et al

• Astrophysical Journal Letters 775(1) L6 (2013 Sep 20) DOI: 10.1088/2041-8205/775/1/L6
  arXiv.org > astro-ph > arXiv:1307.7707 > 29 Jul 2013 (v1), 14 Aug 2013 (v2)

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[bystander]

M60-UCD1: Hubble & Chandra Find Evidence for Densest Nearby Galaxy
NASA | SAO | Chandra X-ray Observatory | STScI | HubbleSite | 2013 Sep 24

Credit: X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI

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The densest galaxy in the nearby Universe may have been found, as described in our latest press release. The galaxy, known as M60-UCD1, is located near a massive elliptical galaxy NGC 4649, also called M60, about 54 million light years from Earth.

This composite image shows M60 and the region around it, where data from NASA's Chandra X-ray Observatory are pink and data from NASA's Hubble Space Telescope (HST) are red, green and blue. The Chandra image shows hot gas and double stars containing black holes and neutron stars and the HST image reveals stars in M60 and neighboring galaxies including M60-UCD1. The inset is a close-up view of M60-UCD1 in an HST image.

Packed with an extraordinary number of stars, M60-UCD1 is an "ultra-compact dwarf galaxy". It was discovered with NASA's Hubble Space Telescope and follow-up observations were done with NASA's Chandra X-ray Observatory and ground-based optical telescopes.

It is the most luminous known galaxy of its type and one of the most massive, weighing 200 million times more than our Sun, based on observations with the Keck 10-meter telescope in Hawaii. Remarkably, about half of this mass is found within a radius of only about 80 light years. This would make the density of stars about 15,000 times greater than found in Earth's neighborhood in the Milky Way, meaning that the stars are about 25 times closer.

The 6.5-meter Multiple Mirror Telescope in Arizona was used to study the amount of elements heavier than hydrogen and helium in stars in M60-UCD1. The values were found to be similar to our Sun.

Another intriguing aspect of M60-UCD1 is that the Chandra data reveal the presence of a bright X-ray source in its center. One explanation for this source is a giant black hole weighing in at some 10 million times the mass of the Sun.

Astronomers are trying to determine if M60-UCD1 and other ultra-compact dwarf galaxies are either born as jam-packed star clusters or if they are galaxies that get smaller because they have stars ripped away from them. Large black holes are not found in star clusters, so if the X-ray source is in fact due to a massive black hole, it was likely produced by collisions between the galaxy and one or more nearby galaxies. The mass of the galaxy and the Sun-like abundances of elements also favor the idea that the galaxy is the remnant of a much larger galaxy.

If this stripping did occur, then the galaxy was originally 50 to 200 times more massive than it is now, which would make the mass of its black hole relative to the original mass of the galaxy more like the Milky Way and many other galaxies. It is possible that this stripping took place long ago and that M60-UCD1 has been stalled at its current size for several billion years. The researchers estimate that M60-UCD1 is more than about 10 billion years old.

Astronomers discover densest galaxy ever
Michigan State University | 2013 Sep 24

[Ann]

Very interesting. M60-UCD1 is slightly similar to M32, the very compact dwarf galaxy orbiting our giant neighbour, the Andromeda galaxy. Obviously M60-UCD1 is a much more extreme case. But both these ultra-compact galaxies are in orbit around a giant, massive galaxy, and both of them have black holes at their centers. It seems likely that that they have been stripped down to their bulges, perhaps even to the inner, densest parts of their bulges, by the interaction with their bullying neighbours.

Even Omega Centauri, the largest globular in the Milky Way, shows some signs of being a stripped-down center of a galaxy, even though Omega Centauri probably lacks a central black hole.

It should be noted that the Wikipedia article about M32 suggests that M32 might be a normal galaxy instead of dwarf galaxy, located three times as far away as the Andromeda galaxy. However, the source for this claim, Young, K. S. et al. (2008), admit in their paper that HST observations of the stellar content of M32 do indeed show it to be a satellite of M31, and two of their principal objections to the idea of M32 being a dwarf galaxy are that M32 is too compact for a dwarf and its stars are orbiting the center of it too fast (as they will if M32 has a central black hole). In other words, a large part of their objections rests on precisely the properties that make M32 an ultra-compact galaxy, similar to M60-UCD1 although not so extreme.

Ann

[bystander]

Smallest Known Galaxy with a Supermassive Black Hole
University of Utah | 2014 Sep 17

Many Black Holes May Hide in Dwarf Remnants of Stripped Galaxies

Formation of Dwarf Galaxy M60-UCD1
Credit: Holger Baumgardt, University of Queensland

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A University of Utah astronomer and his colleagues discovered that an ultracompact dwarf galaxy harbors a supermassive black hole – the smallest galaxy known to contain such a massive light-sucking object. The finding suggests huge black holes may be more common than previously believed.

“It is the smallest and lightest object that we know of that has a supermassive black hole,” says Anil Seth, lead author of an international study of the dwarf galaxy published in Thursday’s issue of the journal Nature. “It’s also one of the most black hole-dominated galaxies known.”

The astronomers used the Gemini North 8-meter optical-and-infrared telescope on Hawaii’s Mauna Kea and photos taken by the Hubble Space Telescope to discover that a small galaxy named M60-UCD1 has a black hole with a mass equal to 21 million suns.

Their finding suggests plenty of other ultracompact dwarf galaxies likely also contain supermassive black holes – and those dwarfs may be the stripped remnants of larger galaxies that were torn apart during collisions with yet other galaxies. ...

Ultracompact Dwarf Galaxy M60-UCD1 ~ Credit: NASA, ESA and A. Seth (Utah)

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Big Surprises Can Come in Small Packages
ESA | HEIC | 2014 Sep 17

Smallest Known Galaxy with a Supermassive Black Hole
NASA | STScI | HubbleSite | 2014 Sep 17

A supermassive black hole in an ultra-compact dwarf galaxy - Anil C. Seth et al

• Nature 513(7518) 398 (18 Sep 2014) DOI: 10.1038/nature13762

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