May 12, 2008

Coma Berenices

Just north of Virgo and east of Leo is a small, faint constellation containing lots of deep sky fireworks, the constellation Coma Berenices. This was once considered an asterism as a part of Leo's tail, but Tycho Brahe first set it apart in his star atlas in 1602 as a separate constellation, and the designation stuck. The name comes from the Egyptian queen Berenice II. When Berenice's husband went on an expedition to Syria in the 3rd century BC, she was so grateful upon his safe return that she promised to sacrifice her famously long, beautiful hair to the goddess Aphrodite. She placed it in the temple, and it was gone the next day, and the court astronomer declared that it was now in the heavens. Coma Berenices translates as the Hair of Berenice.

Alpha Coma Berenices is also known as Diadem, a close binary star with two sun-like main sequence stars, much like the star Porrima in the constellation Virgo. These orbit each other nearly edge-on every 26 years, and their maximum separation on the sky is only about 1 arcsecond from a distance of about 60 light years. Beta is nearly identical to the Sun and only about 30 light years distant, and it has been a target in the search for extrasolar planets and dusty disks, but so far, we've found nothing here.

Also located in this region of the sky is the North Galactic Pole, so we are looking directly up out of the plane of the Milky Way's disk along a line of sight with very little dust and gas. That's one reason so many galaxies are easy to find in this part of the sky. Most of the remaining moderately bright stars in this constellation are in the open cluster about 270 light years away known as Melotte 111 or the Coma Berenices Cluster. The cluster is about twice as far away as the Hyades and so takes up a smaller area on the sky (only about 5 degrees in diameter), and its age is about a half billion years old. Here is a nice photo of this cluster on the sky.

There are seven (!) Messier objects in this small region of the sky, and I'll start with the nearest one to us, the globular cluster M 53. This cluster is orbiting currently up over the top of the galactic nucleus and about 60,000 light years away from us. Typical of halo clusters, this is very metal-poor, meaning it formed early in the history of the galaxy before the interstellar medium was enriched with heavy elements from the first generations of stars. Just one degree southeast is another globular, NGC 5053. These two clusters are fairly close together in space, though far enough apart so as not to gravitationally influence one another. NGC 5053 is moving in the opposite direction compared to M 53, but both are bound to our galaxy.

Among the extragalactic Messier objects, six of them are galaxies located in the Northern half of the Virgo Cluster, about 5 degrees due East from the tail of Leo the Lion, the bright star Denebola. Starting from the Eastern end of the group is Messier 91 (or NGC 4548), a classic barred spiral galaxy about 60 million light years away. This is one of the faintest and most difficult Messier objects, looked like an elongated blob in most telescopes since all you can see is the brightness of the bar.

A little less than a degree to the West is Messier 88, a somewhat brighter and more concentrated spiral. Though M 91 and M 88 are very close together on the sky and both members of Virgo, they are moving through the cluster in opposite directions at very high speed, with M 88 moving away from us and M 91 moving so quickly relative to the cluster center that it almost overcomes the overall expansion speed of the cluster (the Hubble flow) and so is almost stationary for the moment with respect to our own galaxy.

M 88's claim to fame is that it is one of the nearest and brightest Seyfert galaxies. Seyfert galaxies have very active nuclei and strong emission lines from highly ionized gas. The mechanism heating up the gas is likely a supermassive black hole accretion disk in the center of the galaxy. The Doppler broadening of the emission lines is indicating of the very fast rotation that should accompany an accretion disk around a gigantic (20+ million solar mass) black hole with an event horizon roughly the size of our solar system. There are narrow emission lines, too, but their brightness doesn't vary as quickly and so they come from a much larger region. Because the broad-lined emission can vary on short timescales, it must come from a very small region of space (due to light travel time considerations).

Moving about 2.5 degrees North and slightly East from M88 brings us to the next Messier object, Messier 85, a lenticular galaxy (disk-like spiral without the normally associated gas, dust and star formation) on the Northern edge of the Virgo cluster. Two degrees back to the South and a little further East, we find the lovely grand design spiral, Messier 100, seen nearly face-on. You can see only a circular blob of light through a small telescopes, but with photographic techniques and larger telescopes, the spiral arms really come out beautifully. The structure of this galaxy is so intricate that it was used to test the optics of the Hubble Space Telescope after the initial optics fix. This photo shows a type Ia supernova that erupted in this galaxy in 2006, very useful as the HST had already determined the distance to this galaxy via Cepheids.

Now moving a degree or so Southwest, we come to Messier 99, also known as the Coma Pinwheel galaxy, a faint face-on spiral galaxy. Among the Virgo Cluster galaxy, this one has one of the highest peculiar velocities, meaning that perhaps it has recently been accelerated by a close encounter with another galaxy. As you can see in this photo, this galaxy is somewhat asymmetric, which also may indicate a recent interaction with another galaxy, possibly a recently discovered "dark matter galaxy", a large mass of dark matter and Hydrogen gas with very little associated luminosity (detected via 21 cm Hydrogen emission).

About a half degree west from M 99, we find Messier 98, an even fainter mostly edge-on spiral galaxy with a peculiar velocity so large relative to the center of the cluster that its net motion is towards us. Of course, this won't continue, but right now, M 98 is likely passing near the center of gravity of the cluster and so like a pendulum nearing the bottom of its swing is near its maximum speed. As it continues to approach us, it will slow down over the course of millions of years and ultimately remain bound to the cluster.

The final Messier object in Coma Berenices is Messier 64, also known as the Black Eye Galaxy. This galaxy is about 5 degrees Northeast of the group of galaxies associated with the Virgo Cluster, but it is not a member of that cluster at a distance of only about 17 million light years away. The odd structure of this galaxy leads us to believe it is the result of a recent merger as there are two distinct stellar populations in the galaxy rotating in opposite directions! The dark dust band that gives the galaxy its name is easily visible with dark skies and a 6 or 8 inch telescope. In this nice Hubble poster, the inner part of the galaxy is orbiting clockwise while the outer part is moving counterclockwise.

Moving away from the galaxies near the center of the Virgo Cluster toward the Northern half of this constellation, near the Coma Berenices star cluster, we find another, smaller grouping of galaxies, some of which may be associated with the Virgo Cluster. The first of these galaxies we'll study is NGC 4565, about 1-2 degrees East of the center of the star cluster. This is a very pretty edge-on spiral, resembling a needle in a telescope (hence it is sometimes referred to as the Needle Galaxy). It is at the right distance to be a part of Virgo (30 million light years or so), but its overall space motion (in the plane of the sky) can't be determined well enough to be sure that it is bound to that cluster.

Another 2-3 degrees to the East is NGC 4725, seen in the linked image through the new Spitzer Space Telescope with an emphasis on its infrared colors (which brings out the dust in the arms). Here is a nice visible light photograph. This one is about 40 million light years away and may be associated with Virgo. Its claim to fame is that it seems to have only one spiral arm instead of the usual two. Perhaps it is more appropriately a ring galaxy thanks to a recent merger. Another nice nearly edge-on spiral at about the same distance is a degree North of NGC 4565, and that is NGC 4559.

A couple of degrees north of the star cluster is another nice pair of galaxies, the two spirals NGC 4274 and NGC 4314, seen together in this wider view. NGC 4314 is clearly a ringed starbust galaxy, having just undergone a violent interaction in which another galaxy likely passed right through its center. The subsequent shock wave has propagated radially outward through the galaxy, lighting up a ring of luminous starlight.

Finally, we turn to a little patch of sky within about a degree of the North Galactic Pole, about a degree West of Beta Coma Berenices, the Coma Cluster of Galaxies. This cluster is about 10 times further away than the Virgo Cluster, making it about 300 million light years away or more. The only two galaxies in this cluster which may be visible through a large amateur telescope are the two giant ellipticals at the heart of the cluster, NGC 4889 and NGC 4874.

Though the cluster is several million light years in diameter, it takes up an angular space in the sky of a box about one degree on a side, maybe four times larger than the full moon. Perhaps the best visual highlight of the coma cluster is the pair of interacting galaxies known as "the mice" (or NGC 4676). These are two spiral galaxies locked in a gravitational embrace that has lasted hundreds of millions of years and created enormously long tidal tails and massive waves of star formation.

Though only about twice the size of our local group, the Coma Cluster is an unusually rich grouping of galaxies, containing as much as 50 times the mass of our little group of galaxies. The extended halo of this cluster is a circle of about six degrees in radius centered on the two giant ellipticals, and in this great circle, one can find nearly 30,000 galaxies brighter than 19th magnitude, almost all of which are associated with this cluster. If you consider a bubble of the visible Universe centered on us and extending out far enough to encompass the Coma Cluster, that bubble contains hundreds of thousands of galaxies, and this bubble represents about 1/100,000th of the volume of the visible Universe.

Truly staggering when you stop and think about it.

Posted by Observer at May 12, 2008 07:50 AM

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Road Trip!!

Posted by: Humbaba on May 12, 2008 11:48 AM