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Messier Monday: The Most Perfect Elliptical, M89

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The Virgo Cluster of galaxies is the richest nearby treasure trove of ellipticals; this is the final one Messier ever found.

“We sail within a vast sphere, ever drifting in uncertainty, driven from end to end.” –Blaise Pascal

Back in 1758, Charles Messier was looking through a telescope for the return of Halley’s Comet. Initially he saw a faint, fuzzy object that he thought was the comet itself, only to realize that this was a fixed object in the night sky: the Crab Nebula, as it turned out. In order to protect other skywatching comet-hunters from making similar mistakes, he decided to catalogue all the bright deep-sky objects that could potentially be confused for comets. And his detailed and systematic observations of the night sky led to the first large, comprehensive catalogue of night-sky objects: the Messier Catalogue!

Image credit: Tenho Tuomi of Tuomi Observatory, via http://www.lex.sk.ca/astro/messier/index.html.

First published in 1771, Messier (along with his assistant, Pierre Méchain) continued to observe the night sky looking for new objects, winding up with a total of 110 in his catalogue. The date of March 18, 1781 was a particular coup for Messier, who himself found a record eight new galaxies (and one globular cluster) on that night, all in a similar region of sky now identified with the Virgo Cluster. Today’s object in particular, Messier 89, holds the dual distinction of being both the final giant elliptical ever found by Messier himself and also as the most perfectly spherical galaxy in the entire catalogue.

After sunset tonight, here’s how to find it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

The most prominent northern hemisphere asterism — or collection of stars — is probably the Big Dipper, and following the “arc” of its handle will lead you to the brightest star in the northern celestial hemisphere: Arcturus. Instead of “speeding on to Spica” in the south, however, you could instead turn back towards the west a bit, and arrive at the two prominent stars Vindemiatrix and Denebola, which are by far the brightest two stars in their respective regions of sky.

It’s in between these two that Messier 89 — and a trove of Virgo Cluster riches — lies.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

A number of faint (but still clearly visible) naked-eye stars lie in between Vindemiatrix and Denebola, with ρ Virginis lying closer to Vindemiatrix and just below the line connecting them, while 6 Comae Berenices lies closer to Denebola and just above that same imaginary line. If you start at ρ Virginis and navigate just another two degrees (roughly) to the west, you’ll find Messier 89 amidst a sea of other stars and deep-sky objects.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

This galaxy may not be the brightest, most prominent or a very feature-rich galaxy, as Messier himself noted, stating it to be a:

Nebula without star, in Virgo, a little of distance from & on the same parallel as the nebula reported above, No. 87. Its light was extremely faint & pale, & and it is not without difficulty that one can distinguish it.

Through a modest amateur telescope, you’re likely to find something like this.

Image credit: Terry J. Belia of http://www.astrotx.com/Messier%20Images%2073-110.htm.

It’s intrinsically on the faint side, so you’ll need a dark, moonless night (like the first half of tonight ought to provide). Like many giant ellipticals, it has a bright (but not point-like) nucleus, surrounded by a much fainter halo of whitish, fuzzy featureless brightness that dims out and fades even farther as you move away from the center. What you’re looking at, unbeknownst to Messier, is a huge galaxy even larger than our Milky Way, more than 50% larger than our galaxy in diameter and more than double the mass.

But while most elliptical galaxies are, well, ellipsoidal, with one long axis and one short axis, Messier 89 appears to be nearly perfectly spherical!

Image credit: © 2006 — 2012 by Siegfried Kohlert, with M89 (left) and M90 (right) together, via http://www.astroimages.de/en/gallery/M89.html.

No one knows whether this galaxy is truly spherical in nature, or whether its long axis is simply oriented along our line-of-sight, hiding its true form from our eyes. Unfortunately, it will take nearly a hundred million years for this galaxy to rotate a sufficient amount for us to tell! Like most ellipticals, this one was thought to have formed from major mergers of smaller galaxies in the past, including (possibly) previous large spirals not so different from our own.

What we find in this galaxy today is mostly typical of giant ellipticals:

  • A huge, supermassive black hole on the order of a billion solar masses, hundreds of times the mass of our own galaxy’s black hole.
  • Thousands of globular clusters in the extended halo of this galaxy, extending for some 300,000 light-years in all directions from its center. This is about 15 times as many globulars as are found in our own galaxy.
  • And finally, it seems to be both eating material from the intergalactic medium, as well as having gas stripped away as it supersonically passes through it.
Images credit: NASA / Chandra X-ray Telescope.

You can see, in the X-ray, above, that this galaxy looks anything but spherical, and that’s evidenced by the faint outer edges that show a temperature gradient between the leading edge (upper left) and the trailing edge (lower right) of the galaxy as it speeds through the dense intergalactic medium of the Virgo Cluster.

But what you’ll also notice is this strange dual-lobed structure coming from the galaxy’s center, something that isn’t an optical feature in the visible portion of the spectrum.

Image credit: Wikimedia commons user Friendlystar.

These bubble-like features in the X-ray are actually cavities of hot gas caused by a spinning supermassive black hole. For a long time, these features were unknown, but when Chandra uncovered them it was quickly realized what the cause was, and they were discovered to exist in a very large fraction of giant ellipticals!

But, as you may have guessed, these nearby giant ellipticals are only so prominent to us because they’re so massive and full of stars. They’re not, however, the most numerous type of galaxy. A quick glance at an image from the Sloan Digital Sky Survey shows a large number of “other” Virgo Cluster members that are simply washed out by the presence of Messier 89!

Image credit: Sloan Digital Sky Survey (SDSS), via http://cseligman.com/text/atlas/ngc45a.htm#4552.

This galaxy, though, despite its nearly spherical appearance, is actually enveloped by a large, non-symmetrical structure, something discovered by David Malin back in 1979. Have a look at this image!

Images credit: David Malin, via http://messier.seds.org/more/m089_aat.html.

What you’re seeing is not just a large envelope of stars and globular clusters, but also a tiny, much lower-mass galaxy in the process of being devoured by its larger neighbor, M89!

As always (when it’s available), the most spectacular shot of this galaxy comes from the Hubble Space Telescope.

Image credit: NASA / ESA / Hubble Space Telescope, via Wikisky.

The occasional foreground star is visible here, but what I really want to draw your attention to — something that’s available if you look at this picture at its full resolution — is the sheer richness of what lies behind this galaxy.

Take a look for yourself at this slice just to one side of the galactic center, at not only the large but the many smaller “smudges” that exist around this galaxy; everything that’s visible as more than a single point of light is a galaxy in its own right!

Image credit: NASA / ESA / Hubble Space Telescope, via Wikisky.

And with that fun little glimpse into this corner of the Universe, we arrive at the end of another Messier Monday! Including today’s object, here are the ones we’ve profiled so far:

Next week will bring us the final Virgo Cluster galaxy in Messier’s catalogue, so make sure you don’t miss it! (And if you missed our Comments of the Week, check those out, too.) Thanks for joining me, and I’ll see you back here soon for more wonders of the Universe!

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