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Messier Monday: A Most Unusual Elliptical, M105

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“Red and dead” might describe the stars in most ellipticals, but this nearby galaxy tells a different story.

“The line that describes the beautiful is elliptical. It has simplicity and constant change. It cannot be described by a compass, and it changes direction at every one of its points.” –Rudolf Arnheim

There are many giant elliptical galaxies in the Universe: the results of long-ago mergers of many smaller galaxies that formed them. Our own local group will someday become one as well, and the Virgo Cluster — the nearest ultra-rich grouping of over a thousand galaxies — contains a great many of them. But among the Messier objects, there’s only one giant elliptical that isn’t in the Virgo cluster, and that’s the subject of today’s Messier Monday.

Image credit: Ole Nielsen of http://www.ngc7000.org/ccd/messier.html.

In general, galaxies are most densely grouped together in extremely large clusters, with smaller groups on the outskirts, roughly connected by filaments to the largest structures. About halfway from our local group to Virgo lies a couple of smaller groups in the constellation of Leo, and that’s where today’s remarkable object — Messier 105 — lives.

Here’s how to find it for yourself.

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

The constellation of Leo has some bright, prominent stars, and you can find it simply by locating the Big Dipper, orienting yourself so that the “cup” of the dipper is full, and looking “beneath” the dipper itself. The brightest star in that region of sky is Regulus, the brightest star in Leo. You’ll easily be able to see a number of others, including Denebola, the second brightest.

And if you look midway between those two — Denebola and Regulus — you’ll be well on your way to Messier 105.

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

This galaxy also lies midway between two other prominent stars in Leo: Chertan and ρ Leonis. There is, in fact, a sizeable group of galaxies in this region of space: the M96 Group, containing maybe three times as many large galaxies as our own local group. And there’s one particular star shown below — HIP 52683 — that’s just invisible to the naked eye. But if you can find it in binoculars or a telescope, Messier 105 will be unmistakeable.

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

Messier 105 was a late addition to this catalogue, discovered by Messier’s assistant Pierre Méchain, and recorded as this:

[T]wo nebulous stars, which I have discovered in the Leo [M95 and M96]; I find nothing to change for my positions which I have established by comparing these nebulae to Regulus; but there is also a third one to the north; it is a bit more beautiful than the 2 others; I have discovered it on March 24, 1781, 4 or 5 days after the other two.

Through a small telescope, it might look something like this.

Image credit: Gary Clinch, via http://www.prairiehillfarmiowa.com/prairiehill/widefields/widefields.html.

Pretty hard to see anything, isn’t it? This wide-field view has the galaxies M95 and M96 along the right side, with Messier 105 as the slightly “fuzzy star” at the 6 o’clock position. The star HIP 52683 we talked about earlier is just below and to the left of center, and M105 is the “fuzzy” star closest to it.

Let’s take a closer look to find out more about it.

Image credit: Raúl Palma Gallardo, via http://www.cristoraul.com/SPANISH/astromuseo/galerias/messier/M95-M96-M105.htm.

Unlike the other two Messier objects in this substantial group, Messier 105 is located very nearby to two other galaxies: NGC 3384 and NGC 3389, the former of which is another giant elliptical galaxy. Along with Messier 105, these galaxies must have undergone a series of major mergers long ago, forming stars in an intense burst and losing the vast majority of the gas capable of forming new stars.

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M105.html.

Over 80% of the stars in these ellipticals are classified as Population II, meaning that they were formed over a billion years ago, and are much poorer in elements heavier than helium than our Sun is. If you look at galaxies like ours in general, we find that metal-rich stars (like ours) are found concentrated in the plane of the disk, and that the metal-poor ones are found in the halo.

Image credit: Wikimedia Commons user Rursus, based on “Astronomi och Astrofysik” by Gunnar Larsson-Leander, 1970.

Finding so many metal-poor stars in this elliptical galaxy is suggestive that it’s been an elliptical for a long time already!

At 32 million light-years distant, these galaxies are only about halfway to the Virgo Cluster from where we are, are huge, with an estimated mass of about a hundred billion Suns in M105 alone. That’s on the same order as the Milky Way, mind you, but if we look in the X-ray, we find something very different from our Milky Way.

Image credit: Karl Gebhardt (University of Michigan), Tod Lauer (NOAO), and NASA/Chandra.

One of the telltale signs of giant elliptical galaxies seems to be a powerful X-ray source at the center, indicative of a much larger black hole than the supermassive one at our galaxy’s center. Whereas ours weighs in at about 4 million times the mass of our Sun, M105’s black hole is about 200 million times the Sun’s mass!

On the other hand, most elliptical galaxies consist of practically no new, blue stars and are devoid of neutral hydrogen gas, another stark difference from our Milky Way.

Image credit: Rick Beno of Conferring With The Sky Observatory, via http://www.conferringwiththesky.org/displayimage.php?pid=500.

It may be very difficult to see from most optical images, but it turns out that this elliptical galaxy — one of the closest typical giant ellipticals — may be turning that piece of information on its head! You see, even though it does have lots of old, red stars, and very little in the way of hot, new young stars, there’s actually a huge halo of neutral gas centered on this galaxy!

Image credit: NOAO / AURA / NSF, via Twin City Amateur Astronomers at http://tcaa.us/Astronomy/Messier/Messier.aspx?id=M105.

Whereas our Milky Way has a radius of about 50,000 light-years, the halo of neutral gas surrounding M105 extends for about 650,000 light-years, an incredible distance! Despite being a gas-poor galaxy, there are still more than a billion Suns worth of neutral gas surrounding it, a very large amount.

And you might think that it’s rare to have three galaxies grouped together like this, but if we take more detailed observations, we actually find that it isn’t just these three, but there are a great deal more if we include smaller, fainter satellite galaxies of these major ones!

Image credit: © 2008 — 2014 — Dark Horse Observatory, via http://www.darkhorseobservatory.org/index.php?CategoryID=35. And yes, detail-detectives, NGC 3389 is mislabeled!

Yet perhaps most remarkably, we’ve very recently learned that this galaxy’s gas — much like the neutral gas in most galaxies — is actually collapsing to form not only new stars, but new star clusters as well!

Image credits: SDSS (outermost), HST / WFC3 (innermost), University of Michigan / H. Alyson Ford / Joel. N. Bregman (all).

As Alyson Ford and Joel Bregman discovered using ultraviolet observations of this galaxy with the Hubble Space Telescope:

We were confused by some of the colors of objects in our images until we realized that they must be star clusters, so most of the star formation happens in associations.
[…]
This is not just a burst of star formation but a continuous process.

And finally, there are a large number of Hubble observations of M105, but my favorite is a long, 39000-second exposure (in two different filter bands), that actually shows off what’s behind Messier 105! Take an unbelievable look:

Image credit: Hubble Legacy Archive, NASA/Hubble, PI: Gregg, via http://hla.stsci.edu/hlaview.html.

If this reminds you of one of the Hubble Deep Fields, it should! There are literally hundreds of background galaxies visible in the image above, further showing that galaxies are literally everywhere in the Universe, even behind some of the brightest galaxies in our local neighborhood!

So with that, we’ll bring today’s Messier Monday to a close. Take a look back at all our previous Messier Mondays here!

I hope you enjoyed this remarkable object and it’s story, and don’t forget to come back next week for another deep-sky wonder, only here, for another Messier Monday!

Weigh in and have your say at the Starts With A Bang forum on Scienceblogs!

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