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Starts With A Bang

Messier Monday: The Better-You-Look, The Better-It-Gets Galaxy, M90

In the heart of our nearest big galaxy cluster, a massive spiral fights for its life.

“Ali always said I would be nothing without him. But what would he have been without me?” –Joe Frazier

When it comes to the deep-sky wonders of the Universe visible from Earth — clusters, nebulae and galaxies — most of them are relatively quiet, peaceful places, not unlike our own stellar neighborhood. Stars in clusters burning through their fuel slowly, only rarely experiencing a gravitational interaction that threatens to rip them apart; galaxies typically have only a few small regions that are forming new stars, and only uncommonly are found in the midst of gravitational interactions; most nebulae are simple clouds of gas, the majority of which simply either reflects or emits light. But every once in a while, we come across a distant object in the Universe that’s fighting for its life.

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

As the summer months approach and the hours of darkness near their minimum from the Northern Hemisphere, you learn to relish the clear, dark, moonless time you have during nights like tonight. And that makes it the perfect opportunity to check out not only the faint, extended nebulous objects in the Universe, but to go after the ones that reveal even more spectacular details about themselves the deeper you look. For today’s Messier Monday, let’s dive deep inside the Virgo Cluster and check out one of its most spectacular surviving spirals: Messier 90.

Here’s how to find it.

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

From the northern hemisphere, the easily-recognizable Big Dipper now flies high above the horizon even immediately after sundown, with the prominent constellation of Leo located “underneath” the dipper. The brightest and second-brightest stars of Leo are Regulus and Denebola, respectively, and if you follow the line connecting them, you’ll arrive at Vindemiatrix as well: a bit dimmer but still easily seen even from light-polluted skies.

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

It’s in between Denebola and Vindemiatrix that you can locate not only Messier 90, but also the vast majority of the prominent galaxies in the Virgo Cluster. About a third of the way along the imaginary line connecting Vindemiatrix to Denebola — right where another imaginary line connecting the dim-but-naked-eye stars 24 Comae Berenices and ρ Virginis intersects it — there really isn’t anything that stands out to the naked eye. But if you train either a pair of binoculars or a low-powered telescope on that region, a number of faint things, some of them stars and some of them galaxies, will pop out at you.

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

As far as stars go, the brightest one you’re likely to encounter in that patch of space is the 7th-magnitude HIP 61676, a relatively insignificant orange star that’s invisible to the naked eye to all but the keenest observers under supremely dark skies. But — despite what the above illustration shows — stars will definitely stand out more than anything else in a field like this. And Messier 90 is just a single degree away from it!

Here’s what a long-exposure image of (roughly) that same wide-field patch of sky — with the 7th-magnitude star cropped out — looks like. Messier 90 is at the upper left.

Image credit: Blackwater Skies 2013-14, via http://www.blackwaterskies.co.uk/2013/09/widefield-in-virgo-cluster-of-galaxies.html.

As you can see, this is a very dense region of space in terms of galaxies, and these are just the biggest, brightest ones visible to us. In reality, there are an estimated 2,000 galaxies in the Virgo Cluster, but Messier 90 is one of the brightest and largest of all the spirals, significantly brighter than our neighboring big sister: Andromeda. An original discovery by Messier himself in 1781, he described it as:

Nebula without star, in Virgo: its light is as faint as the preceding, No. 89.

But there’s no excuse for having a blasé attitude about this object in light of what we’ve discovered since!

Image credit: Paul and Daniel Koblas / Adam Block / NOAO / AURA / NSF, via http://gallery.rcopticalsystems.com/gallery/m90block.html.

Originally, Messier 90 was a source of controversy; it’s one of the rare galaxies with a blueshift, meaning it’s moving towards us. For a while, there was a contingent of astronomers who argued that it wasn’t a part of the Virgo Cluster at all, but was rather a foreground object, much closer than its presently-estimated distance of 59 million light-years. Instead, we now understand that on average the Virgo cluster recedes from us at a little over 1,000 km/s, but that the individual galaxies inside typically move relative to that number at up to 1,500 km/s, known as peculiar velocity.

Occasionally — such as in the case of M86 and this one as well — these factors conspire so that a giant galaxy appears to be moving towards us, but this is temporary; they’re quite bound inside the Virgo cluster and will reverse their direction within hundreds of millions of years from now.

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M90.html.

You’ll also notice that not only are there other galaxies nearby, but that there are large outstretched spiral arms that look distorted around this galaxy. These two phenomena are related; it appears to be gravitationally interacting with its neighboring smaller galaxy, IC 3583, which is disrupting the structure of the outer spiral arms. These tidal forces are most pronounced along the edges of the galaxy, where deviations in the orbits of stars shows up most prominently.

Image credit: Robert Gendler, via sky-map.org at http://server3.sky-map.org/starview?object_type=2&object_id=6333&locale=EN.

However, unlike most interacting galaxies, Messier 90 does not exhibit signs of intense star formation along its arms. In fact, it seems to be incredibly poor in gas for a spiral galaxy, and contains very few star-formation regions along its arms even relative to a quiet galaxy like our own!

Why could this be happening? Quite simply, when you take a normal galaxy and send it speeding at high velocity through the center of a dense cluster of galaxies, it’s going to be very sensitive to even sparse amounts of matter present in the intracluster medium. Over time, a phenomenon known as ram-pressure stripping has caused the vast majority of interstellar gas and dust to be entirely removed from this galaxy. But there’s one place — where gravity is strongest — that’s held on tightly to its gas: the center!

Image credit: Daniel Duggan of the Faulkes Telescope Project, via http://www.faulkes-telescope.com/node/2185.

There are literally 50,000 of the most massive, bright blue O-and-B-class stars in the central region of this galaxy that have formed over no more than the past 6 million years. As a result, the center of the galaxy is now producing ‘superwinds’ due to a combination of the radiation pressure and from a heightened rate of supernovae that are blowing out even more of the central gas into the intracluster medium. Even though it’s conspicuous, very large and bright, it’s much lower in mass than we’d expect for a galaxy of this size, indicating just how much of this galaxy has been stripped away!

Image credit: Wikimedia Commons user Fabian RRRR using Hubble Legacy Archive data, via http://commons.wikimedia.org/wiki/File:Ngc4569-hst-R814GB555.jpg.

Even though there must have been many relatively recent supernovae in the center of this galaxy to explain the winds (and expected from the intense ultraviolet radiation coming from it), Messier 90 is one of the few galaxies in the catalogue that has never yet had a single supernova observed in it. It may be a highly unusual galaxy that’s fighting for its life and to maintain its spiral integrity, but — at least for this pass through the cluster’s center — it looks like it’s a survivor!

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

And that will wrap up today’s Messier Monday. Take a look back at any or all of our previous entries:

And join us back here next week for another spectacular Messier Monday!


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