A galaxy in a ring

Now here’s something you don’t see every day:

NGC 660, captured with the Gemini Multi-Object Spectrograph on the Fredrick C. Gillett Gemini North telescope on Mauna Kea in Hawai‘i in August of 2012. The optical image, made using g, r, I, and hydrogen alpha filters, were assigned the colors of blue, green, orange and red respectively. The field of view is 9.3×5.6 arcminutes and is oriented 8 degrees clockwise from north at right and east up. The total exposure (integration) time was 1,620 seconds cumulative for all filters.
Color composite produced by Travis Rector, University of Alaska Anchorage.
Credit: Gemini Observatory/AURA Get the 3844×2444 version!

This is NGC 660 as seen by the Gemini North observatory at Mauna Kea, Hawaii and before we go any further, you definitely want to grab the full-resolution version. NGC 660 is an example of a rare polar-ring galaxy – that is, a galaxy surrounded by a ring of stars that rotates over the poles of the galaxy. First we have the main galaxy itself, seen edge-on to us in the middle. The host galaxy has a very thick central bulge, which classifies it as a lenticular galaxy.

Surrounding the central galaxy is a ring of stars and gas. And the ring itself is bursting with star formation! Take a look in that full-resolution version and you’ll see nebulae illuminated by hot young stars, and bubbles blown out by massive stars that went supernova.

Of course, neither the galaxy nor the ring are solid, and both are warped by their mutual tidal forces on each other. Just wow!

So how did this whole thing come to be? Polar-ring galaxies are thought to form in one of two ways: either in a head-on collision or when one galaxy rips apart a passer by, strewing the former galaxy into a ring.

In the merger scenario, one galaxy pierces the heart of another at a right angle, and the “pierced” galaxy ends up as a ring around the intruder galaxy. In such a scenario, you end up with a collapsed core and a burst of star formation, and NGC 660 certainly has both.

But there are some compelling reasons why the piercing scenario may not be the case here. For one, the galaxy and its ring aren’t at right angles to one another, but rather at roughly 45-degree angles. For comparison, here’s NGC 4650, another polar ring galaxy that was probably formed as a result of such a collision:

NGC 4650 – a polar-ring galaxy most likely formed in a collision scenario. Note that the galaxy and its ring are at ~90-degrees to one another. Image credit: Credit:Hubble Space Telescope/NASA/ESA.

Another problem is that in a piercing scenario, the gas gets concentrated into the host galaxy’s central region, while the ring is left largely stripped of this gas. In NGC 660, not only is there a lot of gas and star formation taking place in the ring, but the host galaxy itself is “thick” with gas, and there’s quite a bit of star formation going on in there as well. In addition, ring itself is tilted about 45-degrees. Simulations of piercing mergers don’t get us gas-rich, 45-degree ring that we see.

That leaves us with with another possibility, called tidal accretion. In other words, the host galaxy “shredded” a lower-mass passer-by galaxy into the ring. There’s some compelling evidence for that here. In tidal accretion, you can have a wider distribution of gas both in the ring and in the host galaxy itself, allowing for the star formation we see in both. It is also possible for the ring to be oriented at any angle with respect to the host, since you don’t have to start out with a right-angle collision. Finally, in a right-angle collision, you’d expect the pierced galaxy’s nucleus to fall inward toward the host, giving you a “double nucleus” at the center. But NGC 660 only has its original nucleus.

That means that we’re likely looking at the aftermath of a lower-mass galaxy that passed close to NGC 660 long ago, and lost a lot of its stars and gas to the larger galaxy as it passed by. NGC pulled this “chunk” of the passer-by into a ring and both the ring and the galaxy tidally distorted themselves into the shapes we see today.

There’s one more very cool thing going on here, hidden from our view but bright at radio wavelengths is a compact , 32 light-year diameter source near the center of NGC 660. This source is most likely a super cluster of stars in a dense cloud of dust and gas, hidden from view but transparent at radio. This source contains perhaps a few thousand hot, blue youthful stars.

Whatever caused the ring, it led to a lot of star formation in NGC 660, making this already rare polar-ring galaxy even rarer – a polar-ring starburst galaxy! Very cool!

It’s truly amazing how much we can figure out about the origins and evolution of galaxies “just” by looking at them and critically examining their features. The answers don’t jump out at us, but we can take what we see and propose models to explain them. Those models that don’t explain the observations are discarded, while those that do are refined to come up with a picture that better matches what we see. This is how science – astronomy in particular – works!

An award-winning Whirlpool

Martin Pugh’s award-winning image of M51. If you really want to be blown away, check out the full resolution image. Wow!

M51, aka the Whirlpool Galaxy, is a favorite target for professional and amateur  astronomers alike. It’s a relatively nearby pair of galaxies that are interacting with one another. M51 is the large spiral, seen nearly face-on to us, and its arms are bursting with new star formation. This of course is due to the tidal interaction with its companion NGC 5195, which is a dwarf galaxy passing “underneath” one of the Whirlpool’s spiral arms.

Of course, the Whirlpool has been imaged in exquisite detail by the Hubble Space Telescope and others, but this image is a winner in more ways than one. Martin Pugh’s image was voted the overall winner in the Royal Observatory’s 2012 Astronomy Photographer of the Year competition. And yeah, click that link because even the “als0-rans” images are breathtaking. Seriously, you won’t believe that these images were made by amateur astronomers with backyard telescopes!

What amazes me about Martin’s image is the combination of subtle details and wide range of features. The wisps of scattered gas and dust from the smaller companion are rarely imaged, and yet there they are in the same image as the fine, tightly wound spiral arms near the nucleus of the Whirlpool itself. Then there are of course the fine dust lanes etched into the spiral arms, dotted with the sapphire blue that is the telltale sign of hot new stars recently born in the arms. The colors, the hues, the saturation, and the balance of capturing all of that stunning detail make this one of the best images of the Whirlpool I’ve ever seen – and yes, I’m willing to rank it up there with Hubble’s image.

Not bad at all for a guy with a 17-inch telescope! I can’t imagine the hard work, patience, and skill it must have taken to produce this image. Fortunately, Martin has a few more knockouts on his Flickr stream for our viewing and mind-blowing pleasure. Take some time to check them out, as well as browse through the contest’s photo pool which is chock-a-block of amazing astro-image awesomeness!

A slightly warped edge-on spiral galaxy

NGC 4634 as seen by the Hubble Space Telescope – click for 1280×1093 version, or for full-on warpage, get the 23796×2388 version

Hot on the heels of another beautiful Hubble image of an edge-on spiral galaxy is a Hubble image of NGC 4634. What makes this image so interesting is the slight warp in the disk. If you look closely, you’ll see that the  disk is tilted upright on the right hand side of the image and slightly downward on the left hand side. This is because NG 4634 is not alone in the cosmos, but has a nearby companion tugging on it, just outside of Hubble’s field of view. The companion – NGC 4633 is clearly visible in this wide-field image from the Digital Sky Survey:

Digital Sky Survey image of NGC 4634 & NGC 4633, rotated to match the Hubble image. Get the full field of view or the full resolution version

Even though they’re not interacting yet the Hubble image clearly shows that NGC 4634 is already feeling the gravitational effects of it’s neighbor, NGC 4633. As the two galaxies tug on each other, their shapes begin to distort, as clearly seen by the warp in NGC 4634’s disk. But something else is going on as well – as gas clouds are tugged about the disk of NGC 4634, they collide with slower-moving material. When clouds collide, gravity takes over and stars eventually form within them.

But don’t take my word for it, just look at the Hubble image and you’ll see pink cloud formations that are billowing from the radiation of hot, newly formed stars within. And then there’s all of these jewel-like clusters of very hot, massive stars that have formed all along the length of the galaxy’s disk!

Over the next billion years (or so), these two galaxies will merge, first by passing through one another, and eventually coalesce to form one large elliptical-shaped galaxy. All of the while, new stars will be bursting into life as more and more of the clouds come together and gravity starts working its magic. Should be a very cool show indeed!