Barred Spiral Galaxies

Springtime in the northern hemisphere is the season to observe galaxies, gigantic conglomerations of hundreds of billions of stars, interstellar gas, nebula clouds, and planets. In the center of each big galaxy lurks a massive black hole.

My telescope “Baby” and I spent the night recently at a campground in Emery County, on the lookout for galaxies. The next campsite hosted a huge condominium-size camper with a family that included a sometimes squally child, a puttering generator and a barking dog — but I drowned ’em out when I got my generator going. After sundown the temperature dropped swiftly; I was all right with a heavy jacket, parka and watch cap. I didn’t need the ski mask I had packed.

The sky was supposed to be clear, the National Weather Service had claimed. However, a dark cloud mass hung in the east and I suspect thin filmy clouds hovered overhead. Even after the crescent moon set, the night failed to get truly dark, an effect I attributed to light bouncing off a cloud cover. Mostly, the viewing was all right. I studied some fine celestial objects, including the Ring Nebula (Messier 57), the Dumbbell Nebula (M27) and the Whirlpool Galaxy (M51). I barely detected M100 visually; it is a large, face-on spiral in the Virgo Cluster of galaxies that are prominent in the spring. In my least powerful eyepiece it remained a dim, mottled splotch.

Thanks to Steve Dodds’ wonderful resurfacing of my secondary mirror, and my cleaning the corrector plate with a formula provided by Dr. Clay Sherrod of the Arkansas Sky Observatory, the views were crisper than I had seen in months.

M66, my new favorite galaxy, was beautiful. I easily saw the S shape formed by its central bar and curving arms. I hooked up my astronomical camera and took a few preliminary views in black and white, when a breeze kicked up and began bouncing Baby around.

I took four short (30-second) exposures of M66 before the motion became too nasty and I gave up on taking longer views. Only a couple of the images were good enough to combine, and the result is just barely usable. However, the dramatic bar is visible.

[View of M66 that I took earlier in April]

M66 is one of three galaxies in the constellation Leo that are known as the “Leo Trio” because they’re so close together that their gravity affects one-another. The others are a spiral called M65 and a fat edge-on galaxy dubbed NGC 3628. The biggest, M66, is 35 million light-years away and 100,000 light-years across.

Often galaxies are drawn toward one-another, performing an elaborate slow waltz before colliding; in this case, M66’s arms are distorted from the pull of its neighbors.

Speaking of M66 (also known as NGC 3627), a Jet Propulsion Laboratory web site says, “Its blue core and bar-like structure illustrates a concentration of older stars. While the bar seems devoid of star formation, the bar ends are bright red and actively forming stars. A barred spiral offers an exquisite laboratory for star formation because it contains many different environments with varying levels of star-formation activity, e.g., nucleus, rings, bar, the bar ends and spiral arms.”

Among spiral galaxies, about two-thirds have bars, reports a paper in Astronomy & Astrophysics dated September 2002. Half of the bars are strongly visible and the rest show up “mildly,” adds the article, authored by F. Bournaud of the Ecole Normale Suprieure, Paris, and F. Combes of the Observatoire de Paris.

Using simulations, the researchers found that gravity torques and gas flow help form galactic bars. The bars can channel material inward, to the galaxy’s central bulge, forming new stars. Bars can weaken and become destroyed and form again within the galaxy

“In the course of the simulations, three to four bar episodes have been followed. An interesting discovery is that the pattern speed of the bar is increasing form one bar to the next. This can be considered as a rejuvenating process,” write Combes and Bournaud. ” …. When another bar forms, it is shorter than the previous one.”

A consequence of the gas accretion, they add, “is to maintain an almost permanent (though recurrent) spiral structure through the disk, and also a significant star formation rate ….”

In July 2008, the Space Telescope Science institute, which operates the Hubble Space Telescope, said barred spiral galaxies were far less plentiful 7 billion years ago than now. “The study’s results confirm the idea that bars are a sign of galaxies reaching full maturity as the ‘formative years’ end,” the STScI adds.

Our own Milky Way galaxy is hard to map because Earth is located in the outskirts of one of the spiral arms, not above the plane looking down, and the center is obscured by dust. There were disagreements about whether it has a central ellipse, a bar, or both. If a bar existed, it was believed to be relatively small.

In 2005, astronomers announced that a survey by the Spitzer Space Telescope — which checked infrared light from 30 million stars — showed that the Milky Way sports what NASA calls “a very large central bar some 27,000 light-years long.”

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