NRAO: Lowly Dwarf Galaxy Becomes Star-Forming Powerhouse

[bystander]

How a Lowly Dwarf Galaxy Becomes a Star-Forming Powerhouse
National Radio Astronomy Observatory | 2015 Sep 09

[img3="ALMA discovers an unexpected population of compact interstellar clouds inside the dwarf irregular galaxy WLM. These star-forming clouds provide the necessary nurturing environment to form star clusters. As seen in relation to an optical image of the galaxy taken with the Blanco 4-meter telescope, (box upper left) an overlaying blanket of hydrogen gas (red) imaged with NRAO's VLA telescope provides the pressure necessary to concentrate molecules of carbon monoxide (yellow) as seen with ALMA. These regions correspond to dense cores capable of forming clusters like those found in the Milky Way and other large galaxies.
Credit: B. Saxton (NRAO/AUI/NSF); M. Rubio et al., Universidad de Chile, ALMA (NRAO/ESO/NAOJ); D. Hunter and A. Schruba, VLA (NRAO/AUI/NSF); P. Massey/Lowell Observatory and K. Olsen (NOAO/AURA/NSF)"]https://public.nrao.edu/images/non-gall ... 2_nrao.jpg[/img3][hr][/hr]

A nearby dwarf galaxy poses an intriguing mystery: How is it able to form brilliant star clusters without the dusty, gas-rich environments found in larger galaxies? The answer, astronomers believe, lies in densely packed and previously unrecognized nuggets of star-forming material sprinkled throughout the galaxy.

An international team of astronomers [1] using the Atacama Large Millimeter/submillimeter Array (ALMA) has discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy [2] Wolf—Lundmark—Melotte, more commonly known as WLM.

These clouds, which are nestled within a heavy blanket of interstellar material, help explain how dense star clusters [3] are able to form in the tenuous environs of a galaxy thousands of times smaller and far more diffuse than our own Milky Way.

"For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters," noted Monica Rubio, an astronomer with the University of Chile and lead author on a paper to appear in the scientific journal Nature. "These galaxies are fluffy with very low densities. They also lack the heavy elements that contribute to star formation. Such galaxies should only form dispersed stars rather than concentrated clusters, but that is clearly not the case."

By studying this galaxy with ALMA, the astronomers were able to locate, for the first time, compact regions that appear able to emulate the nurturing environments found in larger galaxies.

These regions were discovered by pinpointing the almost imperceptible and highly localized millimeter wavelength light emitted by carbon monoxide (CO) molecules, which are typically associated with star-forming interstellar clouds. ...

Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM - Monica Rubio et al

• Nature 525(7568):218 (10 Sep 2015) DOI: 10.1038/nature14901

[Ann]

Very interesting! Both the caption and the image are extremely informative.

There is a great BVHa picture of the WLM galaxy by P. Massey on this page. I don't dare link it better than that, because I'm not at all sure that I'm allowed to. But the WLM galaxy is thumbnail number 30 on that page. If you click on it, it takes you to a large picture where you can see that the inner "body" of this galaxy is dominated by blue stars, while the outer parts seem completely dominated by non-blue stars. And while there are several large red emission nebulas in the galaxy, they are even more concentrated to just some parts of the galaxy than the blue stars. But they, too, are "inside" the galaxy.

It is interesting to compare WLM image with a similar BVHa picture by the same photographer of a better-known small galaxy, NGC 6822. You find it on the same page of thumbnails as number 20. P. Massey's closeup of that galaxy shows far fewer blue stars than appear to be there in WLM. Also the emission nebulas are concentrated outside the main body of the galaxy. The stellar populations are quite differently distributed in these two small galaxies, and WLM looks a lot bluer than NGC 6822 to me. But David Malin has said about NGC 6822 that it sits in a vast reservoir of gas, so maybe there is a lot of star formation and stellar fireworks to come for Barnard's galaxy.

And don't miss P. Massey's BVHa image of the Phoenix dwarf galaxy, which is thumbnail number 23 on the same page. The Phoenix dwarf galaxy is altogether devoid of emission nebulas, and it is completely dominated by non-blue stars. But it does have a population of blue stars in its densest part. I have to wonder if the latest episode of star formation in the Phoenix dwarf and its subsequent rounds of supernova explosions blew all the gas out of this little wimp of a galaxy.

I find the star formation history of any galaxy completely fascinating.

Ann