From AAS NOVA: “Forming Stars Near Our Supermassive Black Hole”

AASNOVA

AAS NOVA

24 January 2018
Susanna Kohler

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Eleven bipolar outflows — signatures of star formation — have been discovered in the very center of our galaxy, near the supermassive black hole Sgr A*. [Yusef-Zadeh et al. 2017.]

Is it possible to form stars in the immediate vicinity of the hostile supermassive black hole at the center of our galaxy? New evidence suggests that nature has found a way.

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Infrared view of the central 300 light-years of our galaxy. [Hubble: NASA/ESA/Q.D. Wang; Spitzer: NASA/JPL/S. Stolovy]

Too Hostile for Stellar Birth?

Around Sgr A*, the supermassive black hole lurking at the Milky Way’s center, lies a population of ~200 massive, young, bright stars.

SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

Their very tight orbits around the black hole pose a mystery: did these intrepid stars somehow manage to form in situ, or did they instead migrate to their current locations from further out?

For a star to be born out of a molecular cloud, the self-gravity of the cloud clump must be stronger than the other forces it’s subject to. Close to a supermassive black hole, the brutal tidal forces of the black hole dominate over all else. For this reason, it was thought that stars couldn’t form in the hostile environment near a supermassive black hole — until clues came along suggesting otherwise.

Science as an Iterative Process

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Very Large Array observations of candidate photoevaporative protoplanetary disks discovered in 2015. [Yusef-Zadeh et al. 2015]

NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

Longtime AAS Nova readers might recall that one of our very first highlights on the site, back in August of 2015, was of a study [The Astrophysical Journal Letters] led by Farhad Yusef-Zadeh of Northwestern University. In this study, the authors presented observations of candidate “proplyds” — photoevaporative protoplanetary disks suggestive of star formation — within a few light-years of the galactic center.

While these observations seemed to indicate that stars might, even now, be actively forming near Sgr A*, they weren’t conclusive evidence. Follow-up observations of these and other signs of possible star formation were hindered by the challenges of observing the distant and crowded galactic center.

Two and a half years later, Yusef-Zadeh and collaborators are back — now aided by high-resolution and high-sensitivity observations of the galactic center made with the Atacama Large Millimeter-Submillimeter Array (ALMA).

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

And this time, they consider what they found to be conclusive.

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ALMA observations of BP1, one of 11 bipolar outflows — signatures of star formation — discovered within the central few light-years of our galaxy. BP1 is shown in context at left and zoomed in at right; click for a closer look. [Yusef-Zadeh et al. 2017.]

Unambiguous Signatures

The authors’ deep ALMA observations of the galactic center revealed the presence of 11 bipolar outflows within a few light-years of Sgr A*. These outflows appear as approaching and receding lobes of dense gas that were likely swept up by the jets created as stars were formed within the last ~10,000 years. Yusef-Zadeh and collaborators argue that the bipolar outflows are “unambiguous signatures of young protostars.”

Based on these sources, the authors calculate an approximate rate of star formation of ~5 x 10-4 solar masses per year in this region. This is large enough that such low-mass star formation over the past few billion years could be a significant contributor to the stellar mass budget in the galactic center.

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Locations and orientations of the 11 bipolar outflows found. [Yusef-Zadeh et al. 2017]

The question of how these stars were able to form so near the black hole remains open. Yusef-Zadeh and collaborators suggest the possibility of events that compress the host cloud, creating star-forming condensations with enough self-gravity to resist tidal disruption by Sgr A*’s strong gravitational forces.

To verify this picture, the next step is to build a detailed census of low-mass star formation at the galactic center. We’re looking forward to seeing how this field has progressed by the next time we report on it!

Citation

F. Yusef-Zadeh et al 2017 ApJL 850 L30. http://iopscience.iop.org/article/10.3847/2041-8213/aa96a2/meta

Related Journal Articles

Signatures of Young Star Formation Activity within Two Parsecs of Sgr A* http://iopscience.iop.org/article/10.1088/0004-637X/808/1/97
Sgr A* and Its Environment: Low-mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow http://iopscience.iop.org/article/10.3847/0004-637X/819/1/60
Tidal Distortion of the Envelope of an AGB Star IRS 3 near Sgr A* http://iopscience.iop.org/article/10.3847/1538-4357/aa5ea2/
Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-like Objects Near Sgr A* http://iopscience.iop.org/article/10.1088/2041-8205/801/2/L26/
ALMA Observations of the Galactic Center: SiO Outflows and High-mass Star Formation near Sgr A* http://iopscience.iop.org/article/10.1088/2041-8205/767/2/L32/
Abundant CH3OH Masers but no New Evidence for Star Formation in GCM0.253+0.016 http://iopscience.iop.org/article/10.1088/0004-637X/805/1/72/

See the full article here .

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