From The University of Pittsburgh : “Why do galaxies stop making stars? A huge collision in space provides new clues”

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From The University of Pittsburgh


Six billion years ago, two galaxies were colliding, their combined forces hurling a stream of gas hundreds of thousands of light years away. Reported this week by a team including Pitt astronomers, that unusual feature provides a new possible explanation for why galaxies stop forming stars.

Figure 1. Near-IR and CO emission of SDSS J1448+1010. CO is detected within the prominent stellar tidal tails seen in the HST image, reaching projected distances up to 64 kpc from the central galaxy. The northern (southern) tidal features are blue- (red)shifted from the systemic velocity by ≈150 km s^−1. The gas in the central galaxy is highly turbulent, while the dispersion is <50 km s^−1 at all locations within the tidal features. Contours of the integrated CO emission begin at 3σ and increase in powers of 2 and are repeated in all panels. The ALMA synthesized beam is indicated at the lower left. The dotted circle in the second panel indicates the emission we consider to belong to the central galaxy; all else constitutes the tails.

Figure 2. ALMA CO(2–1) spectra of SDSS J1448+1010 extracted from, in order, the central galaxy and the northern and southern tidal tails. Combined, the tidal tails contain 47% ± 5% of the total CO luminosity in the system.

Figure 3. Left: HST WFC3/F110W image of SDSS J1448+1010, as in Figure 1, shown on log-stretch. Center: after subtraction of the best-fit single Sérsic profile, a second peak ≈0farcs6 northwest of the main galaxy is more easily visible. Right: Gemini FLAMINGOS-2 H-band image with CO(2–1) contours. The white circle shows the photometric aperture used for all imaging data, matched to the SDSS catalog photometry. Note the object just off frame to the northwest is an unrelated foreground galaxy.

Figure 4. The available multiwavelength imaging and SDSS spectroscopic data (upper panels) allow detailed constraints on the star formation history of SDSS J1448+1010. The optical spectrum shows prominent Balmer absorption features, the hallmark of post-starburst galaxies. The reconstructed SFH (lower panels) indicates that the starburst event began ${1.0}_{-0.4}^{+0.3}$ Gyr and ended ${70}_{-30}^{+40}$ Myr ago, and formed ∼40% of the total stellar mass. The photometric data (black points) and best-fit model photometry (red points) are shown in the upper-left panel. The SDSS spectrum (unbinned: gray; binned: black) and best-fit model spectrum (red) are shown in the upper right. Vertical gray bands highlight wavelengths around [O ii] 3727 Å and [O iii] 4959,5007 Å, which are masked in our fitting. Red shaded regions in the lower panels indicate the 16th–84th percentile confidence interval on the reconstructed SFH, and the vertical blue shaded regions indicate the time period of the recent starburst.

Figure 5. The molecular gas in the SDSS J1448+1010 tidal tails is extreme in both scale and magnitude in comparison to all known mergers at any redshift. The low-redshift merger comparison sample is coded by whether or not the molecular gas is thought to have been stripped from the host (solid) or recycled/formed in situ (empty). All objects use the same assumptions for CO excitation and αCO; this figure would appear identical using pure observable quantities.

“One of the biggest questions in astronomy is why the biggest galaxies are dead,” said David Setton, a sixth-year physics and astronomy PhD student in the Kenneth P. Dietrich School of Arts and Sciences. “What we saw is that if you take two galaxies and smash them together, that can actually rip gas out of the galaxy itself.”

In the part of space we inhabit, most large galaxies have long ago stopped making new stars. Only recently have astronomers started looking farther away — and thus farther back in time — with the tools to find recently dead galaxies and figure out how they got that way.

The cold gas that coalesces to form stars might escape from galaxies by several means, blown away by black holes or supernovae. And there’s an even simpler possibility, that galaxies simply quiet down when they’ve used up all the raw materials for creating stars.

Looking for examples of galaxies that recently shut off star formation, the team of researchers used the Sloan Digital Sky Survey, which has catalogued millions of galaxies with a telescope at Apache Point Observatory in New Mexico.
Apache Point Observatory
SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft).

Apache Point Observatory near Sunspot, New Mexico Altitude 2,788 meters (9,147 ft).

Along with observations from the ground-based radio astronomy network ALMA, the researchers found just such a “post-starburst” galaxy seven billion light years away that still showed signs of available star-forming fuel.

“So then we needed an explanation,” said Setton. “If it has gas, why is it not forming stars?”

A second pass with the Hubble Space Telescope then revealed the distinctive “tail” of gas extending from the galaxy.

From that feature, like forensic examiners working through a telescope, the researchers were able to reconstruct the galaxies’ collision and the tremendous gravitational force that tore apart stars and flung a stream of gas a distance more than two Milky Ways laid end to end.

“That was the smoking gun,” said Setton. “We were all so struck by it. You just don’t see this much gas this far away from the galaxy.”

The team, including Pitt Physics and Astronomy Associate Professor Rachel Bezanson and alum Margaret Verrico (A&S ’21) along with colleagues at Texas A&M University and several other institutions, reported their results in The Astrophysical Journal Letters [below] on Aug. 30.

Such an extreme meeting of galaxies is likely rare, Setton said, but because gravity pulls large objects into dense groups, such an event is more common than you might anticipate. “There are all these big voids in space, but all of the biggest galaxies live in the spaces where all of the other big galaxies live,” he said. “You expect to see these sorts of big collisions once every 10 billion years or so for a system this massive.”

Setton’s role on the project was to determine the galaxy’s size and shape, and he discovered that other than the tail, the post-merger galaxy looked surprisingly normal. Once the tail fades in a few hundred million years, it may look just like any other dead galaxy — further suggesting that the process could be more common than it appears, something the team is following up now with another survey.

Along with providing clues for how the universe became the way it is, Setton said such collisions reflect one possibility for the future of our own galaxy.

“If you go to a dark place and look up at the night sky, you can see the Andromeda Galaxy, which in five billion years might do exactly this to our Milky Way,” Setton said.

“It’s helping answer the fundamental question of what’s going to happen to the Milky Way in the future.”

Science paper:
The Astrophysical Journal Letters

See the full article here .


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