NASA’s Hubble Space Telescope shows how black-hole fountain regulates galaxy star birth

By: | Published: August 7, 2015 12:51 PM

Astronomers have seen discovered how the universe's largest elliptical galaxies continue making stars long after their peak years of star birth, thanks to NASA's Hubble Space Telescope's high resolution and ultraviolet-light sensitivity.

Astronomers have seen discovered how the universe’s largest elliptical galaxies continue making stars long after their peak years of star birth, thanks to NASA’s Hubble Space Telescope’s high resolution and ultraviolet-light sensitivity.

Combining Hubble data with observations from a suite of ground-based and space telescopes, two independent teams found that that the black hole, jets, and newborn stars are all parts of a self-regulating cycle. High-energy jets shooting from the black hole heat a halo of surrounding gas, controlling the rate at which the gas cools and falls into the galaxy.

Lead of the first study Megan Donahue of Michigan State University explained the atmosphere can contain material in different states, just like our own atmosphere has gas, clouds, and rain. What we are seeing is a process like a thunderstorm. As the jets propel gas outward from the center of the galaxy, some of that gas cools and precipitates into cold clumps that fall back toward the galaxy’s center like raindrops.

The “raindrops” eventually cool enough to become star-forming clouds of cold molecular gas, explained the lead of the second study, Grant Tremblay of Yale University. These showers are linked to the jets because they’re found in filaments and tendrils that wrap around the jets or hug the edges of giant bubbles that the jets have inflated, and they end up making a swirling “puddle” of star-forming gas around the central black hole.

This discovery explains the mystery of why many elliptical galaxies in the present-day universe are not ablaze with a higher rate of star birth. For many years, the question has persisted of why galaxies awash in gas don’t turn all of that gas into stars. Theoretical models of galaxy evolution predict that present-day galaxies more massive than the Milky Way should be bursting with star formation, but that is not the case.

The study led by Donahue looked at far-ultraviolet light from a variety of massive elliptical galaxies found in the Cluster Lensing And Supernova Survey with Hubble (CLASH), which contains elliptical galaxies in the distant universe. These included galaxies that are raining and forming stars, and others that are not. By comparison, the study by Tremblay and his colleagues looked at only elliptical galaxies in the nearby universe with fireworks at their centers. In both cases, the filaments and knots of star-birth appear to be very similar phenomena.

Along with Hubble, which shows where the old and the new stars are, the researchers used the Galaxy Evolution Explorer (GALEX), the Herschel Space Observatory, the Spitzer Space Telescope, the Chandra X-ray Observatory, the X-ray Multi-Mirror Mission (XMM-Newton), the National Radio Astronomy Observatory (NRAO)’s Jansky Very Large Array (JVLA), the National Optical Astronomy Observatory (NOAO)’s Kitt Peak WIYN 3.5 meter telescope, and the Magellan Baade 6.5 meter telescope. Together these observatories paint the complete picture of where all of the gas is, from the hottest to the coldest.

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