In this comparison of actual observations with simulations, the top
images show Hubble observations of the density of gas in the central
portion of two galaxies. The bottom images are computer simulations
that are remarkably similar to the Hubble observations. Knots of star
formation in the two galaxies show how gas falling into a galaxy's
center is controlled by jets from the central black hole. Credit: NASA, ESA, M. Donahue (Michigan State University), and Y. Li (University of Michigan)
Astronomers have uncovered a unique process for how the universe's
largest elliptical galaxies continue making stars long after their peak
years of star birth. NASA's Hubble Space Telescope's exquisite high
resolution and ultraviolet-light sensitivity allowed the astronomers to
see brilliant knots of hot, blue stars forming along the jets of
active black holes found in the centers of giant elliptical galaxies.
Combining Hubble data with observations from a suite of ground-based
and space telescopes, two independent teams found 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.
"Think of the gas surrounding a galaxy as an atmosphere," explained
the lead of the first study, Megan Donahue of Michigan State
University. "That 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, and the far-ultraviolet capabilities of Hubble
allowed us to directly observe these 'showers' of star formation,"
explained the lead of the second study, Grant Tremblay of Yale
University. "We know that 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," said
Tremblay. "And they end up making a swirling 'puddle' of star-forming
gas around the central black hole."
But what should be a monsoon of raining gas is reduced to a mere
drizzle by the black hole. While some outwardly flowing gas will cool,
the black hole heats the rest of the gas around a galaxy, which
prevents the whole gaseous envelope from cooling more quickly. The
entire cycle is a self-regulating feedback mechanism, like the
thermostat on a house's heating and cooling system, because the
"puddle" of gas around the black hole provides the fuel that powers the
jets. If too much cooling happens, the jets become more powerful and
add more heat. And if the jets add too much heat, they reduce their fuel
supply and eventually weaken.
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.
Now scientists understand this case of arrested development, where a
cycle of heating and cooling keeps star birth in check. A light drizzle
of cooling gas provides enough fuel for the central black hole's jets
to keep the rest of the galaxy's gas hot. The researchers show that
galaxies don't need fantastic and catastrophic events such as galaxy
collisions to explain the showers of star birth they see.
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. An earlier, independent study, led by Rupal Mittal of the
Rochester Institute of Technology and the Max Planck Institute for
Gravitational Physics, also analyzed the star-birth rates in the same
galaxies as Tremblay's sample.
The researchers were aided by an exciting, new set of computer
simulations of the hydrodynamics of the gas flows developed by Yuan Li
of the University of Michigan. "This is the first time we now have
models in hand that predict how these things ought to look," explained
Donahue. "And when we compare the models to the data, there's a
stunning similarity between the star-forming showers we observe and ones
that occur in simulations. We're getting a physical insight that we
can then apply to models."
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. The suite of telescopes
shows how galaxy ecosystems work, including the black hole and its
influence on its host galaxy and the gas surrounding that galaxy.
Donahue's paper was published in the Astrophysical Journal on June 2,
2015. Tremblay's paper was published in the Monthly Notices of the
Royal Astronomical Society on June 29, 2015.
Contact
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu
Megan Donahue
Michigan State University, East Lansing, Michigan
donahue@pa.msu.edu
Grant Tremblay
Yale University, New Haven, Connecticut
grant.tremblay@yale.edu
Source: HubbleSite
