Brightest quasars inhabit galaxies with star-forming gas clouds
Posted: June 10, 2001

A team of scientists at the California Institute of Technology and the State University of New York at Stony Brook has found strong evidence that high-luminosity quasar activity in galaxy nuclei is linked to the presence of abundant interstellar gas and high rates of star formation.

In a presentation at the summer meeting of the American Astronomical Society, Caltech astronomy professor Nick Scoville and his colleagues reported that the most luminous nearby optical quasar galaxies have massive reservoirs of interstellar gas much like the so-called ultraluminous infrared galaxies (or ULIRGs). The quasar nucleus is powered by accretion on to a massive black hole with mass typically about 100 million times that of the sun while the infrared galaxies are powered by extremely rapid star formation. The ULIRG "starbursts" are believed to result from the high concentration of interstellar gas and dust in the galactic centers.

"Until now, it has been unclear how the starburst and quasar activities are related," Scoville says, "since many optically bright quasars show only low levels of infrared emission which is generally assumed to measure star formation activity.

"The discovery that quasars inhabit gas-rich galaxies goes a long way toward explaining a longstanding problem," Scoville says. "The number of quasars has been observed to increase very strongly from the present back to Redshift 2, at which time the number of quasars was at a maximum.

"The higher number of quasars seen when the universe was younger can now be explained, since a larger fraction of the galaxies at that time had abundant interstellar gas reservoirs. At later times, much of this gas has been used up in forming stars.

"In addition, the rate of merging galaxies was probably much higher, since the universe was smaller and galaxies were closer together."

The new study shows that even optically bright quasar-type galaxies (QSOs) have massive reservoirs of interstellar gas, even without strong infrared emission from the dust clouds associated with star formation activity. Thus, the fueling of the central black hole in the quasars is strongly associated with the presence of an abundant interstellar gas supply.

The Scoville team used the millimeter-wave radio telescope array at Caltech's Owens Valley Radio Observatory near Bishop, California, for an extremely sensitive search for the emission of carbon monoxide (CO) molecules in a complete sample of the 12 nearest and brightest optical quasars previously catalogued at the Palomar 200-inch telescope in the 1970s. In particular, the researchers avoided selecting samples with bright infrared emissions, since that would bias the sample toward those with abundant interstellar dust clouds.

In this optically selected sample, eight out of the 12 quasars exhibited detectable CO emission-implying masses of interstellar molecular clouds in the range of two to 10 billion solar masses. (For reference, the Milky Way galaxy contains approximately two billion solar masses of molecular clouds.) Such large gas masses are found only in gas-rich spiral or colliding galaxies. The present study clearly shows that most quasars are also in gas-rich spiral or interacting galaxies, not gas-poor elliptical galaxies as previously thought.

The new study supports the hypothesis that there exists an evolutionary link between the two most luminous classes of galaxies: merging ultraluminous IR galaxies and ultraviolet/optically bright QSOs. Both the ULIRGs and QSOs show evidence of a recent galactic collision.

The infrared luminous galaxies are most often powered by prodigious starbursts in their galactic centers, forming young stars at 100 to 1,000 times the current rate in the entire Milky Way. The quasars are powered by the accretion of matter into a massive black hole at their nuclei at a rate of one to 10 solar masses per year.

The detection of abundant interstellar gas in the optically selected QSOs suggests a link between these two very different forms of galactic nuclear activity. The same abundant interstellar gases needed to form stars at a high rate might also feed the central black holes.

In normal spiral galaxies like the Milky Way, most of the interstellar molecular gas is in the galactic disk at distances typically 20,000 light-years from the center -- well out of reach of a central black hole.

However, during galactic collisions, the interstellar gas can sink and accumulate within the central few hundred light-years, and massive concentrations of interstellar gas and dust are, in fact, seen in the nuclear regions of the ULIRGs. Once in the nucleus, this interstellar matter can both fuel the starburst and feed the central black hole at prodigious rates.

The discovery of molecular gas in the optically selected QSOs that do not have strong infrared emissions suggests that the QSO host galaxies might be similar systems observed at a later time after the starburst activity has subsided, yet with the black hole still being fed by interstellar gas.

For the remaining four quasars where CO was not detected, improved future instrumentation may well yield detections of molecular gas, Scoville says. Even in the detected galaxies the CO emission was extraordinarily faint due to their great distances -- typically over a billion light-years. The remaining four galaxies could well have molecular gas masses only a factor of two below those that were detected.

Future instrumentation such as the CARMA and ALMA millimeter arrays will have vastly greater sensitivity, permitting similar studies out to much greater distances.

Other members of the team are David Frayer and Eva Schinnerer, both research scientists at Caltech, Caltech graduate students Micol Christopher and Naveen Reddy and Aaron Evans at SUNY (Stony Brook).

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