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The European Space Agency (ESA) Herschel Space Observatory, home to the largest single mirror telescope in space, has detected massive amounts of molecular gas gusting at high velocities — in some cases in excess of 1000 kilometers per second — from the centers of a sample of merging galaxies. Herschel was built by a European-led, multi-national team, including U.S. contributions from researchers at NASA’s JPL, Caltech, and the Naval Research Laboratory (NRL). It opens a new terahertz window on the cold and dusty Universe, enabling its scientific objective: to investigate how planets, stars, and galaxies formed and continue to evolve. Driven by star formation and central black holes, these powerful storms are strong enough to sweep away billions of solar masses of molecular gas and to interfere with global galactic processes. The Herschel observations indicate that, in the galaxies hosting the brightest Active Galactic Nuclei, outflows can clear the entire supply for creating stars and feeding the black hole. This finding provides long-sought-after evidence of highly energetic feedback processes taking place in galaxies as they evolve. The discoveries are reported in papers in the journals Astronomy Astrophysics (volume 518, L41, 2010) and Astrophysical Journal Letters (volume 773, L16, 2011).
Together with an international team of investigators, Drs. Eckhard Sturm of the Max-Planck-Institut für extraterrestrische Physik (MPE) in Germany and Jacqueline Fischer of the NRL Remote Sensing Division, the Herschel Optical System Scientist, obtained terahertz spectroscopic observations in order to trace the evolution of merging gas-rich galaxies. The team observed a number of these mergers, which, because they are enshrouded in gas and dust therefore are very luminous in the infrared, are also known as Ultra-Luminous InfraRed Galaxies (ULIRGs). The mergers were observed with the spectrometer of Herschel’s PACS instrument, built by a team led by Dr. Albrecht Poglitsch (MPE), as part of the Survey with Herschel of the ISM in Nearby INfrared Galaxies (SHINING), headed by Dr. Sturm.
Massive outflows of gas from galactic centers are tell-tale signs that powerful, storm-like processes affecting the global galactic balance of mass and energy are underway. Within a galaxy, these storms can be generated in the regions of active star formation, stirred by stellar winds and shock waves from supernova explosions. They can also be triggered close to the central black hole, where radiation pressure from the accretion disc drives the surrounding gas away. When powerful enough, outflows can sweep away the galaxy’s entire reservoir of gas, depleting it of the raw material that creates stars and feeds the central black hole. This inhibits further star formation episodes and additional black hole growth. Thus, galactic outflows cause negative feedback, halting the same mechanisms that produced them in the first place.
Powerful outflows are key features in models of galactic formation and evolution, but while there have been other detections of galactic outflows, almost all previous observations dealt only with neutral and ionized gas. The Herschel discovery is unique in that, for the first time, the outflows were detected in the cool molecular gas from which stars are born, allowing their direct impact on star formation to be studied.
Elliptical galaxies are thought to arise from the merger of gas-rich spiral galaxies, a process in which ULIRGs represent an intermediate stage. Gas outflows develop naturally within this scenario, and they are crucial to explaining some observed characteristics of elliptical galaxies. Elliptical galaxies contain old stellar populations, relatively small amounts of gas and almost no sign of ongoing star formation. This is in contrast with spiral galaxies, which are dominated by young stars and are rich in gas necessary for intense star formation. For elliptical galaxies to derive from spiral galaxies, something must drain the cold gas and halt the production of stars, and outflows such as those observed by Herschel appear as ideal candidates for the job.
Another property that finds a natural explanation in galactic outflows is the strong correlation observed between the mass of black holes and the stellar mass of the spheroidal component of the galaxies hosting them: black holes that are relatively more massive appear to reside in galaxies with spheroids that contain more stars. This empirical relation suggests that black hole growth and star formation are intertwined, both initially drawing from the gas reservoir, and creating feedback mechanisms such as outflows that eventually suppress them.
Herschel’s sensitivity and spectral resolution enabled detection of the Doppler shifted signatures of these gigantic galactic storms, and demonstration for the first time, that they may be strong enough to shut down stellar production entirely. The outflows were traced via spectral lines of the hydroxyl molecule (OH). The excellent spectral resolution of PACS allowed astronomers to clearly identify the characteristic blue- and red-shifted profile caused by the system geometry. With velocities of 1000 kilometers per second and higher, the outflows are able to strip galaxies of gas amounting to several hundred solar masses every year.
The data set suggests that slower outflows may be initiated by star formation regions, whereas those with higher velocity appear to be related to the activity of Active Galactic Nuclei (AGN) powered by central black holes: brighter AGN seem to sweep gas away faster than their less luminous counterparts. However, it will be necessary to analyse a larger sample of galaxies in order to verify this claim that the measured velocity can be used as an indicator of the main mechanism driving the outflow.
Although observations of a larger sample is being collected, the early Herschel observations indicate that the galaxies hosting the strongest signatures of AGN are releasing gas at a much higher pace than their star formation rates, and thus they appear able to provide the mechanism needed exhaust their reservoirs of star-forming gas, as is necessary if they are to evolve into gas-poor elliptical galaxies. In the mergers observed to be undergoing strong feedback, star formation is estimated to cease on timescales shorter than 10 million years. This will produce galaxies with characteristics that match those observed in ellipticals: poor in cold gas and populated by old stars.
By catching molecular outflows ‘in the act,’ Herschel has yielded long-sought-after evidence that powerful processes with negative feedback do take place in galaxies and dramatically affect their evolution.