Monday, September 24, 2012

Uncovering the Role of Black Holes in Galaxy Evolution

Artist impression of a supermassive black hole 
surrounded by an accretion disk of infalling gas 
and twin, highly-collimated plasma jets. 
Credit: Aurore Simonnet (Sonoma State University
Although Active Galactic Nuclei (AGN), and the supermassive black holes (SMBH) that power them, have been studied for more than half a century, their potential importance to the evolution of galaxies has only recently become evident. Observations over the last decade indicate an intimate connection exists between the growth of galaxies and their central SMBHs. Furthermore, computer simulations have shown that highly energetic AGN can drive outflows that disrupt the star formation activity of the AGN's host galaxy.  For these reasons, AGN have become central figures in Astronomy's attempt to understand the evolution of galaxies from star forming to passively evolving systems. However, despite our increasing focus on AGN, it is still unknown how the connection between black holes and their host galaxies is established and maintained. This issue remains one of the key unanswered questions in Astrophysics today.

Brainstorming session during the third annual CANDELS team
meeting recently held at the University of California at Santa Cruz.
Image Credit: Dale Kocevski
One of the goals of the CANDELS AGN working group is to determine how our remaining questions about AGN can be best answered given our current observations and to identify promising directions for future research. Recently the AGN working group gathered to discuss these very issues at the third annual CANDELS team meeting at the University of California at Santa Cruz.  During the meeting, members of the working group not only presented their recent findings to the team, but we also spent a considerable amount of time discussing which areas of AGN-related science still require further study and worked to chart a course for our future work.  Although it may seem odd that scientists would gather to discuss what we don't know about a particular topic, identifying which aspects of AGN are still poorly understood and what areas require further study is key to advancing our understanding of AGN.  The team identified the following three open questions that we believe we now have the potential to answer in the near future with the help of the CANDELS survey.

What Mechanisms Trigger AGN Activity in Galaxies?

Although it is thought that all massive galaxies have a SMBH at their center, only about 10% appear to be experiencing an AGN growth phase at any given time.  The majority of SMBHs simply lie dormant in their host galaxies.  What mechanisms fuel SMBH growth and turn a dormant black hole into an AGN has remained an enduring mystery.  The collision of two galaxies has long been espoused as a possible triggering mechanism since computer simulations have shown that these violent interactions can be extremely effective at funneling gas to the center of a galaxy and into the central black hole.  However our very own research conducted by the CANDELS team suggests galaxy mergers can not be the sole explanation.  Additional work is needed studying the morphologies and environments of galaxies hosting AGN to determine what distinguishes them from non-active galaxies in the hopes of pin-pointing the mechanism that initiates black hole growth in certain galaxies.

What is the Nature of Heavily Obscured AGN?

Artist impression of a thick dust torus surrounding an obscured
supermassive black hole.  When seen edge-on, as in this case,
much of the light emitted by the AGN is blocked from view.
Credit: ESA / V. Beckmann (NASA)
When gas spirals into a black hole, it forms an accretion disk and rapidly heats up. As it does so, it emits immense amounts of energy at optical, ultraviolet and X-ray wavelengths. Since galaxies themselves do not produce strong X-ray emission, X-ray observations have become the most common method that astronomers use to find AGN. However, if the black hole's accretion disk is obscured by interstellar gas and dust, some or all of the emitted X-ray radiation will be absorbed by the surrounding gas.  The AGN will then no longer be visible at X-ray wavelengths and will be missed by AGN surveys relying solely on X-ray observations.  That said, these so-called obscured AGN can be found since the absorbed X-ray emission will be re-radiated at infrared wavelengths.  Only recently have studies starting examining the properties of this population of obscured AGN.  It may be that this long-lost set of AGN are the missing link between dormant SMBHs and X-ray bright AGN and therefore might provide a clue as to what activates AGN activity in galaxies in the first place.

Do AGN Turn Off Star Formation within Galaxies?

The accretion events that power AGN can be extremely energetic and this can have profound effects on a galaxy that harbors a growing SMBH. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that can effectively suppress the surrounding galaxy's star formation activity.  In this way, SMBHs can regulate the growth of their host galaxies by limiting the amount of stars they form.  This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. However, observational evidence that this suppression actually occurs in AGN host galaxies is still tenuous at best.  One of the goals identified by the CANDELS AGN working group is a better understanding of the connection between star formation activity and AGN activity in galaxies.  This may soon be possible as infrared observations from the Herschel Space Observatory are now allowing us to measure the star formation rates of active galaxies far better than previously possible.  This will provide the first clues as to whether star formation activity is indeed suppressed in galaxies harboring highly energetic AGN.

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