Nicholas McConnell: Research

My dissertation research is to measure stellar motions in Brightest Cluster Galaxies (BCGs), using integral-field spectrographs at Keck, Gemini, and McDonald Observatories. By comparing my observations to models of the galaxies, I can determine the mass of the central, supermassive black hole in each galaxy. My collaborators and I recently measured the two most massive black holes in the local Universe (at least, among those that have been discovered so far)! Read more.

My thesis advisors at UC Berkeley are professors Chung-Pei Ma and James Graham.

BCGs and Black Holes
Measurement Techniques
Adaptive Optics
Side Projects (Galactic star clusters, integral-field spectroscopy of super star clusters, supporting observations of A-stars, Kepler targets, and Jupiter)

Homepage
Download my CV, including publications.
For a summary of my undergraduate research, see this page.

Discovery of the Most Massive Black Holes (December 2011)

Working with colleagues at Berkeley, UT Austin, NOAO, and UM Ann Arbor, I measured stellar motions near the centers of two giant elliptical galaxies, each roughly 300 million light years from the Milky Way Galaxy. We determined that the stars were orbiting black holes of 9.7 billion and 21 billion solar masses, and reported our result in the journal Nature. Before our discovery, the most massive known black hole was in a much closer galaxy, M87, and had a mass between 6 and 7 billion solar masses.

In addition to breaking the 10-billion solar mass threshold, our discovery is exciting because it provides a connection between relatively nearby galaxies (in the scheme of the entire Universe, 300 million light years isn't very far) and very distant objects, which we observe as they were billions of years ago when the Universe was just emerging from its infancy. Some of the brightest distant objects are quasars, whose brilliant luminosity comes from gas spiraling into enormous black holes. Astronomers believe that the most massive black holes in quasars were approximately 10 billion solar masses. Over time they ran out of gas and became too faint to observe. Our discovery finally suggests where some of them might be hiding.

Some Media Coverage:
Nature letter
UC Berkeley press release
Gemini Obseratory press release
New York Times article
Nature podcast
Dunlap Institute video interview with James Graham


Brightest Cluster Galaxies and Supermassive Black Holes

Hubble Space Telescope image of a galaxy cluster and its BCG (ESO 325-G004)
Image credit: NASA, ESA, and the Hubble Heritage Team

Brightest Cluster Galaxies (BCGs) are the most luminous galaxies in the Universe. They are giant elliptical galaxies that reside in large galaxy clusters, often anchored near the cluster center. A typical BCG is several times more massive than the Milky Way, and contains over a trillion stars. Unlike the Milky Way, most BCGs stopped forming new stars billions of years ago, and they are almost completely devoid of gas.

Although they are enormous, BCGs are rare and mysterious. Astronomers are not sure whether they formed by tearing apart and consuming hundreds of small galaxies, or from collisions of galaxy clusters that each had a large central galaxy. To gain clues about how BCGs grew into the monsters we observe today, it is important to carefully examine all of their components. One of the most interesting components -- but also one of the hardest to observe -- is the supermassive black hole at the center of each BCG.


Simulated image of light bent around a supermassive black hole (Alain Riazuelo)

Supermassive black holes (SMBHs) reside at the centers of elliptical galaxies and spiral bulges. "Supermassive" means anything from "only" a few hundred thousand times the mass of our Sun, up to 10 billion or more solar masses. Below 1 billion solar masses, the mass of an SMBH correlates with luminosity of its host galaxy or bulge (the MBH-L relation) and also with the velocity dispersion, or the typical speed of orbiting stars, in the host galaxy (MBH-&sigma). To produce the correlations we observe today, galaxies and their central black holes likely evolved together, eating and starving over similar time intervals. This co-evolution may have been driven by violent galaxy collisions. Yet we only partially understand the detailed behavior of colliding and merging galaxies, or the recipes for building different kinds of galaxies from mergers and other processes.

Because of their extreme luminosities, the MBH-L correlation predicts that BCGs host the most massive black holes in the nearby Universe: some could even approach 10 billion solar masses. However, BCGs have similar velocity dispersions to other elliptical galaxies, and so the MBH-&sigma relation predicts that their black holes will not be unusally massive (merely 0.1 to 1 billion solar masses). Directly measuring the masses of SMBHs in BCGs is the only way to resolve the contradicting predictions, and by doing so can provide information about how smaller galaxies and their black holes merged together to eventually build a BCG.

Measuring Black Holes

To determine the mass of a black hole, we need to observe stars that are orbiting in response to the black hole's gravitational pull (in principle we could also measure orbiting gas, but most BCGs don't have any gas). Galaxies beyond the Milky Way are too distant for us to see individual stars -- instead, we see the light from millions of stars blurred together. Fortunately, we can use spectroscopic data (measurements of the light emitted at thousands of different wavelengths) to measure a statistical distribution of stellar velocities. If we can get velocity distributions at different locations in the galaxy -- especially very close to the central black hole -- we can use numerical models of orbiting stars to indicate how massive the black hole is.

My main tool for measuring the orbits of stars in BCGs is integral-field spectroscopy (IFS). An IFS instrument uses an array of small lenses or optical fibers to sub-divide the focal plane into a two-dimensional grid and then disperses the light to create a spectrum for each grid position. I have observed BCGs with IFS instruments on some of the world's largest telescopes:

OSIRIS on the 10-m Keck telescopes
GMOS on the 8-m Gemini North and South telescopes
NIFS on the Gemini North telescope
VIRUS-P on the 2.7-meter telescope at McDonald Observatory

With OSIRIS, GMOS, and NIFS, we try to make measurements on tiny angular scales in order to isolate the black hole's gravitational influence from the rest of the galaxy. However, BCGs have relatively few stars near their centers, making it extremely difficult to obtain high signal-to-noise spectra. Often I will spend an entire observing night pointing the telescope at a single BCG (and watching Youtube videos writing science proposals in the meantime). VIRUS-P gives us information about stars at larger radii, so we can tell how much mass is in stars and dark matter. After analyzing spectra from different instruments to measure stellar motions, I model the galaxies using codes from Karl Gebhardt et al. and the Texas Advanced Computing Center (TACC).

Collapsed data cubes from OSIRIS. Each pixel in the images above has a corresponding near-infrared spectrum.
Above: Center of Brightest Cluster Galaxy NGC 6086. The red square indicates the location and spatial extent of the central spectrum (0.05 x 0.05 arcsec). The field of view is 3.9 x 1.1 arcseconds.
Below: Double star, including sample spectra. The field of view is 3.2 x 0.8 arcseconds.


Adaptive Optics Observations

Observations with OSIRIS and NIFS use laser guide star adaptive optics (LGS-AO) to resolve galaxies more sharply than is typically possible from the ground. Turbulence in Earth's atmosphere bends the direction of starlight in patterns that change rapidly over time. This causes stars to twinkle when viewed with the naked eye, and through a telescope a star will normally appear blurry and jittery. Adaptive optics uses a specially engineered sensor to measure how light is being distorted and a deformable mirror to correct the distortions before the light enters the science camera. For bright stars, the distortions can be measured and corrected thousands of times each second.

However, the night sky has a limited number of bright stars, and we are not so lucky to have one near every BCG. Fortunately, we can create a fake "guide" star by shining a laser into the sky. A powerful laser tuned to the right wavelength (not your dorky cousin's hand-held laser pointer) illuminates a layer of sodium atoms in Earth's ionosphere, and some of the light shines back downwards into the telescope. By correcting the light from our fake star, we can obtain sharp images of a much fainter galaxy.


Laser Guide Star systems at Keck Observatory (left) and Lick Observatory (right)

Other Projects

Embedded star clusters: Star clusters form within clouds of gas and dust, and are therefore invisible to the naked eye at their birth. However, their presence is sometimes betrayed by radio and infrared light produced when the most massive young stars ionize pockets of gas, known as ultra-compact (UC) HII regions. In 2007 and 2008 I hunted for young star clusters in the Milky Way by surveying UCHII regions with the near-infrared camera and laser guid star adaptive optics (LGS-AO) system at Lick Observatory. Below is an infrared image of a gorgeous young cluster, from follow-up observations with the Keck II telescope and LGS-AO system.


Young star cluster, composed from images with two near-infrared filters (courtesy Nate McCrady)

Super star clusters: While supermassive black holes exist in the most luminous galaxies, large star clusters may contain "intermediate-mass" black holes (IMBHs) with thousands or tens of thousands of solar masses. Using OSIRIS at Keck Observatory, my collaborators and I collected spectra of a young, massive star cluster in the dwarf starburst galaxy M82 and an old globular cluster (or possibly the core of a stripped galaxy) orbiting the Andromeda Galaxy. These data are very challenging because the stars in these systems move more slowly than in massive galaxies, and OSIRIS has limited velocity resolution. With some effort, we hope to glean some information about the stellar dynamics and populations of these two systems.

Miscellaneous observations: Sometimes the most interesting observing experiences come when you agree to help a colleague out with a project (s)he is leading. In September 2009, my scheduled observing time at Keck came shortly after a large asteroid impacted the southern hemisphere of Jupiter. During my first night, Jupiter was rotating such that the impact site faced Earth, and I got to point one of Earth's largest telescopes at the Solar System's largest planet and snap away. I've also observed scores of bright stars with adaptive optics to check for companions that would be too faint and close to see with atmospheric blurring. Some of these observations were designed to rule out stellar binaries in targets for the Kepler and GPI missions for detecting extra-solar planets.