I am an astrophysicist who studies extreme deep-space phenomena, mostly quasars, supermassive black holes, and giant galaxies. I was a researcher in the Department of Astronomy at the University of Texas at Austin for 16 years, nine of those years as a post-PhD research fellow. In August 2017, I retired from my position at UT to focus on ministry work.
I was raised atheist in a secular country (Canada), but came to know God and Jesus Christ in large part through my scientific work. In July 2005, halfway through my graduate studies, I was baptized as a Christian. My testimony tells the story of my conversion. Though I was and continue to be passionate about science, after my conversion, my focus began shifting to reconciling Christians with science.
I now spend most of my time researching, writing, and lecturing about the relationship between science and Christianity. However, I remain active in astrophysical research, because I enjoy it and it keeps me current in the field. I continue to do scientific work through a private astrophysics institute, which allows me the freedom to publish articles at my own pace.
My scientific research
My primary research interests are observational study of quasars, supermassive black holes, and the relationship between black holes and their host galaxies.
My publications are available through the NASA Astrophysics Data System and the free arXiv preprint server. My CV is here.
The majority of my scientific research is conducted using archived data from the Sloan Digital Sky Survey, an enormous public astrophysics project funded by the Sloan Foundation. That means I rarely need to operate a telescope and gather my own data. I let the scientists at SDSS make observations on their dedicated telescope, and just download the data I need from their archive.
The SDSS archive hosts telescopic images and spectra for over a billion objects in the night sky, including almost 700,000 quasars. It’s a staggering amount of data, and it allows me to do the kind of statistical work astrophysicists could only dream of just two decades ago.
Quasars are nature’s most extreme phenomena. They belong to a class of objects called active galaxies, which are galaxies with unusually active central regions. Active galaxies come in a variety of luminosities and other properties, with quasars on the extreme end. A single, luminous quasar can outshine a thousand Milky Way-like galaxies, and yet all of this light emanates from a region not much larger than our solar system.
The central “engine” of an active galaxy is almost certainly a supermassive black hole — a black hole that is hundreds of thousands to tens of billions of times the mass of the Sun — actively feeding on gaseous and stellar material within its immediate vicinity. This material reaches speeds close to the speed of light as it careens down to the black hole, becomes super-heated, and shines brightly enough to be observed from billions of light-years away.
Supermassive black holes and galaxies
One of the significant developments of 20th century astrophysics was the discovery that supermassive black holes lurk in the centers of almost all galaxies. These enormous black holes are believed to be relics of ancient quasars long since deprived of their fuel. By the turn of this century, it was found that the masses of these black holes correlate strongly with properties of their host galaxies, but only the spherical part of the galaxies, called the bulge. Black hole mass is closely correlated with the luminosity and mass of the bulge as well as the distribution of velocities of stars in the bulge.
The relationship between the mass of the supermassive black hole and host galaxy properties is known as the “black hole – galaxy relation.” The ubiquity of supermassive black holes in the local universe as well as in distant quasars, coupled with the strong correlations, suggest some kind of cosmic link between the growth of galaxies and the growth of supermassive black holes. The nature of the link is puzzling, because supermassive black holes are typically only about 1/1000th the mass of their host galaxy bulges (see here for a an exception to this), which means they exert effectively no direct gravitational influence on the stars in the galaxies.
The way the black hole – galaxy relations change with time is key to understanding the black hole – galaxy link. The big question is whether black holes grow substantially before, after, or at the same time as their host galaxies. In order to address this question, I’ve been comparing the black hole – galaxy correlations at several different epochs in cosmic history.
In astrophysics, the further back in time you want to investigate something, the further away you have to look. This is because the speed of light, while blazingly fast, is finite. This presents a challenge to anyone who wants to look very far back in time, because it’s exceedingly difficult to obtain detailed observations of very distant objects in space. Most objects are just too dim to be seen beyond a certain distance. Here is where quasars come to the rescue. Given their immense brightness, they are observable over an enormous range of distances. Together with supernovas, they are important markers of deep cosmic history.
The bulk of my work has involved using quasars to investigate the black hole – galaxy relation over billions of years of cosmic history. Instead of trying to measure the black hole mass and galaxy properties directly, these quantities can be inferred using measured properties from quasar spectra.
Frustratingly, the results of various black hole – galaxy evolution studies have painted a confused picture, with many groups disagreeing on the magnitude of the evolution, the direction of the evolution, and even whether evolution has taken place at all. One of the important discoveries in my work has been the role of observational biases in such studies, which can produce a false appearance of evolution. These biases can be significant, and addressing them is a crucial part of ongoing work in this area.
Relevant research publications:
“The Black Hole Mass-Galaxy Luminosity Relationship for Sloan Digital Sky Survey Quasars,” Salviander, Shields, & Bonning, 2015, The Astrophysical Journal [link]
“The Black Hole Mass-Stellar Velocity Dispersion Relationship for Quasars in the Sloan Digital Sky Survey Data Release 7,” Salviander & Shields, 2013, The Astrophysical Journal [link]
“The Black Hole Mass-Galaxy Bulge Relationship for QSOs in the Sloan Digital Sky Survey Data Release 3,” Salviander, Shields, Gebhardt, & Bonning, 2007, The Astrophysical Journal [link]
Other topics of interest
The most massive black holes in the universe weigh in at five billion times the mass of the Sun or more. We expect to find 200 of these big black holes for every billion cubic parsecs of space. The black hole – galaxy correlations suggest a similar number of large galaxies to host the eventual relics of these quasars. However, large-scale searches have failed to detect such galaxies.
As part of the effort to look for these giant host galaxies, I obtained high-quality spectra of six of the largest galaxies in the SDSS archive using the Hobby-Eberly Telescope, the fourth largest telescope in the world. The goal was to determine whether higher quality data would reveal more information about properties of these galaxies than the SDSS data. They did not. However, given that these objects are some of the largest known galaxies, they are still of great interest. One implication of the failed search is that the majority of the most massive black holes in the universe reside in comparatively modest galaxies — I am therefore very interested in what sort of black holes lurk at the centers of these galaxies. I have been working with a collaborator to obtain Chandra X-ray observations and radio observations from the Very Large Array to determine the masses of their central black holes.
“In Search of the Largest Velocity Dispersion Galaxies,” Salviander, Shields, Gebhardt, Bernardi, & Hyde, 2008, The Astrophysical Journal [link]
Recoiling black holes
During major mergers of galaxies, the supermassive black holes residing in each of the galaxies orbit each other for a while and eventually coalesce into a single black hole. Numerical relativity simulations predict that, under certain circumstances, these coalescing black holes will be imparted with a net velocity — a “kick” — and in extreme cases even flung out of the merged galaxies. These kicked black holes are predicted to have distinct observational signatures in the spectra of quasars. Our group conducted a search for these kicked black holes in the spectra of SDSS quasars, but found no significant evidence for their existence. This was unexpected given the frequency with which major mergers occurred during the quasar epoch. This negative result was featured in a press release (see below).
Relevant research publications:
“Comment on the Black Hole Recoil Candidate Quasar SDSS J092712.65+294344.0,” Shields, Bonning, & Salviander, 2009, The Astrophysical Journal [link]
“Recoiling Black Holes in Quasars,” Bonning, Shields, & Salviander, 2007, The Astrophysical Journal [link]
Accretion disk temperatures
The supply of gas fueling quasars is in the form of a disk swirling around the black hole, and it’s the source of most of the ultraviolet and optical emission in a quasar spectrum. Theoretical models of these accretion disks indicate that hotter accretion disks should radiate bluer light than cooler disks. This is not a surprise, since generally speaking the hotter an object becomes, the more it moves from glowing red to white to blue. However, in a study of accretion disk temperatures of SDSS quasars versus the colors with which they radiate light, my research group found deviation from this expected trend, even to the extent that some quasars exhibited the opposite trend — hotter disks tending to be redder in color. The highly-deviant quasars were found to be gobbling up material at a high rate, suggesting that the rate at which material is consumed by black holes may be a factor in the temperature – color relationship for quasar accretion disks.
Relevant research publications:
“Accretion Disk Temperatures of QSOs: Constraints from the Emission Lines,” Bonning, Shields, Stevens, & Salviander, 2013, The Astrophysical Journal [link]
“Accretion Disk Temperatures and Continuum Colors in QSOs,” Bonning, Cheng, Shields, Salviander, & Gebhardt, 2007, The Astrophysical Journal [link]
Iron abundances in quasars
Quasar spectra show a wide range of ionized iron abundances in the gas clouds that orbit the central supermassive black holes. Recent work carried out with collaborators indicates that this range of abundances can be explained by a model in which the gaseous iron is depleted by solidifying into tiny solid grains beyond the point where dust is vaporized by the hot central regions of quasars.
Relevant research publications:
“Fe II Emission in Active Galactic Nuclei: The Role of Total and Gas-Phase Iron Abundance,” Shields, Ludwig, & Salviander, 2010, The Astrophysical Journal [link]
In the press
The negative result for recoiling black holes was sufficiently interesting that McDonald Observatory issued a press release. We presented our result at the 2007 summer meeting of the American Astronomical Society, along with other groups presenting on the same topic. The press covered the event, and it was picked up by Nature, New Scientist, Astronomy magazine, and MSNBC.
In the summer of 2009, I was interviewed by Discovery News (affiliated with the Discovery Channel) about the implications of the unexpectedly large mass for the supermassive black hole in the giant galaxy M87.