Final Report Summary - REVERB_MASS (Advancements in Black Hole Physics with Echo Mapping Experiments)
The over-arching goal of this project was to improve the understanding of black hole mass measurements. Such measurements are of importance because scientists now believe that the growth of black holes throughout the age of the universe have played a role in the overall growth and evolution of galaxies, as we observe them today. So understanding the demographics of black holes and how they grow is of general interest for understanding the evolution of the universe.
The researcher measures the masses of black holes in a specific type of galaxy with an excess of material in the central parts of the galaxy, such that the central “supermassive” black hole (millions to billions times more massive than our sun) is accreting, or feeding on this material. This process, in turn, makes these regions light up to become some of the brightest objects known to exist in the universe, known as quasars.
The researcher uses a method called reverberation mapping to learn about the central parts, or nucleus of these galaxies, and the accretion onto the black hole that lives there. This method makes use of tracing “light echoes” that are created when gas of certain species, such as carbon or hydrogen, first absorbs high-energy, thermal continuum photons emitted by the accretion disk of material falling onto the black hole, and second, re-emits that photon some time later to produce light at a particular wavelength (or frequency), known as an emission line. The brightness of both the accretion disk emission and the gas emission change with time, so when we monitor this brightness over long time periods (months for the nearby sources and years for distant sources) we find that the pattern of brightness variations we see in the accretion disk emission is “echoed” in the brightness pattern of the gas emission. The time it takes the gas to “echo” the accretion disk brightness pattern corresponds to the distance the material is away from the black hole because photons travel at the speed of light, we know the speed of light. Using this measured distance and Newton’s Laws of Motion, we can learn about the central supermassive black hole, including measuring its mass, even though we cannot actually see the black hole, itself.
For this project, the researcher investigated new methods for measuring these black hole masses that increase the accuracy and precision with which we can make these measurements. This will be applied to constraining our understanding of the growth and evolution of galaxies and the universe. One project that was completed on high-redshift (extremely distant) black holes used both new and archival data from telescopes such as the Hubble Space Telescope, the Sloan Digital Sky Survey, and the Large Binocular Telescope. The researcher looked at the quality of data usually used by the community from surveys compared to very high quality data only available from large telescopes or a lot of time spent integrating on a single target. She found that in many cases data quality is very important for making accurate black hole mass estimates. This research has led to significant improvement in the way these black hole masses are estimated from current data. In addition, the researcher looked at specific gas emission from very near the black hole, emitted by ionized carbon atoms. The researcher found that not all the carbon emission echoes the accretion disk brightness, a discovery that has significant implications for our understanding of the emission components in the vicinity of the black hole, which are related to the accretion and feedback mechanisms that grow super-massive black holes and seem to additionally affect galaxy evolution. This work resulted in two first-author publications by the researcher: Denney 2012, ApJ, 759, 44 and Denney et al. 2013, ApJ, 775, 60.
In another project, the researcher helped to develop and update to the calibration that is used to estimate black hole masses for larger samples of objects than can be measured directly with reverberation mapping experiments, since these experiments are very time and resource intensive. This calibration takes advantage of an observed correlation between the line-emitting gas radius, R (measured from the echo time delay), and the source luminosity, L (measured from the average brightness of the accretion disk), which is known as the Radius-Luminosity, or R-L, relationship. The improvement to the calibration suggested by the researcher was to improve the previous measurement of and adoption of the uncertainty in the luminosity by accounting for the fact that the luminosity and radius are connected and change in sync. This simple correction, among other improvements, has improved the calibration of this relationship. The publication that gives this new calibration, Bentz et al. 2013, ApJ, 767, 149 (Denney is 2nd author), is now the most updated version of the R-L relationship, and therefore a significant result, as any new black hole mass estimates will make use of these results, meaning that any studies based on or that make use of black hole masses depend on and will make use of this work.
Another goal for this project was for the researcher to investigate the accuracy with which black hole masses can be measured with reverberation mapping when there are object-to-object differences in the kinematics of the gas producing the emission lines. To do this, the researcher attempted to measure the reverberation response of the hydrogen emission as function of reverberation time delay and the velocity of the gas. This is a velocity-delay map, which will not only further improve the precision of black hole masses, but also inform us about the geometry and kinematics line-emitting gas and help us to understand the accretion processes needed to grow the black hole and the feedback processes, which lead to outflows of gas from the vicinity of the black hole and are thought to impact galaxy evolutionary processes, such as star formation. Unfortunately, the researcher, while making significant progress on this project, was not able to complete it before end of the fellowship. Work on this project is still underway, post-fellowship, and the researcher is hopeful for results within another year.
Finally the last portion of this project is that the researcher is investigating a more resource efficient method for doing reverberation mapping. Traditionally, this method requires spectroscopic observations that divide the light from the quasar by wavelength (like a prism divides white light into colors but with much more precision), which is required to study the line emission from the line-emitting gas that is echoing the accretion disk brightness changes. Significantly more information is gleaned from spectroscopic data than from photometric, which just measures a large “bin” of wavelengths, e.g. it is more like looking at all the light of a specific color. Past studies claim that reverberation mapping can be done with only photometry, which can be obtained from telescopes more easily, but these studies did not have simultaneous spectroscopy with which to compare their photometry. The researcher undertook an observing program at the Nordic Optical Telescope to monitor three objects for photometric reverberation mapping. Unlike past programs, this program was concurrent to a traditional, spectroscopic reverberation mapping program based at MDM Observatory on Kitt Peak, Arizona, USA. Work is still in progress on this project. Qualitative comparison of the accretion disk and line emission brightness signatures from the two objects so far analyzed show general agreement with observations from the spectroscopic program, but the researcher is completing a blind analysis, compared to the spectroscopic program, so cannot yet make a quantitative comparison between the photometric and spectroscopic results. The expectation is that there will be good agreement between the photometric and spectroscopic results. Results of this work will be particularly significant for the future of reverberation mapping experiments in the upcoming LSST era, which will provide thousands of well-sampled quasar continuum light curves spanning multiple years. Results of this program are expected to be published in 2014 and would have been published before the end of the fellowship tenure had the researcher not terminated early to start a new position.
Outreach Project:
Since the researcher was working in a foreign country without good enough knowledge of the local language, directly interacting with school children through education and public outreach in the usual fashion was not possible for her. Instead, she collected from her own experiences or from her own creation a series of inquiry-based astronomy lessons and some presentations, in English, that others could utilize and adapt for their own purpose and/or in whatever language they chose. These are posted on the researcher’s personal website, now maintained at http://www.astronomy.ohio-state.edu/~denney/InquiryAstronomyLessons/index.html since moving institutions. The researcher will continue to expand and modify the lessons and has an outreach program at her new position to personally take these lessons into local classrooms, during which time direct feedback will be gleaned so that modification or adjustment of the current lessons are possible. Plans are also underway to add new lessons to this list. As such, this program will also continue post-fellowship.
Ancillary Projects:
In addition to working on the proposed projects, the researcher was involved in additional research projects during the tenure of the fellowship that led to additional publications, collaborations, mentoring activities, and transfer of knowledge. Some include, but are not limited to (a) working closely with a PhD student on a research project and helping to mentor this student in writing her first paper (A.L. King et al. 2014, MNRAS, in press, arXiv: 1311.2356) (b) planning, proposing, and being awarded time as PI for two reverberation mapping programs on high-redshift, gravitationally lensed quasars – one using the Xshooter spectrograph on the VLT and one using the GMOS IFU on Gemini-S – and being Co-I on a third reverberation program which was awarded 180 HST orbits, and (c) taking part in a collaboration for an Xshooter Large Program to analyze 100 high redshift quasar spectra (also with scientist in charge and to extend collaboration post-fellowship).
The researcher measures the masses of black holes in a specific type of galaxy with an excess of material in the central parts of the galaxy, such that the central “supermassive” black hole (millions to billions times more massive than our sun) is accreting, or feeding on this material. This process, in turn, makes these regions light up to become some of the brightest objects known to exist in the universe, known as quasars.
The researcher uses a method called reverberation mapping to learn about the central parts, or nucleus of these galaxies, and the accretion onto the black hole that lives there. This method makes use of tracing “light echoes” that are created when gas of certain species, such as carbon or hydrogen, first absorbs high-energy, thermal continuum photons emitted by the accretion disk of material falling onto the black hole, and second, re-emits that photon some time later to produce light at a particular wavelength (or frequency), known as an emission line. The brightness of both the accretion disk emission and the gas emission change with time, so when we monitor this brightness over long time periods (months for the nearby sources and years for distant sources) we find that the pattern of brightness variations we see in the accretion disk emission is “echoed” in the brightness pattern of the gas emission. The time it takes the gas to “echo” the accretion disk brightness pattern corresponds to the distance the material is away from the black hole because photons travel at the speed of light, we know the speed of light. Using this measured distance and Newton’s Laws of Motion, we can learn about the central supermassive black hole, including measuring its mass, even though we cannot actually see the black hole, itself.
For this project, the researcher investigated new methods for measuring these black hole masses that increase the accuracy and precision with which we can make these measurements. This will be applied to constraining our understanding of the growth and evolution of galaxies and the universe. One project that was completed on high-redshift (extremely distant) black holes used both new and archival data from telescopes such as the Hubble Space Telescope, the Sloan Digital Sky Survey, and the Large Binocular Telescope. The researcher looked at the quality of data usually used by the community from surveys compared to very high quality data only available from large telescopes or a lot of time spent integrating on a single target. She found that in many cases data quality is very important for making accurate black hole mass estimates. This research has led to significant improvement in the way these black hole masses are estimated from current data. In addition, the researcher looked at specific gas emission from very near the black hole, emitted by ionized carbon atoms. The researcher found that not all the carbon emission echoes the accretion disk brightness, a discovery that has significant implications for our understanding of the emission components in the vicinity of the black hole, which are related to the accretion and feedback mechanisms that grow super-massive black holes and seem to additionally affect galaxy evolution. This work resulted in two first-author publications by the researcher: Denney 2012, ApJ, 759, 44 and Denney et al. 2013, ApJ, 775, 60.
In another project, the researcher helped to develop and update to the calibration that is used to estimate black hole masses for larger samples of objects than can be measured directly with reverberation mapping experiments, since these experiments are very time and resource intensive. This calibration takes advantage of an observed correlation between the line-emitting gas radius, R (measured from the echo time delay), and the source luminosity, L (measured from the average brightness of the accretion disk), which is known as the Radius-Luminosity, or R-L, relationship. The improvement to the calibration suggested by the researcher was to improve the previous measurement of and adoption of the uncertainty in the luminosity by accounting for the fact that the luminosity and radius are connected and change in sync. This simple correction, among other improvements, has improved the calibration of this relationship. The publication that gives this new calibration, Bentz et al. 2013, ApJ, 767, 149 (Denney is 2nd author), is now the most updated version of the R-L relationship, and therefore a significant result, as any new black hole mass estimates will make use of these results, meaning that any studies based on or that make use of black hole masses depend on and will make use of this work.
Another goal for this project was for the researcher to investigate the accuracy with which black hole masses can be measured with reverberation mapping when there are object-to-object differences in the kinematics of the gas producing the emission lines. To do this, the researcher attempted to measure the reverberation response of the hydrogen emission as function of reverberation time delay and the velocity of the gas. This is a velocity-delay map, which will not only further improve the precision of black hole masses, but also inform us about the geometry and kinematics line-emitting gas and help us to understand the accretion processes needed to grow the black hole and the feedback processes, which lead to outflows of gas from the vicinity of the black hole and are thought to impact galaxy evolutionary processes, such as star formation. Unfortunately, the researcher, while making significant progress on this project, was not able to complete it before end of the fellowship. Work on this project is still underway, post-fellowship, and the researcher is hopeful for results within another year.
Finally the last portion of this project is that the researcher is investigating a more resource efficient method for doing reverberation mapping. Traditionally, this method requires spectroscopic observations that divide the light from the quasar by wavelength (like a prism divides white light into colors but with much more precision), which is required to study the line emission from the line-emitting gas that is echoing the accretion disk brightness changes. Significantly more information is gleaned from spectroscopic data than from photometric, which just measures a large “bin” of wavelengths, e.g. it is more like looking at all the light of a specific color. Past studies claim that reverberation mapping can be done with only photometry, which can be obtained from telescopes more easily, but these studies did not have simultaneous spectroscopy with which to compare their photometry. The researcher undertook an observing program at the Nordic Optical Telescope to monitor three objects for photometric reverberation mapping. Unlike past programs, this program was concurrent to a traditional, spectroscopic reverberation mapping program based at MDM Observatory on Kitt Peak, Arizona, USA. Work is still in progress on this project. Qualitative comparison of the accretion disk and line emission brightness signatures from the two objects so far analyzed show general agreement with observations from the spectroscopic program, but the researcher is completing a blind analysis, compared to the spectroscopic program, so cannot yet make a quantitative comparison between the photometric and spectroscopic results. The expectation is that there will be good agreement between the photometric and spectroscopic results. Results of this work will be particularly significant for the future of reverberation mapping experiments in the upcoming LSST era, which will provide thousands of well-sampled quasar continuum light curves spanning multiple years. Results of this program are expected to be published in 2014 and would have been published before the end of the fellowship tenure had the researcher not terminated early to start a new position.
Outreach Project:
Since the researcher was working in a foreign country without good enough knowledge of the local language, directly interacting with school children through education and public outreach in the usual fashion was not possible for her. Instead, she collected from her own experiences or from her own creation a series of inquiry-based astronomy lessons and some presentations, in English, that others could utilize and adapt for their own purpose and/or in whatever language they chose. These are posted on the researcher’s personal website, now maintained at http://www.astronomy.ohio-state.edu/~denney/InquiryAstronomyLessons/index.html since moving institutions. The researcher will continue to expand and modify the lessons and has an outreach program at her new position to personally take these lessons into local classrooms, during which time direct feedback will be gleaned so that modification or adjustment of the current lessons are possible. Plans are also underway to add new lessons to this list. As such, this program will also continue post-fellowship.
Ancillary Projects:
In addition to working on the proposed projects, the researcher was involved in additional research projects during the tenure of the fellowship that led to additional publications, collaborations, mentoring activities, and transfer of knowledge. Some include, but are not limited to (a) working closely with a PhD student on a research project and helping to mentor this student in writing her first paper (A.L. King et al. 2014, MNRAS, in press, arXiv: 1311.2356) (b) planning, proposing, and being awarded time as PI for two reverberation mapping programs on high-redshift, gravitationally lensed quasars – one using the Xshooter spectrograph on the VLT and one using the GMOS IFU on Gemini-S – and being Co-I on a third reverberation program which was awarded 180 HST orbits, and (c) taking part in a collaboration for an Xshooter Large Program to analyze 100 high redshift quasar spectra (also with scientist in charge and to extend collaboration post-fellowship).