1. DISCUSSIONS AND CONCLUSIONS
Michael Xu
Central Virginia Governor’s School
for Science and
Technology
Using water megamasers to trace the temporal evolution of accretion disk structure in active galactic nuclei:
the case of NGC 5765b, UGC 3789 and Mrk 1419
After analyzing each spectrum, I inputted its high velocity features into Microsoft Excel and inputted the formula
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSIONS AND CONCLUSIONS
Alternate Hypothesis: There would be statistically significant difference in the size of the disk radii of AGN over the time scale studied.
A linear regression was run on the data, which showed that there is no change over time in the disk radii of the AGN.
From analyzing the slopes of the lines of best fit, zero slope was always within the standard error for NGC 5765b and UGC For Mrk 1419, the standard error was within two standard deviations of zero slope. The slope that was measured is consistent with being zero because the standard error was equal to the measurement in all of NGC 5765b, UGC 3789 and Mrk 1419 (D. Pesce, personal communication, February 12, 2015). This could be seen in the following scatter plots for NGC 5765b, UGC 3789 and Mrk 1419 (See Figures 2-4).
The alternate hypothesis, stating that there would be statistically significant difference in the size of the disk radii of AGN over time, was not supported because all of the slopes of the regression calculated were consistent with being zero because the standard error was the same size as the measurement in all of NBC 5765b, UGC 3789 and Mrk 1419 over the time observed in this study. (D. Pesce, personal communication, February 9, 2015a).
I made qualitative discoveries during the observation of the accretion disk of AGN. Most notable was the symmetry of the high velocity features disk radii. For all of the observed galaxies, the size of the disk radii was not symmetric. The data suggest that the redshifted features tend to lie at larger radii than the blueshifted features.
I also conducted a root-mean-square deviation (RMSD) to calculate the deviation of the data from the line of best fit. From the results it could be concluded that both NGC 5765b and UGC 3789 had smaller RMSD values when compared to Mrk 1419.
The probable cause of deviation is the channel-to-channel noise in individual spectra, which made it very difficult to distinguish real maser features from spurious noise spikes.
Further research should be done to examine more active galactic nuclei to collect more data to support or challenge astronomy’s current understanding of megamasers.
Rationale: Active galactic nuclei (AGN) are fundamental objects astronomers study in order to gain a deeper understanding of how galaxies form and evolve. However, astronomers know very little about them because they're difficult to observe. They're difficult to observe because they subtend very small angular sizes. AGN are also heavily obscured by all of the material in space that lies in between Earth and the nuclei. Megamasers allow scientists to bypass both issues because they are bright and compact, allowing astronomers to use interferometry to trace the AGN accretion disk very thoroughly. With all the data collected by analyzing megamasers, astronomers will likely be given insight into the physics of the universe along with a greater understanding of how our own galaxy formed and evolved.
Research Objective: With the time-series data of NGC 5765b, UGC 3789 and Mrk 1419, courtesy of the Megamaser Cosmology Project, this project will specifically focus on how the accretion disk structure changes over time.
I was given a series of spectra from the active galactic nuclei NGC 5765b, UGC 3789, and Mrk The spectra were collected by the Megamaser Cosmology Project (MCP) using the Green Bank Telescope located in Green Bank, West Virginia.
I accessed the spectra by using secure shell technology to remotely access the data, located in University of Virginia Astronomy Departments server.
The Green Bank Telescope Interface Description Language (GBTIDL) was then used to display the megamaser spectra (See Figure 1).
BACKGROUND
Megamasers are a form of “maser,” which is an acronym for Microwave Amplification by Stimulated Emission of Radiation (Elitzur, 1992).
Masers arise from “stimulated emission,” which occurs when one photon of a certain frequency is able to stimulate neighboring molecules to emit photons of the same frequency. These photons occur at the same phase (coherence) and they move in the same direction as the original photon which creates a chain reaction, resulting in exponential growth in the number of photons emitted by a region of molecular gas. This process gives masers extreme luminosity (Elitzur, 1992).
The Megamaser Cosmology Project (MCP) uses masers to measure the Hubble constant in order to improve the extragalactic distance scale and constrain the nature of dark energy (Kuo et al., 2011).
A “megamaser” is about 100 million times brighter than a regular maser (Lo, 2005).
Researchers have also used megamasers to measure the size of the accretion disk of supermassive black holes to larger galactic structures (Greene et al., 2013).
When IC 2560 observed in the x- ray spectrum, it was discovered that it hosts a luminous H2O megamaser at its center (Iwasawa et al., 2002).
Figure 1. A radio spectrum of a megamaser disk (the spectrum is in red) from the galaxy NGC 3765b. Note the central group is the “systemic” features, the leftmost group is the “blueshifted” set of features, and the rightmost group is the “redshifted” set of features.
LITERATURE CITED
GBTIDL allowed me to change the settings to the recessional velocity, expressing the velocities relative to a heliocentric frame of reference, and converting the velocity of the maser into kilometers per second.
I analyzed each radio spectrum for the least and most heavily blueshifted features (high velocity features).
I then input all my analyzed data gleaned from the spectra into MS Excel.
After analyzing all the spectra for the three galaxies, I used a series of equations to find the disk radii (parsecs). This was then plotted in a scatterplot in MS Excel over time (Modified Julian Date).
Afterwards, I ran a linear regression test to determine if the disk radii changed over the time period of my data. I followed up with a root mean square deviation (RMSD) to find the deviation for NGC 5765b, UGC 3789 and Mrk 1419.
Elitzur, M. (1992) Astronomical Masers. Astrophysics and Space Science Library. Springer
Greene, J.E., Seth, A., Brok, M.D., Braatz, J.A., Henkel, C., Sun, A.L.,… Lo, K.Y. (2013) Using Megamaser Disks to Probe Black Hole Accretion. The Astrophysical Journal, 771(121), 1-7. doi: / X/771/2/121
Iwasawa, D., Maloney, P. R., Fabian, A. C. (2002) A Chandra observation of the H20 megamaser IC Royal Astronomical Society 336,
Kuo C.Y., Braatz, J.A., Condon, J.J., Impellizzeri, C.M.V., K.Y.Lo., Zaw, I.,... Greene, J.E. (2011). The Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks. The Astrophysical Journal, 727(20), doi: / X/727/1/20
Lo, K.Y. (2005) Mega-Masers and Galaxies. Annual Review Astronomical Astrophysics. 42, Doi: /annurev/astro
Pesce, D. (February 12, 2015). Megamasers, Personal Communication of D. Pesce, University of Virginia, Charlottesville, VA.
Pesce, D. (February 9, 2015a). Megamasers, Personal Communication of D. Pesce, Unversity of Virginia, Charlottesville, VA.
CREDITS
Figure 1 was taken by C.Y Kuo and was accessed by the researcher on GBTIDL.
All other figures created by researcher.