The SDSS Reverberation Mapping Project

The below post was contributed by Dr. Catherine Grier, a postdoctoral researcher at Penn State University (formerly a graduate student from Ohio State University, and the Director of the OSU Planetarium) who has led a recent paper based on results from the SDSS Reverberation Mapping Project (accepted for publication in the Astrophysical Journal; the full text is available at: arXiv:1503.030706 Screen Shot 2015-05-21 at 17.16.00 Supermassive black holes (SMBHs) are present in all massive galaxies and are thought to affect the formation and development of the galaxies themselves. Because of this, understanding SMBHs is important in understanding how galaxies are formed and evolve. Observations of quasars are key to understanding SMBHs and how they affect their host galaxies: Quasars are enormously powerful, observable at great distances, and can potentially regulate the growth of their galaxies through their winds, or outflows. We learn about these winds by observing broad absorption line features (BALs; see the diagram below) in quasar spectra that are created by high-speed winds launched from the quasar accretion disk. These winds are made of gas that blocks the light from the quasar and show up as BALs in the spectra of quasars.


The CIV region of our target showing the CIV Broad Absorption Line (BAL) features investigated in our study.

These absorption features change throughout time, both in strength and in shape. Under the right conditions, we can use the details of the variability to learn about the density of the absorbing gas and the distance of the gas from the SMBH. This information can sometimes be used to determine if the outflow is powerful enough to affect the star formation in their host galaxy. Previous studies have found that BALs are variable on timescales of several years all the way down to timescales of 8-10 days; however, until now, no studies have reported variability on timescales shorter than 8-10 days. In our recent work, we report on very short-timescale (~1 day) BAL variability observed in a SDSS quasar. The spectra used in our study were taken as a part of the SDSS Reverberation Mapping (SDSS-RM) project using the BOSS spectrograph. We monitored 850 quasars with the BOSS spectrograph from January 2, 2014 through July 3, 2014, resulting in 32 observations over this period. The main goal of the SDSS-RM program is the investigation of the broad emission line regions of quasars, but the targets include a number of quasars hosting BALs and can be used for BAL studies too. During our observing campaign, the equivalent width, or strength, of the highest-velocity CIV BAL feature (see above diagram) changed by over a factor of 2. We did not observe similar variations in either the CIV broad emission line or the overall brightness of the quasar, and the shape of the BAL feature stayed roughly the same during the entire campaign. We observed significant changes in the strength of the BAL on timescales as low as 1.20 days in the quasar rest frame (see the graphs below). This is the shortest time frame ever reported over which significant variability in a BAL trough has been observed.


Four different pairs of spectra between which the CIV BAL trough varies significantly.

The most likely cause of the variability is a change in the amount of ionized gas in the outflow. This could be caused by changes in the brightness of the quasar or the amount of energy reaching the absorbing gas for various other reasons. With our observations, we are unable to determine whether this outflow contributes significantly to feedback to the host galaxy, but we do not rule out the possibility. The key to observing this short-term variability was the high cadence of the SDSS-RM campaign, which allowed us to search for BAL variability on shorter timescales than previous studies. This program is still ongoing; we expect to receive more spectra of this target over the next few years with the eBOSS spectrograph, which could shed further light on this topic. The variability properties of this target are similar to those found in other quasars, suggesting that this short-term variability may be common. Further high-cadence spectroscopic campaigns targeting BAL quasars would allow us to learn more about BAL variability in quasars and better understand the possible contributions of BALs to feedback to their host galaxies.

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