The newest class of graduate students have wrapped up their first term and are taking stock of all their milestones that have come along the way. For physics grad Cassidy Metzger, this includes her first paper as lead author. Her work, New TeV-emitting BL Lac Candidates from the eROSITA X-Ray Survey, was published in The Astrophysical Journal in October. Metzger's paper identifies over a hundred potential blazar galaxies, including eight brand-new candidates, for future research that could help illuminate some of the mysteries of our Universe.
Blazars are a unique type of galaxy. They have an active galactic nucleus – or AGN – that is a dense, energetic central region thought to stem from matter accreting onto a supermassive black hole. As gas, dust, and even stars spiral into the black hole, they compact and generate enormous amounts of energy that can be seen from infrared to gamma ray wavelengths.
In addition to AGNs, blazars also produce twin jets of ionized matter that shoot out from the galaxy perpendicular to its central accretion disk. This matter moves at nearly the speed of light, and can be seen streaming into space many times the length of the galaxy itself. When one of these relativistic jets point directly at Earth, the galaxy is considered a blazar. These jets likely originate from the rotating magnetic field associated with the accretion disk around the black hole, although the specific physics are still largely unknown.
Of particular interest to Metzger are a class of blazars known as TeV-emitting high-frequency BL Lac blazars, or TeV-emitting HBLs. She sees these incredibly fascinating sources as keys to unlock some of the great unknowns in the Universe.
TeV-emitting HBLs challenge generally accepted blazar emission models, are a possible source of very high energy cosmic rays and neutrinos, can be used to further test and refine theoretical models of particle acceleration within relativistic jets, and can serve as probes of the extragalactic background light and cosmic magnetic fields.
- Cassidy Metzger, graduate student
While Metzger and her mentors recognized the scientific potential of TeV-emitting HBLs, one big problem stood in their way. There simply weren't enough known blazars to study. Of the specific type of blazar Metzger was after, only 56 confirmed sources existed.
Metzger began her work to identify new blazar candidates during her undergraduate sophomore year at Washington University in St. Louis. She continued this research through her senior year, and recalls "it was an amazing experience to get to follow my own interests and ideas as the lead of the project while having the guidance and support of my mentors".
Metzger began her search by combining the new eROSITA survey with infrared data from the Wide-field Infrared Survey Explorer. Between these two catalogs, Metzger started with about 748,564,229 potential sources. She ultimately identified 121 TeV-emitting HBL candidates. Eight of Metzger's objects are considered completely brand-new blazar candidates!
All 121 blazar candidates add vital data points towards growing the scientific community's sample of research objects. Metzger notes that follow-up observation on high-energy gamma-ray telescopes is needed to confirm any of her candidates. She is hopeful her efforts will create better statistical studies in future blazar research.
Many of Metzger's blazar candidates can be studied in much greater detail using currently or soon-to-be operating telescopes around the world. Metzger and her mentor, Professor Ryan Hickox, recently obtained spectra data on promising candidates with the SALT telescope in South Africa. They aim to retrieve more precise redshift data on these candidates.
Metzger intends to continue researching blazars during her time at Dartmouth; "Personally, I'm excited about the implications this work may have on our understanding of relativistic jets, as we still don't know what causes a relativistic jet to form or what drives it".
Researchers across the astrophysics community are also excited about Metzger's blazar candidates. She shares that "researchers at Columbia have expressed interest in using the VERITAS gamma-ray telescope to observe some of our candidate sources. There has also been talk of investigating a few of our sources with the MDM optical telescope. I'm thrilled to see that this project is spurring further investigations into the fascinating class of TeV-emitting HBLs, and I'm excited to see what may come of gamma-ray and optical observations."
Metzger is a first-year graduate student in the Dartmouth Department of Physics and Astronomy. You can read her full publication here.