Awakening the Beast: Discovery of a Newborn AGN

Title: A Newborn AGN in a Starforming Galaxy

Authors: P. Arévalo, E. López-Navas, M.L. Martínez-Aldama, et al.

First Author’s Institution: Instituto de Física y Astronomía, Universidad de Valparaíso

Status: Accepted to A&A Letters [open access]

Awakening a Supermassive Black Hole

Most supermassive black holes (SMBHs) in the universe are sitting dormant at the center of a galaxy, having a little snooze. But roughly 10% of these SMBHs are alive and hungry, gobbling down lots of gas from their surroundings (through a process we call “accretion”). These hungry SMBHs are called active galactic nuclei (or AGN, for short). They’ve been the subject of so much research, but it is still unclear what causes an SMBH to go from inactive to active, how long this process takes, and how often it occurs throughout cosmic time (i.e. the “duty cycle” of AGN). These questions are so difficult to answer because the AGN duty cycle is probably very long (~ millions of years), meaning that we don’t often see AGN turning on or off on human-observable timescales. However, if we monitor enough galaxies (~ millions), we may be able to get lucky and catch an AGN in the process of turning on or off.

Today’s paper has done exactly this: taking advantage of the vast amount of data available on nearby galaxies to search for turn-on AGN. The most tell-tale sign of a turn-on AGN is one that suddenly forms broad emission lines, which arise from rapidly moving gas very close to the SMBH. However, getting routine follow-up spectroscopy is expensive and takes lots of telescope time – it can’t be done for every single galaxy! Instead, the authors of today’s paper make use of photometric variability (i.e., how the galaxy’s brightness changes with time) to search for turn-on AGN. Most galaxies will not vary much on timescales of decades since the stars that produce most of the light are not changing on these timescales. However, AGN are known to vary rapidly within hours to days, and thus, this variability can be used to spot AGN from a sea of normal, dormant galaxies.

A Needle in the Haystack

The authors started with more than 2 million sources with rapid, AGN-like variability from the Zwicky Transient Facility (ZTF). ZTF is a time-domain optical survey that scans most of the northern sky every 3 days, making it very sensitive to rapid changes from AGN. With these 2.3 million AGN-like sources from ZTF, the authors then looked for any of these sources that were previously classified as inactive with spectroscopy from the Sloan Digital Sky Survey (SDSS). Out of this pool of 2.3 million sources, only 86 sources are promising candidates for turn-on AGN and 18 of which the authors targeted for spectroscopic follow-up! Today’s paper focuses on one of these sources – SDSS J0803 (Figure 1) – which shows the most extreme changes to its SMBH activity (of the 18) and is a promising newborn AGN candidate!

A Hungry SMBH

To confirm the presence of a new AGN, the authors took a new optical spectrum with the SOAR telescope and found that there were now broad emission lines from Hα and Hβ. Figure 3 shows a zoom-in on the Hβ region, comparing the old spectrum from 2003, which showed narrow lines from star formation (left) to the new spectrum from 2022, that now also shows a broad Hβ line from accretion (right). Hα also shows a similar evolution, which, together, provide the smoking gun evidence for an awakening newborn AGN!

A couple of other things can help confirm the presence of a newborn AGN and validate the findings from optical spectra. First, when an SMBH accretes gas from its surroundings, this gas heats up as it gets closer to the SMBH and produces X-ray emission at levels much higher than is seen from stars, neutron stars, and stellar mass black holes in the galaxy. SDSS J0803 is detected in X-rays from 2020-2022 in the eROSITA all-sky X-ray survey, strengthening the case for a newborn AGN! Second, the source also shows a dynamic change to its mid-infrared (mid-IR) brightness with data taken from the WISE all-sky mid-IR survey. The mid-IR color is consistently moving toward a more AGN-like mid-infrared color arising from hot dust near the SMBH. Together, these three lines of evidence point to a recent ignition of accretion onto the SMBH!

SDSS J0803 is a fascinating system that sparks excitement about identifying turn-on AGN for years to come. This study shows that this technique of searching for rapid photometric variability in previously inactive galaxies is promising to identify more turn-on AGN! The authors mentioned that they have more sources in their sample that show similar characteristics to SDSS J0803, with rapid optical photometric variability, yet no past history of AGN activity. Thus, in future work with a sample of newborn AGN with a good understanding of selection biases, we can tackle the questions around the AGN duty cycle and feeding timescales!  

Astrobite edited by Will Golay

Featured image credit: NASA/JPL-Caltech