Who’s Home? A Supermassive Black Hole & its Tiny Friends

Title: A Host Galaxy Morphology Link Between Quasi-Periodic Eruptions and Tidal Disruption Events

Authors: Olivier Gilbert, John J. Ruan, Michael Eracleous, Daryl Haggard, Jessie C. Runnoe

First Author’s Institution: Department of Physics & Astronomy, Bishop’s University

Status: Submitted to ApJ [open access]

Knock knock! Who’s home?

Ever driven past a neighbor’s house and seen many cars parked outside the driveway? This may have then prompted your dad to add in his classic dad joke: “Wow, no one invited me to the party!” How did you know that there was a party going on without seeing the people directly? The cars outside gave it away! Today, we’re going to try to apply the same logic to supermassive black holes (SMBHs). The home in question is the host galaxy, the SMBH is the typical resident, and the party consists of tiny stellar-mass friends in close orbit around the SMBH. 

An SMBH Hosts a Party

Most of the time, we think that SMBHs are rather lonely beasts. The canonical picture, however, is rapidly changing in the era of time-domain astrophysics, with the discovery of behavior that we’ve never seen before around SMBHs! For example, in 2019, a series of rapid and repetitive bursts of X-ray emission were seen coming from the centers of a handful of nearby galaxies. These sources are called Quasi-Periodic Eruptions (QPEs), and they typically brighten by more than a factor of 10 and then fade back to their quiescent level within hours. These bursts recur over and over again in nearly periodic fashion, and the time between bursts typically ranges from hours to days. The X-ray light curves for 2 QPE sources are shown in Figure 1. 

This behavior has puzzled astronomers since the bursts were first discovered (about 5 years ago). One of the leading classes of models proposed to explain this behavior involves a tiny friend (e.g. a star or smaller, stellar-mass black hole) orbiting around the SMBH. Because this friend is extremely small compared to the gargantuan SMBH, we often call this an Extreme Mass Ratio Inspiral (EMRI). These models commonly also invoke a surrounding disk of gas that is being accreted onto the SMBH, which the EMRI must pass through as it orbits the SMBH. These interactions happen twice per orbit (see Figure 1 in this Astrobite for a great diagram of this process) and punch out a cloud of gaseous material with every impact. This cloud then produces X-ray emission, which gets cooler as the cloud expands with time. The orbiting EMRI therefore naturally produces the nearly periodic behavior of the bursts and the shape of the bursts!

The Party Gets Crowded

There is another important piece to the QPE puzzle though; growing observational evidence suggests that QPEs are linked to Tidal Disruption Events (TDEs). TDEs occur when a star gets shredded by an SMBH; unlike QPEs, these stars are on much more eccentric orbits, and they wind up so close to the SMBH that the tidal forces shred the star apart (see a cool video of this here!). The stellar debris that falls back onto the black hole produces a temporary flare of accretion and forms a disk that a pre-existing EMRI then interacts with! The observations of QPEs in TDEs have led theorists to propose that EMRI + TDE = QPE. But are TDEs a requirement for QPEs? How many guests have to be at the party?

Today’s paper investigates the connection between TDEs and QPEs by looking at the morphology of the galaxies they reside in. SMBHs have been shown to have a big impact on their host galaxies despite being >1000 times smaller in mass and even smaller in spatial scale than the galaxy itself. Additionally, there is evidence that TDEs occur in galaxies that have a higher concentration of stars in their centers, suggesting potentially that this higher concentration drives stars onto these shredding orbits. If QPEs are also associated with TDEs, then they should preferentially reside in centrally concentrated galaxies! This is exactly what today’s authors aim to test, with a systematic analysis of the host galaxies of both QPEs and TDEs. 

To assess the central concentration of QPE and TDE host galaxies, the authors collected images of the galaxies from the DESI Legacy Survey. They then fit these two-dimensional images for their surface brightness profiles, using the common Sérsic profile to describe the surface brightness. A Sérsic profile is a function whose central concentration can be adjusted using a parameter n, the Sérsic index. A higher value of n yields a profile that is much more peaky near the center, whereas a lower value of n yields a more gentile, uniform-like profile. The authors also try fitting the surface brightness using two Sérsic profiles with unique Sérsic indices (i.e. two independent n parameters); this allows them to model both the bulge and disk of the galaxy separately to assess the relative contribution of the bulge to the total host galaxy light (another estimate of the central concentration!). Figure 2 shows the Sersic indices and bulge-to-total light ratios (B/T) compared between QPEs, TDEs, and normal galaxies. The top most panels show the histograms – the QPEs (blue) and TDEs (red) both show systematically higher Sérsic indices and B/T compared to the comparison sample of normal galaxies! Thus, the authors conclude QPE host galaxies are very similar to those of TDEs, suggesting that QPEs are indeed linked to TDEs. Connecting back to our party analogy – this means that a SMBH’s home (i.e. its host galaxy) can shed light on the party happening at the small scales in the nucleus!

Finding More SMBH Parties?

These results suggest another exciting connection between QPEs and TDEs. However, the number of QPEs is still quite low – only 9 sources have been detected to date, and that includes two sources which are technically candidate QPEs due to the very limited number of bursts detected. QPE bursts are found only in the X-ray band, which makes discovery quite difficult since there is currently no all-sky X-ray mission that can probe down to the levels of these bursts. The authors note, however, we can potentially improve our detection rate of QPEs by searching specifically with X-ray follow-up of TDEs, which are discovered much more frequently thanks to the large-scale time-domain optical observatories. With more and more QPEs, we can also probe the strong gravity regime around SMBHs in a new “light” – gravitational waves – since QPEs may be detectable with the space-based gravitational wave detectors, like LISA! QPEs may therefore be one of the first electromagnetic precursors to LISA events, and therefore are opening up an entirely new multi-messenger regime around SMBHs!

Astrobite edited by Delaney Dunne 

Featured image credit: Part of Figure 1 of today’s paper with Knock, Knock, You Know from thedailyenglishshow.com