Monday, August 4, 2014

A Tidal Disruption Event in a Dwarf Galaxy

When a star comes too close to a massive black hole, tidal forces may overwhelm the star’s self-binding energy and rip the star apart in what is known as a tidal disruption event (TDE). Some fraction of the stellar material violently accretes onto the massive black hole, giving rise to a luminous tidal disruption flare (TDF). Most if not all galaxies are thought to harbour massive black holes at their centres. Large galaxies such as the Milky Way and Andromeda are hosts to supermassive black holes (SMBHs). Dwarf galaxies have correspondingly less mass black holes known as intermediate-mass black holes (IMBHs). IMBHs have masses that fall between ~100 to 1,000,000 solar-masses. However, IMBHs are difficult to observe as they have much weaker influence on their surroundings compared to SMBHs.

Figure 1: Massive black holes reside in the cores of most if not all galaxies. Large galaxies are host to SMBHs while dwarf galaxies are host to IMBHs.

Two studies by D. Donato et al. (2014) and W. P. Maksym et al. (2013) identified a possible tidal disruption of a star by the IMBH of a dwarf galaxy that is a member of the galaxy cluster Abell 1795. The observed X-ray flux from the TDF seems to be consistent with a TDE. Even so, the possibility that the source of the X-ray flux might be associated with a massive background galaxy or even a flare-up of a more distant active galactic nucleus (AGN) cannot be discounted. A more recent study by W. P. Maksym et al. (2014) with data obtained from 8 hours of observations of Abell 1795 using the Gemini North Observatory determined that the source of the X-ray flux is indeed from a dwarf galaxy in Abell 1795.

The dwarf galaxy is identified as WINGS J1348 and it has an extremely low mass of only ~300 million solar-masses. For comparison, the Milky Way has a mass of well over a trillion solar-masses. WINGS J1348 has a negligible star formation rate and this greatly reduces the probability that the X-ray flux came from a supernova since such events tend to occur in regions with active star formation. Given that a TDE is the most likely explanation for the observed X-ray flux, the IMBH responsible for the TED and its observed TDF is estimated to have a mass of between 20,000 to 70,000 solar-masses.

WINGS J1348 is one of the least massive galaxies known to host a massive black hole, in this case, an IMBH. Over a period of roughly 15 years, the X-ray flux from the TDF in WINGS J1348 has declined from peak luminosity by a factor of over ~10,000. More observations of such events would be necessary to constrain the rate of TDEs by IMBHs. In fact, TDFs from TDEs could be used to probe the properties of black holes in the intermediate-mass regime since such objects seem to be very difficult to study by other means.

Figure 2: Best fit curves and observational data (symbols) of the X-ray flux evolution for WINGS J1348. W. P. Maksym et al. (2013).

- W. P. Maksym et al. (2014), “Deep Spectroscopy of the MV ~ -14.8 Host Galaxy of a Tidal Disruption Flare in A1795”, arXiv:1407.6737 [astro-ph.HE]
- D. Donato et al., “A Tidal Disruption Event in a nearby Galaxy Hosting an Intermediate Mass Black Hole”, 2014, ApJ, 781, 59
- W. P. Maksym et al., “A tidal flare candidate in Abell 1795”, MNRAS (November 01, 2013) Vol. 435, 1904-1927