All the black holes that wander are not lost - and now some may have been found.

All the black holes that wander are not lost – and now some may have been found.

Title: Wandering black hole candidates in dwarf galaxies at VLBI resolution

Author: Andrew J. Sargent, Megan C. Johnson, Amy E. Reines, Nathan J. Secrest, Alexander J. van der Horst, Phil J. Cigan, Jeremy Darling, Jenny E. Greene

First author’s institution: United States Naval Observatory, Washington, DC, USA; Department of Physics, George Washington University, Washington, DC, USA

Status: Approved by ApJ

How to make a black hole billions times the mass of the sun? Also for planet building Magratheansthis seems to be a big order. Many mechanisms have been proposed to explain their formation supermassive black holes (SMBH) located in the center of most galaxies. Some involve mergers of “seeds”, massive black holes (MBH) that weigh only hundreds to hundreds of thousands of solar masses. An easy way to test these theories is to search for relics of MBHs and low mass dwarf galaxies are excellent goals. Since dwarf galaxies have not undergone many mergers, all the MBHs they host should have avoided being swallowed up by growing SMBHs.

Today’s paper studies 13 possible MBH candidates in dwarf galaxies, some of which may have migrated to the edges of their hosts. What’s the matter with that – and are they really MBH? Let’s dive in!

Here is a question. Since SMBHs are usually located near the center of their galaxies, why can we expect some MBHs in dwarf galaxies to be further out? The answer has to do with gravity: because dwarf galaxies are much less massive, their gravitational potential is lower, making it easier for MBHs to “migrate” away from their centers. This means that if you see a radio source that appears far next to the center of a dwarf galaxy, it could be an MBH – or it could be a active galactic core (AGN) which lies in a galaxy far, far away, collects matter and sends energetic jets into the cosmos, which by chance simply happens to lie behind the dwarf galaxy. These unwanted intruders can pose a challenge in identifying MBH.

Another problem with finding MBHs is that they are weak. While going through periods of accretion like regular AGN, their low masses mean that they do not accrete as fast, which reduces their brightness. By the early 2000s, only two growing black holes had been found in dwarf galaxies. Fortunately, this changed with the advent of celestial studies as they are now famous Sloan Digital Sky Survey (SDSS), which has been running since 2000 and has amassed discoveries from nearly 1 billion unique sources.

Optical images of the 13 candidate MBH host galaxies.  Two appear to be spirals, two appear to be edge discs, and the others have morphologies that are less clear.

Figure 1: The 13 dwarf galaxies that host potential MBH candidates, as seen by the Dark Energy Camera Legacy Survey at optical wavelengths. The red crosses show the location of the compact radio sources that may be MBH. While some seem close to the center of their host, others are much further away. Image credit: Fig. 7, Reines et al. 2020.

The 13 candidates in this journal were compiled by some of the same astronomers in a newspaper from 2020. They screened through 43,707 low-mass dwarf galaxies from SDSS, looking for sources that had been detected at radio frequencies of Very Large Array (VLA). After retaining the matches and eliminating the background sources that were background AGNs or could be explained by star formation-related processes, they ended up with 13 MBH candidates, many of whom are not in line with the center of their host galaxies.

In this recent document, they performed follow-up observations using Very long baseline set (VLBA). VLBA is an interferometer that uses radio telescopes thousands of kilometers apart to reach high angular resolutions, allowing astronomers to see fine details. Unfortunately, the VLBA could only detect four of the 13 candidates – and the four, due to their brightness and position, most likely only appeared to be invasive, active galactic nuclei in galaxies far beyond the dwarfs they targeted.

Four images showing radio radiation from the four sources detected with VLBA.  All four appear as small ovals, separate from all other emissions.

Figure 2: The four sources that the team was able to discover with VLBA. Here, S is flux density, a quantity that describes the intensity of radio emission. Since these sources are actually background AGNs, rather than MBHs in the directed dwarf galaxies, the physical scales in the lower right corner are incorrect. Image credit: Fig. 1 from the newspaper.

This can seem like a huge problem! Only four discoveries, all of which appear to be fraudsters? Fortunately, the situation is not as difficult as it may seem. Although VLBA is good at dissolving sources on a small scale, in the configuration used, it is not good at dissolving sources on a large scale – and the radio emission from accumulating black holes may partly come from larger structures such as radiolobes, rather than central point sources.

Multi-wavelength observations confirmed that two of the remaining nine are likely to accrue MBH near the center of their host galaxies. The other seven are still unknown. Five of these are too bright to be from star formation and, based on their positions, they can either be more AGN interlopers in the background – or, attractively enough, migrating MBHs.

Where do we go next? Follow-up observations at other wavelengths may be useful. In particular, the group proposes the Hubble Space Telescope as a way to find out what these seven sources really are. Given the difficulty of detecting MBH, even one can prove valuable when astronomers try to understand the formation of the largest black holes in the universe.

Astrobite edited by Suchitra Narayanan

Selected Image Credit: Reines et al. 2020

About Graham Doskoch

I’m a PhD student at West Virginia University and taking a Ph.D. in radio astronomy. My research focuses on pulsars and efforts to use them to detect gravitational waves as part of pulse timing matrices such as NANOGrav and IPTA. I love to run, hike, read and just enjoy nature.

#black #holes #wander #lost

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