# Is a black hole found in the centre of the low-surface-brightness and dark-matter-free galaxy AGC 114905

Per information found on Internet:

AGC 114905 was discovered while observing the constellation Pisces with the VLA ground-based radio telescope. The galaxy is about 250 million light-years distant from Earth and is a spherical, irregularly shaped object.

Its stellar mass turned out to be about 400 times less than the analogous parameter for the Milky Way, but at the same time it takes up as much space in space as Milky Way galaxy.

The abstract of Mancera Piña (2021) No need for dark matter: resolved kinematics of the ultra-diffuse galaxy AGC 114905 says:

We present new H I (21 cm Hydrogen line) interferometric observations of the gas-rich ultra-diffuse galaxy AGC 114905, which previous work, based on low-resolution data, identified as an outlier of the baryonic Tully-Fisher relation. The new observations, at a spatial resolution ∼2.5 times higher than before, reveal a regular HI disc rotating at about 23 km/s. Our kinematic parameters, recovered with a robust 3D kinematic modelling fitting technique, show that the flat part of the rotation curve is reached. Intriguingly, the rotation curve can be explained almost entirely by the baryonic mass distribution alone. We show that a standard cold dark matter halo that follows the concentration-halo mass relation fails to reproduce the amplitude of the rotation curve by a large margin. Only a halo with an extremely (and arguably unfeasible) low concentration reaches agreement with the data. We also find that the rotation curve of AGC 114905 deviates strongly from the predictions of Modified Newtonian dynamics. The inclination of the galaxy, which is measured independently from our modelling, remains the largest uncertainty in our analysis, but the associated errors are not large enough to reconcile the galaxy with the expectations of cold dark matter or Modified Newtonian dynamics."

Typically in the center of the every galaxy it is expected to find a black hole - is the black hole found to be located in the center of AGC 114905? One of the methods of establishing presence of the black hole in the center of the galaxy is observing the effect of the optical lensing applied to the electromagnetic radiation issued by the source located "behind" the black hole. If there is no black hole in the center of the galaxy, then I would assume that the electromagnetic radiation from the source located "behind" the galaxy would pass through the center of such galaxy without distortion (no observation of the effect of gravitational lensing). In particular, I am applying above stated reasoning to AGC 114905. Is the spatial resolution specified in the above article good enough to detect the presence of any "bright" source of electromagnetic radiation behind the center of AGC 114905 and to see whether such radiation passes with (or without) an effect of gravitational lensing?

PS This question re AGC 114905 has some analogy with the findings related to the Leo I dwarf galaxy per following paper (lead author - Mario José Bustamante, an astronomy doctoral graduate at the University of Texas, Austin) 3:

"The Leo I dwarf galaxy, some 820,000 light-years from Earth, is only about 2,000 light-years across. Until now, astronomers thought the galaxy's mass was about 15 to 30 million times the mass of our sun. That's tiny compared to the Milky Way, which is estimated to weigh as much as 1.5 trillion suns and whose disk is over 100,000 light-years wide.

Unexpectedly, at the heart of the little Leo I sits a black hole that is nearly as large as the one at the heart of the entire Milky Way, a new study found. The discovery defies expectations as astronomers believed giant black holes grow from collisions between galaxies and should correspond with the galaxy's size. Unlike most dwarf galaxies orbiting the Milky Way, Leo I does not contain much dark matter. Researchers measured Leo I's dark matter profile — that is, how the density of dark matter changes from the outer edges of the galaxy all the way into its center. They did this by measuring its gravitational pull on the stars: The faster the stars are moving, the more matter there is enclosed in their orbits. In particular, the team wanted to know whether dark matter density increases toward the galaxy's center. "

PPS With regards to galaxies that don't have dark matter - the NGC 1052-DF4 dark-matter-free dwarf galaxy, which was (similar to NGC 1052-DF2, which is the dwarf satellite galaxy of an elliptical galaxy NGC 1052 in the constellation Cetus) originally thought to be also a satellite galaxy of NGC 1052 (as NGC 1052-DF2 is), but later being identified as located closer to the NGC 1035 galaxy (rather than to NGC 1052), per study led by Mireia Montes, firstly lost its dark matter content, and now is in the last stages of being ripped apart 4.

• I don’t really understand the question— what are you asking, and why? Dec 7, 2021 at 9:16
• @Peter Erwin - Thanks, I edited my question - is it clear for you now what I am asking? With regards to "why" - it looks to me that since the AGC 114905 is "strange" because the research found no presence of the dark matter in it, any additional information - such as presence (or absence) of the black hole in the center of AGC 114905 would be useful.
– Alex
Dec 7, 2021 at 16:23
• I think the question is "Is there a black hole in AGC 114905?" I've edited the title to reflect my understanding of the actual question. (Titles like "I have a question about X" are not so useful) Dec 7, 2021 at 23:11
• @James K - thanks, yes, and also the question whether there currently exists (or doesn't exist) an ability to observe any "bright" source of electromagnetic radiation, located "behind" the center of the AGC 114905?
– Alex
Dec 8, 2021 at 0:06

The short answer is that we have no idea whether there is a massive black hole at the center of this galaxy, and no real hope of finding out (in the absence of possibly detecting, e.g., X-ray emission from an active nucleus).

One of the methods of establishing presence of the black hole in the center of the galaxy is observing the effect of the optical lensing applied to the electromagnetic radiation issued by the source located "behind" the black hole.

This has only ever (sort of) been done for one black hole, the one in the center of M87 which was observed with the Event Horizon Telescope, and the actual presence of that black hole was established more than 20 years earlier via measurements of the velocity of gas and stars orbiting around the black hole in the galaxy center. (It's likely that the -- much closer -- BH in the center of the Milky Way could be another case, but the analysis of the Event Horizon Telescope observations is still ongoing.)

So this isn't really a method for "establishing presence of the black hole in the center of the galaxy".

In any case, we can show that this would be impossible for AGC 114905. There are two general "effect of optical lensing" for an object like a massive black hole. The first (which, to my knowledge, has never been observed for a BH) would be the lensing of a genuine background source (e.g., another galaxy). The scale of the effect is roughly the Einstein radius of the lensing object (if the lensing object is directly between you and the background source, then the result will be an Einstein ring with radius = Einstein radius.) Note that you could get the same general effect from something like a massive star cluster or galactic bulge, so an actual BH isn't required, and you would have trouble isolating the effect due to the BH alone.

Alternately, you might hope to resolve the photon ring due to emission from an accretion disk around the BH, as was done for M87. This is something that's actually due to a BH, but it requires the existence of an accretion disk and is much smaller than the Einstein radius; in physical terms, it's a few times the Schwarzschild radius of the BH.

AGC 114905 is at a distance of roughly 76 Mpc. Let's assume an extreme BH with a mass of $$10^{7} M_{\odot}$$ (five times the BH in Leo I and more than twice the Milky Way's BH mass); in this case the Einstein radius would be 0.02 arcsec. (It would be smaller for smaller, more plausible BH masses.) Since the images used to analyze AGC 114905 had a seeing full-width-half-maximum of $$\sim 1$$ arcsec (Gault et al. 2021), there is no way you could resolve any (hypothetical) lensing effects acting on a conveniently placed background source by a (hypothetical) massive BH in the center of the galaxy. A $$10^{7} M_{\odot}$$ BH would have a Schwarzschild radius of about 300 million km, or $$10^{-5}$$ parsecs. At the distance AGC 114905 this would be smaller than $$10^{-7}$$ arcsec, or 0.1 micro-arcsec. So even if there were an accretion disk around the BH, the corresponding photon ring would be much too small (and no doubt much too faint as well) to be resolved even by the EHT (resolution $$\sim 25$$ micro-arcsec).