Unsure If It’s a Really Massive Black Hole? Take an X-ray Scan!

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Title: X-ray View of Little Red Dots: Do They Host Supermassive Black Holes?
Authors: Tonima Tasnim Ananna (তনিমা তাসনিমঅনন্যা) et al.
First Author’s Institution: Wayne State University
Status: Published in ApJL

Among the fascinating objects observed by JWST are galaxies in the early universe known as little red dots (read more about them here and here!). These galaxies are believed to be rich in dust, which contributes to their distinct red hue. The infrared spectra of little red dots from JWST reveal the presence of strong emission lines. This emission can be a potential indicator of either 1) an active black hole or 2) vigorous star formation. That leaves us with a tantalizing question: which one is it!?

If the little red dots indeed host black holes, research suggests that the masses of these black holes must be between 10 million and 1 billion times the mass of the Sun. This poses a significant challenge — how can these black holes accumulate such large masses in such a short time from their births? This question has left astronomers puzzled, highlighting the need to understand if these little red dots do, in fact, hold such massive black holes.

In astronomy, akin to life, sometimes it helps to take a step back and observe something from a different perspective. Looking at the little red dots at a different wavelength of light can help understand what exactly is happening in these galaxies. Today’s authors set out to harness the power of multiwavelength astronomy and to look at these little red dots at X-ray wavelengths to see if they emit any X-ray signals. Even though both accreting black holes and star formation activity can lead to emission in the X-ray, the difference between the strength of the X-ray emission between the two is greater in the X-ray than in the infrared. So, obtaining the strength of X-ray emission from the little red dots should give us a good idea of whether the X-rays are caused by a black hole or by stellar processes.

The authors select a bunch of little red dots that are hidden behind a big galaxy. Since X-rays can be challenging to detect, a massive galaxy in front can act as a gravitational lens and magnify any X-ray signals from the little red dots. If little red dots indeed host really massive black holes, it is not unreasonable to expect X-ray signals from them.

Lo and Behold! There Are None!

plot of stacked X-ray signals from 21 little red dot galaxies

Figure 1: All 21 little red dots that were stacked to see if they emitted any X-ray signals. There does not appear to be a spike in the X-ray counts anywhere, indicating that there is no strong X-ray emission from the little red dots. [Adapted from Ananna et al. 2024]

The authors do not detect any X-rays from the little red dots. To further improve sensitivity, in case the X-ray signal is too weak to be determined from individual galaxies, the authors also stack the galaxies. X-rays were still undetected with significant confidence (Figure 1).

Although the authors do not detect any X-rays from the little red dots, they can still use the fact that they did not detect any X-rays to obtain conservative limits on the maximum mass of the black hole, if any, that might be present in the little red dots. The authors assume accretion at the Eddington limit, a theoretical maximum rate at which a black hole can grow by accretion of mass, and obtain black hole masses less than 1.5 x 106 M (Figure 2). This is a couple of orders of magnitude lower than the black hole masses in little red dots determined by other works.

plot showing the derived upper limits on the black hole masses of the little red dot galaxies

Figure 2: The derived upper limits to the black hole masses of the little red dots, plotted against their stellar masses. The bottom-most thick dashed green line gives the average value (assuming Eddington limit) of the black hole mass. The black hole masses are not extremely over-massive unless the accretion is a fraction of the Eddington limit. [Ananna et al. 2024]

This mismatch may arise due to the method by which the black hole masses were estimated in these little red dots. Masses were determined using broad emission lines, which have been standardized based on nearby galaxies with active galactic nuclei and thus may not work for galaxies at much higher redshift. It is also possible that the X-rays were not detected because these galaxies are incredibly dusty, but the authors believe that to be very unlikely.

The Mystery of Little Red Dots Continues!

This work suggests that little red dots will not likely host extremely massive black holes. More data coming from JWST, including a detailed study of the spectra of these little red dots, can help us understand them better. This work primarily uses data from the Chandra X-ray Observatory, which has now unfortunately been defunded despite continuing to produce very significant scientific results. (Read here to learn how you can save Chandra!) The mysteries of the universe will ultimately be solved only by looking at it from every possible wavelength, and we need such multiwavelength telescopes to achieve this lofty goal!

Original astrobite edited by Janette Suherli.

About the author, Archana Aravindan:

I am a PhD candidate at the University of California, Riverside, where I study black hole activity in small galaxies. When I am not looking through some incredible telescopes, you can usually find me reading, thinking about policy, or learning a cool language!