Little Red Dots and Big Black Holes

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Title: A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields
Authors: Vasily Kokorev et al.
First Author’s Institution: Kapteyn Astronomical Institute
Status: Accepted to ApJ

Imagine you peek into a kindergarten class and, to your shock, you see that all the children are well over 6 feet tall. That is precisely how astronomers felt when data from JWST showed galaxies with massive black holes just a few hundred million years after the birth of the universe. Some of these black holes have been measured to be a million times more massive than the mass of our Sun, and astronomers are puzzled as to how they could have gained so much mass in such a short time.

The earliest galaxies likely to host these black holes show up as little red dots in the images JWST took of the early universe (as seen in the long exposure images; see the bottom image in Figure 1). They are believed to be compact (with a small radius) galaxies with a Type I active galactic nucleus and obscured (covered by dust), which accounts for their red color and why they are easily observed in the infrared. The spectrum of the galaxy has a “V” shape, a blue continuum from the unobscured part of the active galactic nucleus in the galaxy, and a red continuum from the obscured part (see Figure 1). These little red dots have appeared in several images taken by JWST, hinting that plenty of massive black holes are lurking in the early universe.

spectrum and images of a little red dot galaxy

Figure 1: The characteristic spectrum of little red dots (on the left), with the compact source contributing to the dust-free blue color in the continuum (top right) and the dust-reddened part (bottom right). [Adapted from Kokorev et al. 2024]

Where You Look Matters!

It is vital to systematically look at these little red dot galaxies to understand how many massive black holes were in the early universe. Two factors can introduce biases in the counting of these galaxies. One is the phenomenon of cosmic variance: is JWST just preferentially looking in a direction with many little red dots, or should we expect the same number even when it looks at different parts of the universe? The other is how crowded it is in the direction in which you are observing: if you have a lot of stars in the direction you are looking, they could be misclassified as little red dots or vice versa. If you happen to have plenty of massive galaxy clusters in the direction you are looking, they may create an illusion that more of these little red dots exist than their true numbers (a phenomenon known as gravitational lensing).

To minimize the errors caused by these effects in determining the number of little red dots in the early universe, the authors of today’s research article specifically look at large areas (640 arcminutes2) of the sky by combining JWST data from various programs. This would minimize the effects of cosmic variance as you can measure the numbers over a bigger area of the sky. They also look specifically at data in blank fields (defined here as areas on the sky without galaxy clusters), which helps them determine the accurate number of objects per unit volume. All these galaxies are photometrically selected (i.e., chosen only from looking at images rather than spectra), meaning there is limited spectroscopic data to help confirm what kind of objects they are. Galaxies are determined to be little red dots based on their red colors and how compact they are in the images. Using the obtained fluxes, the authors then construct spectral energy distributions to determine the redshifts (z) of the sample. Limiting the sample to z > 4 (for the early universe), the authors end up with 260 little red dot galaxies

Do Not Judge a Galaxy by Its Size (in Your JWST Image!)

On calculating the total luminosity of the little red dots and comparing it to their redshifts, the authors find a large number of bright little red dots at redshift of z = 5 (around a billion years after the beginning of the universe). The number of little red dots is almost 100 times more than the number of ultraviolet-selected quasars, which are active galactic nuclei identified using another method. They also find that computer models are unable to reproduce the high fraction of the bright galaxies they uncover at high redshifts (left side of Figure 2). The authors derive the mass of the black holes at redshifts of z = 4.5–6.5 to be around 106–108 solar masses, indicating that these black holes were already massive a few hundred million years after the Big Bang. They find deviations from the predicted number densities of massive black holes at these redshifts from galaxy simulations. This is likely because the galaxies that host more gigantic black holes are very dusty, and thus, their spectra do not have any blue continuum. They may then be missed from selections of little red dots as one of the factors it depends on is the characteristic “V”-shaped spectrum, which would need a contribution from the blue continuum (right side of Figure 2).

plots showing the number density of little red dots as a function of luminosity and black hole mass

Figure 2: Left: The number density of the little red dots as a function of luminosity at 6.5 < z < 8.5 with the predicted values from simulations indicated by the blue solid line. Right: The number density as a function of black hole mass at 4.5 < z < 6.5. The observed number density of more massive black holes is lower than the values predicted by simulations. [Adapted from Kokorev et al. 2024]

While spectroscopy is a more reliable method to identify massive black holes, many galaxies that host black holes can still be picked out using near-infrared colors and photometry, which is a much less expensive technique. The challenge lies in ensuring that the photometrically selected sources are reliable, and the authors of today’s article made great use of this technique. Follow-up spectroscopic studies of these photometrically selected samples can help us further understand the exact nature of the black holes. Such studies are already underway, and we can look forward to finally making sense of how these black holes became so massive in such a short time after the formation of the universe!

Original astrobite edited by Delaney Dunne.

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!