Little Red Dots from Low-Spin Galaxies at High Redshifts

Recently, a new population of compact, high-redshift (z>7) galaxies appeared as little red dots (LRDs) in deep JWST observations. The latest spectroscopic data indicates that these galaxies contain an evolved stellar population, reflecting an early episode of high star-formation-rate. The appearance of broad emission lines suggests that a central overmassive black hole also powers these galaxies. I propose that LRD galaxies represent the low-spin tail of the galaxy population. Low-spin galaxies host a more compact gaseous disk with an enhanced star formation rate relative to typical galaxies at the same redshift. The compact disk feeds efficiently a central black hole, as predicted by Eisenstein&Loeb (1995).


INTRODUCTION
One of the surprising discoveries of the James Webb Space Telescope (JWST) involves an early population of compact red galaxies at redshifts z 7, when the Universe was 800 Myr old (Labbé et al. 2023).These galaxies, commonly dubbed as little red dots (LRDs), are redder than expected from their cosmological redshift, indicating additional reddening by dust.Spectroscopy of some LRD galaxies indicates that they may contain as much mass in evolved stars as the Milky Way galaxy or even more, up to ∼ 10 11 M ⊙ (Wang et al. 2024).Nevertheless, LRDs have an effective radius ∼ 10 2 pc, smaller by a factor of 10-100 relative to expectations.
The mass in stars needed to light up LRDs requires, in the context of the expected LCDM abundance of galactic halos, that they convert nearly all their gas into stars rapidly, over hundreds of millions of years (Boylan-Kolchin 2023; Wang et al. 2024).This high level of star formation efficiency is unlikely to be realized because of the inevitable gas loss in supernova-driven winds (Loeb & Furlanetto 2013).This suggests that a significant fraction of the light emitted by LRDs is powered by a central overmassive black hole (Pacucci et al. 2023;Pacucci & Loeb 2024).
The existence of a black hole in LRDs is supported by the spectroscopic detection of broad emission lines, indicating motions of line-emitting gas with a speed of up to ∼ 2.5 × 10 3 km s −1 (Wang et al. 2024), as expected from the broadline-region in the vicinity of a black hole.So far, no X-rays were detected from LRDs (Ananna et al. 2024;Yue et al. 2024).The required mass of the black hole is above expectations based on the stellar mass-black hole mass correlation in the present-day universe (Wang et al. 2024;Pacucci et al. 2023).
Here, I suggest that LRDs are drawn from the low-spin tail in the distribution of specific angular momenta of galaxies (Eisenstein & Loeb 1995b,a).Such an origin would account for their compact disk, high star-formation-rate, enhanced dust opacity, and overmassive black hole.

LOW-SPIN GALAXIES
The standard semi-analytic model for galactic disks relates the disk radius to the angular momentum per unit mass of the host halo, j (Mo et al. 1998).Galactic spin is acquired via tidal torques during turnaround of the protogalactic material (Peebles 1969;Eisenstein & Loeb 1995b;Gao et al. 2004).As the dark matter virializes, the gas cools and settles to a disk of mass M d .The disk radius is dictated by the centrifugal barrier, r d ≈ j 2 /GM d .Since different galaxies form in different environments, the population of disks will have a distribution of j-values at birth. Figure 1 in Eisenstein & Loeb (1995b) suggests that the number density of galactic disks which are 10 to 100 times smaller than a typical radius r d is ∼ 10% to ∼ 1%, respectively, of the abundance of typical galaxies with the same mass at the same redshift.Low-spin galaxies could therefore account for a population of LRD disks with the required abundance and compactness to match the JWST data (Wang et al. 2024).
According to the Kennicutt-Schmidt prescription (Kennicutt & Evans 2012), the star formation rate scales inversely with the dynamical time of the disk, t dyn ∼ 1/ GM d /r 3 d .The reduction in r d for LRDs naturally leads to an enhanced star formation rate, SFR ∝ (M d /t dyn ) ∼ GM 3 d /r 3 d .A reduction in r d by a factor of 10 to 100 leads to an increased SFR by factors of ∼ 32 to 10 3 , respectively.This could account for an early episode of high SFR and the resulting evolved stellar population in LRDs.The early SFR would naturally enrich the disk with dust that together with the disk compactness, would increase the dust opacity and result in enhanced redenning as observed for LRDs.

BLACK HOLE FORMATION
A reduction in r d relative to typical values by factors of 10-100 leads to a decrease in the centrifugal barrier for feeding a central black hole.Consider a halo mass of M h ∼ 10 12 M ⊙ , and a disk mass containing a third of the total baryonic mass in the halo, M d ≈ 0.05M h = 5 × 10 10 M ⊙ .A disk smaller by a factor ∼ 25 relative to the average value r d for the population of galaxies with the same mass and redshift (Eisenstein & Loeb 1995a), would have a radius comparable to LRDs, . (1) The characteristic circular velocity of a disk of this mass and radius, v c = (GM d /r d ) 1/2 ≈ 1.5 × 10 3 km s −1 , is comparable -as needed -to half the velocity width of the observed broad lines of LRDs (Wang et al. 2024).