Little red dots (LRDs) are a class of small, red-tinted galaxies discovered by the James Webb Space Telescope (JWST). [1] [2] [3] Their discovery was published in March 2024, making them poorly understood due to limited data collection. [4] They appear to have existed between 0.6 and 1.6 billion years after the Big Bang (13.2 to 12.2 billion years ago), with a majority found around 600 million years post-Big Bang. [1] [5] Only 341 LRD galaxies have been identified thus far with JWST. [6]
LRDs were first selected by photometric methods because they are blue in ultraviolet and red in the optical spectrum. [4] 80% were found to have very broad Balmer emission lines, suggesting that they are active galactic nuclei (AGN) and host supermassive black holes at their center. [7] Active galactic nuclei are defined as small regions in the centers of galaxies that emit copious amounts of energy in the form of bright jets and winds. [8] [9] Scientists study the properties of AGNs to better understand supermassive black hole formation and how they contribute to the structure and dynamics of LRDs. [4] One property of LRDs explained by the AGN theory is the red color of the galaxies themselves. Astrophysicists have determined that the distinct color can be accredited to the massive amounts of gas, dusk, and electromagnetic energy that surrounds the AGN and supermassive black hole. [10] This region is also known as the accretion disk.
The gas in LRDs spins extremely fast. [1] Scientists argue that the gas is accelerated to these extreme speeds by spinning, supermassive black holes. [1] A team working under the Webb Telescope targeted LRDs in the 'Red Unknowns: Bright Infrared Extragalactic Survey', [11] observing rapid gas orbits of roughly 2 million miles per hour (1,000 km per second)-- A strong indicator of black hole accretion. [5]
On the other hand, LRDs also exhibit properties that are difficult to explain within the AGN scenario. For example, they have a flat infrared spectrum [12] and lack x-ray detection. [13] [14] LRDs also show very weak time variability, often seen in AGN observation. [15]
Several models have been proposed to explain the observed properties of LRDs. [16] [17] [18] The shape of the ultraviolet spectrum can be explained by the scattered AGN light [16] [17] or by the gray dust extinction law. [18]
Research has shown that LRDs do not commonly exist at lower redshifts. One possible reason for this observation is what Webb Space Telescope calls "inside-out growth": When a galaxy evolves and expands outward from its nucleus at lower redshifts, a decreasing amount of gas is deposited near the accreting black hole. Thus, the black hole sheds its outer gas layers, becomes bluer, and is no longer categorized as an LRD. [5]
Most are extremely compact, averaging around 2% of the radius of the Milky Way. [3] A typical LRD has a radius no greater than 500 light-years, though many have radii smaller than 150 light-years. [19]
Likely local analogues of LRDs were discovered in a sample of Green Pea Galaxies (GP). [20] These are broad-line AGN hosting Green Peas (BLGP) with V-shaped rest-frame UV-to-optical spectral energy distribution (SED). Seven such V-shaped BLGPs were identified from a sample size of 2190. These V-shaped BLGPs host over-massive black holes. [20]
