The James Webb Space Telescope's discovery of enigmatic 'little red dots' in the early universe has captivated astronomers, sparking a quest to unravel their true nature. These dots, appearing as small, red, and surprisingly bright objects, were found at redshift values suggesting they existed between 13.2 and 12.2 billion years ago. Initially, their brilliance at such vast distances hinted at quasars, but their light profile more closely resembled bloated stars, primarily in infrared with some ultraviolet, lacking the X-ray signatures expected from active black holes. This led researchers to propose the hypothesis that these dots are 'black hole stars,' or vast clouds of gas heated by concealed, growing supermassive black holes.
However, the population of these little red dots abruptly diminishes at redshifts less than 12 billion years ago, raising questions about their fate. Two possibilities emerged: either they all died off or they evolved into more familiar objects. A recent hypothesis suggests they transformed into large globular clusters, but now, an alternative theory has emerged.
A team led by Pierluigi Rinaldi, previously at the University of Arizona's Steward Observatory and now at the Space Telescope Science Institute (STScI) in Baltimore, believes it has identified a descendant of a little red dot. This descendant appears as a distant galaxy, WISEA J123635.56+621424.2, found at a redshift of 2, or 10.5 billion years ago. The galaxy, nicknamed Saguaro, boasts neat spiral arms around a red core, resembling a cactus species native to the Sonoran desert.
Saguaro's core exhibits characteristics reminiscent of little red dots, shining brightly in infrared and emitting ultraviolet light, as detected by the Hubble Space Telescope. Interestingly, NASA's Chandra X-ray Observatory also detected faint X-rays from Saguaro, indicating an active galactic nucleus, a feature typically associated with quasars. Carys Gilbert, a researcher from the University of Cape Town, suggests that this combination of X-ray weakness and obscuration could explain the lack of X-ray emission observed in other little red dots.
This isn't the first instance of X-rays being detected from a little red dot. Earlier this year, scattered X-rays were observed breaking through from 3DHST-AEGIS-12014, lending support to the black hole star hypothesis. With Saguaro, which existed 1.3 billion years later, being a bit further along in its development, the team simulated its appearance a billion years after the Big Bang, revealing that much of its galactic structure would be too faint or less developed to be visible, leaving only the nucleus, resembling other little red dots.
Rinaldi's team concludes that little red dots are not a unique population but a phase in the development of galaxies with supermassive black holes. Our Milky Way, they speculate, might have once been a little red dot. Saguaro, as a crucial link in the story of little red dots and their evolution into modern galaxies, offers valuable insights into the middle and end of their life cycles, despite the ongoing mysteries surrounding their origins and formation.
The findings, reported in The Astrophysical Journal on July 29, highlight the ongoing quest to understand the life cycle of these enigmatic objects, with Saguaro providing a fascinating glimpse into the transformation of little red dots into the galaxies we observe today.
