An international group of astronomers using NASA’s Hubble Space Telescope has studied some of the smallest and faintest galaxies in our cosmic neighborhood.
Astronomers using the Hubble have unmasked the dim, star-starved dwarf galaxy Leo IV. These images demonstrate why astronomers had a tough time spotting this small-fry galaxy. The image at left shows part of the galaxy, outlined by the white rectangular box. The box measures 83 light-years wide by 163 light-years long. A close-up view of the background galaxies within the box is shown in the middle image. The image at right shows only the stars in Leo IV (NASA / ESA / T. Brown / STScI)
Astronomers have puzzled over why some extremely faint dwarf galaxies spotted in our Milky Way galaxy’s backyard contain so few stars. The galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the Universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey.
But the team needed the Hubble Space Telescope to help solve the mystery of why these galaxies are starved of stars, and why so few of them have been found.
Hubble images of three of these small galaxies – the Hercules, Leo IV and Ursa Major dwarf galaxies – reveal that they all started forming stars more than 13 billion years ago – and then abruptly stopped – all in the first billion years after the Universe was born in the Big Bang.
In fact, the extreme age of their stars is similar to Messier 92, the oldest known globular cluster in the Milky Way.
“These galaxies are all ancient and they’re all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies,” said Dr Tom Brown of the Space Telescope Science Institute in Baltimore and lead author of a paper published in the 1 July issue of the Astrophysical Journal Letters (arXiv). “The most likely explanation is a process called reionization.”
The relic galaxies are evidence for a transitional phase in the early Universe that shut down star-making factories in tiny galaxies. This phase seems to coincide with the time when the first stars burned off a fog of cold hydrogen, a process called reionization. In this period, which began in the first billion years after the Big Bang, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionizing the Universe’s cool hydrogen gas.
The same radiation that sparked universal reionization also appears to have squelched star-making activities in dwarf galaxies, such as those in Dr Brown’s study. The small irregular galaxies were born about 100 million years before reionization began and had just started to churn out stars at that time. Roughly 2000 light-years wide, these galaxies are the lightweight cousins of the more luminous and higher-mass star-making dwarf galaxies near our Milky Way. Unlike their higher-mass relatives, the puny galaxies were not massive enough to shield themselves from the harsh ultraviolet light. What little gas they had was stripped away as the flood of ultraviolet light rushed through them. Their gas supply depleted, the galaxies could not make new stars.
The findings could help explain the so-called “missing satellite problem,” where only a few dozen dwarf galaxies have been observed around the Milky Way while the computer simulations predict that thousands should exist. One possible explanation for the low number discovered to date is that there has been very little, or even no star formation in the smallest of these dwarf galaxies, leaving them virtually invisible.
_______
Bibliographic information: Brown TM et al. 2012. The Primeval Populations of the Ultra-faint Dwarf Galaxies. ApJ 753 L21; doi:10.1088/2041-8205/753/1/L21
