July 11, 2012
Credit: NASA/JPL, Caltech, C.R. Bridge (Caltech), et al. (left); CXC/M.Weiss (right)
Astronomers have combined the power of the Keck I Telescope with that of NASA?s Wide-field Infrared Survey Explorer (WISE) to discover examples of what appears to be one of the missing links in the evolutionary process of galaxies.
Approximately 11 billion light-years away, these galaxies are surrounded by extremely luminous, colossal `blobs? of hydrogen gas. These amoeba-shaped clouds of a gas are emitting ultraviolet light in the spectral line for hydrogen, called the Lyman-alpha emission line. Lyman-alpha blobs were discovered in the late 1990?s (Keel et al., Steidel et al.), and there are only a few dozen, truly bright ones known. They are enormous, having sizes 10 to 20 times that of our own Milky Way galaxy making them amongst the largest galactic structures in the Universe.
?These newly found galaxies are monsters in every sense of the word? said Dr. Carrie Bridge, a senior postdoctoral researcher at Caltech and lead author in a report on the discovery in the July XX edition of Astrophysical Journal Letters. ?They are some of the brightest galaxies in the Universe, with super-massive black holes shrouded in dust and unexpected blobs of gas.? explained Bridge.
What makes these new WISE blobs so unique is that unlike all previously known ?blobs? they also contain massive amounts of heated dust, which suggests that the galaxies are forming stars at rates over a thousand times that of our own galaxy, and that the supermassive black hole at its core is cannibalizing fuel.
?In the past Lyman-alpha blobs were found serendipitously,? Bridge explained. ?Using WISE, we now have a way to find this species of ?hot? Lyman-alpha blob quickly and effectively.?
Among the puzzles she is hoping to solve is how these Lyman-alpha emissions manage to escape the very dusty galaxies where they are found.
?Typically we don?t see Lyman-alpha hand-in-hand with dust,? Bridge said, ?because dust easily absorbs that wavelength of light. So somehow these galaxies have developed dust-free holes for the Lyman-alpha photons to escape from.?
A possible explanation is that these galaxies are undergoing short-lived cycles of feast and famine for the giant black holes at the centers of the dusty galaxies ? something referred to as black hole feedback. This might cause energetic material from feeding black holes to blow material away from the black hole and punch holes through the shell of dust. That same process then starves the black hole of material to feed on. The stellar winds from all those stars being formed is also part of the equation.
?But we don?t fully understand how feedback works,? Bridge cautioned, ?because until now we have primarily studied galaxies before and just after peak feedback, but now we believe we have caught it in the act.?
As for what causes these strange and rare events in the first place, they could be galactic collisions.
?They look like train wrecks,? said Bridge, regarding observations of some of the blobs with the Keck II Telescope?s Near Infrared Camera-2. ?The galaxies appear to be in the intermediate to late stages of a galaxy-galaxy collision.?
This limits how long these blobs could exist to perhaps 500,000 to 1 million years ? a very brief moment on the universal timescale. The relative brevity of the events means they are very rare, which is why only a full-sky survey such as the one WISE provides can find a reasonable sample of them.
Bridge and the international team have also recently been awarded time on the Hubble Space Telescope as well as the Herschel Space Telescope to further study this newly discovery type of ?hot blob?. Bridge notes that ?by combining some of the most sensitive telescopes in world at different wavelengths we hope to expand our understanding of how a galaxy grows, evolves and transforms into the galaxies we see in our celestial neighborhood.?
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The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system which cancels out much of the interference caused by Earth?s turbulent atmosphere. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.