Our own Milky Way and Andromeda, its nearest spiral neighbor, are stunningly beautiful examples of High Surface Brightness (HSB) galaxies.
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Both grand spirals represent the sort of brilliantly-shining specimens that for most of the 20th century, at least, were thought to be the galactic norm.
But since the mid-1980s, astronomers have continued to puzzle over dim so-called Low Surface Brightness (LSB) galaxies, the largest of which has a diameter four times that of the Milky Way.
“Everything we have learned about LSBs suggests that [many] are normal rotating disk galaxies in which the stars are simply spread relatively thin,” said Stacy McGaugh, an astronomer at Case Western Reserve University in Cleveland.
But with surface brightnesses as much as 250 times fainter than the rest of the night sky, LSBs are more diffusely distributed than High Surface Brightness spirals and incredibly difficult to detect. However, several thousand LSBs have been cataloged; ranging from tiny dwarf spirals to supergiant galaxies like Malin 1, which lies more than a billion light years away in the constellation of Coma Berenices.
With a whopping diameter of some 650,000 light years, McGaugh says Malin 1 is a “weird duck” even among LSBs.
“Lit up, Malin 1 would make kind of a boring Christmas tree,” said McGaugh. “There are spots of star formation here and there, but I don’t think it’s ever experienced a big starburst.”
Some LSBs appear to be sitting on as much as 90 percent of their neutral hydrogen gas. Yet most of this gas is spread so thin that galactic theorists remain puzzled as to how it gravitationally collapsed into such a giant spiral.
Although in recent years, Malin 1 has been found to have a fairly normal looking stellar disk at its core, McGaugh says that even when “active,” such LSBs have low star-forming rates, and produce very few if any supernovae.
“Being spread so thin, it is hard for them to get material together in order to form stars,” said McGaugh. “On top of that, they are largely bereft of molecular gas, which is the phase of dense gas from which stars are thought to form.”
As James Schombert, an astronomer at the University of Oregon, points out, below a certain gas density, current models indicate star formation altogether halts.
Even so, Schombert says recent Hubble Space Telescope images of three nearby LSB galaxies reveal an “amazing widely-spaced blue population,” differing from “tight star-forming clusters” in normal galaxies.
“The blue implies lots of recent star formation,” said McGaugh. “But this blue color is often spread very thinly over huge areas; the opposite of dense, intense regions of star formation that we’re use to. So, it isn’t clear how recently-formed stars get so widely- dispersed.”
Without question, LSBs — which appear to lie at all cosmic distances — are easiest to spot with with wide-field telescopes. But astronomers have often been hamstrung in confirming such distances, primarily due to their inherent faintness.
“We need larger samples of LSB to break them into dwarfs, normal disks, large Malin-[type] objects,” said Schombert. “But we are moving out of LSB “discovery” mode and into the “recover and classify” phase.”
Schombert says future LSB detection will require deep, dark sky surveys, for which a space telescope sky survey might prove faster.
But in lieu of a dedicated space survey, astronomers will take advantage of the planned LSST (Large Synoptic Survey Telescope), tentatively scheduled for “first light” in Chile by 2020.
By making repeated observations, the LSST will offer astronomers the ability to build up very deep images of the sky, and McGaugh says, may represent the best opportunity to “break through the wall” limiting the surface brightness depth at which galaxies can currently be discovered.
“We haven’t yet been able to observe LSB [stellar] chemical abundances,” said McGaugh.
Thus far, if LSBs don’t appear to be rife with the kinds of heavy elements that make it possible to form earthlike planets, making complex life in LSBs a longshot.
Even so, LSBs do have longevity on their side.
“Many have neutral hydrogen reservoirs comparable to, or greater in mass, than their stars,” said McGaugh. “So, many of these LSBs will be slowly forming stars still long after the Milky Way runs through its gas and sputters out.”
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