BY DR EMILY BALDWIN
Posted: 17 April, 2009
A sample of massive galaxies with properties suggesting they formed relatively recently could challenge classical theories of galaxy formation.
It is currently widely believed that massive luminous galaxies, like our own Milky Way Galaxy, began their formation shortly after the Universe was born in the Big Bang 13 billion years ago. But in a new study lead by John Salzer of Indiana University, 15 galaxies of a 2,400 sample of the Kitt Peak National Observatory International Spectroscopic Survey (KISS) displayed the characteristics of much younger galaxies.
While their luminosities are characteristic of other massive galaxies, their chemical signatures display relatively low abundances of heavy elements, or metals. Taking some basic assumptions relating stellar evolution to chemical enrichment, the galaxies may be as young as just three or four billion years old, placing them outside the bracket for when massive galaxies are expected to form.
The observations were carried out as part of the Kitt Peak National Observatory International Spectroscopic Survey.
“These objects may represent a unique window on the process of galaxy formation, allowing us to study relatively nearby systems that are undergoing a phase in their evolution that is analogous to the types of events that, for most galaxies, typically occurred much earlier in the history of the Universe,” says Salzer.
While this interpretation is provocative, Salzer concedes that it is not the only possible explanation for their existence. The same chemical measurements could result from a merger of two smaller galaxies, which would cause a reduction of metal abundances due to dilution from unprocessed gas and a simultaneously brief but large increase in luminosity caused by a bout of rapid star formation.
The galaxies could also represent “straggler” galaxies, some of the last massive objects to collapse and form stars. In this scenario, the progenitor gas clouds initially resisted gravitational collapse due to a locally low gas density, which inhibited evolutions until an event - such as collision with another gas cloud, or gravitational interactions with a passing galaxy - raised the density enough to trigger collapse.
“Is it interesting that not all massive galaxies started forming within one billion years of the big bang?” Answering his own question, Salzer says, “Yes, since it implies a larger diversity of initial conditions and environmental triggers than is usually assumed. But all indications are that these event are rare, so it does not require a major modification in our models.”
Salzer tells Astronomy Now that perhaps most interesting about these objects is that they afford a marvelous opportunity to study the early phases of galaxy formation in unprecedented detail. “If the late-forming galaxy scenario is correct, then we are viewing these objects from a modest distance and seeing processes that, for most galaxies, occurred 12-13 billion years ago and which are currently beyond our ability to observe,” he says. “So these rare objects potentially provide a window into the past.”
In order to distinguish between the competing theories for the existence of these apparently young galaxies, Salzer and his team will request observing time on NASA’s Hubble Space Telescope to use high-resolution imaging to determine whether or not the systems might be products of merging.
The findings are discussed in the 19 February 2009 issue of Astrophysical Journal Letters.