* Astronomy

A new method to detect small, faint galaxies that spring up in the wake of violent galactic collisions has been devised by astronomers using the Spitzer Space Telescope. The method could shed light on how most of the galaxies near our own formed.

The vast majority of local galaxies are "dwarfs" – our galaxy, the Milky Way, has 1000 times more mass in stars. But it is not clear how these dwarfs form.
Some may have condensed directly from primordial gas soon after the big bang. But astronomers are not sure these lightweight galaxies would have been able to survive unscathed the relatively common galactic smash-ups that occurred in the early universe.
Now, a team of astronomers from Cornell University in Ithaca, New York, US, is trying to answer that question by studying a population of dwarf galaxies that were definitely born when two larger galaxies slammed into each other.


Position(2000): RA 13h47m24.5s Dec -30d24m25s


"It's important to compare the properties of the ones we know with those that may have lasted since the big bang" - Jim Higdon, team member.

The objects studied by the team are called tidal dwarf galaxies (TDGs), because they are created when two galaxies interact gravitationally. They form in long, tail-like structures but are so small and faint that only about a dozen had previously been observed.



But now the Cornell team has used Spitzer's exquisite sensitivity at infrared wavelengths to identify 15 TDGs around a pair of merging galaxies, together called NGC 5291, that lie 200 million light years from Earth. The galaxies are strung along two arcs of stars and gas stretching about 240,000 light years behind each of the two larger galaxies.

Previous ultraviolet measurements had suggested these arcs contained "knots" of star formation. Spitzer confirmed this, revealing these modest TDGs are actually powerhouses of star formation – boasting about 200 million sun-like stars that are just a few million years old.

The researchers also discovered the merger-induced dwarf galaxies have a telltale infrared spectrum. They are now searching for this signature in large infrared sky surveys to understand what fraction of other dwarf galaxies formed this way.

"We want to understand how the universe was built up" - Sarah Higdon, team leader.

The research will be published in a future issue of the Astrophysical Journal.

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Title: NGC 5291: Implications for the Formation of Dwarf Galaxies
Authors: Malphrus, Benjamin K.; Simpson, Caroline E.; Gottesman, S. T.; Hawarden, Timothy G.

The possible formation and evolution of dwarf irregular galaxies from material derived from perturbed evolved galaxies is addressed via an H I study of a likely example, the peculiar system NGC 5291.
This system, located in the western outskirts of the cluster Abell 3574, contains the lenticular galaxy NGC 5291 which is in close proximity to a disturbed companion and is flanked by an extensive complex of numerous knots extending roughly 4 min north and 4 min south of the galaxy.
In an initial optical and radio study, Longmore et al. (1979, MNRAS, 188, 285) showed that these knots have the spectra of vigorous star-forming regions, and suggested that some may in fact be young dwarf irregular galaxies.
High resolution 21-cm line observations taken with the VLA are presented here and reveal that the H I distribution associated with this system encompasses not only the entire N-S complex of optical knots, but also forms an incomplete ring or tail that extends approximately 3 min to the west.
The H I associated with NGC 5291 itself shows a high velocity range; the Seashell is not detected. The formation mechanism for this unusual system is unclear and two models - a large, low-luminosity ram-swept disk, and a ram-swept interaction-are discussed.
The H I in the system contains numerous concentrations, mostly along the N-S arc of the star-forming complexes, which generally coincide with one or more optical knots; the larger H I features contain several x 10^9 solar mass of gas. Each of the knots is compared to a set of criteria designed to determine if these objects are bound against their own internal kinetic energy and are tidally stable relative to the host galaxy.
An analysis of the properties of the H I concentrations surrounding the optical star-forming complexes indicates that at least the largest of these is a bound system; it also possesses a stellar component. It is suggested that this object is a genuinely young dwarf irregular galaxy that has evolved from the material associated with the system and that this entire complex contains several proto- or young dwarf irregular galaxies in various stages of development. We are therefore witnessing the early evolution of a number of genuinely young galaxies.
Given the evident importance of the NGC 5291 system as a 'nursery' for young galaxies, careful modelling is required if we are to understand this remarkable galaxy.