Figure 1: Three-color archival Spitzer images of (a) He 2-10, (b) the Antennae, (c) NGC 4214, and (d) NGC 5253 with RGB mapped to 24, 8, and 3.6 µm. In these images red colors pinpoint the most active regions of star formation. The locations of the 33'' GBT beam are indicated with green circles; the positions of the two Antennae nuclei are also indicated. The GBT beam corresponds to a physical scale in these galaxies of 1.7, 3.2, 0.47, and 0.53 kpc, respectively.
The cocoons of dusty, dark, molecule-rich gas clouds found in our own Milky Way galaxy and beyond are nurseries for new stars. Star formation in the Milky Way is slow and steady, but in other galaxies great bursts of star formation can and do occur. These events generate large populations of stars at once, consuming or dissipating much of the gas in the process. Such starburst galaxies range in type and size from small dwarf starbursts to massive colliding galaxies. Starbursts are an important part of galaxy evolution and star formation in the universe and were the dominant means of new star formation when the universe was half its current age.
Bright water masers are a common signature of stellar nurseries in our own galaxy. In fact, most regions containing young stars in the Milky Way show signs of water. A water maser is a laser-like beam of microwaves that naturally forms when water molecules are excited by shockwaves or strong infrared (thermal) radiation. Water masers occur naturally and at the same frequency used by microwave ovens.
In a recent survey, we discovered water in every starburst galaxy we observed and anticipate water to be a ubiquitous signature of star formation. Pointing the giant Green Bank Telescope (GBT) deep into space, we detected water masers in three dwarf starburst galaxies and the famous Antennae Galaxy. The Antennae is a dramatic collision of two massive spiral galaxies, resulting in tremendous bouts of star formation.
These discoveries were only possible with the exquisite sensitivity of the GBT. Some of the masers are as much as 100 times less luminous than Milky Way water masers, suggesting that water masers should be observable in nearly all nearby star-forming galaxies. The masers we observed are not exceptional cases; instead they are commonplace.
Stellar nurseries are very dusty places that visible light cannot penetrate — they are hidden from our view. However, radio waves can pass through the dusty veils surrounding new stars to reveal a clear view. Radio telescopes also provide extremely sharp pictures of these regions because we can form Earth-sized telescopes with radio dishes strung across the globe. These images can be so sharp that it is possible to track the motions and gyrations of galaxies over years of steady observations.
The main utility is a high brightness temperature probe of extragalactic star-forming regions that can be observed with interferometers including VLBI. We can also identify sites of highly obscured star formation and possibly do proper motion measurements of very nearby systems. This also tells us that water is present where stars form — not surprising, but not directly confirmed in very many extragalactic cases.
This discovery opens a new field of research and a new high-resolution window on star formation in other galaxies. We have known for some time that water is present at most sites of star formation in our Galaxy, but we now know that it is often and perhaps always present in the stellar nurseries of other galaxies as well.
Our results are reported in the September 20 issue of Astrophysical Journal Letters, and are also available at http://arxiv.org/abs/0808.2643.