28 January 2015
The dwarf spheroidal galaxy Segue I (K. Spekkens/Capella Observatory)
Stars in galaxies are formed from the interstellar medium, and recycle material to their interstellar medium through stellar winds and mass loss after a star turns off the main sequence. The interstellar medium of a galaxy thus reflects its current ability to form stars, its history of stellar mass loss, and factors such as accretion, supernovae, and interactions.
The Milky Way is surrounded by a number of lower-mass dwarf spheroidal galaxies and it has long been known that those within the Milky Way’s virial radius, about 300 kpc, tend to be deficient in neutral hydrogen (HI) compared to dwarf galaxies at a greater distance. Because these galaxies lack the young, massive stars that are the progenitors of supernovae, this suggests that environmental factors, such as gas stripping through interaction with the Milky Way’s extended hot corona, may play an important role in their evolution.
Recent 21 cm HI observations using the NRAO Green Bank Telescope (GBT) have produced the most stringent limits to date on the presence of neutral gas in a set of nearby dwarf spheroidal galaxies, limits that are a factor of more than 20 below previous measurements.
The limits can be astonishingly small. GBT observations of the galaxy Seque I, which has a luminosity of 340 L⊙, limit the HI content to no more than 11 M⊙ (5σ limit). For the much larger Ursa Minor dwarf, with a luminosity of 2.8 x 105 L⊙, the 5σ limit on the HI mass is only 63 M⊙. The GBT observations show that five dwarfs have < 100 M⊙ of HI and several others have < 400 M⊙. For comparison, a “standard Spitzer” diffuse cloud in the Milky Way contains 400 M⊙ of HI.
The mass loss expected from stellar evolution in several of these galaxies exceeds the detection limit by more than a factor of 10. The Ursa Minor dwarf is particularly interesting, as its orbit takes it beyond the hot corona of the Milky Way for nearly 1 Gyr, during which time it should have accumulated 30 times more HI than the new limit. Thus, either the accumulating interstellar gas in these galaxies is not in the atomic phase, or some mechanism beyond interaction with the hot corona has removed it.
Reference: K. Spekkens et al., The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies, 2014, ApJ Letters, 795, L5.
See the full article here.
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The NRAO operates a complementary, state-of-the-art suite of radio telescope facilities for use by the scientific community, regardless of institutional or national affiliation: the Very Large Array (VLA), the Robert C. Byrd Green Bank Telescope (GBT), and the Very Long Baseline Array (VLBA)*.
The NRAO is building two new major research facilities in partnership with the international community that will soon open new scientific frontiers: the Atacama Large Millimeter/submillimeter Array (ALMA), and the Expanded Very Large Array (EVLA). Access to ALMA observing time by the North American astronomical community will be through the North American ALMA Science Center (NAASC).
*The Very Long Baseline Array (VLBA) comprises ten radio telescopes spanning 5,351 miles. It’s the world’s largest, sharpest, dedicated telescope array. With an eye this sharp, you could be in Los Angeles and clearly read a street sign in New York City!
Astronomers use the continent-sized VLBA to zoom in on objects that shine brightly in radio waves, long-wavelength light that’s well below infrared on the spectrum. They observe blazars, quasars, black holes, and stars in every stage of the stellar life cycle. They plot pulsars, exoplanets, and masers, and track asteroids and planets.