Jean L. Turner
UCLA
Physics & Astronomy
Research Interests
Early in its history, the Milky Way formed many massive and luminous star clusters, many of which have survived to this day in the form of globular clusters. Currently the disk of our Milky Way does not seem to be able to
form such
massive clusters. However, young clusters that appear to be as massive as globular clusters do appear to be forming in the present universe, and are visible in nearby galaxies. Young, forming super star clusters are examples of
extreme star formation
taking place in luminous and ultraluminous infrared galaxies and starburst galaxies, and this is the area of my research.
My targets are the very youngest star-forming regions. Like the youngest Galactic star-forming regions, these are typically still "embedded" in their natal dusty gas clouds. While this allows us to study their pristine natal environments, before the stars have had a chance to disperse the gas, it also means that they are hidden from optical view. To see them, one observes
in the infrared, millimeter/submillimeter and radio portions of the
spectrum. In many cases the star clusters themselves are completely hidden
within dusty cocoons of gas, invisible even in the near and mid-infrared. They are indirectly detectable at longer wavelengths, through the HII regions that are excited by these young clusters and their surrounding gas clouds.
The nearby molecular clouds from which they formed can be detected in millimeter and submillimeter-wave emission from lines of molecules such as CO.
Where are the regions of extreme star formation? How big are the star -forming regions? Are they composed of large clusters? What environments favor the formation of large clusters?
My approach has been to use radio
continuum imaging to detect free-free emission from HII regions, using the
Very Large Array
of the National Radio Astronomy
Observatory.
Radio continuum emission is emitted by hot nebulae surrounding
the most massive young stars, stars less than a few million years
old. The great advantage of radio emission is that it is unaffected by the dust extinction, which can be extremely high, up to thousands of magnitudes, in the
youngest star-forming regions. For example, 7mm imaging with the VLA shows that the very youngest HII regions in M82 are completely hidden in the optical and even in the near-infrared (Tsai et al. 2009). The VLA at its highest frequencies is a great way to measure the energetics of the youngest massive clusters. The new and improved high frequency capabilities of the VLA and the GBT are proving most useful for characterizing extragalactic star formation (e.g., Murphy et al. 2011).
M82 in radio continuum at high (~0.2") resolution. Magenta contours are 2cm emission, green contours are 7mm. Nearly all of the radio emission comes from the obscured region, shown by comparing to the false color VRI image from Spitzer SINGS, Tsai et al. 2009.
Is extreme star formation a different kind of star formation from what we
observe in the Milky Way?
Star formation in the Milky Way is slow and inefficient; at any given time only a percent or less of a giant molecular cloud is collapsing to form stars. The existence of globular clusters implies that this efficiency was much greater in the past, since efficiencies of closer
to 50% are required to form such clusters. What conditions favor efficient star formation? I am presently using data from the Submillimeter Array (SMA), and in the future, the Atacama Large Millimeter-Submillimeter Array (ALMA), to study the efficiency of star formation in the youngest super star clusters. The high resolution of array telescopes allows the imaging of molecular clouds on the scales of individual giant molecular clouds, or better. ALMA will image down to the scales of star clusters themselves, resolving regions only a few light years across. Here is a picture of star formation being fed by a molecular streamer
originating outside the galaxy.
Dwarf spheriodal galaxy, NGC 5253. This tiny galaxy has been forming
super star clusters for a least a gigayear. How does it do it? Part of it
could be the gas streamer falling into the galaxy... a molecular
gas streamer. Most of the molecular gas in this galaxy is located
outside the galaxy, origin unknown. Weird.
Definitely an unusual star-forming environment.
Optical + IR image showing embedded IR cluster
from Turner et al. 2003
and CO map from
Meier, Turner, & Beck 2002 The streamer has been confirmed and cloud
associated with the "supernebula" detected in CO 3-2 with the SMA, Turner et al. 2012 (in prep.)
What is the direct effect of star clusters on their environment?
Extreme star forming regions contain many stars, and where there are many stars there are O stars. O stars have enormous potential to inflict mayhem on their environments. In the Galaxy, having more than a few O stars in a single region is rare. This is not the case in super star clusters, which may contain a million stars and a few thousand O stars within a region only a few light years across. What are the effects of stellar winds on the surrounding gas clouds in young super star clusters? How and when do these clusters halt
accretion; what do they do to the nearby molecular clouds? I am currently studying the kinematics of gas near super star clusters. The NIRSPEC instrument
on the Keck
Telescope allows high resolution spectra in the near-infrared, including
the Brackett alpha and gamma recombination lines of hydrogren. The resolution is sufficient to resolve recombination lines in nebulae, which led us to discover the first gravity-bound HII region around a super star cluster. In general, the linewidths of super star clusters are not so different from the linewidths of Galactic HII regions containing only one or two O stars, which is a little surprising. I am also working with data from the TEXES mid-infrared spectrometer, observations of heavy element ions from HII regions. Heavy elements have smaller intrinsic linewidths and allow precision kinematical analysis. We have evidence of systematic flows in super star cluster nebulae that suggest either a champagne or blister flow or outflows.
The Keck telescope image from the Keck Observatory webpage as
modified by J.T.
What can chemistry tell us about extragalactic star-forming environments?
The relatively high spatial resolution of millimeter
interferometers revealed that the chemistry of molecular clouds is strongly
dependent on their location within a galaxy. We are able to determine if clouds
are affected by interstellar shocks, or radiation fields, or are relatively
quiescent, just from their chemistry. This is a promising new field. The first
imaging chemistry
study of
a galaxy was done in collaboration with (then) student David S. Meier, and is part of a series of studies of chemistry in
nearby galaxies. This field has a very promising future.
Molecules in the central arcminute (300 pc or so) of the nucleus of the nearby spiral galaxy Maffei 2. From
Meier & Turner 2012.
What next? ALMA
The new instrument, ALMA, is just coming online and is still being commissioned. This instrument will transform our knowledge of dust and gas around extragalactic star-forming regions. It will be ten times more sensitive than current millimeter arrays, and it works in the submillimeter too. I have served on many of the committees leading to the design of this telescope, starting when it was the U.S.-only project, the Millimeter Array. I spent three months in 2011 in Chile
helping out with commissioning, reducing science verification
data. I can attest that the early data from this instrument is already amazing!
There is no telling what is coming next, expect to be surprised.
ALMA at the beginning of early science, September, 2011. Sixteen antennas at 16,400 ft! (Latest count, about 40)
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Last modified August 2012
