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NOAO Annual Report FY 1999


Scientific Program - Cerro Tololo Inter-American Observatory (CTIO)

Scientific projects at CTIO cover the whole Universe---from the study of the sizes and distribution of asteroids in our own Solar System, through measuring the properties and distribution of stars in our own and nearby galaxies, to determining the scale of structures formed by groupings of clusters of galaxies and distant quasars. Aided by recent advances in detector technology, these projects are increasingly surveying greater areas of sky in order to obtain larger samples of objects. We present four such projects that are presently underway at CTIO.

The Search for Stars Near the Lower Stellar Mass Limit

The lower limit for stellar masses is uncertain, as are the luminosity and mass functions below 0.2 solar masses. Recent discoveries of sub-stellar companions to nearby stars, and particularly the first results from the new infrared sky surveys (DENIS, 2MASS, of which the southern telescope is at CTIO) have shown that there is a significant population of stars (L dwarfs) cooler, and less massive, than the coolest M dwarfs. Some of these stars are clearly sub-stellar due to the detection of lithium in their spectra, showing that hydrogen-burning has never initiated. A team led by John Giziz of the University of Massachusetts and including James Liebert of the University of Arizona, Neil Reid of the University of Florida, and Davy Kirkpatrick (IPAC) has been following up on candidate low-mass stars from the 2MASS survey with optical photometry on CTIO telescopes, as they have found that the 2MASS photometric catalog, when combined with optical data in the R and I bands, is an extremely effective way of identifying these cool stars in the general field. The initial 2MASS follow-up found some 20 L dwarfs per 400 square degrees surveyed, and the new survey has already covered over 800 square degrees. Comparing the observed space density with models simulating the solar neighborhood surface densities and temperature distributions shows that the data can be represented by a power-law mass function, and indicates that the local space density of stars with masses in the range 0.01--0.075 solar could be as high as 0.1 per cubic parsec. This preliminary result suggests that brown dwarfs are twice as common as main-sequence stars, but contribute no more than ~ 15% of the total mass of the Galactic disk.

The Structure of the Sagittarius Stream

The discovery of the Sagittarius dwarf galaxy five years ago, and the immediate realization that it is presently interacting with our own Galaxy, has allowed us to view in situ a process that may have taken place several times in the life of our Galaxy. This has profound implications for stellar population studies in galaxies in general and our own in particular. Mario Mateo of the University of Michigan, Heather Morrison of Case Western Reserve University, and Edward Olszewski of the University of Arizona have been using the Blanco 4-m telescope, initially with the BTC and now with Mosaic II, to survey large areas of sky in the vicinity of the Sagittarius dwarf to very low equivalent surface brightness. They have already found very clear evidence for Sagittarius stars far from the main body of the galaxy, with a change in structure some 20 degrees from the center which they suggest corresponds to a transition from a dynamically distinct inner zone of the dwarf galaxy to a ``stream." All interaction models predict that the galaxy should be extended along a very long stream both trailing and leading the main body. Recent work by Zhao proposes that Sagittarius was injected into its present orbit by a close encounter with the Large Magellanic Cloud; this has profound implications for the present structure of the LMC, as well as for Sagittarius. Mateo et al. have extended their survey this year to trace the Sagittarius major axis out a further 30 degrees, and have obtained a series of cross-cuts to map the structure, curvature, and tilt of the Sagittarius stream. Their data, consisting of accurate photometry for hundreds of thousands of stars, should also be able to constrain the distance and stellar populations along the stream, which together with the structural information, should allow selection of the correct interaction model.

Gravitational Lenses and the Distance Scale

Gravitational mirages (more commonly, but somewhat misleadingly called gravitational lenses) occur on the rare occasions when an intervening mass (e.g., a galaxy) lies along the line of sight to a distant object. As in the case of terrestrial mirages, multiple, distorted images are formed by the variations in the effective index of refraction that the intervening gravitational potential introduces. Such lenses offer opportunities to carry out a variety of interesting astrophysical measurements, including the mapping of dark matter and estimation of the Hubble constant and the vacuum energy density. Among the most distant quasars, roughly one in 1000 undergoes such multiple imaging. Graduate student Nicholas Morgan and Paul Schechter of MIT, José Maza of the University of Chile, and Lutz Wisotzki of the University of Hamburg have been carrying out a program to search for new mirage systems with the CTIO 1.5-m, taking advantage of the increased probability of observing mirages in relatively shallow but wide field surveys. Two such mirages have been discovered and a third candidate is being studied further. One of the mirages has all the properties required to make a distance estimate which bypasses the traditional cosmological distance ladder. Interestingly, the candidate system may not be a mirage but instead the closest known pair of quasars. Kochanek and collaborators have suggested that there is an excess of quasar pairs beyond what might be expected if quasars were uncorrelated events at the centers of galaxies.

Weak Lensing and Cosmological Models

Direct measurements of the amount and spatial distribution of dark matter in clusters of galaxies can strongly constrain cosmological structure formation scenarios. A new generation of models is making specific and testable predictions regarding the mass distribution in clusters, the degree of sub-clustering, and the relationship between light and mass and how these vary when cosmological parameters are varied. Weak gravitational lensing is the most direct method presently available to determine the mass distribution of a galaxy cluster. The method has been pioneered by J. Anthony Tyson (Lucent Technologies) who, in the past two years, together with collaborators David Wittman and David Kirkman (Lucent Technologies) and Ian Dell'Antonio (NOAO) have been using weak lensing to conduct a rigorous survey of 10 X-ray-selected clusters at redshift z ~ 0.2, using the CTIO 4-m Blanco telescope and BTC Imager. Using the shear of background galaxies around each cluster, they measure M/L, the radial profile of mass and light, and the mass anisotropy for each cluster. The group has already discovered that the cluster profiles are isothermal to r ~ 1 Mpc, further out than expected from numerical simulations of these relatively low-mass systems, before falling off as predicted by N-body Cold Dark Matter models. A further key prediction, differentiating between models of CDM, is the spatial density of mass concentrations for a given redshift limit. Tyson et al. have serendipitously found a number of mass concentrations in their two blank comparison fields, and are presently extending this aspect of the project, using Mosaic II.


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