Phoenix: Spectra from the commissioning and Demonstration Science run

photo: Sample Phoenix spectrum of a K=11 mag field red giant in the Large Magellanic Cloud. This spectrum consists of a combination of three 20-minute integrations using a 4-pixel slit. This spectral region is dominated by CO 2-0 vibration-rotation lines from both carbon-12 and -13, although two atomic lines are identified, as well as the R9 line from HF (1-0).

2002 February 22

Phoenix, a high resolution infrared spectrograph, originally built for use on the NOAO 4-m telescopes at Kitt Peak and Cerro Tololo, underwent commissioning on Gemini South in December 2001. The first science observing programs were performed in early February 2002. They comprised a Demonstration Science program (described below) and individual science projects from the Gemini queue. Both the commissioning and the science runs were highly successful.

Introduction to Demonstration Science

Our universe initially was virtually entirely hydrogen and helium, as a result of the Big Bang. The heavier elements (which astronomers call "metals") that now make up a small fraction of the universe's mass and our sun's mass (but much of the mass of the earth, and our bodies) were made inside previous generations of stars, which exploded and ejected their material into space. Studies have shown that spiral galaxies, including our Milky Way, have higher abundances of "metals" in their centers than at their edges and that the overall metallicities of galaxies are greater in brighter galaxies than in fainter ones. Our galaxy's metal abundances are primarily the result of two processes: evolution of its own stars; and its consumption of smaller and fainter galaxies and their stars, by virtue of its greater size and gravity. The science described below concerns the abundances in the Large Magellanic Cloud, a close neighbor galaxy to our Milky Way, which is already in the process of being incorporated into the Milky Way.

Large telescopes make it possible to make detailed studies of the chemical abundances in individual stars in external galaxies. Perhaps Gemini's most important contribution to this field will be its ability to make infrared measurements of the chemical abundances of certain elements which cannot be easily measured at optical wavelengths. This advance promises to help astronomers paint a complete picture of galactic chemical evolution from the Big Bang to the present.

Demonstration Science

The aims of Demonstration Science are to demonstrate the capabilities of Phoenix on Gemini South and verify the system performance by executing a specific scientific program. The program was selected from 12 responses to a solicitation in June 2001; the data will be made publicly available after ~2 months following verification and quality assessment by the team.

The main goal of this program is to determine the Oxygen to Iron abundance ratio (O/Fe) in the Large Magellanic Cloud, a key parameter for understanding the chemical history of the LMC in comparison to the Milky Way. The few existing observations suggest that O/Fe is lower in LMC stars than in stars with comparable Fe abundance in our own Galaxy. As the abundance of Oxygen and Iron are sensitive to the chemical yields from two different types of supernovae (SNII and SNIa, respectively), which arise from very different types of stellar systems with different evolutionary timescales, the O/Fe ratio would be a measure of different types and rates of star formation in separate galactic systems. Field K- and M-giants in the LMC were observed in two wavelength regions containing samples of OH, CO, and CN lines, as well as atomic lines from Fe, Na, Si, Sc, Ti, and Ni. The molecular lines allow for C, N, and O abundances to be derived, and thus, any mixing-induced abundance changes in C, N, or O can be determined and accounted for, resulting in accurate oxygen to iron abundances being measured over a range in metallicity in the LMC.

Phoenix on Gemini has the sensitivity to observe many individual late-type stars in globular clusters and nearby galaxies. The excellent image quality of Gemini not only directs most of the light down the Phoenix slit but allows the selection of objects in crowded fields. Objects as faint as K=13 were observed in the Demonstration Science Program.


Early Science

Owing to their faintness and red color, there are few very high resolution spectra of cool dwarf stars and brown dwarfs. The sensitivity of Phoenix on Gemini allows high resolution infrared spectroscopy to be performed on a large sample of these objects and to become an important tool in understanding them. For example, from 2.3 micron spectroscopy, as in the illustration below, carbon and oxygen abundances can be measured using the CO and H2O lines. The widths of the spectral lines also allow measurement of v sin i, which will permit insight into the formation of these objects. Comparison of observed and synthetic spectra will give insight into the atmospheric opacity sources and atmospheric structures of these objects.



photo: The figure shows the spectrum of a K=12.7 mag early L-type dwarf . In the figure the raw count rate spectrum is shown in red (in electrons per summed row) and the continuum normalized, telluric corrected spectrum is shown in black. The latter has been convolved with a 5 pixel wide Gaussian averaging over the slit width. The strong CO lines are labeled across the top of the plot. There are also strong H2O features present which are not labeled.

Gemini Observation Deepens Mystery of Local Active Galaxy

2001 October 29

In the deepest ground-based mid-infrared image ever, the Gemini North Telescope reveals that the mysterious environment around one of the most massive black holes in the Universe is missing a key feature predicted by astronomical theory."Contrary to what most theories predict, our Gemini observations show that the giant elliptical galaxy M87 either lacks a torus around its central black hole, or else this doughnut-shaped ring of material is extremely faint. With the unparalleled resolution and depth of the Gemini mid-infrared observations, the torus should have been easy to detect," says Principal Investigator Dr. Eric Perlman of the University of Maryland, Baltimore County.

The Gemini data elaborate on earlier high-resolution optical observations by the Hubble Space Telescope, which hinted at the unique conditions inside this famous galaxy. "This will definitely cause some head-scratching among theorists about how much we really understand regarding the cores of active galaxies like M87," adds Perlman, lead author of a paper published in the November 1, 2001 issue of the Astrophysical Journal Letters.

"In other, similar objects, we see large quantities of warm dust surrounding the nucleus, which we think may be associated with the flow of fuel toward the central black hole," says theorist Dr. Julian Krolik of the Johns Hopkins University, in response to this finding. "Now that we know M87 is missing this, we'll have to look for other ways that its activity may be powered."

The galaxy commonly known as M87 is a popular target for astronomers, and most famous for the high-energy jet of material that extends from its central region. Although the galaxy is 50 million light-years from Earth, it is one of the closest galaxies of its type and hence lends itself extremely well to detailed studies. Hubble Space Telescope observations have shown that at its heart is a black hole, containing the mass of about three billion stars compressed into a region approximately the size of our Solar system.

Such a massive black hole (sometimes called a super-massive black hole) at the center of a galaxy can spark a huge outpouring of energy, driven by gas and dust that the black hole slowly devours. The sheer amount of energy released by this process staggers even astronomers who have been studying these galaxies for years, as the energy released can outshine the billions of stars making up the entire galaxy! A galaxy with such extreme nuclear emission is called an active galactic nucleus (AGN). Ironically, a black hole in a galaxy often reveals its presence by the very bright and concentrated optical and infrared emission in the region around the black hole.

Astronomers have long postulated that the extreme infrared emission in the center of these active galaxies must be produced by a substantial doughnut-shaped torus of dusty material surrounding the black hole. The dusty torus absorbs high-energy radiation - from material that is heated to extremely high temperatures immediately before falling into the black hole - and re-emits it at infrared wavelengths.

Earlier, high-resolution optical images by the Hubble Space Telescope had revealed a disk of hot gas, rotating around the black hole, and thin, filamentary dust in M87's nuclear regions, but not massive obscuration, as hinted at in observations of other AGN.

To search for the torus, astronomers have used the latest in infrared detector technology to observe light that the human eye cannot see. Now, with these technologies on large telescopes like Gemini, it has become possible to peer into the nuclear cauldron at mid-infrared wavelengths with enough clarity to conclusively test for the existence of the torus.

The Gemini observations of M87 were about a factor of 10 times deeper than had previously been published by any other ground-based telescope at mid-infrared wavelengths.

Due to its small size, the torus region of an AGN has never before been resolved, so that its real shape and geometry are not fully understood. The combination of M87's extremely massive black hole and its proximity to Earth led theorists to predict that the torus could be seen at mid-infrared wavelengths at radii of between 0.3-3 arcseconds - within the grasp of the new telescopes such as Gemini North. To put this in perspective, one arcsecond corresponds to the angular size of a golf ball on the ground as seen by a passenger in a commercial airliner travelling at an altitude of 30,000 feet.

"Counter to our expectations, we did not see the torus structure nor could we detect bright thermal emission, which would have given away its presence." Perlman explains. "These data show that the torus in M87 is at least a thousand times fainter compared to its radio jet than in other well-known radio galaxies, where bright mid-infrared radiation has been observed."

Other co-authors on the paper are W. B. Sparks and J. A. Biretta (from the Space Telescope Science Institute), J. T. Radomski, C. Packham and R. K. Piña (from the University of Florida), and R. S. Fisher (from the Gemini Observatory).

The observations of M87 were made in May 2001 using the Gemini North Telescope and the University of Florida's OSCIR mid-infrared imager/spectrometer, which was built with funding from the National Science Foundation and NASA.

"Making mid-infrared observations such as these is an extremely challenging endeavor," says the University of Florida's Dr. Charles Telesco, who led the effort to build the instrument OSCIR that was used to make these observations.

"The power of observing in the mid-infrared is that we can peer through much of the gas and dust that obscures our view at shorter wavelength radiation," Telesco adds. "When combined with Gemini, OSCIR can do things that have never been possible before in the mid-infrared. I expect that this is just the beginning of many new discoveries made possible by these cutting edge technologies combined with Gemini."

In addition to the mid-infrared observations of the central regions of the galaxy, the new Gemini data also contains images of the galaxy's jet that help to paint a more complete picture of this particle accelerator, which extends for thousands of light years.

The Gemini images of the jet also complement recently released X-ray observations with the Chandra X-ray observatory and earlier optical observations with Hubble and radio observations by the VLA.