Jacob Peter Fugal and Dr. J. Ward Moody, Physics and Astronomy
Cosmology has always attracted the minds of those who pursue the burning question of how the universe began. Since Edwin Hubble proved not only the existence of galaxies outside the Milky Way but also that the further away they are, the faster they are moving away, more and more research has given support to the “Big Bang” theory of the birth of the universe. This research includes the discovery of the cosmological microwave background radiation and the agreement of modeled and observed ratios of Hydrogen, Deuterium, Helium, and Lithium seen in primitive areas of galaxies.
Beyond the basics of the Big Bang Theory, there are processes not yet understood and parameters yet to be determined. Several burning questions at the focus of late observational research include the evolution of galaxies, their star formation histories, their distribution and clustering and the cause and mechanism of any unusual activity. Unusual activity is most often discovered and studied in “abnormally” strong emission lines (light emitted at a distinctive color or wavelength) present in a galaxy’s spectrum. Galaxies with these unusually strong emission lines are called Emission-Line Galaxies (ELGs), but are more often known by their subclass names such as quasars, Seyferts, radio galaxies or starburst galaxies (galaxies undergoing a “burst” of star formation). These galaxies are useful to study because they are easily identified by their spectra, and they hold unique information about the evolution and formation of galaxies.
Methods of obtaining data useful in answering cosmological questions include case studies and surveys. A case study on a galaxy or a particular cluster or class of galaxies offers in-depth detail about a particular galactic property but has no appreciable generality. The survey method is useful in finding statistically significant and general conclusions. Unless this sample of objects is homogeneous, uniformly selected, and well defined, the sample loses its generality and significance. Among the two methods, surveys are the method of choice to answer the broadest and most general cosmological questions.
Because of the importance of large uniform samples of galaxies to cosmological research, many galaxy surveys are in progress or have recently released galaxy catalogs. Surveys covering the visible region of the electromagnetic spectrum (the light spectrum from radio waves to visible to x rays) include the Sloan Digital Sky Survey (SDSS), the 2dF (2-degree Field) Galaxy Redshift Survey (2dFGRS), and the Deep Extragalactic Evolutionary Probe II survey (DEEP II). Other surveys include the Faint Images of the Radio Sky at Twenty-centimeters survey (FIRST) in the radio, and the 2 Micron All Sky-Survey (2MASS) in the near infrared (near to the visible light region). Each of the foregoing surveys have an imaging component where they take digital pictures of the sky. SDSS, 2dFGRS, and DEEP II also include a spectroscopic component which adds spectra of galaxies to their catalogs. The spectra are needed to determine the redshift (the lengthening or reddening of the wavelength of light due to the expanding universe) and hence the distance to the galaxy as well other properties of the galaxy including star formation history, stellar populations, gas cloud content and in the case of ELGs the presence, location and source of unusual galactic activity.
These surveys obtain their spectra by placing fiber optic cables at the focus of a telescope where the image of a galaxy lies then running the fibers to a spectrograph that measures each galaxy’s spectrum. The fiber optic cable acts as a “slit” in that it limits the light that goes to the spectrometer to a certain area of the galaxy. The slit has the advantage of limiting background light and noise as well as confining the source of the spectrum to the area of interest. However, smaller slits allow less light to enter and require longer times to get a good spectrum. The other limitation of this method is that it requires the knowledge of the location of every galaxy. Locations are found by imaging the sky first, then locating and selecting galaxies from the images and finally taking their spectra. It is critical that the selection process is well researched as it is what makes the sample well-defined, uniform and homogeneous.
Two other visible region surveys, the KPNO (Kitt Peak National Observatory) International Spectroscopic Survey (KISS) and the Palomar Transit Grism Survey (PTGS) each used a different method of gathering spectra. They placed a prism (with no slits or fiber optic cables) in front of the imaging camera which produced a coarse spectrum of every object in the frame. This allowed a direct method of selecting for emission-line objects, most of which were galaxies. There are a few downfalls of this spectroscopic method including increased background radiation, frame crowding due to too many objects in the frame, and contamination from stellar spectra or image glitches that make spectra appear to be ELGs. However, the advantages of this method are its ability to obtain spectra, distances to objects and strengths of emission lines faster and more efficiently than the fiber-optic method.
My project was to determine the feasibility of a slitless spectroscopic survey of ELGs in the near infrared. No large-scale, near-infrared spectroscopic survey has ever been attempted, nor has there been any large-scale ( > 104 ELGs) ELG survey. PTGS and KISS provided a proof of concept of the slitless spectroscopic method but obtained only a few thousand ELGs each. Surveying in the near infrared offers the advantage of being sensitive to objects which are further away. This is because farther galaxies have some of their prominent emission lines redshifted out of the visible region into the near infrared. Surveying deeper objects allow studies on the determination of galaxy evolution and cosmological parameters.
To determine the feasibility, I calculated the density of stars per square degree out to a given certain faintness limit from which I found that there were few enough stars that frame crowding was not a problem. I also determined how faint an ELG that a typical telescope (2 meter) could detect with a reasonable exposure time. At first, my advisor and I thought the limit we found would be faint enough to detect enough ELGs to justify the expense of the survey. However, my final step was to determine how many ELGs we would be able to detect at that limit. We found out that we would have to look significantly fainter to find any appreciable number of ELGs which would require a much larger telescope (30 meter), much more telescope time (decades) or perhaps a telescope in space. My conclusion is that such a survey would be cost prohibitive despite the scientific advantages it offers.
This finding is significant in showing that slitless spectroscopic surveys should not be extended into the near infrared without some advancement in imaging technology. KISS and PTGS have proven the utility of the slitless spectroscopic method in the red end of the visible region, and such surveys may be effective in the gap between the red and near-infrared regions.