PLOTTING
AND
ANALYZING GALACTIC DATA
AS
UNDERGRADUATE RESEARCH
Daniel M. Smith, Jr., South Carolina State University
Rayshad Ali*, South Carolina State University
Camille Denalli*, Morgan State University
Teon Wiles*, Virginia State University
*Students
I. INTRODUCTION
Astronomical data from the Sloan Digital Sky Survey (SDSS) are publicly available, affording undergraduates the opportunity to explore the frontiers of astronomy for themselves. We report on the use of the Early Data Release (EDR) from SDSS to (1) make a limited map of the large scale structure of the universe, and (2) calculate the two-point correlation function that characterizes the clustering of galaxies.
II. DATA ACQUISITION
The SDSS will collect data for a million galaxies in the northern sky at wavelengths 3540Å, 4770 Å, 6230 Å, 7630 Å, 9130 Å (designated u, g, r, i, and z). The EDR used in the work is 5% of the total. Cuts on the data, 14.5<r<17.6 and 0.019<z<0.13, result in a total of 21,707 galaxies for analysis. (The z range corresponds to a distance range of 85-567 Mpc, or 11-13 Gyr after the big bang.)
III. DATA ANALYSIS
A. Plots
Galaxies are sorted into elliptical or spiral catagories according to their u–r color (see below) and plotted using the software Mathematica. In two-dimensions
r = z
f = ra,
and in three-dimensions
x’ = zsin(90°— dec)cos(ra)
y’ = zsin(90°— dec)sin(ra)
z’ = zcos(90°— dec).
B. Correlation Functions
The correlation function is defined as
After calculating the comoving distance between every galaxy pair by using W _{m} = 0.3 and WL = 0.7, the correlation function is computed from
.
IV. RESULTS
10,610 spiral galaxies satisfying u—r < 2.2 in color
23,108 elliptical galaxies satisfying u—r > 2.2 in color. Clustering density is greater than for spiral galaxies but there are noticeably fewer elliptical galaxies at zero redshift.
Combination of previous two plots.
Ellipticals and spirals are separated according to spectral type for the correlation function analysis (but Zehavi's criterion is u—r color as in the above plots). Data are fit to
Sample |
N_{gal} |
r_{o} |
g |
A |
8099 |
6.2 |
1.94 |
B |
10667 |
7.5 |
2.47 |
Elliptical |
7653 |
17.5 |
1.23 |
Spiral |
3104 |
7.2 |
1.99 |
Zehavi Full |
29300 |
6.14 |
1.75 |
Zehavi Elliptical |
19603 |
6.78 |
1.86 |
Zehavi Spiral |
9532 |
4.02 |
1.41 |
Comparisons between the first four entries above (student analyses), and Zehavi's entries show that the results are comparable except in the case of elliptical galaxies.
Plot of correlation functions for sub-samples A and B
Plot of correlation functions for elliptical and spiral galaxies
V. SUMMARY
Undergraduate students can analyze SDSS data using off-the-shelf software, and obtain results that are of the same order of magnitude as the published results. A more careful analysis than that demonstrated here (completed within six weeks) should yield more accurate results. As all of the SDSS data become publicly available, the variety and sophistication of possible undergraduate projects shall become limitless.
VI. REFERENCES
http://skyserver.sdss.org/en/proj/
http://astro.uchicago.edu/%7Esubbarao/chautauqua.html
http://archive.stsci.edu/sdss/
Zehavi, et. al., astro-ph/0106476
Supported by NASA MU-SPIN Cooperative Agreement NCC 5-534