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

January 14, 2003 Poster at American Association of Physics Teachers Meeting, Austin, TX

 

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

Ngal

ro

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