Stellar Densities in the Southern Pinwheel Galaxy Spring 2010 Symposium Tucson, Arizona Intern: Stevie Dunn Mentors: Rolf Jansen, Rogier Windhorst School of Earth & Space Exploration Arizona State University Image © Robert Gendler and Stephane Guisard 2008

How Do We Count Star Populations? Direct star count is not possible fainter stars are always more crowded and more difficult to discern than brighter, more sparsely distributed ones Synthetic star count is possible Isochrones are tracks in the Hertsprung-Russell diagram for a stellar population of a given age convert into a number of stars originally born per unit light in a given filter or wavelength interval sum all the different-age isochrones and fit to the actual amount of light emitted by that galaxy in each wavelength interval Extremely complicated, time consuming, way too many assumptions required, so instead… Goal: to use a function to describe star populations and integrate over the surface of the galaxy

Methods Stars in a galaxy have number density that differs with their distance from center A function can describe varying number densities We can integrate over the surface of the galaxy to find total number of stars in the galaxy Detecting number density with images of galaxy Assumption: brightness depends on stellar density Brightness ALSO can vary with luminosity (metallicity, star size, star color) Record and plot the brightness values Plot values for both perpendicular and parallel to bar Find the function to describe the graph Integrate over the entire galaxy to get total population

Database Development Use a program to view pixel values of images taken with various instruments NASA/IPAC Extragalactic Database Record values into database program Graph as a function Determine mathematical model of function Use polar integrals double integrals include both the radius and circumference of a circle If used with a density function, it gives the total population But we must relate pixel value to a density value

Mathematical Analysis Each pixel value represents about 430,000 stars Density function: Integrate over the 30,000 light-year width Totals 25,500,000,000 stars

Conclusions Gauge accuracy of the surface integral results The face-on barred spiral-galaxy M83 is about 30,000 light-years in diameter which makes M83 about half the size of the Milky Way Galaxy. Estimated 100 billion stars in Milky Way Galaxy Therefore, M83 must have roughly 50 billion stars; on an order of 1010 stars Future Work: fine-tuning the function that describes a stellar population Accounting for arms Translating overall brightness to stellar densities Luminosity factors: metallicity, colors, sizes, etc.

Thank You! Credit: ASU/NASA Space Grant Program Staff and Administration European Southern Observatory: www.eso.org NASA/IPAC Extragalactic Database: nedwww.ipac.caltech.edu Richard Reynolds – School of Mathematical & Statistical Analysis, College of Liberal Arts and Sciences, Arizona State University, Tempe AZ Rogier Windhorst - School of Earth & Space Exploration, College of Liberal Arts and Sciences, Arizona State University, Tempe AZ Rolf Jansen – School of Earth & Space Exploration, College of Liberal Arts and Sciences, Arizona State University, Tempe AZ SAOImage DS9 Version 6.0; Authors: William Joye (SAO), Eric Mandel (SAO), Steve Murray (SAO), John Roll (SAO)