Astronomy Lab

9 Exploring Galaxies: Colors and Stellar Popula- tions

9.1 Background

In astronomy, color has a di!erent meaning than in everyday life. When we talk about colors in astronomy, we are looking at the di!erence in brightness between two di!erent parts of the spectrum. For the purposes of this lab, we will limit ourselves to visible colors. The standard filters that cover the visible light part of the spectrum are U, B, V, R, and I. The most standard color that is seen in astronomy is the B-V color. We measure the brightnesses in each filter in units of magnitude, then take the di!erence between those 2 magnitudes. For the B-V color, a color of 1.0 mag is extremely red and a color of 0.0 is extremely blue. When we use the terms red and blue, we mean that the color is towards the red and blue ends of the spectrum, respectively. This does not mean the galaxy would actually look blue or red to our eyes if our eyes were actually sensitive enough to detect the colors.

The purpose of determining galaxy colors is to try to ascertain the stellar populations present in the galaxy under question. Recall the spectral types: O, B, A, F, G, K and M. O stars are blue, have high masses, and correspondingly short lives. M stars are red, have low masses, and correspondingly long lives. Therefore, if a galaxy is more blue in color, then it has younger stars. We know blue galaxies have experienced recent star formation since the blue stars do not stay around for long (relatively speaking). Unfortunately, just because a galaxy is red does not mean it has older stars. Since the red stars can fuse hydrogen in their cores for such long time periods, it is impossible to say based on color alone if the stars present were recently formed or if they have been there for millions or billions of years. Still, looking at galaxy colors is useful in characterizing the properties of the di!erent galaxy types.

The Hubble classification scheme for galaxies uses three main classes: spiral, elliptical, and irregular. Spiral galaxies have spiral arms that can be seen when viewed at a certain angle (face-on). When spirals are viewed from another angle (edge-on), the spiral arms cannot be seen and the galaxies take on a disk-like appearance. Ellipticals generally look spherical in shape (like the centers of spirals), although they may also appear elongated in one direction. The irregulars have no definite shape, and may or may not have any discernible symmetry. While galaxies were first classified based on appearance alone, it turns out that these three types of galaxies di!er by more than just their general shapes.

In this lab, we will be using the Sloan Digital Sky Survey (SDSS) to determine the properties of certain galaxies. SDSS uses a di!erent filter system. The SDSS filters are u, g, r, i, and z. In order to determine a galaxy’s color, we will use the g and r filters.

15

9.2 Purpose

To determine the di!erences in stellar populations between the three main types of galaxies: spirals, ellipticals, and irregulars.

9.3 Procedure

For each galaxy listed in the table below, you will use the Sloan Digital Sky Survey to determine (based on appearance) what type of galaxy it is (spiral or elliptical), and to look up the g and r magnitudes. The table below has a space for you to record your observations of each galaxy’s appearance. You will need to calculate the g-r color (literally the di!erence between the g and r magnitudes) and answer the questions that follow.

To access each galaxy’s information on the SDSS, we will make use of a tool called NED. Go to the website http://nedwww.ipac.caltech.edu/. On the left hand side of the page, there is a column labeled Objects, under which you will find a link for By Name. Click on this link. When this page opens, there should be a text box where you can enter an object’s name. For each object listed in the table, you will enter the name (e.g. ngc 201) and hit enter. The page that this pulls up will have various information about the galaxy. Scroll toward the bottom of the page until you see the title External Archives and Services (the box will be yellowish). Click on the link to Query the SDSS Server. This will open a new window, where the SDSS information on this object will be displayed. In this window, read o! the g and r magnitudes and record in the table below. Also, make note of the galaxy’s appearance and record your observations in the table. Indicate whether the galaxy is spiral (S) or elliptical (E). You also need to compute the g-r color, and record that in the table.

Once you have completely filled in the information for each galaxy in the table below, answer the questions that follow.

16

T a b le

1 .

G a la

x y

A p p ea

ra n ce

N o te

s T

y p e

(E o r

S )

g r

g -r

N G

C 2 0 1

N G

C 1 6 3

N G

C 2 1 9

N G

C 4 2 6

N G

C 2 2 3

N G

C 2 3 4

N G

C 2 3 7

N G

C 4 3 0

N G

C 5 4 8

N G

C 2 9 1

17

(1) Which type of galaxy (spiral or elliptical) appears to be bluer in color?

(2) Which type of galaxy is likely to have undergone recent star formation? How did you determine this?

(3) In which type of galaxy will you most likely find O type stars? How did you determine this?

9.4 References

The Sloan Digital Sky Survey can be explored online at their website www.sdss.org

NED can be accessed at the following address http://nedwww.ipac.caltech.edu/

18