Problem set 5 for ASTR 323: The Local Universe

Due 5pm Friday November 17

(1) Find a paper which uses the Baade-Wesselink method to measure the distance to an open or globular cluster. Then find a paper which uses main-sequence fitting to obtain an independent estimate for the cluster distance. Compare them, and comment on possible reasons for any discrepancy. The authors of your papers may have done this already. You should write about two paragraphs.

(2) Sparke and Gallagher Problems 2.7 and 4.7 (there is a copy in the library: make sure you use the second edition)

(3) In this problem you will measure the 4000 Angstrom break and the strength of the H delta line in the spectrum of 5 galaxies observed by the SDSS.
The table below gives basic information on each galaxy: the plate, MJD and fiber identify its spectrum, the RA and Dec its image. NB: you need to include MJD (modified Julian date of the last observation of the galaxy with the SDSS spectrographs). If you leave this out you will find yourself trying to analyze the spectrum of a QSO. I have also given you the redshift of each galaxy and its total magnitude and g-r₀ color.

plate	fiber	mjd	ra	dec	redshift z	petroMag_g	gr0
266	545	51630	146.79909	0.70274154	0.03053254	15.61597	0.2200199
266	619	51630	147.02167	0.75447111	0.06200635	15.77394	0.9227763
266	174	51630	146.23184	0.06822915	0.02160817	15.24219	0.8749458
266	1	51602	146.71421	-1.0413043	0.02127545	15.6145	0.7124085
266	628	51602	147.19292	0.27191843	0.02018126	15.83588	0.3826969

(a) Using the SDSS Navigator feature you used in Homework 2, look at the image of each galaxy and print out a copy. Navigator gives a first-look spectrum, but you will need to use a more sophisticated tool to zoom in on the spectral features of interest.
Use the Basic Optical Spectra Search to get the spectrum, and turn off the model spectrum overlaid in red using a button named "Best Fit" on the left below the spectral plot.
In a few sentences, describe the spectrum of each galaxy, discussing absorption features and mentioning emission lines if they are visible.
Class notes on Spectral Synthesis include Figure 1 from Kauffmann et al (MNRAS 341, 33, 2003), which shows how the D_n4000 and H delta_A indices are calculated. You can click and drag to zoom in on the region of the spectrum you are interested in. Zoom to produce a spectrum from about 3700 to 4400 Angstroms, as shown in Kauffman's figure. Print this spectrum out.
(b) Calculate the amount that wavelengths of interest in the spectrum have been redshifted using the definition of redshift (Carroll and Ostlie p99). Check that this calculation is correct by looking up the rest wavelength of one of the features marked on the spectrum and comparing with your result. Give both values.
(c) Calculate D_n4000 and H delta_A for each spectrum at the galaxy's rest wavelengths for these features, giving a careful error estimate for each value. Use the Bruzual et al (83) continuum bands for the Dn(4000) estimate. The H delta_A index is a "pseudo-equivalent width: the continuum is measured using the average value in the red and blue side-bands, then the equavalent width is calculated, as explained here. Some of your values will be negative ..... explain how that happens. A prize will be given to the student who gives values which are closest to the truth and also have realistic error estimates.
For the galaxies with emission lines, you will need to account for their presence in the spectrum: what you should be measuring is the strength of H delta in the absorption spectrum from the stars in the galaxy. You may make this calculation using pencil and ruler or by using splot in IRAF. Graduate students should use splot. The fits files with the spectra can be downloaded here and here. If you use splot you should also make a rough sanity check with pencil and paper. Describe your measurement procedure, including the way you choose to compensate for the emission lines, in a few sentences.
(d) Plot each galaxy on the D_n4000 vs H delta_A plot from class notes (Figure 3 from Kauffmann et al 2003) and discuss whether its position on this plot makes sense with (i) the model lines from Kauffman et al's Figure 3 (ii) the galaxy's g-r₀ color and spectrum and (iii) the morphology that you see on the SDSS Navigate image.

(4) (Grad students only): Give two reasons why the very first stars to form might be more massive than current-day stars. Search the literature for a recent (last 20 years) observational paper on the metallicity distribution of the Milky Way halo at the low-metallicity end, and give a 1-2 paragraph summary of the paper's findings, discussing whether you think this convincingly demonstrates that there are no zero-metallicity low mass stars existing today.