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Galaxy Properties from the MPA-JHU Group


There are some special reduction results for galaxies, kindly provided by the MPA-JHU group, and available directly from them as well. The reductions are described below, and more fully in a set of papers (Brinchmann et al. 2004, Kauffmann et al. 2003 and Tremonti et al. 2004). These are the "galSpec" results and can be found in the following files, or in the corresponding CAS tables described on the spectroscopic data access page. The "galSpec" analyses have not been performed on the SDSS-III DR9 spectra.

These files are line-by-line matched with the specObj file. However, no SDSS-III spectra have been analyzed by galSpec. The galSpec pipelines have not been run on all data; if the PLATE, FIBERID, MJD values are -1 then there is no result for that spectrum. Furthermore, if RELIABLE in the galSpecInfo file is set to zero, then the parameters are not considered reliable.

The code has been run on previous SDSS data releases and the resulting measurements used for a variety of scientific applications. These data have been publicly available since DR4; we are making them accessible through the SDSS data release for the first time with DR8. We also refer to this set of line measurements as the MPA-JHU measurements, after the Max Planck Institute for Astrophysics and the Johns Hopkins University where the technique was developed. We provide MPA measurements for all objects that idlspec2d calls a galaxy in run2d=26 (used for the DR7 plates). This code has not been run on the new SEGUE-2 plates in run2d=103 and 104 or on any BOSS spectra. We briefly describe the technique here; details can be found in the papers referenced above.

As noted above, these files contain measurements of emission lines in galaxies, Lick and other indices, and derived quantities. The sections below describe the methods in more detail.

Galaxy emission lines

In measuring the nebular emission lines of galaxies, it is important to properly account for the galaxy continuum which is very rich in stellar absorption features. In DR8 we offer a set of emission line measurements for galaxy spectra which makes use of stellar population synthesis models to accurately fit and subtract the stellar continuum.

We first scale each galaxy spectrum to match its r-band fiber magnitude, and correct each spectrum for Galactic extinction following SFD and the O'Donnell (1994) attenuation curve. We adopt the basic assumption that any galaxy star formation history can be approximated as a sum of discrete bursts. Our library of template spectra is composed of single stellar population models generated using the population synthesis code of Bruzual & Charlot (2003). We have used a new version kindly made available by the authors which incorporates the MILES empirical spectral library (Sanchez-Blazquez et al. 2006; these spectra cover the range 3525-7500 Angstroms with 2.3 Angstrom FWHM). The spectral-type and metallicity coverage, flux-calibration accuracy, and number of stars in the library represent a substantial improvement over previous libraries. Our templates include models of ten different ages (0.005, 0.025, 0.1, 0.2, 0.6, 0.9, 1.4, 2.5, 5, 10 Gyr) and four metallicities (1/4, 1/2, 1, 2.4 solar). For each galaxy we transform the templates to the measured redshift and velocity dispersion and resample them to match the data. To construct the best-fitting model we perform a non-negative least squares fit to a linear combination of our ten single-age populations, with internal dust attenuation modeled as an additional free parameter following Charlot & Fall (2000). Given the S/N of the spectra, we model galaxies as single metallicity populations and select the metallicity that yields the minimum χ2.

After subtracting the best-fitting stellar population model of the continuum, we remove any remaining residuals (usually of order a few percent) with a sliding 150-pixel median, and fit all the nebular emission lines simultaneously as Gaussians. In doing so, we require that the Balmer lines (Hδ, Hγ, Hβ, and Hα) have the same line width and velocity offset, and likewise for the forbidden lines. We take into account the wavelength-dependent instrumental resolution of each fiber, which is measured by the idlspec2d pipeline from the arc lamp images.

Derived galaxy parameters

The files and tables above also include a number of galaxy parameters derived by the MPA-JHU group available.