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Example of a research project using the Virtual Observatory: the sodium absorption line in galaxies

In connection with a so-called research summer school about the Virtual Observatory in 2008 (organised by the US National Virtual Observatory, US NVO) Marcel Bergmann & Bo Milvang-Jensen carried out a research project that used different tools from the Virtual Observatory to study the sodium absorption line in galaxies. The project had the title: "Is the Na D Absorption Line Useful for Integrated Light Stellar Population Studies in Galaxies?"

The project is described below. In addition the electronic slides from the presentation of the project at the summer school are available (PDF or Powerpoint). The project was later presented at a meeting of the American Astronomical Society; abstract is available from the ADS: Bergmann & Milvang-Jensen (2009).

Abstract / summary of the project

The Sodium Na D absorption line at 5895 Angstroms is one of the strongest absorption features in stellar photospheres, but has been rarely used in integrated light stellar population studies of galaxies. A principal reason why it has not been used is the suspicion that interstellar absorption within the galaxies may enhance or alter the absorption profile of the combined stellar light, thus giving an errant description of the stellar population. As a project undertaken during the National Virtual Observatory Summer School, we have investigated to what extent ISM absorption seems to alter the measurements. We use VO tools to create multiple galaxy samples: a sample expected to have little ISM (cluster galaxies, which are mainly ellipticals), and two samples with higher expected levels of ISM (HI-detected galaxies and morphologically late-type galaxies). After culling the samples to match the same distribution of (older) ages and (higher) metallicities, we find that the Na D vs. velocity dispersion correlation is not significantly different for the samples with and without ISM, and all have similar levels of scatter. Consequently, the Na D line seems like a promising tool for evolutionary studies comparing high and low redshift galaxy samples.

Background and project design

Galaxies are full of stars, and sometimes gas and dust too. The optical spectrum of a galaxy is the composite sum of the emission from all its stars & hot gas, with possible absorption or extinction by its dust and gas. We are interested in deriving the star formation and chemical enrichment history of galaxies, and a common tool used is the study of stellar absorption lines (or line indices). The strongest and most often studied features are the Balmer lines, the Mgb triplet around 5170 Angstroms, and some Fe line features around 5200-5500 Angstroms. One very strong line which has not been much utilized is the Sodium Na D doublet at 5895 Angstroms. While this is a very strong stellar absorption line, it may also be absorbed (or emitted) by the ISM of the galaxy, and this potential contamination has kept investigators from using it for stellar population analysis work in the past. As part of the National Virtual Observatory (NVO) Summer School in September 2008, we designed a project to test whether this concern about ISM influence on the stellar Na D line strength was warranted. Using various NVO tools we constructed three galaxy samples: one composed of galaxies expected to have no ISM (cluster galaxies), and two samples of galaxies expected to have ISM (an HI selected sample, and a sample of late-type spirals). We would then compare correlations of Na D strength and other observables between the samples, to see if Na D is a useful diagnostic tool.

Definition of 3 samples of galaxies

All galaxies must have SDSS spectra with S/N > 35 and velocity dispersion 100 < sigma < 300 km/s (from DR4)
  1. Cluster Sample (expected to not have ISM): Choose all galaxies within 1 Mpc of the center of any cluster in the C4 (Miller et al. 2005) cluster catalog, using a cone search in the NVO and the STILTS tool. Total ~700 galaxies.
  2. Late type spiral sample: Select all galaxies with concentration index C > 0.4. C was derived in TOPCAT using the R50 & R90 radii from SDSS DR4 extracted with CASJobs. Total ~500 galaxies.
  3. HI sample: Select all galaxies with an HI detection in the FIRST survey, using openskyquery.net. Total ~700 galaxies.

Below is shown images from the SDSS of 5 examples from each of the 3 samples. From the top and down the 3 rows show samples 3, 2 and 1, respectively.

Examples of galaxy spectra

SDSS spectra of two galaxies from the sample, with the major spectral features marked. Note the strong NaD absorption feature at restframe 5895 Angstroms.

Plots comparing the 3 samples of galaxies

Plots showing all the galaxies in the 3 samples (each sample is shown in a different colour).
x-axis: log(age), where the age is in Gyr
y-axis: log(metallicity)

Plots showing only the old and metal rich galaxies in the 3 samples (each sample is shown in a different colour).
x-axis: log(age), where the age is in Gyr
y-axis: log(metallicity)

Plots showing all the galaxies in the 3 samples (each sample is shown in a different colour).
x-axis: velocity dispersion of the stars in the galaxy, in km/s
y-axis: strength of the sodium NaD absorption line

Plots showing only the old and metal rich galaxies in the 3 samples (each sample is shown in a different colour).
x-axis: velocity dispersion of the stars in the galaxy, in km/s
y-axis: strength of the sodium NaD absorption line

Results

The NaD absorption strength correlates well with velocity dispersion (which is a proxy for mass).

Above, left, the full samples are shown, and above, right, are just the old, metal rich galaxies. All three samples follow the same correlation. Restricting the age/metallicity reduces the scatter (as expected).

We conclude that for comparative studies of samples of galaxies (say, high redshift vs. low redshift), the NaD line is a useful indicator, not significantly affected by ISM absorption.

References and useful links

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Bo Milvang-Jensen 2008-2009. Page last updated: 25-Jun-2009