Galaxy Zoo Starburst Talk

How can a spectrum clearly show H-alpha and H-beta in absorption, but ...

  • JeanTate by JeanTate

    ... the DR9 galSpecLine ("Emission line measurements (from MPA-JHU spectroscopic re-analysis) for this spectrum") says they're both in emission?

    I have come across many examples, among both the QS and QC objects.

    Here's a particularly obvious one, a QS object, AGS00001xa, SDSS J114159.39+265700.0 (DR7 ObjId 587741601491517466, SpecObjID 2500685054489946112):

    DR9 image: enter image description here

    enter image description here

    And two screen-shots from the interactive spectrum (the red line is "Best Fit"):

    enter image description here

    enter image description here

    The halpha_flux is given as 65.1764±8.19592, and the hbeta_flux as 0.063206±6.038076. Yes, "Warnings: MANY_OUTLIERS"; however, there are plenty of examples without such a warning.

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  • mlpeck by mlpeck

    I'll take a stab at this. The MPA pipeline makes a correction for stellar absorption in measuring emission line fluxes as described here, and their method is relatively sophisticated compared to the SDSS spectro pipeline. Just eyeballing the example spectrum you posted their flux estimates look fairly reasonable to me.

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  • JeanTate by JeanTate in response to mlpeck's comment.

    Thanks! 😃

    Here's what that webpage says, about 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.

    Two parts stand out for me:

    1. we transform the templates to the measured redshift and velocity dispersion
    2. with internal dust attenuation modeled as an additional free parameter following Charlot & Fall (2000)

    So spectra contaminated by foreground stars (or galaxies), or background galaxies should throw the models off. And if the velocity dispersion is poorly estimated, likewise.

    And there is an estimate of internal dust attenuation! 😄 (It's in galSpecIndx)

    I wonder if a member of Science Team could comment?

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  • JeanTate by JeanTate in response to JeanTate's comment.

    I wonder if a member of Science Team could comment?

    Nothing 😢

    I've just posted a new thread, on a similar topic: How to decide which objects' MPA-JHU derived parameters are unreliable because ...

    Posted