MAPS Corrections

The DAP MAPS file provide values that must be corrected by the user using provided corrections. For MaNGA, it is recommended that you take advantage of the convenience methods in Marvin to apply these corrections. Regardless, the required corrections and how to apply them are described below.

Velocity-Dispersion Measurements

Note that the stellar and gas velocity dispersions must be corrected for instrumental resolution effects to obtain the astrophysical Doppler broadening.

The corrected gas velocity dispersion is:

\[\sigma_{\rm gas}^2 = \sigma_{\rm gas,obs}^2 - \sigma_{\rm inst}^2\]

where \(\sigma_{\rm gas,obs}\) and \(\sigma_{\rm inst}\) are provided in, respectively, the EMLINE_GSIGMA and EMLINE_INSTSIGMA extensions of the DAP MAPS file.

The corrected stellar velocity dispersion is:

\[\sigma_\ast^2 = \sigma_{\ast,{\rm obs}}^2 - \delta\sigma_{\rm inst}^2\]

where \(\sigma_{\ast,{\rm obs}}\) and \(\delta\sigma_{\rm inst}\) are provided in, respectively, the STELLAR_SIGMA and STELLAR_SIGMACORR extensions of the DAP MAPS file.

In both cases, beware of imaginary numbers. That is, when the correction is larger than the provided value, the above equations result in taking the sqrt of a negative number. Specifically for the stellar velocity dispersions, we recommend you consult Section 7.7 of Westfall et al. (2019, AJ, 158, 231) for some usage guidelines and discussion.

In particular, we have found that it is important to understand the error-convolved distribution of stellar-velocity-dispersion measurements when analyzing the data. For example, ignoring anything that has a converged pPXF fit, even at low S/N and low dispersion, will yield a biased determination of the mean (or median) dispersion as a function of radius. Further assessments of the reliability of the data is an ongoing process: Any additional assessments of the data along these lines from the collaboration is more than welcome.

Stellar velocity dispersions are currently provided for two approaches to the calculation: A nominal correction is calculated using the quadrature difference between the instrumental dispersion of the template and galaxy spectra over the fitted wavelength range. This is the correction provided in MPL-5, MPL-7/DR15. In MPL-8 and later, we also provide a correction based on a fit of the optimal template with and without the resolution matched to the MaNGA data. For now, use the correction in the first channel of the STELLAR_SIGMACORR extension until the data in the second channel can be vetted.

The Marvin method that applies these corrections is

Spectral-Index Measurements

Corrections that account for the effect of the velocity dispersion on the spectral indices are provided, as discussed in Section 10.1 of the Westfall et al. (2019, AJ, 158, 231). Unlike, e.g., the Firefly VAC, these corrections must be applied by the user. To apply the corrections, you have to know the unit of each index. For angstrom units (or for unitless bandhead/color indices):

\[\mathcal{I}^c = \mathcal{I}\ \delta\mathcal{I}\]

and for magnitude units:

\[\mathcal{I}^c = \mathcal{I} + \delta\mathcal{I}\]

where the raw index measurements, \(\mathcal{I}\), and the correction, \(\delta\mathcal{I}\) are provided in, respectively, the SPECINDEX and SPECINDEX_CORR extensions of the DAP MAPS file. Correction are identical for both index definitions, \({\mathcal I}_{\rm WT}\) and \({\mathcal I}_{\rm BF}\); see Spectral Indices. Corrections for the weights should only be applied when aggregating corrected indices, and the weight corrections are multiplicative:

\[w^c = w\ \delta w .\]

The Marvin method that applies these corrections is

Usage Example

A Usage example is included as our discussion of Getting started.