# MAPS Corrections¶

The DAP MAPS file provide values that *must be corrected by
the user* using provided corrections. It’s recommended that you take
advantage of the convenience methods in Marvin to apply these
corrections.

## 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:

where \(\sigma_{\rm gas}\) 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:

where \(\sigma_{\rm obs}\) 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 marvin.tools.quantities.map.Map.inst_sigma_correction.

## 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):

and for magnitude units:

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:

The Marvin method that applies these corrections is marvin.tools.quantities.map.Map.specindex_correction.

## Usage Example¶

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