MaNGA Caveats

The Secondary sample was designed to have a higher density of targets than is required to give the desired 2:1 ratio of Primary+ to Secondary galaxies. Therefore, before allocating IFUs we randomly sample the Secondary sample to the desired target density. Due to a bug in the target selection code this random sampling is not truly random and in fact samples in such a way as to make the number density distribution flat as a function of stellar mass. This is a small change, since the density distribution was already quite close to being flat with stellar mass. However, it does mean the observed Secondary sample is no longer selected purely by i-band absolute magnitude and redshift, but also has a weak dependence on stellar mass. This has consequences for calculating the appropriate weights to apply to any sample containing the Secondary sample (see the weights FAQ).

For more information, see Wake et al. 2017 (AJ, 154, 86).

While MaNGA does not usually reobserve the same targets, the DR17 sample contains a number of galaxies that have been observed more than once, either deliberately for data quality assessment purposes or due to early issues with targeting and/or plate drilling. In almost all cases the duplicate observations have the same mangaid but different plate-ifu identifiers, and thus are easy to differentiate. Some galaxies, however, have been observed more than once but with different mangaids. The following table lists the galaxy data cubes with their two assigned mangaids:

PLATE-IFU	MANGAID	PLATE-IFU	MANGAID
9029-12702	1-135093	11941-1901	54-71
9872-3703	1-199287	7443-1902	12-49536
10504-9102	1-231891	10506-12702	1-605770
8625-9101	1-24179	11025-12704	54-11
9194-12702	1-289729	9193-12702	1-37822
9872-1901	1-322159	7443-1901	12-84620
9872-3702	1-322172	7443-3701	12-193534
8983-3703	1-458396	7495-12704	12-129618
8952-12703	1-458515	7495-12703	12-129646
11744-3702	54-80	10217-12703	59-80

So, what happened here? In some cases this was a targeting bug; galaxies with mangaid 12-XXX correspond to an early sample selection done using the NSA catalogue v1b_0_0_v2 and were originally intended only for a commissioning run in March 2014. Given that the data obtained for those 12- galaxies is of comparable quality to the main sample MaNGA galaxies, DR17 includes those galaxies but not as part of the main samples. Some of those galaxies were reobserved later as part of the final sample selection and were assigned new 1- mangaids (1- indicates the v1_0_1 version of the NSA catalogue). We therefore suggest omitting any galaxies with mangaid of the form 12- in any statistical population analyses.

In other cases an ancillary program deliberately re-observed a galaxy from the main sample again, potentially centering on a slightly different region of the galaxy (e.g., to reobserve a supernova host environment after a supernova in the original data had faded).

We also note that the above list was obtained using an RA/DEC cone search around each object; in addition to finding genuine duplicates however, such a search can also find very-close galaxy pairs, in which multiple galaxies fall within the IFU field of view and may have been observed once for each component with a slightly different centering. Such cases are often (although not always) indicated by one of the PAIR flags in MANGA_TARGET3.

Although the vast majority of cosmic rays and other transient features are detected by the DRP and flagged (either for removal or for masking), lower-intensity glitches are sometimes missed and can make it into the final datacubes where they show up as unmasked hot pixels.It is particularly important to be wary of this when searching for isolated emission features in the data cubes.

If you find a data cube in the DRP data products that does not have the associated DAP output files, there are two reasons why this may have happened.

• In the automated run of the DAP, any observation staged to be analyzed must satisfy the following criteria:
  • MANGAID is not 'NULL',
  • MANGA_TARGET1 > 0 or MANGA_TARGET3 > 0,
  • and cz > -500 km/s .

  These items basically mean that the target must be part of a main-survey or an ancillary target with a valid initial estimate of its cz redshift; this is allowed to be negative for some very local targets.

• The DAP data could be missing because the code faulted on a specific data cube; see below.

In summary: There are 11273 DRP-produced data cubes. Of these, 491 did not have an input redshift, and were therefore not analyzed by the DAP; most (if not all) of these are the result of allocating bundles to empty sky for special observations (such as the mosaic of IC342).

Of the 10782 cubes that the DAP attempted to analyze, 47 caused faults during the analysis. These failures are mostly because the S/N required for either the stellar kinematics or emission-line fit was insufficient for any spectrum in the datacube. Of these, two data cubes successfully finished the SPX-MILESHC-MASTARSSP analysis, but were unsuccessful for the other three types. The 41 cubes that failed all DAPTYPEs are:

8312-6101, 8479-6101, 8479-6102, 8479-12703, 8480-6102, 8480-12703,
8587-12701, 8587-12702, 8626-9102, 8953-6104, 9051-3704, 9051-6104,
9051-12704, 9673-1901, 9673-1902, 9673-3701, 9673-3702, 9673-3703,
9673-3704, 9673-6101, 9673-6103, 9673-6104, 9674-1901, 9674-1902,
9674-3701, 9674-3702, 9674-3703, 9674-3704, 9674-6101, 9674-6104,
9675-1901, 9675-1902, 9675-3701, 9675-3702, 9675-3703, 9675-3704,
9675-6101, 9675-6103, 9675-6104, 11828-1902, 11939-1901, 11949-1901,
11950-6102, 11987-1901, 11987-6104

The cubes that passed SPX-MILESHC-MASTARSSP, but failed the other two are:

8158-3703, 8953-9102

Errors are "formal" errors determined either by the covariance (inverse Hessian) matrix provided by the optimization algorithm (stellar and emission-line kinematics) or a direct propagation of the error based on the inverse variances provided by the DRP (as for the emission-line moments and spectral indices). Idealized experiments and analysis of repeat observations have shown that these formal errors are generally within a factor of two of the statistical error; see the DAP technical papers listed here. Specifically note, however, that the discrepancies with the statistical error become worse toward high S/N; this is effectively a propagation of the astrometric error to the error in, e.g., the flux of an emission line in a given data cube spaxel.

There are still severe deficiencies in the flagging, in general. Measurements can exhibit pixel-to-pixel variations that are inconsistent with random error (because of the strong covariance between neighboring spaxels) or fiber-level deviations that are inconsistent with physical intuition. These issues tend to occur at low S/N, however, they might not all be caught by a simple S/N cut. Please consider the limited robustness of the flagging for your science goals.

As mentioned above, the DRP will inevitably miss some cosmic rays. Occasionally cosmic rays will land on or near emission lines, which will play havoc with the emission-line fitter. Beyond possibly affecting the flux of the nearest line, it can also pull off all of the lines because the lines are forced to have the same velocity. An example of this is 8134-9102 where a cosmic ray lands on [SII]6718 for e.g., spaxel (j,i) = (22,33). The cosmic ray is slightly blueward of the [SII] line center and leads to all the remaining lines being too far to the blue with incorrect fluxes and dispersions.

Both the spectral indices and their velocity-dispersion corrections are measured using the input (NSA) redshift, not the fitted stellar velocity. Specifically for the velocity-dispersion corrections, this means that there will be a velocity dependence of the correction that can be more significant that the correction for the velocity dispersion itself.

See ReadTheDocs for a more detailed description of known issues and minor inconsistencies in the DAP data products.