How do I select a sample of galaxies from MaNGA?
MaNGA targets are split into main sample galaxies and ancillary targets. The main sample galaxies are further split into Primary, Secondary and Color-Enhanced selections. Details of these selections are given at the MaNGA target selection page, and their differences may make some samples more or less useful for your given science analysis.
How to select a sample
During the early phases of the observations we were still modifying the sample selection definitions and so there were several similar yet slightly different selections. The final selection is v1_2_0, which makes up the vast majority of observed targets. It is therefore advisable to restrict any analysis to v1_2_0 selected galaxies. These can be identified in MANGA_TARGET1 as PRIMARY_v1_2_0, SECONDARY_v1_2_0, and COLOR_ENHANCED_v1_2_0, bits 10, 11, 12 respectively.
Examples of how to select subsets of MaNGA galaxies using these bits as well as by other properties are shown at the MaNGA data cubes tutorial on SkyServer.
Important tips for sample selection
There are three particularly important tips to bare in mind when selecting and analyzing subsets of galaxies.
1) Never use the Color-Enhanced sample on its own
The Color-Enhanced sample is designed to add galaxies to the Primary sample in under-sampled regions of NUV-i vs Mi color-magnitude space, e.g. faint red galaxies, bright blue galaxies etc. As a result it does not fully sample the full NUV-i vs Mi plane and so used on its own is highly biased and unrepresentative of the full population in an un-correctable way. The Color-Enhanced sample is designed to be used in conjunction with the Primary sample to make the Primary+ sample.
2) Avoid selecting by IFU size
Since we have a range in IFU sizes, some targets will have more spatial resolution elements than others. As a result it may be tempting to restrict your analysis to those galaxies observed with the largest IFUs. However, doing so will introduce a strong bias into your sample. We have a range in IFU sizes to match the range in galaxy sizes at a given absolute magnitude. If you select only the larger IFUs you will be preferentially selecting lower surface brightness galaxies, and so more disks, more star formation etc.
3) Use Volume Weights
The selection methodology applied to the MaNGA sample means that weights must be applied to any statistical population analysis to correct for the selection function. The main MaNGA sample galaxies are selected to lie within a redshift range, zmin < z < zmax, that depends on absolute i-band magnitude (Mi) in the case of the Primary and Secondary samples, and absolute i-band magnitude and NUV-i color in the case of the Primary+ (Primary + Color-Enhanced (CE)) sample. zmin and zmax are chosen to yield both the same number density of galaxies and angular size distributions, matched to the IFU sizes, at all absolute i-band magnitudes (or magnitudes and colors for the Primary+ sample). This results in lower and narrower redshift ranges for less luminous galaxies and higher and wider redshift ranges for more luminous galaxies. At a given Mi (or Mi and NUV-i color for the Primary+ sample) the sample is effectively volume limited in that all galaxies with zmin(Mi) < z < zmax(Mi) are targeted irrespective of their other properties. However, that volume varies with Mi. Therefore in any analysis of the properties of MaNGA galaxies as a function of anything other than Mi we must correct for this varying selection volume. Details of how to calculate these weights are given in the MaNGA FAQ at How do I weight galaxies…?.
Wake et al. in press at AJ (arXiv:1707.02989)