ALFALFA-SDSS cross tablew            last update 12Jul2010 by martha

Stern warnings are meant to get your attention:
POINT #1:    These crosslists may be very useful to you but you didn't make them. 
             It has taken considerable time, effort and thought to generate them;
             guess who is responsible for that?
             You should treat them as intellectual property. 
             Do not pass them on to others or use it for non-ALFALFA-related
             work without permission. 
             And thank the person(s) who got you this far.
    
POINT #2.    As is the case with every large database, you need to understand
             the good and bad aspects of this crosslist. Both the ALFALFA 
             and SDSS datasets have their quirks. It is YOUR RESPONSIBILITY
             to understand them. If you find this crosslist faulty, then don't 
             use it. On the other hand, if you find errors that should be corrected
             discuss with Martha how best to go about doing that.

POINT #3     The AGC is a great database. But it is EXTREMELY inhomogeneous and
             incomplete in random ways. So, if you plan to use samples drawn from
             it, investigate how its quirks affect your data. In particular,
                 -- the accuracy of positions are of variable quality
                 -- it has a tendency to include more spirals than early types
                    especially for galaxies without redshifts or beyond 18,000 km/s
                 -- For small/faint objects most morphologies, mags and sizes are
                    ballpark only.

POINT #4     The SDSS is a great database. But the pipeline used to process SDSS images,
             is meant to be optimized for galaxies as the SDSS median redshift (z~0.11)
             NOT nearby objects. So there are serious issues with extended objects,
             especially ones with low surface brightness or clumpy structure (i.e. just
             the objects ALFALFA detects by the truckload). So watch out.
                  -- The PhotoObjs listed here are meant to be the brightest "child"
                     but that depends on whether the positional accuracy was good when
                     the match was made. There is also in many cases a "ParentObjID"
                     which could be a better magnitude (the sum of all the "children").
                     However, sometimes foreground stars get added in. So watch and
                     test for outliers!
                  -- We have tried to identify spectra which are either clearly bogus
                     or lie off-center from the galaxy but we haven't checked every
                     case. So likewise, watch out for bad stuff. Surely, there are
                     still some skeletons lurking about.


             These are works in progress. Always more to do. Never perfect. Caveat emptor.
             You get what you pay for.

Each cross list begins with a header which records its origin; we suggest that you do **not** remove 
what's there but add to it if you want. Here is an example

#   ALFALFA-SDSS cross table           GROUPsdss.cross100707.txt    last update:   07Jul2010 by Martha                         
#   ALFALFA catalog:    master100627.txt                (this is Martha's version)                                             
#   Documentation:      alfalfasdsscrossnotes.txt                                                                              
#AGCnr     HI source       OC position   I P       SDSSname           PhotObjID             SpecObjID                cz        


Columns are:
      AGCnr     
      HI source       From ALFALFA src/catalog file if applicable
      OC position     From ALFALFA src/catalog file if applicable
      ICODE           HI det code from src/catalog file
      PCODE           Code relevant to SDSS match    
                      U = no optical counterpart
                      J = PhotoID set
                      O = out of range
                      N = no photometric source identified (should probably be checked)
                          Can be due to bright star, satellite trails, incomplete coverage
                          or sometimes, no apparent reason
                      P = photometry suspect for some reason
                          Note: if set, someone has done it by intentionally by hand
                             presumably for some good reason.
                      D = clear displacement of Photo/SpectID from optical center
                          Note: if set, someone has done it intentionally by hand
                             PhotoID may be ok (often this is brightest child), but you 
                             should check. 
                      T = displacement of Photo vs SpectID, i.e. they do not coincide
                          but the spectrum probably arises from within (offcenter) the
                          galaxy.
                          Note: if set, someone has done it intentionally by hand;
                          cases like this need to be checked.  
                      S = redshift is of superposed star or QSO
                      B = redshift is bogus for some reason

                      Notice that the PCODE is set to O,N,P,D,T,S,B by hand. Not all objects have
                      been checked, so the setting of PCODE should not be considered complete or
                      uniform. It is only intended to record where a person has noted something.  
      SDSSname        J+position
      PhotObjID       This of the individual source (CHILD); use this to find the PARENTOBJID for 
                      photometric parameters
      SpecObjID       The spectroscopic target closest to the center of the galaxy    
      cz(SDSS)        cz from above; these don't always have required precision so don't use
                      this value.

Note: At this point, galaxies with HI code = 4,5 have not been checked. Do not use them!