Details regarding the updated just posted in the description of this RFC.
The DM System Science Team proposes to add to the DIAObject table a total of four new elements that will provide more scientifically useful associations for transients and their potential host galaxies.
Currently, the two DIAObject table elements that are relevant to the association of transients with their potential host galaxies are:
- nearbyObj (unit64) “Closest Objects (3 stars and 3 galaxies) in DR database.”
- nearbyObjDist (float) “Distances to nearbyObj.” [arcsec]
Scientifically, this is inadequate for two main reasons:
- The radial distance in arcsec matters less than a separation distance which accounts for the galaxies’ spatial luminosity profile (e.g., Sullivan+06).
- Transients in the local volume might not be associated with their large extended host galaxies, especially those in the outskirts of their hosts.
To relieve the first issue, the following two elements should be computed during Alert Production and added to the DIAObject table:
- potentialHost (unit64) – the objectId for the three Object catalog extended sources with the lowest separation distances
- potentialHostSeparation (float) – the separation distances for each potentialHost
The separation distance should be calculated with respect to the transient location using the second moments of each Object's luminosity profile, as described in DMTN-151. All parameters required for the calculation of the separation distances are already planned to be in the DIAObject and Object tables.
To relieve the second issue, DM will adopt an established catalog of low-redshift galaxies (e.g., from the Revised New General Catalogue and Index Catalogue; TBD) and do a cross match during Alert Production in order to report the name and the radial sky separation of the nearest catalog galaxy:
- potentialLowzHost (str) – the catalog name of the nearest “low-z” potential host
- potentialLowzHostSeparation (float) – the distance to potentialLowzHost [arcsec]
If the community provides a catalog of names and coordinates that they would prefer to be used, it will be adopted. If that catalog includes more information about the galaxy then the value of potentialLowzHostSeparation could be more sophisticated than the radial distance in arcseconds. For example, if distances are included then the offset in kpc could be provided; if sizes are included the number of effective radii could be reported; or if the second moments are included then the separation could be the same as used for potentialHostSeparation.