A Galaxy Sample Free of Fiber Collisions


Photo of Zheng Zheng
Zheng Zheng
University of Utah


Galaxies that had previously been missed from CMASS and LOWZ target selection due to fiber collisions were observed as part of this ancillary target program

Finding Targets

Because the galaxy and quasar targets were identified in primary sample selection, no ancillary science target bit is defined for these targets. Instead, they can be identified by matching the collided galaxies in the SDSS DR9 spectroscopic catalog to the new spectroscopic catalog in DR12. A total of 1,282 targets were included in this program.


The finite size of the BOSS fiber ferrules means that no two fibers can be placed closer than 62″ apart on a given plate. These fiber collisions affect measurements of the small-scale clustering of galaxies chosen from the CMASS and LOWZM samples.

This program targeted CMASS and LOWZ galaxies that had not been observed in the main BOSS survey due to fiber collisions with other primary targets. Targets were added to ancillary plates 6373-6398 (in the North Galactic Cap), 6780-6782 (in Stripe 82), 6369, and 6717. Fibers were also assigned to CMASS and LOWZ targets that had suffered redshift failures (identified by ZWARNING_NOQSO = 0; Bolton et al. 2012) in previous observations in the data reduction pipeline.

Unlike other BOSS ancillary targets, these objects are not identified by flag values in ANCILLARY_TARGET2. Instead, they are identified with the CMASS or LOWZ target flags in the database.

This ancillary target program significantly increases the completeness of BOSS galaxy samples in the regions covered by these plates. It therefore provides a useful dataset for testing the fiber-collision correction methods currently used in BOSS clustering analyses (e.g. Guo et al. 2012).

Target Selection

Galaxies with failed spectra were identified in cases with ZWARNING NOQSO > 0 and where BOSS TARGET1 was satisfied by the 0th, 1st, 2nd, 3rd, or 7th target bit in the DR9 catalog. Because the target selection in Stripe 82 evolved after 2009, galaxies that were uniquely found in the BOSS1 chunk were excluded from re-observation.

The fiber-collided objects were selected if the TRIMMED = 0 bit was set and BOSSTILE_STATUS is not equal to 0 or 216, indicating that a redshift was known from SDSS-I or SDSS-II. The tile status was tracked in the tiling output file final-bossN.fits. This file records the final tiling results, provides a status flag for each object, and tracks which (if any) tile(s) an object was assigned to.

Additionally, that file provides information about the collision group to which a target had been assigned. The collision groups are friends-of-friends-linked groups with a length equal to the collision angle (normally 62″).


Bolton et al. 2012, AJ, 144, 144
Guo et al. 2012, ApJ, 756, 127