On the distance of PG 1553+11. A lineless BL Lac object active in the TeV band

TL;DR

This paper estimates the redshift of the BL Lac object PG 1553+11 using HST archival images, suggesting it is likely between 0.3 and 0.4, making it the most distant TeV-detected extragalactic source.

Contribution

It provides a novel lower limit estimate for PG 1553+11's redshift based on host galaxy brightness constraints, aiding understanding of its distance.

Findings

Redshift of PG 1553+11 is ≥ 0.25.

Likely redshift range is 0.3-0.4.

Object is the most distant TeV source detected.

Abstract

Context: The redshift of PG 1553+11, a bright BL Lac object (V~14), is still unknown. It has been recently observed in the TeV band, a fact that offers an upper limit for the redshift z<0.4. Aims: We intend to provide a lower limit for the distance of the object. Methods: We used a chi^2 procedure to constrain the apparent magnitude of the host galaxy in archived HST images. Supposing that the host galaxy is typical of BL Lac objects (M_{R} -22.8), a lower limit to the distance can be obtained from the limit on the apparent magnitude of the host galaxy. Results: Using the 3 sigma limit on the host galaxy magnitude, the redshift is found to be greater or equal to 0.25. Conlusions: The redshift of PG 1553+11 is probably in the range z=0.3-0.4, making this object the most distant extragalactic source so far detected in the TeV energies. We suggest that other bright BL Lac objects of unknown redshift and similar spectroscopic characteristics may be interesting targets for TeV observations.