The Cosmic Evolution of Fermi BL Lacertae Objects

TL;DR

This study analyzes a large sample of Fermi-detected BL Lac objects to understand their cosmic evolution, luminosity functions, and jet properties, revealing diverse evolutionary behaviors and supporting the blazar sequence.

Contribution

It provides the largest and most complete redshift and luminosity function analysis of BL Lacs, including their evolution and jet Lorentz factors, with implications for blazar models.

Findings

Most BL Lac classes show positive evolution peaking at z~1.2.

Low-luminosity HSP BL Lacs exhibit negative evolution, increasing at low redshift.

BL Lacs have an average Lorentz factor of about 6, with jets aligned within 10 degrees.

Abstract

Fermi has provided the largest sample of gamma-ray selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by it Fermi during its first year of operation. We have obtained redshift constraints for 206 out of the 211 BL Lacs in our sample making it the largest and most complete sample of BL Lacs available in the literature. We use this sample to determine the luminosity function of BL Lacs and its evolution with cosmic time. We find that for most BL Lac classes, the evolution is positive with a space density peaking at modest redshift (z~1.2). The low-luminosity, high-synchrotron peaked (HSP) BL Lacs are an exception, showing strong negative evolution, with number density increasing for z$\lesssim$0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end-state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so called `blazar sequence'. Finally, estimating the beaming corrections to the luminosity function, we find that BL Lacs have an average Lorentz factor of $\gamma=6.1^{+1.1}_{-0.8}$, and that most are seen within 10$^{\circ}$ of the jet axis.