[Paper Review] The Cosmic Evolution of Fermi BL Lacertae Objects

M. Ajello, D. Gasparrini+15 more|Sep 30, 2013

Astrophysics and Cosmic Phenomena27 citations

TL;DR

This study presents the largest and most complete sample of Fermi-selected BL Lac objects (211 sources, 206 with redshifts) to date, using luminosity function modeling to reveal cosmic evolution patterns. It finds positive evolution for most BL Lac classes peaking at z ≈ 1.2, except high-synchrotron-peaked (HSP) BL Lacs, which show strong negative evolution peaking at z ≲ 0.5, suggesting they may represent an accretion-starved end-state of merger-driven evolution.

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 $γ=6.1^{+1.1}_{-0.8}$, and that most are seen within 10$^{\circ}$ of the jet axis.

Motivation & Objective

• To determine the cosmic evolution of Fermi-selected BL Lac objects using a uniformly selected sample.
• To measure the luminosity function (LF) of BL Lacs and its evolution with cosmic time.
• To investigate the relationship between BL Lac evolution and that of flat-spectrum radio quasars (FSRQs).
• To assess the impact of observational selection effects on the luminosity–spectral index correlation ('blazar sequence').
• To estimate beaming corrections and infer jet properties such as Lorentz factor and viewing angle.

Proposed method

• Used a uniformly selected sample of 211 Fermi-detected BL Lac objects from the first year of operation.
• Obtained redshift constraints for 206 sources, enabling robust luminosity function modeling.
• Applied both pure luminosity evolution (PLE) and luminosity-dependent density evolution (LDDE) models to the LF.
• Conducted Monte Carlo simulations to estimate parameter uncertainties and confidence intervals.
• Corrected for observational selection effects using simulated source distributions and flux limits.
• Estimated beaming corrections via comparison with isotropic luminosities and inferred jet Lorentz factors.

Experimental results

Research questions

• RQ1 How does the space density of BL Lac objects evolve with cosmic time across different synchrotron peak frequency classes?
• RQ2 What is the nature of the luminosity function of BL Lacs, and how does it evolve in luminosity and density?
• RQ3 Why do high-synchrotron-peaked (HSP) BL Lacs show negative evolution, and how does this compare to the evolution of FSRQs?
• RQ4 To what extent do observational selection effects affect the observed correlation between luminosity and photon spectral index?
• RQ5 What are the inferred jet properties (e.g., Lorentz factor, viewing angle) based on beaming corrections to the luminosity function?

Key findings

• Most BL Lac classes exhibit positive cosmic evolution, with space density peaking at redshift z ≈ 1.2.
• High-synchrotron-peaked (HSP) BL Lacs show strong negative evolution, with number density increasing at z ≲ 0.5, suggesting a population distinct from higher-redshift BL Lacs.
• The observed correlation between luminosity and photon spectral index persists after correcting for selection effects, supporting the existence of the 'blazar sequence'.
• The luminosity function implies an average Lorentz factor of γ = 6.1⁺¹.¹₋₀.⁸, indicating most BL Lacs are viewed within 10° of the jet axis.
• The drop in HSP BL Lac density at higher redshift correlates with a rise in FSRQ density, suggesting HSPs may represent an accretion-starved end-state of a merger-driven evolutionary phase.
• The LDDE model provides the best fit to the data, with best-fit parameters indicating a turnover in space density at z ≈ 1.4–1.6 for HSPs and lower redshifts for other classes.