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Optical spectroscopy of high redshift BL Lac objects

Renato Falomo, Aldo Treves, Simona Paiano, Riccardo Scarpa

Abstract

BL Lac objects (BLL) are defined by the presence of very weak (typically $<$ 5 Å) or even absent spectral lines. This makes determining their distance particularly challenging, especially at high redshift, where the sources are fainter and the host galaxy contribution in the optical band becomes negligible. Yet measuring their distance is crucial for deriving and modelling their luminosity, notably in the gamma-ray band, where BLLs dominate the extragalactic sky. In this work, we re-examine the reported high-redshift (z $>$ 0.6) BLL, many of which are commonly cited in the literature despite appearing questionable. We present new spectra for 52 objects obtained with the 10.4 m GTC. For 16 of them we propose a new redshift, or provide a spectroscopic lower limit, while for 14 sources we confirm previously published values. In 22 cases the spectra remain featureless, even with high S/N observations. These objects are likely to lie at 0.3 $<$ z $<$ 1.4 : the lack of host-galaxy features sets a lower limit to their distance, while the absence of intervening absorption systems argues against substantially higher redshifts. We compare our findings with the previous robustly established cases of BLLs at z $>$ 0.6 that meet our selection criteria.

Optical spectroscopy of high redshift BL Lac objects

Abstract

BL Lac objects (BLL) are defined by the presence of very weak (typically 5 Å) or even absent spectral lines. This makes determining their distance particularly challenging, especially at high redshift, where the sources are fainter and the host galaxy contribution in the optical band becomes negligible. Yet measuring their distance is crucial for deriving and modelling their luminosity, notably in the gamma-ray band, where BLLs dominate the extragalactic sky. In this work, we re-examine the reported high-redshift (z 0.6) BLL, many of which are commonly cited in the literature despite appearing questionable. We present new spectra for 52 objects obtained with the 10.4 m GTC. For 16 of them we propose a new redshift, or provide a spectroscopic lower limit, while for 14 sources we confirm previously published values. In 22 cases the spectra remain featureless, even with high S/N observations. These objects are likely to lie at 0.3 z 1.4 : the lack of host-galaxy features sets a lower limit to their distance, while the absence of intervening absorption systems argues against substantially higher redshifts. We compare our findings with the previous robustly established cases of BLLs at z 0.6 that meet our selection criteria.

Paper Structure

This paper contains 14 sections, 15 figures, 5 tables.

Table of Contents

  1. Introduction
  2. The sample
  3. Observations
  4. Results
  5. Redshift and spectral features
  6. The continuum spectral shape
  7. Discussion
  8. Optical spectra
  9. Notes to individual sources
  10. Extra targets
  11. 5BZBJ1107+5010
  12. 4FGL J1824.1+5651
  13. 4FGL2134.2-0154
  14. Tables

Figures (15)

  • Figure 1: Distribution of the redshift and the spectroscopic lower limits from the distribution of intervening systems of BL Lac objects at z $>$ 0.6. See Table \ref{['tab:results']} for details.
  • Figure 1: GTC optical spectra of high z blazar candidates (see details in Table \ref{['tab:targets']} and Table \ref{['tab:results']} and notes in Appendix \ref{['sec:notes']}). The redshift of featureless spectra is marked by an asterix. Flux units :10$^{-16}$ erg cm$^2$ s$^{-1}$ Å$^{-1}$
  • Figure 1: Continued.
  • Figure 1: Continued.
  • Figure 1: Continued.
  • ...and 10 more figures