The BL Lac object 3C 66A is one of the most luminous extragalactic sources at TeV-rays (VHE, i.e. E >100 GeV). Since TeV-ray radiation is absorbed by the extragalactic background light (EBL), it is crucial to know the redshift of the source in order to reconstruct its original spectral energy distribution, as well as to constrain EBL models. However, the optical spectrum of this BL Lac is almost featureless, so a direct measurement of z is very difficult; in fact, the published redshift value for this source (z = 0.444) has been strongly questioned. Based on EBL absorption arguments, several constraints to its redshift, in the range 0.096 < z < 0.5, were proposed. Since these AGNs are hosted, typically, in early type galaxies that are members of groups or clusters, we have analysed spectro-photometrically the environment of 3C 66A, with the goal of finding the galaxy group hosting this blazar. This study was made using optical images of a 5.5x5.5 arcmin² field centred on the blazar, and spectra of 24 sources obtained with Gemini/GMOS-N multi-object spectroscopy. We found spectroscopic evidence of two galaxy groups along the blazar’s line of sight: one at z~0.020 and a second one at z~0.340. The first one is consistent with a known foreground structure, while the second group here presented has six spectroscopically confirmed members. Their location along a red sequence in the colour-magnitude diagram allows us to identify 34 additional candidate members of the more distant group. The blazar’s spectrum shows broad absorption features that we identify as arising in the intergalactic medium, thus allowing us to tentatively set a redshift lower limit at z_3C66A ≥ 0.33. As a consequence, we propose that 3C 66A is hosted in a galaxy that belongs to a cluster at z = 0.340.

SExtractor classifier (CLASS_STAR) as a function of the apparent magnitude i' of all the sources detected in the field of 3C 66A (green circles). CLASS_STAR = 1: point sources; CLASS_STAR = 0: extended sources. Galaxies detected in the field having FWHM > 8 pixels in the i' band (plus the blazar, the i'~14 object with CLASS_STAR ~ 1) are shown with red crosses.

GMOS i'-band image of the field centred on 3C 66A. Spectroscopically observed objects are labelled following their slit number and marked according to their redshift (see Table 1), with respect to the detected z = 0.340 group (see Section 3.1): triangles (z > 0.340), circles (z ~ 0.340), squares (z < 0.340), star shape (Galactic stars), and straight lines underneath (objects with unknown redshift). The large boxes mark galaxies G1 (slit #22) and G2 in Bowen et al.(1997). Contours show the projected density of candidate members of the z = 0.340 group (see text). The frame is 5.5 arcmin on a side, corresponding to 1.61 Mpc at z = 0.340.

Redshift distribution of observed sources in a field of radius 7 arcmin centred on 3C 66A. Each bar represents the total number of objects with spectroscopic redshifts, both within (red) and outside (grey) the field studied in this paper. The histogram shows three galaxy groupings, at z ~ 0.34, z ~ 0.067 and at z ~ 0.02 in the close (projected) environment of the blazar.

Optical spectra (B600_G5307 grating) of the six sources identified as group members at z ~ 0.34 within the field of 3C 66A, in units of counts per Å (no flux calibration was applied). Spectral features used to determine the redshifts of the sources are labelled. A few “emissions” are residuals from poorly corrected sky lines.

Colour-magnitude diagram (CMD) for detected galaxies in the field of 3C 66A (red crosses). Framed crosses within blue circles indicate the position of cluster members confirmed through spectroscopic redshifts (z = 0.34±0.001). The blue solid line shows the Virgo cluster CRS shifted to z = 0.34. Framed crosses within green circles indicate galaxies with spectroscopic redshifts z ~ 0.02. The green solid line shows the Virgo cluster CRS shifted to z = 0.02. Dashed lines show the adopted colour scatter in each case. The black solid line shows the Virgo cluster CRS shifted to z = 0.444.

Wavelength branches of 3C 66A’s spectrum. The branch obtained with the B600_G5307 grating (blue line) covers a spectral range between 3500 Å and 6500 Å, while the branch obtained with the R400_G5325 grating (red line) covers a spectral range between 6100 Å and 10,000 Å.

The linearised 3C 66A spectrum, compared with a Bluered synthetic spectrum of a stellar model atmosphere with T = 12 000K shifted within the range 0.27 <z< 0.33 (blue line). The Balmer sequence of the Hγ, Hβ and Hα absorption lines might be recognised to match the blazar spectrum. See text for a discussion.