2017A&A...606A..15C

Context. Ultra-compact structure in radio sources (especially in quasars that can be observed up to very high redshifts), with milliarcsecond angular sizes measured by very-long-baseline interferometry (VLBI), is becoming an important astrophysical tool for probing both cosmology and the physical properties of AGN.
Aims. We present a newly compiled data set of 120-milliarcsec. compact radio sources representing intermediate-luminosity quasars covering the redshift range 0.46<z<2.76 and check the possibility of using these sources as independent cosmological probes. These quasars observed at 2.29GHz show negligible dependence on redshifts and intrinsic luminosity, and thus represent a fixed comoving-length of standard ruler.
Methods. For a cosmological ruler with intrinsic length l_m, the angular size-redshift relation can be written as θ(z)=l_m/D_A(z, where θ(z) is the angular size at redshift z, and D_A(z) is the corresponding angular diameter distance. We use a compilation of angular size and redshift data for ultra-compact radio sources from a well-known VLBI survey, and implement a new cosmology-independent technique to calibrate the linear size of this standard ruler, which is also used to test different cosmological models with and without the flat universe assumption.
Results. We determine the linear size of this standard ruler as l_m=11.03±0.25pc, which is the typical radius at which AGN jets become opaque at the observed frequency ν∼2GHz. Our measurement of this linear size is also consistent with the previous and recent radio observations at other different frequencies. In the framework of flat ΛCDM model, we find a high value of the matter density parameter, Ω_m=0.322^+0.244_–0.141, and a low value of the Hubble constant, H₀=67.6^+7.8_–7.4km/s/Mpc, which is in excellent agreement with the cosmic microwave background (CMB) anisotropy measurements by Planck. We obtain Ω_m=0.309^+0.215_–0.151, w=-0.970^+0.500_–1.730 at 68.3% CL for the constant w of a dynamical dark-energy model, which demonstrates no significant deviation from the concordance ΛCDM model. Consistent fitting results are also obtained for other cosmological models explaining the cosmic acceleration, like Ricci dark energy (RDE) or the Dvali-Gabadadze-Porrati (DGP) brane-world scenario. While no significant change in w with redshift is detected, there is still considerable room for evolution in w and the transition redshift at which w departing from -1 is located at z∼2.0. Our results demonstrate that the method extensively investigated in our work on observational radio quasar data can be used to effectively derive cosmological information. Finally, we find the combination of high-redshift quasars and low-redshift clusters may provide an important source of angular diameter distances, considering the redshift coverage of these two astrophysical probes.