Astrometric properties of reference frame sources as a function of redshift

Abstract

Previous studies based on the latest realisation of the International Celestial Reference Frame (ICRF3) have suggested a correlation between astrometric properties (such as the radio-optical offset) and redshift for active galactic nuclei (AGNs). We extend these investigations by using a large, all-sky sample of approximately 22,000 compact radio sources from the Radio Fundamental catalogue (RFC) to examine this relationship in a systematic and statistically robust manner. We compiled redshifts for about 10,000 RFC sources over the range 0 < z < 5 by combining data from the Dark Energy Spectroscopic Instrument Data Release 1 and the Sloan Digital Sky Survey Data Release 17/19 with additional datasets from the NASA/IPAC Extragalactic Database. Cross-matching with Gaia Data Release 3 yielded a clean sample of 4,068 RFC objects with reliable spectroscopic redshifts and classifications (galaxies and quasi-stellar objects; QSOs). We analysed the redshift dependence of their radio astrometric properties from very long baseline interferometry (VLBI) and their optical astrometric properties from Gaia. We find that the VLBI astrometric properties show no significant dependence on redshift within the achieved level of precision. In contrast, several optical astrometric quantities exhibit clear redshift-dependent behaviour. The median absolute radio-optical offsets decrease markedly over 0 < z < 0.5, where galaxies dominate the sample, decline more gradually over 0.5 < z < 1.3, and exhibit a mild increase at z > 1.3, where QSOs dominate. Similar behaviour is observed for several Gaia astrometric quantities, including astrometric uncertainties, proper motions, and G magnitudes. These behaviours can be largely explained by the dependence of Gaia astrometric performance on G magnitude and by the evolution of the G magnitude with redshift.

Figures (13)

• Figure 1: Redshift distribution of objects in this work. Panel a: The grey histogram shows sources with spectroscopic redshifts from DESI/SDSS, the blue and red histograms show the distributions of DESI and SDSS redshifts, respectively. Panel b: The same sample as panel a separated by spectral classification: green for galaxies, orange for QSOs, and pink for sources classified as stars or lacking a reliable classification.
• Figure 2: Positional uncertainty derived from RFC VLBI observations (left) and Gaia optical measurements (right) as functions of redshift. For each panel, the orange data points indicate QSOs, the green data points indicate galaxies in the sample, and the grey data points indicate sources that are either unclassified or lack a reliable classification. The blue and red solid lines represents the median values of the galaxies and QSOs, respectively, whereas the purple line represents the median value of the total sample. In addition, to ensure the reliability of the results, median values are not shown for bins containing fewer than 20 data points.
• Figure 3: Absolute radio-optical offset (left) and relative radio-optical offset (right) as functions of redshift. The meanings of the colours, data points, and lines are the same as those in Fig. \ref{['fig:uncer']}.
• Figure 4: Parallax (left) and parallax uncertainty (right) as functions of redshift. The meanings of the colours, data points, and lines are the same as those in Fig. \ref{['fig:uncer']}.
• Figure 5: Proper motion (left) and proper motion uncertainty (right) as functions of redshift. The meanings of the colours, data points, and lines are the same as those in Fig. \ref{['fig:uncer']}.