Compact Radio Sources with the Steepest Spectra

We used the 1.4 GHz NRAO VLA Sky Survey (NVSS) and 365 MHz Texas survey catalogs to select relatively compact (θ ≤ 25'') radio sources having exceptionally steep spectral indices α ≥ 1.5 (where the flux density S_ν at frequency ν scales as S_ν ∝ ν^-α). Our sample of 74 sources (of ~30,000 total sources common to both surveys) with S(365 MHz) ≥ 200 mJy in the 10 sr between δ = -35^°30' and δ = 71^°30' should be complete for spectra as steep as α = 3.2, but we found only four sources with α > 2.5. Most known sources with such steep spectra are pulsars (coherent radio sources), relic radio galaxies, and high-redshift radio galaxies (synchrotron sources with spectra steepened by radiation losses). In fact, six of our sources have been identified with known pulsars and one with a high-redshift (z = 3.79) radio galaxy. In order to find possible new populations of steep-spectrum sources and classify the rest, we made VLA images of our sources with 15 and 05 resolution at 1.4 and 5 GHz, respectively. There are 25 doubles, 24 multicomponent sources, 16 unresolved (θ ≲ 02) sources, and six known pulsars. Three sources have undetermined morphologies as they were not reobserved with the VLA. The smallest sources have exceptionally steep spectra for their angular sizes. New pulsars, if any, are likely to be of special interest since they are strong but were missed by pulse-detection surveys. Preliminary radio, infrared, and optical observations suggest that most are high-redshift extragalactic sources.