Is the optically unidentified radio source first J121839.7+295325 a dark lens?

R. E. Ryan, S. H. Cohen, Rogier Windhorst, C. R. Keeton, T. J. Veach

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


We present evidence that the optically unidentified radio source FIRST J121839.7+295325 may be strongly lensing a background galaxy. We estimate the redshift of the assumed gravitational arc, discovered in parallel imaging with HST, from MMT-Blue Channel spectroscopy to be zarc = 2.48 -0.05+0.14. We present lens models with an Einstein radius of RE = 1.3" which contains a mass of Mdyn = 10 12±0.5 M, where the uncertainty reflects the range of possible lens redshifts. The putative lens is not detected to J lim = 22.0 mag and Hlim = 20.7 mag in our MMT-SWIRC imaging. Using the flux limits from WFPC2 and SWIRC, we estimate that the dynamical mass-to-light ratio of J121839.7+295325 is Mdyn/L B gsim; 10 ML-1 for AV= 1 mag, and this lower limit could be as high as 30 M ̇L̇-1 for AV = 0 mag. Since the radio source is optically unidentified (Vlim = 25.5 mag) and has a radio flux of S1.4 GHz = 33 mJy, it is likely a massive early-type galaxy which hosts a radio-loud AGN at 0.8 ≲ zradio ≲ 1.5. However, the present data cannot uniquely determine the mass-to-light ratio of the lensing galaxy, and hence the possibility that this system may be a reasonably dark lens is not ruled out.

Original languageEnglish (US)
Pages (from-to)43-47
Number of pages5
JournalAstrophysical Journal
Issue number1
StatePublished - Nov 20 2008


  • Dark matter
  • Gravitational lensing

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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