SDSS-III Observations of LOFAR Sources


Photo of Bob Nichol
Bob Nichol
University of Portsmouth


This ancillary program was intended to target radio sources identified in deep LOFAR observations of the ELAIS-N1 region

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers
ELAIS_N1_LOFAR 58 LOFAR source selected with RMS noise criteria 413
ELAIS_N1_FIRST 59 Source detected in the FIRST radio catalog 314
ELAIS_N1_GMRT_GARN 60 Source identified from deeper GMRT radio data 357
ELAIS_N1_GMRT_TAYLOR 61 Fainter source identified from deeper GMRT radio data 1,020
ELAIS_N1_JVLA 62 Very faint source detected in JVLA radio data 56


This ancillary program was intended to target radio sources identified in deep observations of the ELAIS-N1 region by by the Low Frequency Array (LOFAR; van Haarlem et al. 2013).

LOFAR observations were planned with the high-band antenna (HBA: 110-250 MHz) time for roughly ten hours over 9 deg2, to eventually reach a depth of 100 µJy at 150 MHz.

Spectroscopic confirmation of these sources will provide insight into the nature of the LOFAR radio population, and aid in the science exploitation of new radio surveys. The LOFAR ELAIS N1 region is well-studied by optical surveys and contains deep Jansky Very Large Array (JVLA) and Giant Metre-Wave Radio Telescope (GMRT) imaging data near the center of the field.

Target Selection

The LOFAR sample goes considerably deeper near the center of the spectroscopic field, concentrating targets there and making it impossible to assign sky fibers uniformly over the focal plane of the camera.

Instead, there were a large number of fiber bundles that did not contain a sky fiber, and the usual sky interpolation routine in the automated BOSS reductions could not be applied to the four plates designed for this program. For these four plates, the data reduction pipeline was modified to apply a constant sky model across each spectrograph (i.e. fibers 1-500 and 501-1000 respectively). This modification results in larger sky residuals than in typical calibrated BOSS spectra.

With this in mind, users of spectra obtained by this ancillary target program should treat the automated redshift classification and narrow emission line measurements with caution.

All LOFAR radio sources were matched to SDSS optical counterparts found within 2″ of the radio source position. The SDSS position was used for fiber placement. The target classes selected for this program are as follows:

  • ELAIS_N1_LOFAR: These targets were selected from a preliminary image of the ELAIS-N1 HBA data (115 to 190 MHz) that reached an rms noise level of 333 µJy. Approximately 800 sources were detected to a threshold of 1650 µJy, and an additional 400 sources were detected to a threshold of 1000 µJy. These sources are distributed over a field of radius approximately three degrees, for a total surface area of roughly 30 deg-2. In addition, 387 fainter LOFAR sources that could be clearly identified by eye in the ELAIS-N1 field were targeted.
  • ELAIS_N1_FIRST: These sources lacked a detection by LOFAR but appeared in the catalog of the Faint Images of the Radio Sky at Twenty cm (FIRST) survey (Becker, White, & Helfand 1995), and had an SDSS optical counterpart with rmodel < 23.0. Fibers were placed at the SDSS position.
  • ELAIS_N1_GMRT_GARN: These sources were identified from deeper GMRT data at 610 MHz (rms depth of 40-70 µJy) from the Garn, Green, Riley, & Alexander (2008) source catalog. These sources are expected to be dominated by AGN.
  • ELAIS_N1_GMRT_TAYLOR: These targets were also selected from GMRT data (Taylor et al. 2014), which are even deeper (rms depth of 10 µJy) than that used in the ELAIS_N1_GMRT_GARN target class. The deep GMRT radio catalog includes 2800 sources over 1.2 deg2. The positional accuracy from the radio data appears to be better than 0.5″.
  • ELAIS_N1_JVLA: These sources were also selected to be much fainter than the other samples. The deep JVLA radio catalog includes 483 sources over 0.13 deg2 at an angular resolution of 2.5″ and RMS noise of 1 µJy (Taylor et al. 2014). The positional accuracy is similar to the ELAIS_N1_GMRT_TAYLOR sample. Both this sample and the ELAIS_N1_GMRT_TAYLOR sample should include a significant fraction of star forming galaxies at z < 1.


    Becker, R.H., White, R.L., & Helfand, D.J., 1995, ApJ, 450, 559

    Garn, T., Green, D.A., Riley, J.M., & Alexander, P. 2008, MNRAS, 383, 75

    van Haarlem, M. P., et al. 2013, A&A, 556, 2

    Taylor, A. R., et al. 2014, arXiv:1405.0117