Lyα -EMITTING GALAXIES AT 0.2 < z < 0.35 FROM GALEX SPECTROSCOPY

摘要

We have used the GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of ～8 A in the far-ultraviolet (FUV; 1350-1750 A) and ～20 A in the near-ultraviolet (NUV; 1950-2750 A) for a systematic search of Lyα-emitting galaxies at low redshift. Our aim is to fill a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in five deep exposures (5.65 deg~2) has resulted in 96 Lyα-emitting galaxy candidates in the FUV domain after accounting for broad-line AGNs. The Lyα equivalent widths (EWs) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, with the exception of a possible decrease in the most luminous objects that may be due to small-number statistics. The objects' distribution in EW is similar to that at z ～ 3, but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a subsample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Lyα luminosity function (LF). The incompleteness due to objects with significant Lyα emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with Ha LFs in the same redshift domain is consistent with an average Lyα/Hα of ～1 in about 15% of the star-forming galaxies. A comparison with high-redshift Lyα LFs implies an increase of the Lyα luminosity density by a factor of about 16 from z ～ 0.3 to z ～ 3. By comparison with the factor of 5 increase in the UV luminosity density in the same redshift range, this suggests an increase of the average Lyα escape fraction with redshift.