Lyα excess in high-redshift radio galaxies: a signature of star formation★

Abstract

About 54 per cent of radio galaxies at z 3 and 8 per cent of radio galaxies at 2 z < 3 show unusually strong Lyα emission, compared with the general population of high-redshift (z 2) radio galaxies. These Lyα-excess objects (LAEs) show Lyα/He II values consistent with or above standard photoionization model predictions.We reject with confidence several scenarios to explain the unusual strength of Lyα in these objects: shocks, low nebular metallicities, high gas densities and absorption/scattering effects. We show that the most successful explanation is the presence of a young stellar population which provides the extra supply of ionizing photons required to explain the Lyα excess in at least the most extreme LAEs (probably in all of them). This interpretation is strongly supported by the tentative trend found by other authors for z 3 radio galaxies to show lower ultraviolet rest-frame polarization levels, or the dramatic increase in the detection rate at submm wavelengths of z > 2.5 radio galaxies. The enhanced star formation activity in LAEs could be a consequence of a recent merger which has triggered both the star formation and the active galactic nucleus/radio activities.The measurement of unusually high Lyα ratios in the extended gas of some high-redshift radio galaxies suggests that star formation activity occurs in spatial scales of tens of kpc.We argue that, although the fraction of LAEs may be incompletely determined, both at 2 z < 3 and at z 3, the much larger fraction of LAEs found at z 3 is a genuine redshift evolution and not due to selection effects. Therefore, our results suggest that the radio galaxy phenomenon is more often associated with a massive starburst at z > 3 than at z < 3.

UBIQUITOUS GIANT Lyα NEBULAE AROUND THE BRIGHTEST QUASARS AT z ∼ 3.5 REVEALED WITH MUSE∗

“…Nonetheless, the relative comparison between different objects is still informative. This figure clearly shows the main difference between radio-quiet and radio-loud systems: radio-quiet nebulae are narrower (500-700 km s −1 ) than radio-loud systems (FWHM > 1000 km s −1 ) in agreement with previous results (e.g., Villar-Martín et al 2007). There is only one radio-quiet nebula with broad-line emission, i.e., #6, and this is also the nebula that showed a clearly distinct SB profile (see Section 4.3).…”

“…high-redshift radio galaxies (HzRG) from Villar-Martín et al (2007). In all but three radio-quiet systems, we have detected neither C IV nor He II emission.…”

“…However, the much broader a Ly line profiles of the nebulae associated with these radio-loud sources (with a line full width half maximum FWHM > 1000 km s −1 ), the alignment between the extended a Ly emission and the radio-loud lobes, and the higher metallicities all suggest a different origin with respect to radio-quiet systems, e.g., outflows rather than intergalactic filaments, at least for the inner parts of the a Ly emission (Heckman et al 1991a;Villar-Martín et al 2003;Humphrey et al 2007, but see also Villar-Martín et al 2007).…”

“…These are typically the brightest UV lines after hydrogen a Ly for AGN-photoionized nebulae and their line ratios have traditionally been used to constrain the origin and physical properties of the emitting gas (see e.g., Dey et al 2005;Villar-Martín et al 2007, Humphrey et al 2008Prescott et al 2009;Scarlata et al 2009;Arrigoni Battaia et al 2015a. In particular, the strengths of He II and C IV with respect to a Ly may provide information on the ionization parameter, gas density and metallicity, and eventually on the mechanism responsible for the gas emission.…”

Lyα emitters with very large Lyα equivalent widths, EW0(Lyα) ≃ 200–400 Å, atz∼ 2

“…We obtain the 3σ lower limit of the flux ratio, fLyα/fCIV > 19.0, where fLyα and fCIV are the Lyα and Civ fluxes, respectively. The flux ratio is significantly larger than that for z ∼ 2 − 3 radio galaxies, fLyα/fCIV = 6.9 (Villar-Martín et al 2007). …”