Discovery of a Ly{\alpha} blob photo-ionised by a super-cluster of massive stars associated to a z = 3.49 galaxy

TL;DR

This study reports the discovery of a Lyα blob at z=3.49, likely photo-ionised by a nearby super-cluster of massive, young stars, with detailed analysis of its luminosity and spatial relationship to the galaxy.

Contribution

It provides the first detailed characterization of a Lyα blob associated with a super-cluster of massive stars at high redshift, including luminosity measurements and ionisation source analysis.

Findings

Lyα luminosity of the blob is 1.0x10^43 erg/s.

The galaxy's Lyα equivalent width is 111 Å (rest-frame).

The blob is spatially offset by 5.7 kpc from the galaxy.

Abstract

We report the discovery and characterisation of a Lya blob close to a galaxy at redshift z=3.49. We present our analysis to check whether the companion galaxy could be the source of the ionised photons responsible for the Lya emission from the blob. We use images obtained from the 10.4 m GTC telescope that are part of the SHARDS project. The blob is only visible in the F551W17 filter, centred around the Lya line at the redshift of the galaxy. We measure the luminosity of the blob with a two-step procedure. First, we describe the radial surface brightness (SB) profile of the galaxy using a Sersic function. We then remove this model from the SB profile of the blob and measure the luminosity of the blob alone. We also estimate the Lya continuum of the galaxy using an ACS image from the HST in the filter F606W, that is wider than the SHARDS one and centred at about the same wavelength. In this image the galaxy is visible, but the blob is not detected, since its Lya emission is diluted in the larger wavelength range of the F606W filter. We find that the Lya luminosity of the blob is 1.0x1043 erg s-1, in agreement with other Lya blobs reported in the literature. The luminosity of the galaxy in the same filter is 2.9x1042 erg s-1. The luminosity within the HST/ACS image is Lcont=1.1x1043 erg s-1. With these values we are able to estimate the Lya equivalent width (EW), that is found to be 111 {\AA} (rest-frame). This value suggests that a super-cluster of massive (1-2x107 Msun) and young (2-4 Myr) stars could be responsible for the ionisation of the blob. We also use two other methods to estimate the luminosity of the galaxy and the blob, both supporting our conclusions. It is worth noting that the Lya blob is spatially decoupled from the galaxy by 5.7 kpc. This misalignment could suggest the presence of an ionised cone of escaping material, as found in nearby galaxies such as M82.