Abstract
Context. One of the major science cases of the Multi Unit Spectroscopic Explorer (MUSE) integral field spectrograph is the detection of Lyman-alpha emitters at high redshifts. The on-going and planned deep fields observations will allow for one large sample of these sources. An efficient tool to perform blind detection of faint emitters in MUSE datacubes is a prerequisite of such an endeavor. Aims. Several line detection algorithms exist but their performance during the deepest MUSE exposures is hard to quantify, in particular with respect to their actual false detection rate, or purity. The aim of this work is to design and validate an algorithm that efficiently detects faint spatial-spectral emission signatures, while allowing for a stable false detection rate over the data cube and providing in the same time an automated and reliable estimation of the purity. Methods. The algorithm implements (i) a nuisance removal part based on a continuum subtraction combining a discrete cosine transform and an iterative principal component analysis, (ii) a detection part based on the local maxima of generalized likelihood ratio test statistics obtained for a set of spatial-spectral profiles of emission line emitters and (iii) a purity estimation part, where the proportion of true emission lines is estimated from the data itself: the distribution of the local maxima in the “noise only” configuration is estimated from that of the local minima. Results. Results on simulated data cubes providing ground truth show that the method reaches its aims in terms of purity and completeness. When applied to the deep 30 h exposure MUSE datacube in the Hubble Ultra Deep Field, the algorithms allows for the confirmed detection of 133 intermediate redshifts galaxies and 248 Lyα emitters, including 86 sources with no Hubble Space Telescope counterpart. Conclusions. The algorithm fulfills its aims in terms of detection power and reliability. It is consequently implemented as a Python package whose code and documentation are available on GitHub and readthedocs.
Highlights
Spectroscopic observations of galaxies at high redshift have recently been revolutionized by the Multi Unit Spectroscopic Explorer (MUSE) instrument in operation at the VLT (Very Large Telescope) since 2014
Its unique capabilities of providing three-dimensional (3D) deep field observations have been demonstrated in the early observations of the Hubble Deep Field-South (Bacon et al 2015) and more recently in the Hubble Ultra Deep Field (HUDF, Bacon et al 2017) and the CANDELS – GOOD South area (Urrutia et al 2019)
The most spectacular increase is at high redshift (z > 3), where MUSE was able to detect a large number of Lyα emitters
Summary
Spectroscopic observations of galaxies at high redshift have recently been revolutionized by the Multi Unit Spectroscopic Explorer (MUSE) instrument in operation at the VLT (Very Large Telescope) since 2014. For example in the HUDF, which achieves a 5σ depth of 29.5 in the F775W filter, MUSE was able to detect Lyα emitters without an HST counterpart (Bacon et al 2017; Maseda et al 2018). These observations have led to a breakthrough in our understanding of the high redshift Universe, which includes, for example, the discovery of Lyα emission from the circumgalactic medium around individual galaxies (Wisotzki et al 2016, 2018; Leclercq 2017) or the role of the low mass galaxies and the evolution of the faint-end Lyα luminosity function (Drake 2017)
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