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AstroSat detection of Lyman continuum emission from a z = 1.42 galaxy


One of the outstanding problems of current observational cosmology is to understand the nature of sources that produced the bulk of the ionizing radiation after the Cosmic Dark Age. Direct detection of these reionization sources1 is practically infeasible at high redshift (z) due to the steep decline of intergalactic medium transmission2,3. However, a number of low-z analogues emitting Lyman continuum at 900 Å restframe are now detected at z < 0.4 (refs. 4,5,6,7,8) and there are also detections in the range 2.5 < z < 3.5 (refs. 9,10,11,12,13,14). Here we report the detection of Lyman continuum emission with a high escape fraction (>20%) from a low-mass clumpy galaxy at z = 1.42, in the middle of the redshift range where no detection has been made before and near the peak of the cosmic star-formation history15. The observation was made in the Hubble Extreme Deep Field16 by the wide-field Ultraviolet Imaging Telescope17 onboard AstroSat18. This detection of extreme ultraviolet radiation from a distant galaxy at a restframe wavelength of 600 Å opens up a new window to constrain the shape of the ionization spectrum. Further observations with AstroSat should substantially increase the sample of Lyman-continuum-leaking galaxies at cosmic noon.

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Fig. 1: Detection of the source in the AstroSat.
Fig. 2: Emission-line mapping and clumps.
Fig. 3: Modelling of SED.
Fig. 4: Comparison of AUDFs01 with other confirmed LyC detection and IGM distribution.

Data availability

The HST data are available at and The VLT/ISAAC H- and Ks-band data are available at ESO Science Archive Facility ( The Spitzer GOODS-South data used in the analysis are available from The SDSS data are available at the Sloan Digital Sky Survey ( The MUSE spectroscopic data for AUDFs01 is available The other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

We have used standard data reduction tools in Python, IDL, IRAF and the publicly available code SExtractor ( for this study. For SED fitting and analysis, we have used publicly available code CIGALE (, EASY ( and BPASS ( The photoionization code CLOUDY used in this paper is in the public domain ( The pipeline used to process the level 1 AstroSat/UVIT data can be downloaded from


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The deep field imaging data in the FUV and NUV wavelengths are based on a proposed observation carried out by the AstroSat/UVIT, which was launched by the Indian Space Research Organization (ISRO). We thank ISRO for providing such observing facilities. K.S. and F.C. acknowledge the support of CEFIPRA-IFCPAR grant through the project number 5804-1. K.S. thanks D. Sobral for kindly providing the code to make in Extended Data Fig. 2d.

Author information




K.S. led the project, and the writing of the manuscript; reduction of the the UVIT data (pipeline, photometry, astrometry) and analysis, SED modelling, dust extinction and escape fraction calculation. S.N.T. contributed to the UVIT data reduction pipeline and interpretation of the photometry. C.S., A.V. and D.S. performed the BPASS modelling as well as interpretation of the result in terms of IGM distribution. A.V. and D.S. wrote the contamination hypothesis part. A.P. performed the spectral fitting of the grism data and emission-line mapping, BPT diagram. A.B. contributed to the FUV image analysis, noise estimation. A.K.I. performed Monte Carlo simulations of the IGM at the FUV band and at the redshift of the object. M.R., B.E., F.C. and D.E. participated actively in the scientific discussion, interpretation throughout the project and contributed to the final version of the manuscript. M.P. contributed in the MUSE analayis.

Corresponding author

Correspondence to Kanak Saha.

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Extended data

Extended Data Fig. 1 HST grism (G141) image.

a, Coloured rectangular regions (1,2,3,4) marked on the grism image are used to extract spectra for the clumps. North-East directions are marked on the grism image. b, 1D spectrum for the full galaxy. Red solid line represents the fitting of the spectrum. Redshift measurement is based on the fitting of Hα+[N II] line alone.

Extended Data Fig. 2 SF-AGN diagnostic diagram.

a, location of the clumpy galaxy AUDFs01 on the Hα - [O III] plane. The line fluxes are measured from HST grism G141 data20. AUDFs01 being the only galaxy having highest [O III] flux in the XDF region; the color bar indicates the stellar masses of the galaxies. b, c, Mass Excitation and BPT diagram using the SDSS galaxies. d, location of AUDFs01 on the Sobral et al. (2009) plot. The line ratios for all galaxies except AUDFs01 are taken from z-COSMOS survey57 at z ~ 0.84. The error bars represent 1σ uncertainties on the flux measurements.

Extended Data Fig. 3 Postage stamp images.

GALEX (FUV, NUV), HST (UV, Optical, IR), VLT/ISAAC (H, Ks), Spitzer/IRAC (3.4, 4.5 micron)-bands. The radius of the blue circle in each panel is 1.6”.

Extended Data Fig. 4 Distributions of LyC escape fractions.

a, For the first method, calculating the LyC escape fraction from the Hα luminosity following Eq. (8). b, Following Eq. (10), for the best-fit BPASS model with metallicity Z = 0.004, and an age of the stellar burst of ~ 4.5 × 106 years. Other BPASS models, varying ages, are shown with faded lines for comparison. On both panels, the vertical dashed line shows the value of the escape fraction assuming a transparent IGM (following Eq. (8) and Eq. (10)).

Extended Data Fig. 5 Emission-line fluxes and luminosities.

Col2: line flux as measured in the HST grism G141; [O ii] is from MUSE catalogue. col3: line fluxes after foreground dust plus internal extinction correction using Balmer decrement. Col4: same as col3 but internal extinction (E(B-V)=0.13) due to UV beta slope; the value of Hβ (marked bold-face in the bracket) is what would be expected as per the internal Balmer decrement given the measured Hα. Col5: line luminosity following UV beta slope.

Extended Data Fig. 6 SED fitting parameters for CIGALE and BPASS.

The best-fit parameters for cigale modelling are indicated by the bold-face letters. Dn4000 represents the ratio of the average flux density in two two narrow bands, 3850 − 3950 Å and 4000 − 4100 Å. IRX refers to the infrared excess.

Extended Data Fig. 7 Magnitudes of the galaxy AUDFs01 and its clumps.

Magnitudes of the galaxy AUDFs01 and its clumps at different passband. All magnitudes are aperture and foreground dust corrected.

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Saha, K., Tandon, S.N., Simmonds, C. et al. AstroSat detection of Lyman continuum emission from a z = 1.42 galaxy. Nat Astron 4, 1185–1194 (2020).

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