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Extended far-ultraviolet emission in distant dwarf galaxies


Blue compact dwarf (BCD) galaxies are low-luminosity (absolute K-band magnitude, MK > −21 mag)1, metal-poor (1/50 ≤ Z/Z ≤ 1/2, where Z is the metallicity in terms of the solar metallicity Z)2, centrally concentrated3 galaxies with bright clumps of star formation4. Cosmological surface-brightness dimming5 and the small size of BCDs limit their detection at high redshifts, making their formation process difficult to observe. Observations of BCDs are needed at intermediate redshifts, where they are still young enough to show their formative stages, particularly in the outer regions where cosmic gas accretion should drive evolution. Here we report the observation of excess far-ultraviolet (FUV) emission in the outer regions of 11 BCDs in the GOODS South field at redshifts between 0.1 and 0.24, corresponding to look-back times of 1.3–2.8 billion years in standard cosmology. These observations were made by the Ultra-Violet Imaging Telescope6 on AstroSat7. For ten BCDs, the radial profiles of the intrinsic FUV emission, corrected for the instrument point spread function, have larger scale lengths than their optical counterparts observed with the Hubble Space Telescope. Such shallow FUV profiles suggest extended star formation in cosmically accreting disks. Clumpy structure in the FUV also suggests that the outer FUV disks are gravitationally unstable. Dynamical friction on the clumps drives them inwards at an average rate exceeding 106 solar masses per billion years.

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Fig. 1: Detection of XUV disks in two BCDs.
Fig. 2: One-dimensional surface-brightness profile fitting.
Fig. 3: Comparison of the star-formation properties of our BCDs with those of local galaxies.
Fig. 4: Relation between excess UV light and disk scale lengths.

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Data availability

The HST imaging data are available at and the 3D-HST catalogue is available at The HAWK-I Ks band data are available at the ESO Science Archive Facility ( The original level 1 far-UV data observed by UVIT/AstroSat are available for download from the ISSDC site at

Code availability

We have used standard data reduction tools in Python, IRAF, and the publicly available code SExtractor ( and PROFILER ( for this study. We also have used the MPFIT routine translated to Python language here ( The pipeline used to process the Level 1 AstroSat/UVIT data can be downloaded from


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This paper uses the data from the AstroSat mission of the Indian Space Research Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC). The far- and near-UV observations were carried out by UVIT, which was built in collaboration between IIA, IUCAA, TIFR, ISRO and CSA. A.B. acknowledges the support of the DST-INSPIRE Fellowship programme by the Government of India. A.B. and R.G. thank the IUCAA associateship programme for their support and hospitality. K.S. and F.C. acknowledge the support of CEFIPRA-IFCPAR grant through the project number 5804-1 during its initial phase.

Author information

Authors and Affiliations



A.B. contributed to the data acquisition, analysis, figures and writing. K.S. conceptualized the overall project, reduced the UVIT raw data with the L2 pipeline, and contributed to analysis and writing. B.E. contributed to the analysis and writing. R.G., F.C. and S.N.T. participated actively in the scientific discussion and interpretation throughout the project, and provided critical inputs to the manuscript.

Corresponding authors

Correspondence to Anshuman Borgohain or Kanak Saha.

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Nature thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Imaging of BCD sample.

Same as Fig. 1 (a,b,d,e) but for the rest of the BCDs in our sample (see ED Fig. 1 for the rest of the BCDs). Individual panel sizes are 15”x15”, except GS12–18”x18”. We include the XUV disk type, if qualified, for each BCD in the lower left corner of each panel. We find a total of 6 sources (orange boxes) having S/N ≥ 3 and without any HST counterpart outside the XUV region of the BCDs. Colour bar label is flux in counts per second.

Extended Data Fig. 2 1D Surface brightness profile fitting of BCD sample.

Same as in Fig. 2 but for the rest of the BCDs in our sample (see ED Fig. 2 for the rest of the BCDs). In each panel, we include the XUV types and ΔLx values for each BCD. All data points have 1σ error bars.

Extended Data Fig. 3 Test for clump significance.

a. Patch of sky of size 341 × 341 pixels devoid of bright and large sources in the GOODS South field. Red circles denote sources having S/N ≥ 3 and their number is 49. Of these, 13 are marked with black circles that do not have any HST counterpart . b. S/N histogram of sources detected in the patch shown in a. Panel c, e GS3 and GS6 with the same size as presented in Fig. 1. Cyan circles and boxes mark the SExtractor-detected sources. Sources having S/N < 3 are marked with cyan circles. Sources having S/N ≥ 3 and HST counterparts are marked with cyan dashed squares. The orange square has S/N ≥ 3 and no HST counterpart. Magenta circles mark noise dips below a flux value of zero. Panel d, f represent flux (measured within a fixed aperture of size 0.7” radius) histograms corresponding to panel c, e.

Extended Data Table 1 Photometric and structural parameters of the BCDs
Extended Data Table 2 Clump detection parameters
Extended Data Table 3 S/N of SExtractor-detected sources around GS3 and GS6
Extended Data Table 4 Full FUV-clump analysis in our sample of BCDs
Extended Data Table 5 Various metrics derived for the XUV disks of the BCDs
Extended Data Table 6 XUV ages based on observed FUV – NUV colour and stellar population synthesis

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Borgohain, A., Saha, K., Elmegreen, B. et al. Extended far-ultraviolet emission in distant dwarf galaxies. Nature 607, 459–462 (2022).

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