Recovering the HII region size statistics from 21-cm tomography

Koki Kakiichi, Suman Majumdar, Garrelt Mellema, Benedetta Ciardi, Keri L. Dixon, Ilian T. Iliev, Vibor Jelić, Léon V.E. Koopmans, Saleem Zaroubi, Philipp Busch

Research output: Contribution to journalArticlepeer-review


We introduce a novel technique, called 'granulometry', to characterize and recover the mean size and the size distribution of HII regions from 21-cm tomography. The technique is easy to implement, but places the previously not very well-defined concept of morphology on a firm mathematical foundation. The size distribution of the cold spots in 21-cm tomography can be used as a direct tracer of the underlying probability distribution of HII region sizes. We explore the capability of the method using large-scale reionization simulations and mock observational data cubes while considering capabilities of SquareKilometreArray 1 (SKA1) lowand a future extension to SKA2. We show that the technique allows the recovery of the HII region size distribution with a moderate signal-to-noise ratio from wide-field imaging (SNR ≲ 3), for which the statistical uncertainty is sample variance dominated. We address the observational requirements on the angular resolution, the field of view, and the thermal noise limit for a successful measurement. To achieve a full scientific return from 21-cm tomography and to exploit a synergy with 21-cm power spectra, we suggest an observing strategy using widefield imaging (several tens of square degrees) by an interferometric mosaicking/multibeam observation with additional intermediate baselines (~2-4 km) in an SKA phase 2.

Original languageEnglish
Pages (from-to)1936-1954
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - 2017
Externally publishedYes

Bibliographical note

Funding Information:
KK acknowledge support from Richard Ellis and the European Research Council Advanced Grant FP7/669253. KK would also like to thank for the hospitality offered by the Department of Astronomy at Stockholm University during one visit, where this work was initiated. Some of the work was also conducted during the workshop supported by the Munich Institute for Astro-and Particle Physics (MIAPP) of the DFG cluster of excellence 'Origin and Structure of the Universe'. SM would like to acknowledge financial assistance from the European Research Council under ERC grant number 638743-FIRSTDAWN. We acknowledge PRACE for awarding us computational time under PRACE4LOFAR grants 2012061089 and 2014102339 and access to resource Curie based in France at CEA and to resource SuperMUC at LRZ. This work was supported by the Science and Technology Facilities Council (grant numbers ST/F002858/1 and ST/I000976/1); and the Southeast Physics Network (SEPNet).

Publisher Copyright:
© 2017 The Authors.


  • Cosmology: theory
  • Dark ages
  • First stars
  • Intergalactic medium
  • Methods: data analysis
  • Radiative transfer
  • Reionization
  • Techniques: image processing


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