A methodology to select galaxies just after the quenching of star formation

Annalisa Citro, Lucia Pozzetti, Salvatore Quai, Michele Moresco, Livia Vallini, Andrea Cimatti

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


We propose a new methodology aimed at finding star-forming galaxies in the phase which immediately follows the star-formation (SF) quenching, based on the use of high- to lowionization emission line ratios. These ratios rapidly disappear after the SF halt, due to the softening of the UV ionizing radiation. We focus on [O III] λ5007/Hα and [Ne III] λ3869/[O II] λ3727, studying them with simulations obtained with the CLOUDY photoionization code. If a sharp quenching is assumed, we find that the two ratios are very sensitive tracers as they drop by a factor of ~ 10 within ~10 Myr from the interruption of the SF; instead, if a smoother and slower SF decline is assumed (i.e. an exponentially declining SF history with e-folding time τ = 200 Myr), they decrease by a factor of ~2 within ~80 Myr. We mitigate the ionization- metallicity degeneracy affecting our methodology using pairs of emission line ratios separately related to metallicity and ionization, adopting the [N II] λ6584/[O II] λ3727 ratio as metallicity diagnostic. Using a Sloan Digital Sky Survey galaxy sample, we identify 10 examples among the most extreme quenching candidates within the [O III] λ5007/Hα versus [N II] λ6584/[O II] λ3727 plane, characterized by low [O III] λ5007/Hα, faint [Ne III] λ3869, and by blue dust-corrected spectra and (u - r) colours, as expected if the SF quenching has occurred in the very recent past. Our results also suggest that the observed fractions of quenching candidates can be used to constrain the quenching mechanism at work and its time-scales.

Original languageEnglish (US)
Pages (from-to)3108-3124
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Aug 11 2017
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2017 The Authors.


  • Galaxies: ISM
  • Galaxies: evolution
  • ISM: H II regions
  • ISM: lines and bands


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