科研之友
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摘要
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 low-ionization 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]lambda 5007/H alpha and [Ne III]lambda 3869/[O II]lambda 3727, 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 similar to 10 within similar to 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 tau = 200 Myr), they decrease by a factor of similar to 2 within similar to 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]lambda 6584/[OII]lambda 3727 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]lambda 5007/H alpha versus [N II]lambda 6584/[O II]lambda 3727 plane, characterized by low [O III]lambda 5007/H alpha, faint [Ne III]lambda 3869, 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.
- 出版日期2017-8
