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تسجيل مستخدم جديدThe Atomic Gas Mass of Green Pea Galaxies
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We have used the Arecibo Telescope and the Green Bank Telescope to carry out a deep search for H{sc i}~21,cm emission from a large sample of Green Pea galaxies, yielding 19 detections, and 21 upper limits on the H{sc i} mass. We obtain H{sc i} masses of $rm M_{HI} approx (4-300) times 10^8 , rm M_odot$ for the detections, with a median H{sc i} mass of $approx 2.6 times 10^9 , rm M_odot$; for the non-detections, the median $3sigma$ upper limit on the H{sc i} mass is $approx 5.5 times 10^8 , rm M_odot$. These are the first estimates of the atomic gas content of Green Pea galaxies. We find that the H{sc i}-to-stellar mass ratio in Green Peas is consistent with trends identified in star-forming galaxies in the local Universe. However, the median H{sc i} depletion timescale in Green Peas is $approx 0.6$~Gyr, an order of magnitude lower than that obtained in local star-forming galaxies. This implies that Green Peas consume their atomic gas on very short timescales. A significant fraction of the Green Peas of our sample lie $gtrsim 0.6$~dex ($2sigma$) above the local $rm M_{HI} - M_B$ relation, suggesting recent gas accretion. Further, $approx 30$% of the Green Peas are more than $pm 2sigma$ deviant from this relation, suggesting possible bimodality in the Green Pea population. We obtain a low H{sc i}~21,cm detection rate in the Green Peas with the highest O32~$equiv$~[O{sc iii}]$lambda$5007/[O{sc ii}]$lambda$3727 luminosity ratios, O32~$> 10$, consistent with the high expected Lyman-continuum leakage from these galaxies.
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Green Peas are a class of extreme star-forming galaxies at intermediate redshifts, originally discovered via color-selection using multi-filter, wide-field survey imaging data (Cardamone et al. 2009). They are commonly thought of as being analogs of
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Compact starburst galaxies are thought to include many or most of the galaxies from which substantial Lyman continuum emission can escape into the intergalactic medium. Li and Malkan (2018) used SDSS photometry to find a population of such starburst
galaxies at z~0.5. They were discovered by their extremely strong [OIII]4959+5007 emission lines, which produce a clearly detectable excess brightness in the i bandpass, compared with surrounding filters. We therefore used the HST/COS spectrograph to observe two of the newly discovered i-band excess galaxies around their Lyman limits. One has strongly detected continuum below its Lyman limit, corresponding to a relative escape fraction of ionizing photons of 20+/-2%. The other, which is less compact in UV imaging, has a 2-sigma upper limit to its Lyman escape fraction of <5%. Before the UV spectroscopy, the existing data could not distinguish these two galaxies. Although a sample of two is hardly sufficient for statistical analysis, it shows the possibility that some fraction of these strong [OIII] emitters as a class have ionizing photons escaping. The differences might be determined by the luck of our particular viewing geometry. Obtaining the HST spectroscopy, revealed that the Lyman-continuum emitting galaxy differs in having no central absorption in its prominent Ly{alpha} emission line profile. The other target, with no escaping Lyman continuum, shows the more common double-peaked Ly{alpha} emission.
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