We use integral-field spectroscopy from the SAMI Galaxy Survey to identify galaxies that show evidence of recent quenching of star formation. The galaxies exhibit strong Balmer absorption in the absence of ongoing star formation in more than 10% of their spectra within the SAMI field of view. These Hd-strong (HDS) galaxies (HDSGs) are rare, making up only ∼2% (25/1220) of galaxies with stellar mass log(MM)>10. The HDSGs make up a significant fraction of nonpassive cluster galaxies (15%; 17/115) and a smaller fraction (2.0%; 8/387) of the nonpassive population in low-density environments. The majority (9/17) of cluster HDSGs show evidence of star formation at their centers, with the HDS regions found in the outer parts of the galaxy. Conversely, the HDS signal is more evenly spread across the galaxy for the majority (6/8) of HDSGs in low-density environments and is often associated with emission lines that are not due to star formation. We investigate the location of the HDSGs in the clusters, finding that they are exclusively within 0.6R200 of the cluster center and have a significantly higher velocity dispersion relative to the cluster population. Comparing their distribution in projected phase space to those derived from cosmological simulations indicates that the cluster HDSGs are consistent with an infalling population that has entered the central 0.5r200,3D cluster region within the last ∼1 Gyr. In the eight of nine cluster HDSGs with central star formation, the extent of star formation is consistent with that expected of outside-in quenching by ram pressure stripping. Our results indicate that the cluster HDSGs are currently being quenched by ram pressure stripping on their first passage through the cluster.
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The above analysis provides support for ram pressure stripping in the HDSGs with central star formation. However, inspection of the Rstrip ellipses for the eight HDSGs with no evidence for ongoing star formation indicates that the predicted ram pressure at their current positions is insufficient to completely strip their gas. In this regard, it is important to note that the stripping radius estimates given by Equation (5) do not account for past stripping (e.g., if those galaxies have previously passed pericenter where they experienced peak ram pressure). Figure 16 shows that six of eight of the NSF HDSGs are located at larger projected distances and smaller LOS velocities when compared with the remaining HDSGs. We speculate that the majority of these galaxies have passed pericenter and are currently outbound toward apocenter. This interpretation is supported by the simulations presented in Figure 16, which shows that the expected position of the postpericenter, outbound satellites in the 0.6–1.0 Gyr panels is consistent with the positions of the NSF HDSGs. In this scenario, the strong ram pressure experienced during pericenter would have completely stripped the gas from the galaxies, leading to their completely quenched state.
We thank the anonymous referee for their comments that have helped to improve this paper. M.S.O. acknowledges the funding support from the Australian Research Council through a Future Fellowship (No. FT140100255). M.H. acknowledges support from an NSERC Discovery Grant, from the Australian Astronomical Observatory Distinguished Visitor Scheme and from the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) also as a Distinguished Visitor. K.O. received support from VICI grant No. 016.130.338 of the Netherlands Foundation for Scientific Research (NWO). J.B.H. is supported by an ARC Laureate Fellowship that funds Jesse van de Sande and an ARC Federation Fellowship that funded the SAMI prototype. S.B. acknowledges the funding support from the Australian Research Council through a Future Fellowship (No. FT140101166). J.J.B. acknowledges support of an Australian Research Council Future Fellowship (No. FT180100231). L.C. is the recipient of an Australian Research Council Future Fellowship (No. FT180100066) funded by the Australian Government. J.vd.S. is funded under Bland-Hawthorn’s ARC Laureate Fellowship (No. FL140100278). C.F. acknowledges funding provided by the Australian Research Council (Discovery Projects No. DP150104329 and No. DP170100603, and Future Fellowship No. FT180100495), and the Australia-Germany Joint Research Cooperation Scheme (UA-DAAD). B.G. is the recipient of an Australian Research Council Future Fellowship (No. FT140101202). Support for A.M.M. is provided by NASA through Hubble Fellowship grant No. HST-HF2-51377 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract No. NAS5-26555. N.S. acknowledges support of a University of Sydney Postdoctoral Research Fellowship. S.K.Y. acknowledges support from the Korean National Research Foundation (grant No. 2017R1A2A1A05001116) and by the Yonsei University Future Leading Research Initiative (grant No. 2015-22-0064). This study was performed under the umbrella of the joint collaboration between Yonsei University Observatory and the Korean Astronomy and Space Science Institute.
The SAMI Galaxy Survey is based on observations made at the Anglo-Australian Telescope. The Sydney-AAO Multi-object Integral-field spectrograph (SAMI) was developed jointly by the University of Sydney and the Australian Astronomical Observatory. The SAMI input catalog is based on data taken from the Sloan Digital Sky Survey, the GAMA Survey, and the VST/ATLAS Survey. The SAMI Galaxy Survey is supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project No. CE170100013, the Australian Research Council Centre of Excellence for All Sky Astrophysics (CAASTRO), through project No. CE110001020, and other participating institutions. The SAMI Galaxy Survey website is http://sami-survey.org/.
GAMA is a joint European-Australasian project based on a spectroscopic campaign using the Anglo-Australian Telescope. The GAMA input catalog is based on data taken from the Sloan Digital Sky Survey and the UKIRT Infrared Deep Sky Survey. Complementary imaging of the GAMA regions is being obtained by a number of independent survey programs, including GALEX MIS, VST KiDS, VISTA VIKING, WISE, Herschel-ATLAS, GMRT, and ASKAP providing UV to radio coverage. GAMA is funded by the STFC (UK), the ARC (Australia), the AAO, and the participating institutions. The GAMA website is http://www.gama-survey.org/. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID No. 177.A-3016. Based on data products (VST/ATLAS) from observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID No. 177.A-3011(AJ). This paper includes data that have been provided by AAO Data Central (datacentral.org.au). Facility: AAT (SAMI).
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science