The Bright Void Galaxy Population in the Optical and Mid-IR

University of Louisville ThinkIR: The University of Louisville's Institutional Repository Faculty Scholarship 11-2015 Galaxy And Mass Assembly (GAMA) : the bright void galaxy population in the optical and mid-IR. S. J. Penny University of Portsmouth M. J. I. Brown Monash University K. A. Pimbblet University of Hull M. E. Cluver The University of Western Cape D. J. Croton Swinburne University of Technology See next page for additional authors Follow this and additional works at: https://ir.library.louisville.edu/faculty Part of the Astrophysics and Astronomy Commons Original Publication Information Penny, S. J., et al. "Galaxy And Mass Assembly (GAMA): The Bright Void Galaxy Population in the Optical and Mid-IR." 2015. Monthly Notices of the Royal Astronomical Society 453(4): 3519-3539. This Article is brought to you for free and open access by ThinkIR: The University of Louisville's Institutional Repository. It has been accepted for inclusion in Faculty Scholarship by an authorized administrator of ThinkIR: The University of Louisville's Institutional Repository. For more information, please contact [email protected]. Authors S. J. Penny, M. J. I. Brown, K. A. Pimbblet, M. E. Cluver, D. J. Croton, M. S. Owers, R. Lange, M. Alpaslan, I. K. Baldry, J. Bland-Hawthorn, S. Brough, S. P. Driver, Benne W. Holwerda, A. M. Hopkins, T. H. Jarrett, D. Heath Jones, L. S. Kelvin, M. A. Lara-Lopez, J. Liske, Angel R. Lopez-Sanchez, J. Loveday, M. Meyer, P. Norberg, A. S. G. Robotham, and M. Rodrigues This article is available at ThinkIR: The University of Louisville's Institutional Repository: https://ir.library.louisville.edu/ faculty/187 MNRAS 453, 3519–3539 (2015) doi:10.1093/mnras/stv1926 Galaxy And Mass Assembly (GAMA): the bright void galaxy population in the optical and mid-IR S. J. Penny,1,2,3‹ M. J. I. Brown,1,2 K. A. Pimbblet,1,2,4 M. E. Cluver,5 D. J. Croton,6 M. S. Owers,7,8 R. Lange,9 M. Alpaslan,10 I. Baldry,11 J. Bland-Hawthorn,12 S. Brough,7 S. P. Driver,9,13 B. W. Holwerda,14 A. M. Hopkins,7 T. H. Jarrett,15 D. Heath Jones,8 L. S. Kelvin,16 M. A. Lara-Lopez,´ 17 J. Liske,18 A. R. Lopez-S´ anchez,´ 7,8 J. Loveday,19 M. Meyer,9 P. Norberg,20 A. S. G. Robotham9 and M. Rodrigues21 Affiliations are listed at the end of the paper Accepted 2015 August 18. Received 2015 August 4; in original form 2015 June 4 ABSTRACT We examine the properties of galaxies in the Galaxies and Mass Assembly (GAMA) survey located in voids with radii >10 h−1 Mpc. Utilizing the GAMA equatorial survey, 592 void galaxies are identified out to z ≈ 0.1 brighter than Mr =−18.4, our magnitude completeness limit. Using the WHα versus [N II]/Hα (WHAN) line strength diagnostic diagram, we classify their spectra as star forming, AGN, or dominated by old stellar populations. For objects more massive than 5 × 109 M, we identify a sample of 26 void galaxies with old stellar populations classed as passive and retired galaxies in the WHAN diagnostic diagram, else they lack any emission lines in their spectra. When matched to Wide-field Infrared Survey Explorer mid-IR photometry, these passive and retired galaxies exhibit a range of mid-IR colour, with a number of void galaxies exhibiting [4.6] − [12] colours inconsistent with completely quenched stellar populations, with a similar spread in colour seen for a randomly drawn non-void comparison sample. We hypothesize that a number of these galaxies host obscured star formation, else they are star forming outside of their central regions targeted for single-fibre spectroscopy. When matched to a randomly drawn sample of non-void galaxies, the void and non-void galaxies exhibit similar properties in terms of optical and mid-IR colour, morphology, and star formation activity, suggesting comparable mass assembly and quenching histories. A trend in mid-IR [4.6] − [12] colour is seen, such that both void and non-void galaxies with quenched/passive colours <1.5 typically have masses higher than 1010 M, where internally driven processes play an increasingly important role in galaxy evolution. Key words: galaxies: evolution – galaxies: general – infrared: galaxies. voids are not empty, and contain a sizeable galaxy population (e.g. 1 INTRODUCTION Rojas et al. 2004; Croton & Farrar 2008; Kreckel et al. 2012;Pan Redshift surveys reveal a remarkable amount of structure, with et al. 2012). Nevertheless, cold dark matter cosmology predicts the majority of galaxies located in groups, clusters, and along the more dark matter haloes in voids than we currently observe galaxies filaments linking these massive structures (e.g. Huchra et al. 1983; (Peebles 2001; Tikhonov & Klypin 2009; Peebles & Nusser 2010), York et al. 2000; Colless et al. 2001). However, it is the voids that and a better understanding the evolution of these galaxies will help represent most of the volume of the Universe. Spanning tens of us better understand this discrepancy. Void galaxies of all masses megaparsecs, these void regions are extremely underdense, with a are therefore ideal objects in which to examine intrinsic versus galaxy density less than 20 per cent of the cosmic mean. These extrinsic effects on galaxy evolution, having formed far from the nearest cluster mass dark matter halo. Galaxies can be broadly divided into two categories based on their E-mail: [email protected] optical colours: blue cloud galaxies with ongoing star formation, C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 3520 S. J. Penny et al. and red sequence galaxies with colours consistent with a non-star- de Weygaert 2004;Zitrin&Brosch2008;Alpaslanetal.2014), forming stellar population, though there is colour overlap between continually supplying them with the fuel for star formation, and these two populations (e.g. Taylor et al. 2015). Clearly, at some point increasing the chance of minor mergers. in their histories all galaxies were star forming – it is how and where Examining the void galaxy population is impossible without the quenching of star formation takes place that is one of the most deep, wide-field redshift surveys. Such surveys are vital not only to active areas of research today. These quenching mechanisms fall provide a statistically significant sample of void galaxies, but also to into two classes: intrinsic (driven by galaxy mass) versus extrinsic identify the void regions themselves via accurate positional and dis- (environmentally driven) evolution. tance measurements. In this paper, we examine void galaxies in the Galaxies follow a strong morphology–density relation, such that Galaxy and Mass Assembly (GAMA) survey (Driver et al. 2009), early-type galaxies with little or no star formation are primarily utilizing optical and infrared data. In addition to data at optical found in high-density regions of the Universe such as groups and wavelengths provided by GAMA and the Sloan Digital Sky Sur- clusters, while late-type galaxies dominate in low-density environ- vey (SDSS), we furthermore use mid-IR data to search the GAMA ments such as the field (Dressler 1980; Smith et al. 2005;Park void galaxy population for obscured star formation. Sensitive to et al. 2007). Similar environmental trends are seen for colour and the emission from dust-reprocessed star formation, data from the star formation, such that red galaxies with no star formation favour mid-IR is crucial in establishing if a population of truly passive void high-density environments (Lewis et al. 2002; Kauffmann et al. galaxies exists (intrinsic versus extrinsic evolution). 2004; Ball, Loveday & Brunner 2008; Bamford et al. 2009). This Uncovering the nature of mass quenching first requires obtaining is reflected in the colour–magnitude relation, such that a strong red a population of well-isolated galaxies with no ongoing star for- sequence is observed for cluster members, with the fraction of red mation, spanning the stellar mass range at which the fraction of galaxies decreasing out to the field. The red cluster galaxies typi- quenched galaxies increases (≈3 × 1010 M; Kauffmann et al. cally have reduced star formation rates relative to galaxies in the 2003). The most basic way to do this is via the colour–mass re- field (e.g. Balogh et al. 1997, 1998), along with little to no gas (e.g. lation, selecting galaxies that are passive in terms of their optical di Serego Alighieri et al. 2007). colours (they lie on the red sequence), and that do not exhibit spec- At a naive first glance, it appears that environment is the clear tral features consistent with ongoing star formation, via emission driver in galaxy evolution: when a galaxy enters a high-density line diagnostics such as the Baldwin, Phillips & Terlevich (BPT; region of the Universe (e.g. a cluster), it is stripped of star-forming Baldwin, Phillips & Terlevich 1981; Kewley et al. 2006)orWHα material, and it ceases star formation. Processes such as galaxy versus [N II]/Hα (WHAN, Cid Fernandes et al. 2011)diagrams. harassment (Moore et al. 1996), viscous stripping (e.g. Rasmussen Examining the stellar populations of these optically passive et al. 2008;Cluveretal.2013), strangulation (Larson, Tinsley & galaxies in other wavelengths can reveal a different picture to the Caldwell 1980), tidal stripping, and ram-pressure stripping (Gunn optical. For example, galaxies residing on the optical red sequence &Gott1972), can all disrupt galaxies and remove their fuel for star may exhibit a UV excess consistent with star formation in the past formation (e.g. Wetzel, Tinker & Conroy 2012). However, it has 1Gyr(Yietal.2005;Schawinskietal.2007; Crossett et al. 2014). been observed that the current level of star formation (or colour) The mid-IR provides further evidence that not all early-type galax- in a galaxy is more strongly correlated to galaxy mass than local ies or optically red galaxies are passively evolving.

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