The SAMI Galaxy Survey: Stellar Population and Structural Trends Across the Fundamental Plane
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MNRAS 000,1{36 (2021) Preprint April 22, 2021 Compiled using MNRAS LATEX style file v3.0 The SAMI Galaxy Survey: stellar population and structural trends across the Fundamental Plane Francesco D'Eugenio1;2? Matthew Colless2;3, Nicholas Scott4;3, Arjen van der Wel1, Roger L. Davies5, Jesse van de Sande4;3, Sarah M. Sweet6, Sree Oh2;3, Brent Groves7 Rob Sharp2, Matt S. Owers8;9, Joss Bland-Hawthorn4;3, Scott M. Croom4;3, Sarah Brough3;10, Julia J. Bryant4;3;11, Michael Goodwin11, Jon S. Lawrence11, Nuria P. F. Lorente11 and Samuel N. Richards12 1Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium 2Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia 3ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia 4Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW, 2006, Australia 5Sub-department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 6School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia 7International Centre for Radio Astronomy Research (ICRAR), University of Western Australia, Crawley, WA 6009, Australia 8Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia 9Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia 10School of Physics, University of New South Wales, NSW 2052, Australia 11Australian Astronomical Optics, Macquarie University, Sydney, NSW 2109, Australia 12SOFIA Science Center, USRA, NASA Ames Research Center, Building N232, M/S 232-12, P.O. Box 1, Moffett Field, CA 94035-0001, USA Accepted 2021 April 20. Received 2021 April 14; in original form 2021 March 4 ABSTRACT We study the Fundamental Plane (FP) for a volume- and luminosity-limited sample of 560 early-type galaxies from the SAMI survey. Using r−band sizes and luminosities from new Multi-Gaussian Expansion (MGE) photometric measurements, and treat- ing luminosity as the dependent variable, the FP has coefficients a = 1:294 ± 0:039, b = 0:912±0:025, and zero-point c = 7:067±0:078. We leverage the high signal-to-noise of SAMI integral field spectroscopy, to determine how structural and stellar-population observables affect the scatter about the FP. The FP residuals correlate most strongly (8σ significance) with luminosity-weighted simple-stellar-population (SSP) age. In contrast, the structural observables surface mass density, rotation-to-dispersion ra- tio, S´ersicindex and projected shape all show little or no significant correlation. We connect the FP residuals to the empirical relation between age (or stellar mass-to-light ratio Υ?) and surface mass density, the best predictor of SSP age amongst parameters based on FP observables. We show that the FP residuals (anti-)correlate with the residuals of the relation between surface density and Υ?. This correlation implies that part of the FP scatter is due to the broad age and Υ? distribution at any given surface arXiv:2104.10167v1 [astro-ph.GA] 20 Apr 2021 mass density. Using virial mass and Υ? we construct a simulated FP and compare it to the observed FP. We find that, while the empirical relations between observed stellar population relations and FP observables are responsible for most (75%) of the FP scatter, on their own they do not explain the observed tilt of the FP away from the virial plane. Key words: galaxies: elliptical and lenticular, cD { galaxies: spiral { galaxies: for- mation { galaxies: evolution { galaxies: stellar content 1 INTRODUCTION ? E-mail: [email protected] The Fundamental Plane (FP) is a 2-dimensional empirical relation between three galaxy observables: physical size (R), © 2021 The Authors 2 F. D'Eugenio et al. root mean square velocity along the line of sight (σ) and functions of both σe and Re. Empirically, the FP is com- surface brightness (I; Djorgovski & Davis 1987; Dressler monly expressed as et al. 1987). By connecting distance-independent σ and I log R = a log σ + b log I + c (3) to distance-dependent R, the FP can be used as a distance e e e indicator in cosmology (e.g. Hudson et al. 1999; Colless et al. where Ie is the mean surface brightness inside one Re (in 2001; Beutler et al. 2011; Johnson et al. 2014). Tension be- this formulation, the virial prediction is a = 2 and b = −1). tween different determinations of the cosmological parame- In this work however, following C13, we use the alternative ters (Planck Collaboration et al. 2016; Riess et al. 2016), as expression well as its use to map peculiar velocities, mean that the FP remains a critical tool for cosmology (e.g. Springob et al. log L = a log σe + b log Re + c (4) 2014; Scrimgeour et al. 2016; Said et al. 2020). In addi- because it reduces correlated noise between Re and L and tion, there are several reasons why the FP remains a critical is easier to interpret. While the observed mass plane is con- benchmark for galaxy evolution studies. The tightness of sistent with the virial plane (a = 2 and b = 1, C13), the the FP (scatter of 20{25%, Jørgensen et al. 1996, Hyde & FP has systematically different coefficients (Djorgovski & Bernardi 2009, Magoulas et al. 2012, hereafter: M12) pro- Davis 1987; Dressler et al. 1987). Geometrically, this differ- vides a strong constraint to theory, limiting the rate and ence means that the FP is tilted (or rotated) with respect strength of physical processes that drive galaxies away from to the virial plane. By comparing equations (1) and (2), the the plane (e.g. Kobayashi 2005). Moreover, the FP enables FP tilt must arise from systematic variations of κ,Υ?, f? us to probe - albeit not directly - the scaling relations of and/or ΥIMF=Υ? with σe and Re. dark matter and initial mass function (IMF; e.g. Prugniel & In addition to its tilt, the observed FP also differs Simien 1997; Graves & Faber 2010), thus constraining the from the mass plane in that the latter is consistent with present-day structure of local galaxies. Finally, the advent no intrinsic scatter (C13), whereas the FP has finite scatter of large-aperture (8-10 m) telescopes opened a window to (Jørgensen et al. 1996). The FP intrinsic scatter is critical to study how the FP changes over cosmic time (van Dokkum & cosmology, because it represents a hard limit to its precision Stanford 2003; van der Wel et al. 2004; Wuyts et al. 2004), a as a distance estimator (M12). Therefore, understanding the subject that continues to this day, with studies reporting ei- origin of this scatter is important to understanding if it can ther evolution (Saracco et al. 2020), weak evolution (Saglia be reduced, thereby improving the FP as a tool for cosmol- et al. 2010, 2016; Oldham et al. 2017; Dalla Bont`aet al. ogy. From equation (2), the FP scatter must originate from 2018) or no evolution (Holden et al. 2010; Prichard et al. galaxy-to-galaxy variations in κ,Υ?, f? and/or ΥIMF=Υ? at 2017; de Graaff et al. 2020). For all these reasons, a better fixed σe and Re. Therefore, the fact that the FP is `tight' understanding of the FP will improve our understanding of requires either that these galaxy observables have narrow galaxies and potentially increase its precision and accuracy distributions at fixed σe and Re, or, alternatively, that these as a tool for cosmology. distributions are correlated in such a way as to decrease the The FP is physically rooted in the virialised nature of FP scatter (FP ‘fine-tuning’, Ciotti et al. 1996 - but see Chiu galaxies: the scalar virial theorem links dynamical mass, size et al. 2017 for a different view). This fine-tuning requirement and specific kinetic energy, thus constraining these observ- provides an additional constraint to galaxy evolution theory. ables on the virial (or mass) plane ( Djorgovski & Davis A satisfactory understanding of the FP requires (i) de- 1987, Dressler et al. 1987, Cappellari et al. 2006, hereafter: termining κ,Υ?, f? and ΥIMF=Υ? as functions of σe and Re C06, Cappellari et al. 2013a, hereafter: C13). If we use pro- and (ii) using these functions in equation (2) to reproduce jected half-light radius (Re) to measure size and root mean the observed FP. The FP intrinsic scatter must also be con- square velocity along the line of sight inside an aperture of sistent with the combined (and possibly correlated) scatter radius Re (σe) to measure kinetic energy, the virial mass can of these input galaxy properties at fixed σe and Re. Unfortu- be expressed by nately, measuring f? and ΥIMF=Υ? is challenging, because dark matter (in the definition of f?) cannot be observed log Mvir = log κ + 2 log σe + log Re − log G (1) directly and because low-mass stars (for ΥIMF=Υ?) require where G is the gravitational constant and κ is a parameter high quality observations (cf. Conroy & van Dokkum 2012). which encodes the possibility of `non-homology', that is of A more practicable approach is to combine measurements of systematic differences in galaxy structure along and orthog- κ and Υ? with the observed FP and to infer, by subtraction, onal to the FP (cf. Bender et al. 1992; Graham & Colless the missing contribution due to dark matter and IMF trends 1997; Prugniel & Simien 1997). Equation (1) defines a ge- (e.g. Prugniel & Simien 1997; Graves & Faber 2010); these inferred trends are a useful benchmark for theory. ometric plane in the logarithmic space of (σe;Re;Mvir). To obtain the FP, we further introduce the stellar mass-to-light Single-fibre spectroscopy surveys enabled us to measure the relation between the FP observable σ and stellar popu- ratio (Υ?, assuming a fiducial Chabrier initial mass func- tion, IMF; Chabrier 2003), the stellar mass-to-light ratio as- lation age and metallicity (which, together, determine Υ?).