A Structural and Dynamical Study of Late-Type, Edge-on Galaxies. II. Vertical Color Gradients and the Detection of Ubiquitous Thick Disks

By J. J. Dalcanton, R. Bernstein. 2002

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We present an analysis of optical (B-R) and optical-infrared (R-Ks) color maps for 47 extremely late-type edge-on unwarped, bulgeless disk galaxies spanning a wide range of mass. The color maps show that the thin disks of these galaxies are embedded in a low surface brightness red envelope. This component is substantially thicker than the thin disk (a : b ∼ 4 : 1 vs. >8 : 1), extends to at least 5 vertical disk scale heights above the galaxy midplane, and has a radial scale length that appears to be uncorrelated with that of the embedded thin disk. The color of the red envelope is similar from galaxy to galaxy, even when the thin disk is extremely blue, and is consistent with a relatively old (>6 Gyr) stellar population that is not particularly metal-poor. The color difference between the embedded thin disk and the red stellar envelope varies systematically with rotation speed, reflecting an increasing age difference between the thin and thick components in lower mass galaxies, driven primarily by changes in the age of the thin disk. The red stellar envelopes are similar to the thick disk of the Milky Way, having common surface brightnesses, spatial distributions, mean ages, and metallicities. We argue that the ubiquity of the red stellar envelopes implies that the formation of the thick disk is a nearly universal feature of disk formation and is not necessarily connected to the formation of a bulge. Our data suggest that the thick disk forms early (>6 Gyr ago), even within galaxies where the bulk of the stars formed very recently (<2 Gyr). We argue that several aspects of our data and the observed properties of the Milky Way thick disk argue in favor of a merger origin for the thick disk population. If so, then the age of the thick disk marks the end of the epoch of major merging, and the age difference between the younger thin disk and the older thick disk can become a strong constraint on cosmological constants and models of galaxy and/or structure formation.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/8648a2d1aefc501712fc79c1d45b437649e7bf76

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Citations in the corpus

Numerical simulation of a possible origin of the positive radial metallicity gradient of the thick disk (2013, background): “Thick disks have been observed in many other disk galaxies (e.g. Dalcanton & Bernstein 2002; Yoachim & Dalcanton 2006) suggesting that they may be ubiquitous.”
THE METALLICITY DISTRIBUTION FUNCTIONS OF SEGUE G AND K DWARFS: CONSTRAINTS FOR DISK CHEMICAL EVOLUTION AND FORMATION (2011, background): “…our own Galaxy to disentangle the mechanisms behind disk development because the thick disk appears to be a regular feature in numerous galaxies (Burstein 1979; Dalcanton & Bernstein 2002; Yoachim & Dalcanton 2008a, 2008b), including those at redshifts as high as z ∼ 3 (Elmegreen & Elmegreen 2006).”
Oxygen trends in the Galactic thin and thick disks (2003, background): “However, evidence from extragalactic studies of edge-on spiral galaxies (e.g. Reshetnikov & Combes 1997; Schwarzkopf & Dettmar 2000; Dalcanton & Bernstein 2002) appear to indicate that thick disks are more common in galaxies that are experiencing or have experienced mergers or interaction.”
On the Accretion Origin of a Vast Extended Stellar Disk around the Andromeda Galaxy (2005, background): “Several edge-on galaxies have been found to possess a flattened component with a much larger scale-height than the thin disk (Dalcanton & Bernstein 2002) probably indicating the presence of a thick disk.”
Impacts of a flaring star-forming disc and stellar radial mixing on the vertical metallicity gradient (2016, background): “Both the Milky Way (Yoshii 1982; Gilmore & Reid 1983) and external galaxies (e.g. Burstein 1979; Dalcanton & Bernstein 2002) show geometrically thick discs in addition to the geometrically thin disc.”
The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution (2005, background): “…of the formation of realistic galaxies in a CDMhierarchical merger picture context (Abadi et al. 2003; Brook et al. 2004), the observation of properties of thick disks (Dalcanton & Bernstein 2002; Yoachim &Dalcanton 2006), and even a counterrotating thick disk in FGC 227 (Yoachim & Dalcanton 2005).”
The effect of galaxy mass ratio on merger-driven starbursts (2007, result): “Furthermore, there is an increasing amount of evidence that these theoretical expectations are consistent with observations in the local universe (e.g., Ibata et al. 2001; Dalcanton & Bernstein 2002; Navarro et al. 2004; Bullock & Johnston 2005).”
Structure of Disk-dominated Galaxies. II. Color Gradients and Stellar Population Models (2004, background): “In Figures 10 and 11 we divide the Courteau et al. and BdJ00 samples, respectively, at Vrot k120 km s 1, the threshold above which all edge-on galaxies have prominent central dust lanes (Dalcanton & Bernstein 2002).”
THE THICK DISKS OF SPIRAL GALAXIES AS RELICS FROM GAS-RICH, TURBULENT, CLUMPY DISKS AT HIGH REDSHIFT (2009, background): “The structural properties of thick disks that are relevant for comparisons with our models are: • The thick-disk sech2 scale height, z0, is generally between 1.4 and 2.8 kpc, with a large scatter (Dalcanton & Bernstein 2002, YD06).”
RECONSTRUCTING THE STELLAR MASS DISTRIBUTIONS OF GALAXIES USING S4G IRAC 3.6 AND 4.5 μm IMAGES. II. THE CONVERSION FROM LIGHT TO MASS (2014, methodology): “…typical uncertainties on total masses are 0.1-0.2 dex using global colors to assign the stellar M/L (Bell & de Jong 2001), although colors are expected to remain reliable in 1D for all but the most massive (and therefore dustiest; i.e. Dalcanton & Bernstein 2002) disks (MacArthur et al. 2004).”
Halo Substructure and the Power Spectrum (2002, background): “…merger/disruption phase in each halo’s past is intriguing, as this phase is approximately coincident with the estimated ages of galactic thick disks, ttd ∼ 8 − 10 Gyr (e.g., Quillen & Garnett 2000 for the Milky Way), which seem to be ubiquitous and roughly coeval (Dalcanton & Bernstein 2002).”
The age structure of stellar populations in the solar vicinity Clues of a two-phase formation history of the Milky Way disk (2013, background): “…parameters may give us a hint of the existence of such a population in the Milky Way (Gilmore & Reid 1983) as well as in external galaxies (Burstein 1979; Dalcanton & Bernstein 2002; Pohlen et al. 2004; Comerón et al. 2011), but no clue as to its origin or to its link to the other populations.”
Constraining the thick disc formation scenario of the Milky Way (2014, unknown): “Moreover, it should be noted that in external galaxies, most thick discs do not show outer flares (Dalcanton & Bernstein 2002), also favouring the scenario of Bournaud et al. (2009).”
A RADIAL AGE GRADIENT IN THE GEOMETRICALLY THICK DISK OF THE MILKY WAY (2016, background): “Their red colors (and the absence of radial color gradients) have led to the tentative conclusion that they are made of uniformly old stellar populations (Dalcanton & Bernstein 2002; Rejkuba et al. 2009).”
Merger Histories of Galaxy Halos and Implications for Disk Survival (2007, background): “More detailed simulations will be required to test whether disks are destroyed or overly thickened by the predicted infall of m ∼ 5×1010h−1M⊙ objects, and whether these thickening events happen too late to explain thick disks as old as those observed (Dalcanton & Bernstein 2002).”
Dissecting simulated disc galaxies - I. The structure of mono-age populations (2014, background): “The stellar disc of the Milky Way, as well as of many nearby galaxies, is traditionally viewed as consisting of two distinct components: a thin and a thick disc (e.g., Burstein 1979; Tsikoudi 1979; Gilmore & Reid 1983; Dalcanton & Bernstein 2002; Yoachim & Dalcanton 2006; Comerón et al. 2011).”
Thin disc, thick disc and halo in a simulated galaxy (2012, background): “…and evolution throughout the Universe, as such thick discs appear C© 2012 The Authors Monthly Notices of the Royal Astronomical Society C© 2012 RAS ubiquitous (e.g. Dalcanton & Bernstein 2002; Yoachim & Dalcanton 2005) and contain the oldest disc stars, born during the earliest formation of discs.”
Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars (2003, background): “Extra-galactic evidence of thick disks in edge-on galaxies is continuously growing (e.g. Reshetnikov & Combes 1997; Schwarzkopf & Dettmar 2000; Dalcanton & Bernstein 2002).”
Simulations of minor mergers - I. General properties of thick discs (2008, background): “Dalcanton & Bernstein (2002) have found a vertical colour-gradient in their sample of galaxies, in the sense that the thick disks are redder, and consistent with a relatively old (> 6 Gyr) stellar population.”