The unusual vertical mass distribution of NGC 4013 seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G)
Sébastien Comerón, Bruce G. Elmegreen, Johan H. Knapen, Kartik Sheth, Joannah L. Hinz, Michael W. Regan, Armando Gil de Paz, Juan-Carlos Muñoz-Mateos, Karín Menéndez-Delmestre, Mark Seibert, Taehyun Kim, Trisha Mizusawa, Eija Laurikainen, Heikki Salo, Jarkko Laine, E. Athanassoula, Albert Bosma, Ronald J. Buta, Dimitri A. Gadotti, Luis C. Ho, Benne Holwerda, Eva Schinnerer, Dennis Zaritsky
aa r X i v : . [ a s t r o - ph . C O ] A ug The unusual vertical mass distribution of NGC 4013 seen through the SpitzerSurvey of Stellar Structure in Galaxies (S G) S´ebastien Comer´on, Bruce G. Elmegreen, Johan H. Knapen, , Kartik Sheth, Joannah L. Hinz, Michael W. Regan, Armando Gil de Paz, Juan-Carlos Mu˜noz-Mateos, Kar´ın Men´endez-Delmestre, Mark Seibert, Taehyun Kim, Trisha Mizusawa, Eija Laurikainen, , Heikki Salo, Jarkko Laine, E. Athanassoula, Albert Bosma, Ronald J. Buta, Dimitri A. Gadotti, Luis C. Ho, BenneHolwerda, , Eva Schinnerer and Dennis Zaritsky ABSTRACT
NGC 4013 is a nearby Sb edge-on galaxy known for its “prodigious” H i warp and its “giant”tidal stream. Previous work on this unusual object shows that it cannot be fitted satisfactorilyby a canonical thin+thick disk structure. We have produced a new decomposition of NGC 4013,considering three stellar flattened components (thin+thick disk plus an extra and more extendedcomponent) and one gaseous disk. All four components are considered to be gravitationallycoupled and isothermal. To do so, we have used the 3 . µ m images from the Spitzer Survey ofStellar Structure in Galaxies (S G).We find evidence for NGC 4013 indeed having a thin and a thick disk and an extra flattenedcomponent. This smooth and extended component (scaleheight z EC ∼ ∼
20% of the total mass of all threestellar components. We argue it is unlikely to be related to the ongoing merger or due to theoff-plane stars from a warp in the other two disk components. Instead, we favor a scenario in Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 305-348, Republic of Korea IBM T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA Instituto de Astrof´ısica de Canarias, E-38200 La Laguna, Spain Departamento de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain National Radio Astronomy Observatory / NAASC, 520, Edgemont Road, Charlottesville, VA 22903, USA Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Departamento de Astrof´ısica, Universidad Complutense de Madrid, 28040, Madrid Spain The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA Astronomy Division, Department of Physical Sciences, P. O. Box 3000, FIN-90014 University of Oulu, Finland Finnish Centre of Astronomy with ESO (FINCA), University of Turku, V¨ais¨al¨antie 20, FI-21500, Piikki¨o, Finland Laboratoire d’Astrophysique de Marseille (LAM), UMR6110, Universit´e de Provence/CNRS, Technopˆole de Marseille ´Etoile,38 rue Fr´ed´eric Joliot Curie, 13388 Marseille C´edex 20, France Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487, USA European Southern Observatory, Casilla 19001, Santiago 19, Chile European Space Agency, ESTEC, Keplerlaan 1, 2200, AG, Noordwijk, the Netherlands Astrophysics, Cosmology and Gravity Centre (AC
GC) Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, 69117 Heidelberg, Germany
Subject headings: galaxies: individual (NGC 4013) — galaxies: photometry — galaxies: spiral— galaxies: structure
1. Introduction
Thick disks, first detected by Burstein (1979) and Tsikoudi (1979), are seen in edge-on galaxies as ex-cesses of light a few thin disk scaleheights above the galaxy mid-planes. They are known to be ubiquitous(Yoachim & Dalcanton 2006; Comer´on et al. 2011a) and their properties give important clues for under-standing galaxy formation and evolution (Comer´on et al. 2011b and references therein; hereafter CO11b).Recent studies (Robertson et al. 2006; Elmegreen & Elmegreen 2006; Brook et al. 2007; Richards et al. 2010)suggest an in situ formation mechanism for a significant fraction of the thick disk mass. This was recentlysupported further by our result that the masses of thick and thin disks are of the same order (CO11b).Other formation mechanisms such as disk kinematical heating due to its own overdensities (Villumsen 1985;H¨anninen & Flynn 2002; Sch¨onrich & Binney 2009; Bournaud et al. 2009) and the accretion of stars frominfalling satellites (Statler 1988; Gilmore et al. 2002; Abadi et al. 2003; Navarro et al. 2004; Martin etal. 2004; Read et al. 2008) also contribute to the thick disk mass (CO11b).In CO11b we made thin+thick disk decompositions of 46 edge-on galaxies using images from the SpitzerSurvey of Stellar Structure in Galaxies (S G; Sheth et al 2010). However, two galaxies, ESO 079-003and NGC 4013, could not be successfully fitted down to low surface brightness due to the presence of anextra light component not accounted for in our fits [affecting significantly the luminosity profiles starting at µ . µ m (AB) = 23 mag arcsec − ]. In CO11b, NGC 3628 also presents an obvious third component, but at alower surface brightness [ µ . µ m (AB) = 24 . − ]. The aim of this Letter is to study the propertiesof these extra components. As ESO 079-003 is located near a bright star which makes its study at lowsurface brightness levels difficult and NGC 3628 presents a disturbed morphology due to a recent interactionwe focused on NGC 4013.NGC 4013 is an Sb galaxy (Buta et al. 2007), at 18 . ± . D = 294 ′′ (HyperLEDA; Paturel et al. 2003). It is often describedusing superlative adjectives: it has a “prodigious” H i warp (Bottema et al. 1987; Bottema 1995; 1996) anda “giant” stellar tidal stream with an age of a few Gyr (Mart´ınez-Delgado et al. 2009; hereafter MD09). TheH i warp starts just at the optical edge of the galaxy (Bottema 1995) and it is one of the largest warps everobserved. It may have been triggered by the minor merger event which caused the tidal stream (MD09). Inaddition, NGC 4013 has a boxy bulge (Jarvis 1986) which could be tracing an edge-on bar (Athanassoula &Misiriotis 2002; Mart´ınez-Valpuesta et al. 2006) or be the consequence of a merging event (Binney & Petrou1985).The S G 3 . µ m-band image used for the study presented in this Letter is shown in Fig. 1. 3 –Fig. 1.— 3 . µ m-band S G image of NGC 4013 with two different luminosity stretches. The vertical linesindicate the limits of the bins for which luminosity profiles have been produced, from left to right, atgalactocentric distances of − . r < R < − . r , − . r < R < − . r , − . r < R < . r ,0 . r < R < . r and 0 . r < R < . r . In order to avoid the influence of the bulge we have notproduced fits for the central bin. 4 –
2. Fitting procedure
In CO11b, the observed luminosity profiles were fitted with synthetic profiles resulting from couplingtwo stellar plus one gaseous isothermal disks (using the equations in Narayan & Jog 2002). The solutionsare not analytic, so the fit was done by comparing the luminosity profile to a grid of pre-computed modelswith different thick-to-thin central mass density ratios ( ρ T0 /ρ t0 ) and thick-to-thin velocity dispersions inthe vertical direction ratios ( σ T /σ t ) where T stands for the thick disk and t stands for the thin disk. Bothobserved luminosity and pre-computed profiles were scaled to have a mid-plane luminosity equal to unityand to have ρ ( z = 200) /ρ ( z = 0) = 0 .
1. In addition the synthetic profiles were convolved with a Gaussiankernel with a full width at half maximum (FWHM) equal to 2.2 ′′ in order to account for the point spreadfunction of the 3 . µ m-band image (S G “super-PSF”; Sheth et al. 2010). The mass-to-light ratios (Υ) forboth thin and thick disk stars had to be assumed in order to make the computation of the grid of models,and reasonable limiting cases with Υ T / Υ t = 1 . T / Υ t = 2 . m , which was definedas the range for which either the square root of the χ of the fit was smaller than p χ < . − ordown to a limiting magnitude µ l (AB) = 26 mag arcsec − . The discussion on how we chose this goodness-of-fitparameter and this limiting magnitude appear in CO11b.In CO11b, for the NGC 4013 luminosity profiles at 0 . r < | R | < . r the fits could only beperformed down to µ (AB) ∼
23 mag arcsec − , at which point our criterion for the quality of the fit, p χ > . − , was not met. The reason for that was the presence of a third component which made ourtwo stellar disk decomposition insufficient at describing NGC 4013. This component was called “halo” byvan der Kruit & Searle (1982) and by MD09 who reported that it is box-shaped (although the boxiness isprobably caused by the location of the tidal streams and not by the intrinsic shape of the component).We adapted the code used in C011b to include a third component in equilibrium with the two stellardisks and the gaseous disk. This is a natural approach, because the luminosity profile decays exponentiallywith height once in the range of heights for which the more extended component dominates, and because thesolution of the equations for a set of coupled flattened components can be approximated as exponential athigh distances above the mid-plane. However, the fact of adding a third flattened component does not implythat it is a disk; this is because, without kinematical information, a squashed ellipsoidal halo nature for thiscomponent cannot be discarded. Arguments in favor of this component being a disk are (i) its relativelyhigh surface brightness, and (ii) its luminosity profile parallel to the mid-plane direction (not shown here)which is that of a typical edge-on disk, with a shallow slope for low R and an exponential slope at larger R . Arguments in favor of the extended component being an squashed elliptical halo are (i) its isophotesare not disky (MD09) and may be very close to elliptical if the light of the tidal streams could be removed,and (ii) its ellipticity ( ǫ = 0 .
63 when measured in an ellipse fit between radius 120 ′′ , which is the truncationradius of the thin disk, and 170 ′′ , where the extended component starts to be highly affected by noise) iscompatible with that of simulated elliptical haloes (see, e.g., Lee et al. 2005). Due to its uncertain naturewe will hereafter term this component as “extended component” (EC).The grid of models we used for the fits had ρ T0 /ρ t0 , ρ T0 /ρ EC0 , σ T /σ t and σ T /σ EC as free parameters,where EC denotes the extended component. Our grid of models has been computed including a gaseous diskand using the same normalizations as in CO11b. We considered the cases Υ T / Υ t = 1 . T / Υ t = 2 . T / Υ EC = 1 .
0. This is justified by the fact that once above the mid-plane dust lane 5 –( z > ′′ , a region mainly influenced by the thick disk and the EC), the colors remain roughly constant,as can be seen in Fig. 2. Fig. 2 provides extra evidence that the thick disk and the EC contain old starpopulations since the average color for z > ′′ is g ′ − r ′ ( z > ′′ ) = 0 .
73 mag, which is compatible with theaverage colors predicted for S0 galaxies ( g ′ − r ′ = 0 .
68 mag; Fukugita et al. 1995).The reddening for z < ′′ is caused by the mid-plane dust lane. Since we found no significant influence of this mid-plane dust on the3 . µ m-band profile for this galaxy in CO11b, we considered the mid-plane dust lane to have no effect on ourfits.
3. Results
The addition of a third stellar component allows us to fit the surface brightness profile down to µ (AB) =26 . − . The fit is significantly better than that published in CO11b (Fig. 3 and Table 1) in thesense that it goes far deeper. The fact that for 0 . r < | R | < . r ∆ m is 3 . − larger thanwhat was achieved in CO11b and that the outermost fitted z goes from 25 ′′ − ′′ in CO11b to 70 ′′ − ′′ inthe present work argues for the correctness of the fit, and thus for the physical reality of the third component.The numbers exclude a possible alternative, namely that the better fit is achieved merely by the addition ofmore free parameters.The fitted properties of the thin and the thick disk are fairly similar to those reported in CO11b (Table 1),and the ratio of the column mass densities Σ T / Σ t is equal, within 10%, to that obtained for the three binsfor which we succeeded to obtain a fit in CO11b. The mid-plane stellar densities are not much affected bythe inclusion of the EC, and its main effect is to reduce the scaleheights of the thin and thick disks. Thesmall effect introduced by the EC in Σ T / Σ t is due to the fact that the mid-plane density of the EC is ∼ ∼
10) times smaller than that of the thick disk and ∼
20 ( ∼
10) times smaller than that of the thin diskfor Υ T / Υ t = 1 . T / Υ t = 2 . z < ′′ , the thick disk dominates for 5 ′′ < z < ′′ and the EC dominates for z > ′′ . The only problem in the fit is that the thin disk scaleheight is poorly constrained due to poorsampling ( z t is around 1.5 ′′ , which is smaller than the FWHM).The EC, if considered to be a disk, has an exceptionally large scaleheight, z EC ∼ − . r < R < − . r bin probably because its luminosity profile is affected by the brightestloop of the tidal stream (MD09). The ρ T0 /ρ EC ratio for 0 . r < | R | < . r is significantly higher (afactor of two) than it is for 0 . r < | R | < . r implying that the scalelength is significantly larger thanthat of the thin and the thick disk.Using the weightings in Eq. 5 from CO11b we find that the ratios of masses of the stellar components are M T /M t = 1 .
22 ( M T /M t = 2 .
19) and M T /M EC = 2 .
74 ( M T /M EC = 2 .
69) for Υ T / Υ t = 1 . T / Υ t = 2 . M T + M EC ) /M t = 1 .
67 (( M T + M EC ) /M t =3 . v c = 181 . − (HyperLEDA), NGC 4013 would not fit inthe M T /M t − v c relationship discovered by Yoachim & Dalcanton (2006) and shown in Fig. 12 of CO11bboth when considering M T /M t and ( M T + M EC ) /M t . Furthermore, if we consider the thick disk as partof the thin disk and the EC to be the only thick disk in NGC 4013, we get M EC / ( M t + M T ) = 0 .
20 6 – z (arcsec)0.40.60.81.01.2 g ’- r ’ ( AB m ag ) Fig. 2.— g ′ − r ′ color profiles made for 0 . r < | R | < . r . The profile has been produced using SDSSDR7 data (Abazajian et al. 2007). Points for z ≤ ′′ are calculated every three pixels ( ∼ . ′′ ) and thosewith z > ′′ are calculated every nine pixels ( ∼ . ′′ ). The error bars represent the 2 σ statistical errors.The vertical lines separate the regions dominated by the luminosity of the thin disk, the thick disk and theEC for Υ T / Υ t = 1 . z (arcsec)2624222018 µ . µ m ( AB m ag a r cs e c - ) z (kpc)-0.5 r < R <-0.2 r thin+thick+extended 0 20 40 60 80 100 120 14026242220 thin+thick Fig. 3.— Example of a fit to the vertical luminosity profiles of NGC 4013 considering Υ T / Υ t = 1 .
2. Datapoints with error bars (2 σ statistical errors) represent the observed luminosity profile, and the dashed greencurve the best fit. The dotted red curves indicate the contributions of the three stellar components. Thedash-dotted vertical lines indicate the limits of the range in vertical distance above the mid-plane used forthe fit. The horizontal line represents the µ (AB) = 26 . − level down to which the fits have beendone. The inset shows the thin+thick disk best fit obtained for NGC 4013 in CO11b. 7 –( M EC / ( M t + M T ) = 0 . M T /M t − v c relationship.The luminosity profiles could not be satisfactorily fitted by a sum of two sech ( z/z ) functions, but wereacceptably fitted by a sum of two exponential functions. The p ( χ ) was, however, worse than in the threedisk fit and misses a substantial amount of light corresponding to the thinner component of the three-diskfit (0 . − . − for the inner bins). This fit, although simpler than that made with three coupledstellar components, should be regarded as unphysical.
4. Discussion
Just as causal links have been sought between the “prodigious” warp and the merging event causing the“giant” stream, one may also be tempted to find links between a minor merger and the EC of NGC 4013.If the EC is linked to the minor merger, then it can be made of tidal debris. According to the images inMD09, the tidal loops should not affect the luminosity profiles at R = 0. Although producing a fit for theminor axis using our procedure is impossible due to the presence of the bulge, we produced the luminosityprofile (Fig. 4). It is clear that in the bin with − . r < R < . r the EC is present and probablyslightly brighter than in the 0 . r < | R | < . r as predicted in the case of a disk or an ellipsoid. Thecolor analysis made by MD09 shows that the tidal stream is much redder than the EC, although the largeerror bars make a common origin still possible. Other arguments against a tidal origin for the EC are itssmoothness, its symmetry and the uniformity of its scaleheight with varying radial distance R .Another possibility is that the EC has been formed by the dynamical heating of the disks by the crossingof the dwarf galaxy causing the tidal stream. However, as the disk self-gravity which counteracts the diskheating is higher at low galactocentric radii, disks formed in this way appear flared and, as a consequence,the EC would form with a relatively small scaleheight at low R . Simulations (Quinn et al. 1993; Walker etal. 1996; Kazantzidis et al. 2008; Bournaud et al. 2009) show how this mechanism could reasonably producethe observed scaleheights at high galactocentric radius, but would fail to produce such an EC for the innerkiloparsecs. Additionally, it seems difficult to create a component containing 20% (26%) of the mass ofthe galaxy if we assume Υ T / Υ t = 1 . T / Υ t = 2 .
4) in a recent event without disturbing the disks. Theregularity and the symmetry of the galaxy argues for an old origin for the EC, thus confirming that the µ . µ m ( AB m ag a r cs e c - ) -0.2 r < R <0.2 r Fig. 4.— Luminosity profile for the − . r < R < . r bin (solid line) compared to that obtained fromthe 0 . r < | R | < . r bins (dotted line). 8 –assumption of equilibrium is reasonable. Finally, the stream in NGC 4013 is Monoceros-like (MD09), thusprobably a very minor merger ( ∼ z > .
8% of disk mass at z > i warp and the stellar warp were decoupledand with different lines of nodes the stellar warp would be unlikely to mimic an EC unless the stellar warpstarted at a radius smaller than the H i warp.Thus, we deduce from our findings and from literature data that the EC is a real feature and not sometidal feature or warp seen in projection. We also deduce that both the thick disk and the EC are old.A possible scenario for the formation of a three component system would be having a disk formed thickwith stars formed before and during the build-up of the galaxy from small fragments (Robertson et al. 2006;Brook et al. 2007; Richards et al. 2010) and/or a disk heated by the internal thickening caused by kinematicalheating due to giant clumps (Elmegreen & Elmegreen 2006). Then a merger event would further thickenthe disk, which would become what we know as the EC. After the merger, the canonical thick disk wouldform with the same mechanisms as the EC formed prior to the merger event. Finally, the remaining gas,plus that coming from cold flows would settle in the mid-plane and form the thin disk. This disk formationmechanism is not very frequent, since only two galaxies over 30 exhibit a bright EC in CO11b. The differencein the formation mechanisms may explain why these two galaxies fall outside the M T /M t − v c relationshipdiscovered by Yoachim & Dalcanton (2006).
5. Conclusions
NGC 4013 is not adequately described by the canonical thin+thick disk description (CO11b). In thisLetter, the luminosity profiles of NGC 4013 are fitted satisfactorily using the solutions of three stellarflattened components plus one gaseous disk in equilibrium. The newly described extended component (EC)has a relatively low surface brightness, but due to its vertical extent, contains a significant fraction of thedisk mass (between 20% and 26% depending on the assumed Υ t / Υ T ). The EC has a longer scalelength thanthe galaxy disks, is smooth and its properties do not depend strongly on varying galactocentric radii.The nature of the EC is unknown and could be a second thick disk or a squashed elliptical halo. Thesmoothness of the EC makes it unlikely to be related to the ongoing minor merger of NGC 4013. We alsodiscard the EC to be made of off-plane stars of a warped disk, since the warp has been modeled to haveits line of nodes in the direction of the line of sight (Bottema 1996). We favor a scenario in which the ECwas formed in a two-stage process, in which an initially thick disk was dynamically heated by a merger soonenough in the galaxy history to have a new thick disk formed within it. 9 – Acknowledgments
The authors wish to thank the entire S G team for their efforts in this project. This work is basedon observations and archival data made with the Spitzer Space Telescope, which is operated by the JetPropulsion Laboratory, California Institute of Technology under a contract with NASA. We are gratefulto the dedicated staff at the Spitzer Science Center for their help and support in planning and executionof this Exploration Science program. We gratefully acknowledge support from NASA JPL/Spitzer grantRSA 1374189 provided for the S G project. EA and AB thank the CNES for support. KS, J-CMM, TKimand TMizusawa acknowledge support from the National Radio Astronomy Observatory, which is a facilityof the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.Funding for the SDSS has been provided by the Alfred P. Sloan Foundation, the Participating Institutions,the National Science Foundation, the U.S. Department of Energy, NASA, the Japanese Monbukagakusho,the Max Planck Society, and the Higher Education Funding Council for England. This research has made useof the NASA/IPAC Extragalactic Database (NED) which is operated by JPL, CALTECH, under contractwith NASA.
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This preprint was prepared with the AAS L A TEX macros v5.2.
11 –Table 1: Result of the fits.Fitting range Υ T / Υ t = 1 . T / Υ t = 2 . − . r < R < − . r µ l = 25 .
96 mag arcsec − ∆ m = 6 . − ρ T0 /ρ t0 = 0 . ρ T0 /ρ t0 = 0 . ρ T0 /ρ EC0 = 7 . ρ T0 /ρ EC0 = 6 . σ T /σ t = 2 . σ T /σ t = 2 . σ T /σ EC = 0 . σ T /σ EC = 0 . T / Σ t = 1 .
21 Σ T / Σ t = 2 . T / Σ EC = 1 .
54 Σ T / Σ EC = 1 . z t = 130 pc z t = 120 pc z T = 610 pc z T = 580 pc z EC = 3660 pc z EC = 3500 pc − . r < R < − . r µ l = 25 .
99 mag arcsec − ∆ m = 7 . − ρ T0 /ρ t0 = 0 . ρ T0 /ρ t0 = 0 . ρ T0 /ρ EC0 = 12 . ρ T0 /ρ EC0 = 12 . σ T /σ t = 2 . σ T /σ t = 2 . σ T /σ EC = 0 . σ T /σ EC = 0 . T / Σ t = 1 .
19 Σ T / Σ t = 2 . T / Σ EC = 2 .
91 Σ T / Σ EC = 3 . z t = 110 pc z t = 110 pc z T = 510 pc z T = 530 pc z EC = 2780 pc z EC = 2890 pc0 . r < R < . r µ l = 25 .
93 mag arcsec − ∆ m = 7 . − ρ T0 /ρ t0 = 0 . ρ T0 /ρ t0 = 0 . ρ T0 /ρ EC0 = 11 . ρ T0 /ρ EC0 = 10 . σ T /σ t = 2 . σ T /σ t = 2 . σ T /σ EC = 0 . σ T /σ EC = 0 . T / Σ t = 1 .
25 Σ T / Σ t = 2 . T / Σ EC = 3 .
13 Σ T / Σ EC = 2 . z t = 110 pc z t = 100 pc z T = 520 pc z T = 500 pc z EC = 2540 pc z EC = 2470 pc0 . r < R < . r µ l = 25 .
67 mag arcsec − ∆ m = 5 . − ρ T0 /ρ t0 = 0 . ρ T0 /ρ t0 = 0 . ρ T0 /ρ EC0 = 5 . ρ T0 /ρ EC0 = 5 . σ T /σ t = 2 . σ T /σ t = 2 . σ T /σ EC = 0 . σ T /σ EC = 0 . T / Σ t = 1 .
19 Σ T / Σ t = 2 . T / Σ EC = 1 .