Comerón, S.

Knapen, J. H.

Peletier, R. F.

Salo, H.

English

University of Groningen

   University of GroningenThick discs in galaxies were most likely not accretedComerón, S.; Salo, H.; Knapen, J. H.; Peletier, R. F.Published in:Highlights  on  Spanish  Astrophysics  X,  Proceedings  of  the  XIII  Scientific  Meeting  of  the  SpanishAstronomical Society held on July 16 –20, 2018, in Salamanca, SpainIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2019Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Comerón, S., Salo, H., Knapen, J. H., & Peletier, R. F. (2019). Thick discs in galaxies were most likely notaccreted. In B. Montesinos, A. Asensio Ramos, F. Buitrago, R. Schödel, E. Villaver, S. Pérez-Hoyos, & I.Ordóñez-Etxeberria (Eds.), Highlights on Spanish Astrophysics X, Proceedings of the XIII ScientificMeeting of the Spanish Astronomical Society held on July 16 –20, 2018, in Salamanca, Spain (pp. 104-109). Spanish Astronomical Society. https://ui.adsabs.harvard.edu/abs/2019hsax.conf..104CCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 19-11-2022Highlights on Spanish Astrophysics X, Proceedings of the XIII Scientific Meeting of the Spanish Astronomical Society held on July 16 – 20, 2018, in Salamanca, Spain. B. Montesinos,  A. Asensio Ramos, F. Buitrago, R. Schödel, E. Villaver, S. Pérez-Hoyos, I. Ordóñez-Etxeberria (eds.), 2019  Thick discs in galaxies were most likely notaccreted.Sébastien Comerón1, Heikki Salo1, Johan H. Knapen2,3,4 and ReynierF. Peletier51 University of Oulu, Astronomy Research Unit, P.O. Box 3000, FI-90014 Oulu, Finland2 Instituto de Astrof́ısica de Canarias E-38205, La Laguna, Tenerife, Spain3 Departamento de Astrof́ısica, Universidad de La Laguna, E-38205, La Laguna, Tenerife,Spain4 Astrophysics Research Institute, Liverpool John Moores University, IC2, LiverpoolScience Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK5 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, NL-9700 AVGroningen, the NetherlandsAbstractThe origin of thick discs in galaxies remains shrouded in mystery. A variety of formationscenarios has been proposed. Here we aim to test one such scenario where the thick discstars are proposed to be accreted from satellite galaxies. In this scenario, in at least somegalaxies a fraction of thick disc stars would rotate in a retrograde way, which would causea large thick disc velocity lag. Here, we compare the rotation curves of the thin and thethick discs of eight edge-on galaxies observed with MUSE at the VLT. We find that thevelocity lags of the thick discs are compatible with those expected from asymmetric drift. Ifwe consider the galaxies with thick disc rotation curves in the literature, only one in aboutfifteen shows clear signs of an accreted thick disc. Based on simulations in the literaturewe estimate that if thick discs were accreted, at least one in six would show clear signsof retrograde material. Thus, there is a growing tension between the observations and thehypothesis that thick discs are made of accreted stars.1 IntroductionThe formation and early evolution epoch of galaxies is hard to study due to observationalissues such as cosmological dimming and the lack of means to properly resolve objects atlarge angular diametre distance. An alternative approach to unveil how galaxies evolved is tostudy with an exquisite level of detail the oldest stars in local objects and deduce their origin104Comerón, S. et al. 105(this approach is called galactic archæology). The oldest stars of a galaxy are typically foundin the halo and in the thick disc. In this paper we will focus on the thick discs to advance inour understanding of galaxy evolution.In edge-on galaxies thick discs are seen as roughly exponential excesses of light thatbecome apparent a few thin disc scale-heights above the mid-plane. They contain a significantfraction of the baryons in a galaxy and, in low-mass galaxies, can be as massive as the thindiscs[30, 6, 7, 10].The origin of thick discs has been a source of debate. Some authors have argued foran internal origin of the thick disc stars. Either a pre-existing thin disc would have beenthickened by dynamical heating[28, 23, 24, 19], or the thick disc would have been born thickin an early phase of the galaxy evolution when the interstellar medium was very turbulent andthe specific star formation rate was much larger than it is nowadays[12, 2, 8]. Alternatively,the thick discs could have been accreted through minor mergers[1].The stars of the Milky Way thick disc are now known to be mostly of internal origin[22,18, 15]. However, it is known that at least one external thick disc – that in FGC 227 – hasstellar rotation velocities close to zero[29], which implies that about half of the light is emittedby retrograde stars. This strongly suggests an external origin for a large fraction of its thickdisc stars. Unfortunately, the number of thick disc rotation curves in the literature is small– about ten[29, 30, 9, 11, 14, 16] –, so it is not possible to say whether the case of FGC 227is unique or not.Here we aim to obtain additional thick disc rotation curves to determine whether ac-cretion is a plausible origin for the majority of stars in thick discs.2 Sample, observations, and data processingWe observed eight nearby edge-on galaxies with MUSE at the VLT. MUSE is an integralfield unit with a 1′× 1′ field of view and a simultaneous wavelength coverage between 4750 Åand 9300 Å. All the observed galaxies had previously been observed within the Spitzer Surveyof Stellar Structure in Galaxies (S4G)[25] and have photometric decompositions available in3.6µm[10]. The selected galaxies are relatively small with circular velocities below 180 km s−1.All but one are bulgeless.The galaxies were exposed for almost three hours. They were reduced using the stan-dard ESO pipelines. We used ZAP[26] to remove the sky residuals. The datacubes whereVoronoi-binned[3] to a signal-to-noise ratio of S/N ∼ 25 in the wavelength range between5490 Å and 5510 Å. Then PPXF[4] was applied to the spectra obtained for each Voronoi bin.PPXF fits the spectra with a linear combination of spectral energy distribution templates andobtains the line of sight velocity distribution. We used the templates from the MIUSCATlibrary[27]. The templates and the spectra where Gauss-convolved to match their spectralfull width at half maximum at every wavelength.106 The rotation curves of thin and thick discs as seen with MUSE-30 -20 -10 0 10 20 30-30-20-100102030-30-20-100102030                                          NWSE              -30-20-100102030-30-20-100102030VelocityNWSE-100-50050100V (km/s)Velocity dispersion-30 -20 -10 0 10 20 30       -30-20-100102030-30-20-100102030NWSE020406080100σ (km/s)ESO 157-49-100-50050100V (km/s)       -2 -1 0 1 2x (kpc)NWz=0.00′′z=1.53′′z=3.05′′z=4.58′′z=6.10′′z=7.63′′z=9.15′′       -2 -1 0 1 2x (kpc)          SEz=0.00′′z=-1.53′′z=-3.05′′z=-4.58′′z=-6.10′′z=-7.63′′z=-9.15′′-30 -20 -10 0 10 20 30x (arcsec)       -40-2002040∆V (km/s)     -30 -20 -10 0 10 20 30x (arcsec)                 020406080100120V (km/s) RRBB-10 -5 0 5 10z (arcsec)-0.5 0.0 0.5z (kpc)Figure 1: Top-left: Image of ESO 157-49. Middle-left: Velocity map of ESO 157-49. Bottom-left: Velocity dispersion map of ESO 157-49. The values in the axes are in arcseconds. Northis up and East is left (shown by the reverse-L symbol). The lines parallel to the mid-planedenote the heights for which the rotation curves (top-right panels) were computed. The thincontinuous line indicates the mid-plane and the thick continuous lines indicate the heightwhere the thick disc starts to dominate the surface brightness[10]. The middle-right panelsshow the difference between the velocity at a given height and that at the mid-plane. Thedashed vertical lines indicate where the rotation curve becomes flat. The bottom-right panelshows the velocity as a function of height for the flat part of the rotation curve for the blue-shifted and red-shifted sides of the galaxy (indicated by “B” and “R”, respectively). Thevertical lines indicate the height above which the thick disc dominates the surface brightness.3 ResultsWe produced velocity and velocity dispersion maps for the eight galaxies in our sample. Weused the velocity maps to compute rotation curves at different heights above and below themid-planes. In Fig. 1 we show the maps and the rotation curves extracted for ESO 157-49.In Fig. 2 we show, for the eight galaxies in our sample, the circular velocity as afunction of height for the regions where the rotation curve of the galaxies is flat. We findComerón, S. et al. 107ESO 443-21 vc=151 km/s0 2 4 6z (arcsec)0.0 0.5 1.0 1.5z (kpc)050100150V (km/s)    IC 1553 vc=115 km/s0 2 4 6 8 10z (arcsec)0.0 0.5 1.0 1.5z (kpc)        PGC 28308 vc=111 km/s0 2 4 6 8z (arcsec)0.0 0.5 1.0 1.5z (kpc)        ESO 544-27 vc=96 km/s0 2 4 6 8z (arcsec)0.0 0.5 1.0 1.5z (kpc)        ESO 157-49 vc=90 km/s0 5 10 15 20z (arcsec)0.0 0.5 1.0 1.5z (kpc)050100150V (km/s)    ESO 469-15 vc=83 km/s0 2 4 6 8 10 12z (arcsec)0.0 0.5 1.0 1.5z (kpc)        PGC 30591 vc=82 km/s0 2 4 6 8 10z (arcsec)0.0 0.5 1.0 1.5z (kpc)        IC 217 vc=63 km/s0 5 10 15z (arcsec)0.0 0.5 1.0 1.5z (kpc)        Figure 2: Velocity as a function of height for the flat part of the rotation curve for thegalaxies in our sample. The horizontal axes are scaled so all the plots cover the same heightrange. The grey bands indicate the dispersion in the velocity for the Voronoi bins used tocompute each data point. The maximum velocity at the mid-plane is denoted by vc.that the maximum velocity is lower at increasing distances from the mid-plane, with a lagat large heights of a few tens of kilometres per second for all the galaxies in the sample.This is compatible with the combined effects of asymmetric drift and a not perfectly edge-onorientation. Indeed, in the past we have shown that retrograde thick disc fractions largerthan 10−20% would result in lags larger than those that we detect[9]. Also, large retrogradematerial fractions would result in a velocity dispersion comparable to the circular velocity ofthe galaxy, vc. Therefore, our data indicate a very small fraction – compatible with zero –of retrograde stars in the thick discs. We shall however note that only five out of our eightgalaxies have massive thick discs that dominate the luminosity at some of the observed heightrange[10].4 Discussion and conclusionsOur observations increase the number of available thick disc rotation curves to about fifteen.Only one of them (FGC 227[29]) shows clear evidence for retrograde stars. One may thus108 The rotation curves of thin and thick discs as seen with MUSEnaively conclude that accreted thick discs are rare. This is, however, not necessarily truebecause accretion is more efficient for prograde encounters than it is for retrograde ones. Thereason for that is how dynamical friction works. Dynamical friction in its linear regime actsover an infalling satellite as described by the Chandrasekhar formula[5]F ∝M2ρ/V 2, (1)where M is the mass of the satellite, ρ the density of the main galaxy, and V the relativevelocity between the satellite and the particles of the main galaxy. When the satellite is faraway from the disc of the main galaxy, V does not depend on whether the merger is progradeor retrograde because the dark matter halo is pressure-supported and, hence, has a meanvelocity of zero. However, as the satellite falls and becomes influenced by the disc, progradeobjects feel a larger force than retrograde ones. The detailed calculation on the decay timesof satellites requires numerical simulations, and it can be estimated that a retrograde satellitetakes at most two times longer than a prograde one to be accreted[20]. Given a halo mergerrate proportional to ∝ (1 + z)2[13], isotropic accretion, and a galaxy age of 10 Gyr, we obtainthat about a third of the satellites accreted by now had retrograde orbits.Another effect to be taken into account is in-plane dragging. Prograde satellites withorbital inclinations smaller than i = 20o are dragged into the plane of the main galaxyby dynamical friction and dissolve into the thick disc[21]. Satellites with larger incominginclinations dissolve into the inner halo. The effect of in-plane dragging is diminished forretrograde satellites and we estimate that only those with inclinations smaller than i = 10oare accreted into the thick disc. Hence, an accreted retrograde satellite is two times less likelyto end in a thick disc than a prograde satellite.In summary: 1) one third of the satellites accreted by now by disc galaxies were retro-grade and 2) a retrograde merger is two times less likely to contribute to a thick disc than aprograde one. This implies that in the limit where all thick discs have been created in singlemerger events, we expect ∼ 1/6 of the thick discs to be retrograde. However, the observedfigure is ∼ 1/15. Hence, our results point to a tension between the observations and thehypothesis of accreted thick discs. If thick discs were created in a small number of mergers(two or three), the tension would worsen. We can therefore conclude that an accretion originfor thick discs is not favoured by the observations.AcknowledgmentsSC and HS acknowledge support from the Academy of Finland. HS, JHK, and RFP acknowl-edge financial support from the European Union’s Horizon 2020 research and innovation pro-gramme under Marie Sk lodowska-Curie grant agreement No. 721463 to the SUNDIAL ITNnetwork. JHK acknowledges additional support from the Spanish Ministry of Economy andCompetitiveness (MINECO) under grant number AYA2016-76219-P, as well as from the Fun-dación BBVA under its 2017 programme of assistance to scientific research groups, for theproject “Using machine-learning techniques to drag galaxies from the noise in deep imaging”,and from the Leverhulme Trust through the award of a Visiting Professorship at LJMU.Comerón, S. et al. 109References[1] Abadi, M. G., Navarro, J. F., Steinmetz, M., et al. 2003, ApJ, 597, 21[2] Bournaud, F., Elmegreen, B. G., & Martig, M. 2009, ApJ, 707L, 1[3] Cappellari, M., & Copin, Y. 2003, MNRAS, 342, 354[4] Cappellari, M., & Emsellem, E. 2004, PASP, 116, 1[5] Chandrasekhar, S. 1943, ApJ, 97, 255[6] Comerón, S., Elemgreen, B. G., Knapen, J. H., et al. 2011, ApJ, 741, 28[7] Comerón, S., Elemgreen, B. G., Salo, H., et al. 2012, ApJ, 759, 98[8] Comerón, S., Elemgreen, B. G., Salo, H., et al. 2014, A&A, 571, 58[9] Comerón, S., Salo, H., Janz, J., et al. 2015, A&A, 584, 34[10] Comerón, S., Salo, H., Knapen, J. H. 2018, A&A, 610, 5[11] Comerón, S., Salo, H., Peletier, R. F., et al. 2015, A&A, 593L, 6[12] Elmegreen, B. G., & Elmegreen, D. M. 2006, ApJ, 650, 644[13] Fakhouri, O. & Ma, C.-P. 2008, MNRAS, 386, 577[14] Guérou, A., Emsellem, E., Krajnović, D. 2016, A&A, 591, 143[15] Haywood, M., Di Matteo, P., Snaith, O., et al. 2015, A&A, 579, 5[16] Kasparova, A. V., Katkov, I. Y., Chilingarian, I. 2016, 460L, 89[17] Kazantzidis, S., Bullock, J. S., Zentner, A. R., et al. 2008, ApJ, 688, 254[18] Lehnert, M. D., Di Matteo, P., Haywood, M., et al. 2014, ApJ, 789L, 30[19] Loebman, S. R., Roŝkar, R., Debattista, V. P, et al. 2011, ApJ, 737, 8L[20] Peñarrubia, J., Kroupa, P., & Boily, C. M. 2002, MNRAS, 333, 779[21] Read, J. I., Lake, G., Agertz, O., et al. 2008, MNRAS 389, 1041[22] Ruchti, G. R., Read, J. I., Feltzing, S., et al. 2014, MNRAS, 444, 515[23] Scönrich, R., & Binney, J. 2009, MNRAS, 396, 203[24] Scönrich, R., & Binney, J. 2009, MNRAS, 399, 1145[25] Sheth, K., Regan, M., Hinz, J. L. 2010, PASP, 122, 1397[26] Soto, K., Lilly, S. J., Bacon, R., et al. 2016, MNRAS, 458, 3210[27] Vazdekis, A., Ricciardelli, E., Cenarro, A. J., et al. 2012, MNRAS, 424, 157[28] Villumsen, J. V. 1985, ApJ, 290, 75[29] Yoachim, P., & Dalcanton, J. J. 2005, ApJ, 624, 701[30] Yoachim, P., & Dalcanton, J. J. 2006, AJ, 131, 226

Thick discs in galaxies were most likely not accreted

The origin of thick discs in galaxies remains shrouded in mystery. A variety of formation scenarios has been proposed. Here we aim to test one such scenario where the thick disc stars are proposed to be accreted from satellite galaxies. In this scenario, in at least some galaxies a fraction of thick disc stars would rotate in a retrograde way, which would cause a large thick disc velocity lag. Here, we compare the rotation curves of the thin and the thick discs of eight edge-on galaxies observed with MUSE at the VLT. We find that the velocity lags of the thick discs are compatible with those expected from asymmetric drift. If we consider the galaxies with thick disc rotation curves in the literature, only one in about fifteen shows clear signs of an accreted thick disc. Based on simulations in the literature we estimate that if thick discs were accreted, at least one in six would show clear signs of retrograde material. Thus, there is a growing tension between the observations and the hypothesis that thick discs are made of accreted stars

ARTS repository - University of Groningen

   University of GroningenThick discs in galaxies were most likely not accretedComerón, S.; Salo, H.; Knapen, J. H.; Peletier, R. F.Published in:Highlights  on  Spanish  Astrophysics  X,  Proceedings  of  the  XIII  Scientific  Meeting  of  the  SpanishAstronomical Society held on July 16 –20, 2018, in Salamanca, SpainIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2019Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Comerón, S., Salo, H., Knapen, J. H., & Peletier, R. F. (2019). Thick discs in galaxies were most likely notaccreted. In B. Montesinos, A. Asensio Ramos, F. Buitrago, R. Schödel, E. Villaver, S. Pérez-Hoyos, & I.Ordóñez-Etxeberria (Eds.), Highlights on Spanish Astrophysics X, Proceedings of the XIII ScientificMeeting of the Spanish Astronomical Society held on July 16 –20, 2018, in Salamanca, Spain (pp. 104-109). Spanish Astronomical Society. https://ui.adsabs.harvard.edu/abs/2019hsax.conf..104CCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-2022Highlights on Spanish Astrophysics X, Proceedings of the XIII Scientific Meeting of the Spanish Astronomical Society held on July 16 – 20, 2018, in Salamanca, Spain. B. Montesinos,  A. Asensio Ramos, F. Buitrago, R. Schödel, E. Villaver, S. Pérez-Hoyos, I. Ordóñez-Etxeberria (eds.), 2019  Thick discs in galaxies were most likely notaccreted.Sébastien Comerón1, Heikki Salo1, Johan H. Knapen2,3,4 and ReynierF. Peletier51 University of Oulu, Astronomy Research Unit, P.O. Box 3000, FI-90014 Oulu, Finland2 Instituto de Astrof́ısica de Canarias E-38205, La Laguna, Tenerife, Spain3 Departamento de Astrof́ısica, Universidad de La Laguna, E-38205, La Laguna, Tenerife,Spain4 Astrophysics Research Institute, Liverpool John Moores University, IC2, LiverpoolScience Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK5 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, NL-9700 AVGroningen, the NetherlandsAbstractThe origin of thick discs in galaxies remains shrouded in mystery. A variety of formationscenarios has been proposed. Here we aim to test one such scenario where the thick discstars are proposed to be accreted from satellite galaxies. In this scenario, in at least somegalaxies a fraction of thick disc stars would rotate in a retrograde way, which would causea large thick disc velocity lag. Here, we compare the rotation curves of the thin and thethick discs of eight edge-on galaxies observed with MUSE at the VLT. We find that thevelocity lags of the thick discs are compatible with those expected from asymmetric drift. Ifwe consider the galaxies with thick disc rotation curves in the literature, only one in aboutfifteen shows clear signs of an accreted thick disc. Based on simulations in the literaturewe estimate that if thick discs were accreted, at least one in six would show clear signsof retrograde material. Thus, there is a growing tension between the observations and thehypothesis that thick discs are made of accreted stars.1 IntroductionThe formation and early evolution epoch of galaxies is hard to study due to observationalissues such as cosmological dimming and the lack of means to properly resolve objects atlarge angular diametre distance. An alternative approach to unveil how galaxies evolved is tostudy with an exquisite level of detail the oldest stars in local objects and deduce their origin104Comerón, S. et al. 105(this approach is called galactic archæology). The oldest stars of a galaxy are typically foundin the halo and in the thick disc. In this paper we will focus on the thick discs to advance inour understanding of galaxy evolution.In edge-on galaxies thick discs are seen as roughly exponential excesses of light thatbecome apparent a few thin disc scale-heights above the mid-plane. They contain a significantfraction of the baryons in a galaxy and, in low-mass galaxies, can be as massive as the thindiscs[30, 6, 7, 10].The origin of thick discs has been a source of debate. Some authors have argued foran internal origin of the thick disc stars. Either a pre-existing thin disc would have beenthickened by dynamical heating[28, 23, 24, 19], or the thick disc would have been born thickin an early phase of the galaxy evolution when the interstellar medium was very turbulent andthe specific star formation rate was much larger than it is nowadays[12, 2, 8]. Alternatively,the thick discs could have been accreted through minor mergers[1].The stars of the Milky Way thick disc are now known to be mostly of internal origin[22,18, 15]. However, it is known that at least one external thick disc – that in FGC 227 – hasstellar rotation velocities close to zero[29], which implies that about half of the light is emittedby retrograde stars. This strongly suggests an external origin for a large fraction of its thickdisc stars. Unfortunately, the number of thick disc rotation curves in the literature is small– about ten[29, 30, 9, 11, 14, 16] –, so it is not possible to say whether the case of FGC 227is unique or not.Here we aim to obtain additional thick disc rotation curves to determine whether ac-cretion is a plausible origin for the majority of stars in thick discs.2 Sample, observations, and data processingWe observed eight nearby edge-on galaxies with MUSE at the VLT. MUSE is an integralfield unit with a 1′× 1′ field of view and a simultaneous wavelength coverage between 4750 Åand 9300 Å. All the observed galaxies had previously been observed within the Spitzer Surveyof Stellar Structure in Galaxies (S4G)[25] and have photometric decompositions available in3.6µm[10]. The selected galaxies are relatively small with circular velocities below 180 km s−1.All but one are bulgeless.The galaxies were exposed for almost three hours. They were reduced using the stan-dard ESO pipelines. We used ZAP[26] to remove the sky residuals. The datacubes whereVoronoi-binned[3] to a signal-to-noise ratio of S/N ∼ 25 in the wavelength range between5490 Å and 5510 Å. Then PPXF[4] was applied to the spectra obtained for each Voronoi bin.PPXF fits the spectra with a linear combination of spectral energy distribution templates andobtains the line of sight velocity distribution. We used the templates from the MIUSCATlibrary[27]. The templates and the spectra where Gauss-convolved to match their spectralfull width at half maximum at every wavelength.106 The rotation curves of thin and thick discs as seen with MUSE-30 -20 -10 0 10 20 30-30-20-100102030-30-20-100102030                                          NWSE              -30-20-100102030-30-20-100102030VelocityNWSE-100-50050100V (km/s)Velocity dispersion-30 -20 -10 0 10 20 30       -30-20-100102030-30-20-100102030NWSE020406080100σ (km/s)ESO 157-49-100-50050100V (km/s)       -2 -1 0 1 2x (kpc)NWz=0.00′′z=1.53′′z=3.05′′z=4.58′′z=6.10′′z=7.63′′z=9.15′′       -2 -1 0 1 2x (kpc)          SEz=0.00′′z=-1.53′′z=-3.05′′z=-4.58′′z=-6.10′′z=-7.63′′z=-9.15′′-30 -20 -10 0 10 20 30x (arcsec)       -40-2002040∆V (km/s)     -30 -20 -10 0 10 20 30x (arcsec)                 020406080100120V (km/s) RRBB-10 -5 0 5 10z (arcsec)-0.5 0.0 0.5z (kpc)Figure 1: Top-left: Image of ESO 157-49. Middle-left: Velocity map of ESO 157-49. Bottom-left: Velocity dispersion map of ESO 157-49. The values in the axes are in arcseconds. Northis up and East is left (shown by the reverse-L symbol). The lines parallel to the mid-planedenote the heights for which the rotation curves (top-right panels) were computed. The thincontinuous line indicates the mid-plane and the thick continuous lines indicate the heightwhere the thick disc starts to dominate the surface brightness[10]. The middle-right panelsshow the difference between the velocity at a given height and that at the mid-plane. Thedashed vertical lines indicate where the rotation curve becomes flat. The bottom-right panelshows the velocity as a function of height for the flat part of the rotation curve for the blue-shifted and red-shifted sides of the galaxy (indicated by “B” and “R”, respectively). Thevertical lines indicate the height above which the thick disc dominates the surface brightness.3 ResultsWe produced velocity and velocity dispersion maps for the eight galaxies in our sample. Weused the velocity maps to compute rotation curves at different heights above and below themid-planes. In Fig. 1 we show the maps and the rotation curves extracted for ESO 157-49.In Fig. 2 we show, for the eight galaxies in our sample, the circular velocity as afunction of height for the regions where the rotation curve of the galaxies is flat. We findComerón, S. et al. 107ESO 443-21 vc=151 km/s0 2 4 6z (arcsec)0.0 0.5 1.0 1.5z (kpc)050100150V (km/s)    IC 1553 vc=115 km/s0 2 4 6 8 10z (arcsec)0.0 0.5 1.0 1.5z (kpc)        PGC 28308 vc=111 km/s0 2 4 6 8z (arcsec)0.0 0.5 1.0 1.5z (kpc)        ESO 544-27 vc=96 km/s0 2 4 6 8z (arcsec)0.0 0.5 1.0 1.5z (kpc)        ESO 157-49 vc=90 km/s0 5 10 15 20z (arcsec)0.0 0.5 1.0 1.5z (kpc)050100150V (km/s)    ESO 469-15 vc=83 km/s0 2 4 6 8 10 12z (arcsec)0.0 0.5 1.0 1.5z (kpc)        PGC 30591 vc=82 km/s0 2 4 6 8 10z (arcsec)0.0 0.5 1.0 1.5z (kpc)        IC 217 vc=63 km/s0 5 10 15z (arcsec)0.0 0.5 1.0 1.5z (kpc)        Figure 2: Velocity as a function of height for the flat part of the rotation curve for thegalaxies in our sample. The horizontal axes are scaled so all the plots cover the same heightrange. The grey bands indicate the dispersion in the velocity for the Voronoi bins used tocompute each data point. The maximum velocity at the mid-plane is denoted by vc.that the maximum velocity is lower at increasing distances from the mid-plane, with a lagat large heights of a few tens of kilometres per second for all the galaxies in the sample.This is compatible with the combined effects of asymmetric drift and a not perfectly edge-onorientation. Indeed, in the past we have shown that retrograde thick disc fractions largerthan 10−20% would result in lags larger than those that we detect[9]. Also, large retrogradematerial fractions would result in a velocity dispersion comparable to the circular velocity ofthe galaxy, vc. Therefore, our data indicate a very small fraction – compatible with zero –of retrograde stars in the thick discs. We shall however note that only five out of our eightgalaxies have massive thick discs that dominate the luminosity at some of the observed heightrange[10].4 Discussion and conclusionsOur observations increase the number of available thick disc rotation curves to about fifteen.Only one of them (FGC 227[29]) shows clear evidence for retrograde stars. One may thus108 The rotation curves of thin and thick discs as seen with MUSEnaively conclude that accreted thick discs are rare. This is, however, not necessarily truebecause accretion is more efficient for prograde encounters than it is for retrograde ones. Thereason for that is how dynamical friction works. Dynamical friction in its linear regime actsover an infalling satellite as described by the Chandrasekhar formula[5]F ∝M2ρ/V 2, (1)where M is the mass of the satellite, ρ the density of the main galaxy, and V the relativevelocity between the satellite and the particles of the main galaxy. When the satellite is faraway from the disc of the main galaxy, V does not depend on whether the merger is progradeor retrograde because the dark matter halo is pressure-supported and, hence, has a meanvelocity of zero. However, as the satellite falls and becomes influenced by the disc, progradeobjects feel a larger force than retrograde ones. The detailed calculation on the decay timesof satellites requires numerical simulations, and it can be estimated that a retrograde satellitetakes at most two times longer than a prograde one to be accreted[20]. Given a halo mergerrate proportional to ∝ (1 + z)2[13], isotropic accretion, and a galaxy age of 10 Gyr, we obtainthat about a third of the satellites accreted by now had retrograde orbits.Another effect to be taken into account is in-plane dragging. Prograde satellites withorbital inclinations smaller than i = 20o are dragged into the plane of the main galaxyby dynamical friction and dissolve into the thick disc[21]. Satellites with larger incominginclinations dissolve into the inner halo. The effect of in-plane dragging is diminished forretrograde satellites and we estimate that only those with inclinations smaller than i = 10oare accreted into the thick disc. Hence, an accreted retrograde satellite is two times less likelyto end in a thick disc than a prograde satellite.In summary: 1) one third of the satellites accreted by now by disc galaxies were retro-grade and 2) a retrograde merger is two times less likely to contribute to a thick disc than aprograde one. This implies that in the limit where all thick discs have been created in singlemerger events, we expect ∼ 1/6 of the thick discs to be retrograde. However, the observedfigure is ∼ 1/15. Hence, our results point to a tension between the observations and thehypothesis of accreted thick discs. If thick discs were created in a small number of mergers(two or three), the tension would worsen. We can therefore conclude that an accretion originfor thick discs is not favoured by the observations.AcknowledgmentsSC and HS acknowledge support from the Academy of Finland. HS, JHK, and RFP acknowl-edge financial support from the European Union’s Horizon 2020 research and innovation pro-gramme under Marie Sk lodowska-Curie grant agreement No. 721463 to the SUNDIAL ITNnetwork. JHK acknowledges additional support from the Spanish Ministry of Economy andCompetitiveness (MINECO) under grant number AYA2016-76219-P, as well as from the Fun-dación BBVA under its 2017 programme of assistance to scientific research groups, for theproject “Using machine-learning techniques to drag galaxies from the noise in deep imaging”,and from the Leverhulme Trust through the award of a Visiting Professorship at LJMU.Comerón, S. et al. 109References[1] Abadi, M. G., Navarro, J. F., Steinmetz, M., et al. 2003, ApJ, 597, 21[2] Bournaud, F., Elmegreen, B. 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Thick discs in galaxies were most likely not accreted

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