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NGC 7457: Evidence for Merger-Driven Cylindrical Rotation in Disc the first Official Application of the Schwpy Code!

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NGC 7457: Evidence for Merger-Driven Cylindrical Rotation in Disc the first Official Application of the Schwpy Code! NGC7457: A challenge to our understanding of the evolutionary mechanisms in disc galaxies Alireza Molaeinezhad (IAC, Spain) Dynamics Workshop, Vienna, 8 - 11 October 2019 Formation Theories for disc galaxies Protogalactic Galaxy mergers, collapse Internal versus External RAM-pressure stripping of gas star formation, gas recycling, metal enrichment, energy feedback via supernovae, etc. Fast versus Slow Fast versus Slow Internal Internal versus External Environmental from Kormendy and Kennicutt (2004) Taken Secular evolution Secular evolution driven by bar instability, driven by prolongated gas infall, by dark matter haloes, by minor mergers, by spiral structures, by galaxy harassment, etc. etc. NGC7457: A peculiar low-mass lenticular galaxy in the filamentary group NGC 7331 NGC7457 is a highly inclined S0 galaxy (i = 74 deg) resides in one the 3 subgroups of filamentary group NGC 7331 (Ludwig et al. 2012). It is accompanied by UGC 12311, a spindle-shaped galaxy at a distance of 5.7′ from the centre of NGC 7457. Parameter Value Source Distance (Mpc) 12.9 Alabi et al. (2017) Morphological type SA(rs)0-? de Vaucouleurs et al. (1991) PA (N–E) (deg.) 35 Molaeinezhad et al. (2016) Inclination (deg.) 74 Cappellari et al. (2013) Effective radius (arcsec) 32 de Vaucouleurs et al. (1991) Photometric bulge radii (xB , zB ) (arcsec) 11.0, 9.5 Molaeinezhad et al. (2016) M/L (Using HST/WFPC2 in the I–band) 1.86 Cappellari et al. (2006) V/σ (within 1Re ) 0.62 Cappellari et al. (2007) log(M∗ ) (M⊙ ) 10.13 Alabi et al. (2017) λRe (within 1Re ) 0.570 Cappellari et al. (2007) Gebhardt et al. (2003) MBH (M⊙) 3.5 × 106 Stellar Kinematics Stellar Populations Molaeinezhad et al. 2016, 2017, 2019 NGC7457: A peculiar low-mass lenticular galaxy in the filamentary group NGC 7331 The photometric, kinematics and stellar populations of this galaxy has been a matter of debate: A very small bar-like distortion in the centre (Michard & Marchal 1994). No evidence in favour of a bar-like structure in this galaxy (Molaeinezhad et al. 2016). A significantly high level of cylindrical rotation and unusually low measured velocity dispersion in the bulge (Silchenko et al. 2002; Emsellem et al. 2004; Molaeinezhad et al. 2016). A possible disc-like bulge on the basis of observed deviations from the Faber–Jackson L–σ relation (e.g. Kormendy 1993; Pinkney et al. 2003). Burstein (1979) classified this galaxy as a disc-dominated system with D/B = 1.6 Andredakis et al. (1995) argued the slope of luminosity profile for bulges of this galaxy is much steeper (nSersic = 6) than one would expect for a disc-dominated galaxy, even steeper than namely normal lenticulars and of pure ellipticals. Morphology and photometric features and photometric Morphology Erwin et al. (2015) claim that, the isophotal structure of this galaxy at both inner and outer regions could be very well described with a round classical bulge embedded within a highly inclined disc, with no evidence in favour of any bar-like stellar structure in the isophotes and surface brightness profiles. A counter-rotating core (Sil’chenko et al. 2002, Molaeinezhad et al. 2016, 2019) A chemically distinct nuclei (r < 1. 5) younger than the surrounding bulge (Silchenko et al. 2002) associated with AGN (Gebhardt et al. 2003). populations and Kin. populations Considering the estimated halo assembly epoch of this galaxy, along with the low density environment of this system and discy distribution of its GCs, Zanatta et al. (2018) suggest a secular evolution scenario is the best mechanism to explain the observed properties of this system. Based on a detailed chromodynamical analysis of globular clusters (GCs) in this galaxy, Hargis et al. (2011) conclude that the formation scenario for this galaxy is most-likely through a merger event involving galaxies with unequal masses. 4 Formation scenarios Formation Cylindrical rotation in disc galaxies Cylindrical rotation (Kormendy and Illingworth 1982) is an unusual kinematics feature, observed in B/P bulges. Definition: lines of constant velocity are parallel to each other and perpendicular to the major axis. It means, within the bulge, the rotational velocity does not change with height: ∂ V/∂ |z| ~ 0 5 NGC7457: Very high level of cylindrical rotation with no BAR!!! -band image -band i Exponential disc subtracted disc Exponential Line-of-sight (LOS) Velocity mapand profiles Velocity (LOS) Line-of-sight The velocity map of NGC 7457 clearly exhibits a cylindrical rotation pattern within the photometric bulge region. NGC 7457 has been classified as a SA0-(rs) (de Vaucouleurs et al. 1991) galaxy with no observational evidence supporting the presence of a B/P bulge or an strong bar in the photometric data. The level of cylindrical rotation within the bulge dominated region is remarkably high (mcyl = 0.83 ± 0.06), even higher than that measured for strong B/P bulges in side-on bars with perfectly edge-on orientation!!! Answering the question that ”What is the origin of such high level of cylindrical rotation in bulge of NGC7457?” could offer new insights into our understanding of the formation and evolution of this peculiar galaxy. 6 Schwarzschild’s orbit-superposition method Schwarzschild’s (1979) orbit-superposition method is a powerful dynamical modelling technique that builds galactic models, using a representative library of stellar orbits in a gravitational potential and weighting them to reconstruct the observed surface brightness and kinematics of galaxies. 2D Multiple Gaussian Expansion Intrinsic shape param 2D image 3D triaxial MGE luminosity density space orientation of the galaxy Free param: p, q, u Assume constant stellar M/L (Free Param) Gravitational Potential + A spherical NFW halo + central BH mass (Free param) Intrinsic mass density model DM distribution Free param: C, M200 For all possible combination of free params: (p, q, u, M/L, M200, M_BH) Characterised orbits by r and λz λz Generating Orbits library Comparing orbit-superposition models Select orbits that admit E, I2, I3 With the observed SB and Kinematics Find orbits weights Based on ρ (λz , r) Orbital decomposition (Based on λz) Best triaxial orbital model (cold, warm, hot and CR components) Short-axis tube orbits Box orbits Inner-long axis tube orbits Modeled galaxy + + = The reconstructed kinematics map of the best orbital model for NGC7457 NGC7457: Orbital decomposition Orbital Components: 1. Cold: 0.8 ︎ < λz ︎ < 1.0 (7 %) 2. Warm: 0.25 < λz < 0.8 (43 %) 3. Hot: −0.25 ︎ < λz ︎< 0.25 (46 %) 4. Counter rotating (CR): λz < −0.25 (< 4%) Califa data: Zhu et al. 2018 The reconstructed SB and kinematics maps of the orbital components Cold component Remarkably high Sersic index and concentration (n=2.7, C=0.9) in the central parts. It cannot be fit by a single Sersic profile . It represents a perturbed disc with stars on nearly circular orbits. The reconstructed SB and kinematics maps of the orbital components Warm component Warm orbits contribute in both inner and outer parts of the surface brightness map. The SB profile of the warm orbits are well described by an exponential profile (nSersic = 1.2), constitute a thick disc. The reconstructed SB and kinematics maps of the orbital components Hot component Morphology: Elongated (and triaxial) spheroid. This orbital component with the highest contribution in the total luminosity of the best-fitting model, demonstrates significantly high level of Sersic index (n=3.6) and intrinsic flattening. The surface brightness of the hot component fairly matches the photometric bulge, with comparable scale length (∼ 10′′). The reconstructed LOSVD map of this component shows clear rotation around the major photometric axis of this galaxy, known as ”prolate rotation” The reconstructed SB and kinematics maps of the orbital components CR component The CR component of the best fitting model is highly concentrated (C=1.4) in the central regions (r < 2. 5). The scale length of the CR component matches quite well the measured diameter of the KDC, observed in the centre of NGC 7457 (e.g. Emsellem et al. 2004). NGC 7457’s disc: born hot or dynamically heated? Possible formation scenarios: 1. Disc stars were dynamically hot at birth (e.g. Bird et al. 2013). • Discs being formed originally thick in situ at high redshift by the merger of gas-rich protogalactic fragments and later thin discs may be formed. • This scenario should produce old stellar population in the thick disc, which is in contrast to the generally young population of stars, observed over the entire SAURON field of NGC 7457! 2. Disc stars born in a very thin layer of gas with cold orbits and observed thickening have appeared more recently (e.g. Merrifield et al. 2001). • Stars were born dynamically cold in a primordially thin discs and can get dynamically heated and form a thick disc through different mechanisms e.g. satellite flybys, disturbances by satellite galaxies or mergers and secular thin disc flaring. • Given that, NGC 7457 is a relatively low-mass and dynamically young galaxy, any long-term secular scenario is unlikely. Moreover, there is no observational evidence in favour of possible progenitor of thick disc in our kinematical, chemical properties and dynamical modelling of this galaxy. Comparing the velocity ellipsoid (σz /σR ) of the best-fitting model for NGC 7457 and those predicted by the simulation studies, we suggest that the thick disc in NGC 7457 is most likely a dynamically heated structure, formed through the interactions and accretion of satellite(s) with near-polar initial inclinations (see Villalobos & Helmi (2008) For more details) Polar Merger • This feature has been observed in few massive early-type galaxies, mostly belong to galaxy groups or clusters • Given the rarity of observations for this dynamical feature, its origin is still under debate and not well understood.
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