Composite Bulges -- II. Classical Bulges and Nuclear Discs in Barred Galaxies: The Contrasting Cases of NGC 4608 and NGC 4643

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Peter Erwin (1), Anil Seth (2), Victor P. Debattista (3), Marja Seidel (4), Kianusch Mehrgan (1), Jens Thomas (1), Roberto Saglia (1), Adriana de Lorenzo-Cáceres (5), Witold Maciejewski (6), Maximilian Fabricius (1), Jairo Méndez-Abreu (5), Ulrich Hopp (1), Matthias Kluge (1), John E. Beckman (5), Ralf Bender (1), Niv Drory (7), Deanne Fisher (8) ((1) Max-Planck-Institut fuer extraterrestrische Physik, Garching, Germany, (2) Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah, USA, (3) Jeremiah Horrocks Institute, University of Central Lancashire, Preston, UK, (4) IPAC, California Institute of Technology, Pasadena, California, USA, (5) Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain, (6) Astrophysics Research Institute, Liverpool John Moores University, Birkenhead, UK, (7) McDonald Observatory, The University of Texas at Austin, Austin, Texas, USA, (8) Center for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Australia)

We present detailed morphological, photometric, and stellar-kinematic analyses of the central regions of two massive, early-type barred galaxies with nearly identical large-scale morphologies. Both have large, strong bars with prominent inner photometric excesses that we associate with boxy/peanut-shaped (B/P) bulges; the latter constitute ~ 30% of the galaxy light. Inside its B/P bulge, NGC 4608 has a compact, almost circular structure (half-light radius R_e approx. 310 pc, Sérsic n = 2.2) we identify as a classical bulge, amounting to 12.1% of the total light, along with a nuclear star cluster (R_e ~ 4 pc). NGC 4643, in contrast, has a nuclear disc with an unusual broken-exponential surface-brightness profile (13.2% of the light), and a very small spheroidal component (R_e approx. 35 pc, n = 1.6; 0.5% of the light). IFU stellar kinematics support this picture, with NGC 4608's classical bulge slowly rotating and dominated by high velocity dispersion, while NGC 4643's nuclear disc shows a drop to lower dispersion, rapid rotation, V-h3 anticorrelation, and elevated h4. Both galaxies show at least some evidence for V-h3 correlation in the bar (outside the respective classical bulge and nuclear disc), in agreement with model predictions. Standard 2-component (bulge/disc) decompositions yield B/T ~ 0.5-0.7 (and bulge n > 2) for both galaxies. This overestimates the true "spheroid" components by factors of four (NGC 4608) and over 100 (NGC 4643), illustrating the perils of naive bulge-disc decompositions applied to massive barred galaxies.


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