Composite bulges – II. Classical bulges and nuclear discs in barred galaxies: the contrasting cases of NGC 4608 and NGC 4643

Creators: Erwin, Peter; Seth, Anil; Debattista, Victor P.; Seidel, Marja; Mehrgan, Kianusch; Thomas, Jens; Saglia, Roberto; de Lorenzo-Cáceres, Adriana; Maciejewski, Witold; Fabricius, Maximilian; Méndez-Abreu, Jairo; Hopp, Ulrich; Kluge, Matthias; Beckman, John E.; Bender, Ralf; Drory, Niv; Fisher, Deanne

Abstract

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 per cent of the galaxy light. Inside its B/P bulge, NGC 4608 has a compact, almost circular structure (half-light radius R_e ≈ 310 pc, Sérsic n = 2.2) we identify as a classical bulge, amounting to 12.1 per cent 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 per cent of the light), and a very small spheroidal component (R_e ≈ 35 pc, n = 1.6; 0.5 per cent 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 two-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 4 (NGC 4608) and over 100 (NGC 4643), illustrating the perils of naive bulge-disc decompositions applied to massive barred galaxies.

Additional Information

© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2021 January 12. Received 2021 January 11; in original form 2020 March 23. Published: 16 January 2021. We thank the referee for their comments on this paper, which have led to several improvements. We are also happy to thank Dimitri Gadotti, Matias Blaña Diaz, and Mattia Sormani for comments on earlier drafts of this paper. VPD was supported by Science and Technology Facilities Council Consolidated grant ST/R000786/1. AdLC acknowledges support from grant AYA2016-77237-C3-1-P and JMA acknowledges support from grant AYA2017-83204-P, both from the Spanish Ministry of Economy and Competitiveness (MINECO). This research is based on observations made with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope Science Institute. Support for Program number GO-15133 was provided by NASA through a grant from the Space Telescope Science Institute. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. This work is based in part on observations made with the Spitzer Space Telescope, obtained from the NASA/IPAC Infrared Science Archive, both of which are operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration. This paper also makes use of data obtained from the Isaac Newton Group Archive that is maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge. This paper is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 097.B-0640(A). Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the US Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS website is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington. This research also made use of both the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and the Lyon-Meudon Extragalactic Database (LEDA; http://leda.univ-lyon1.fr). Data Availability: The raw HST, Spitzer, and MUSE data for both galaxies are publicly available from the respective archives. Reduced, mosaic Spitzer images for individual observations of both galaxies are also available from the Spitzer archive, and the reduced S4G mosaic image for NGC 4643 is available at the NASA/IPAC Infrared Science Archive (https://irsa.ipac.caltech.edu/data/SPITZER/S4G/). The reduced, combined MUSE datacube for NGC 4643 is available at http://archive.eso.org/wdb/wdb/adp/phase3_spectral/form? collection_name= MUSE_DEEP; our kinematic analyses of this datacube are available on request. Finally, the SAURON kinematic data for both galaxies are available at the ATLAS3D site (http://www-astro.physics.ox.ac.uk/atlas3d/. Reduced images of our WFC3-IR F160W observations of both galaxies, along with sky-subtracted versions of the IRAC1 images, mask and PSF images, ellipse-fit files, IMFIT config files, and PYTHON code for generating the figures in this paper can all be found in a Github repository at https://github.com/perwin/n4608-n4643 and also at https://doi.org/10.5281/zenodo.4235501.

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