High-resolution MEGARA Integral-field Unit Spectroscopy and Structural Analysis of a Fast-rotating, Disky Bulge in NGC 7025

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Disky bulges in spiral galaxies are commonly thought to form out of disk materials (mainly) via bar-driven secular processes. They are structurally and dynamically distinct from “classical bulges,” which are built in violent merger events. We use high-resolution GTC/MEGARA integral-field unit spectroscopic observations of the Sa galaxy NGC 7025, obtained during the MEGARA commissioning run, together with detailed 1D and 2D decompositions of this galaxy’s Sloan Digital Sky Survey i-band data to investigate the formation of its disky (bulge) component, which makes up ∼30% of the total galaxy light. With a Sérsic index n ~ 1.80  0.24, a half-light radius R_(e) ~ 1.70  0.43 kpc, and stellar mass M_(*) ~ (4.34  1.70) x 10^(10)M_(☉), this bulge dominates the galaxy light distribution in the inner R ~ 15 (∼4.7 kpc). Measuring the spins (λ) and ellipticities (Є) enclosed within nine different circular apertures with radii R ≤ R_(e), we show that the bulge, which exhibits a spin track of outwardly rising λ and Є, is a fast rotator for all the apertures considered. Our findings suggest that this inner disky component is a pseudo-bulge, consistent with the stellar and dust spiral patterns seen in the galaxy down to the innermost regions but in contrast to the classical bulge interpretation favored in the past. We propose that a secular process involving the tightly wound stellar spiral arms of NGC 7025 may be driving gas and stars out of the disk into the inner regions of the galaxy, building up the massive pseudo-bulge.
© 2019 The American Astronomical Society. We thank the referee for their careful reading of the paper and many suggestions that improved the paper. B.T.D. acknowledges support from a Spanish postdoctoral fellowship, “Ayudas 1265 para la atracción del talento investigador. Modalidad 2: jóvenes investigadores,” funded by Comunidad de Madrid under grant no. 2016-T2/TIC-2039. We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness under grant no. AYA2016-75808-R, which is partly funded by the European Regional Development Fund, and from the Excellence Network MægNet (AYA2017-90589-REDT). We acknowledge financial support from SpaceTec-CM: Desarrollo de nuevas tecnologías para instrumentación espacial en la Comunidad de Madrid, S2013/ICE-2822. Funding for SDSS-IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for HighPerformance Computing at the University of Utah. The SDSS website is located at SDSS-IV is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration, which include the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, the Harvard–Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, Johns Hopkins University, the Kavli Institute for the Physics and Mathematics of the Universe/University of Tokyo, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam, MaxPlanck-Institut für Astronomie (Heidelberg), Max-PlanckInstitut für Astrophysik (Garching), Max-Planck-Institut für Extraterrestrische Physik, the National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatário Nacional/ MCTI, Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, the United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.
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