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

Abstract

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.