Evidence for Secular Evolution in Late-Type Spirals

Abstract

We combine deep optical and IR photometry for 326 spiral galaxies from two recent galaxy samples and report that the surface brightness profiles of late-type spirals are best fit by two exponentials. Moreover, the ratio of bulge and disk scale lengths takes on a restricted range of values and is uncorrelated with Hubble type. This suggests a scale-free Hubble sequence for late-type spirals. Careful numerical simulations ensure that our results are not affected by seeing or resolution effects. Many of these galaxies show spiral structure continuing into the central regions with a previously undetected small bar and slowly changing colors between the inner disk and the bulge. We invoke secular dynamical evolution and interpret the nature of disk central regions in the context of gas inflow via angular-momentum transfer and viscous transport. In this scenario, galaxy morphologies in late-type spirals are not imprinted at birth but are the result of evolution.Comment: Accepted for publication in the Astrophysical Journal Letters 11 pages LaTeX file, 2 PostScript figures, uses aaspp4.sty, also available at: ftp://sagittarius.tuc.noao.edu/pub/bulges95.ps.Z http://www.astro.su.se/~broeils/papers.html http://star-www.dur.ac.uk/~rdejong/papers.htm

The Bulge/Disk Connection in Late-Type Spirals

“…Recently, the groups of Peletier, Balcells, and Andredakis (Andredakis & Sanders 1994, Andredakis, Peletier & Balcells 1995hereafter APB95), and Courteau, de Jong, Broeils, andHoltzman (de Jong &van der Kruit 1994, de Jong 1995, hereafter dJ95;Courteau, de Jong & Broeils 1996, hereafter CdJB96;Broeils & Courteau 1996, hereafter BC96;, hereafter CH96) have used their high-quality surface brightness (SB) pro les to show that central regions of disks are generally best described by an exponential luminosity pro le.…”

“…The evidence in favor of secular evolution in late-type spirals is compelling which is why I have chosen to focus on it. This article is based in part on a Letter by Courteau, de Jong, and Broeils (1996). Recommended reading material on the topic of exponential disks, secular evolution, or bar dynamics include Struck-Marcell (1991), Sellwood & Wilkinson (1993), Pfenniger (1993), Martinet (1995), and Pfenniger (1996, this conference).…”

GHASP: an Hα kinematic survey of spiral galaxies – X. Surface photometry, decompositions and the Tully–Fisher relation in theRcband

“…Courteau et al (1996) argue that disks should have been formed earlier than bulges and, therefore, the properties of the bulges are linked to their host disks. Due to the relatively low Sérsic indices of the bulges, these are indeed likely to be pseudobulges (Fisher and Drory 2008), which are formed by secular evolution of the disks and, therefore, correlations among these parameters naturally arise in a scenario of secular evolution, disfavoring scenarios of decoupled size relation such as bulges formed by mergers.…”

Internal Dynamics of Local Group Galaxies

“…Putman et al (1998) For comparison, our Galaxy has a smaller boxy bulge with a near-exponential light distribution on the minor axis. This is common for small bulges (Courteau et al 1996). The outer halo of the Galaxy follows the luminous density distribution PL rv r-3 .…”