Revisiting the structure of galactic disks with deep imaging

TL;DR

This study reconsiders the canonical three-Type framework for disk galaxy surface brightness profiles by leveraging deep IAC Stripe82 imaging and stellar mass density profiles. By analyzing a mass-selected sample (~$10^{10}$ M$_⊙$) of 44 spirals, the authors show that galaxies of Types I, II-CT, and III-d share a common edge in their stellar mass distributions, located near $μ_g(R_{edge}) \sim 26.5$ mag arcsec$^{-2}$ with $Σ(R_{edge}) \sim 0.5$–$1$ M$_⊙$ pc$^{-2}$, suggesting the edge marks the end of in situ star formation. Type III-d galaxies, while lacking a clear edge in elliptically averaged mass profiles, reveal a defined edge when analyzed with wedge directions, underscoring the role of outer-disk asymmetry in shaping mass profiles. The results imply a unifying view where differences in surface brightness shapes mainly reflect bulge prominence and outer-disk assembly, with deeper imaging likely motivating a revised Type nomenclature such as I+II, II+II, or III-d+II. Overall, the work demonstrates how deeper mass-density information refines our understanding of disk structure and galaxy evolution, pointing to future surveys (e.g., LSST) to further test these edges and their physical origins.

Abstract

Thanks to new advances in astronomical imaging, we can now routinely explore disk galaxy profiles about two magnitudes deeper than the data available 20 years ago. In this regard, it is an opportune time to reevaluate the past classifications of different surface brightness Types. In this paper, we explore the stellar mass profiles of a sample of disk galaxies with similar stellar masses ($\sim$10$^{10}$ M$_{\odot}$) using IAC Stripe82 Legacy Project data. We find that Type I, II-CT, and III-d galaxies exhibit edges at surface brightnesses $μ_g$(R$_{edge}$)$\sim$26.5 mag/arcsec$^2$ and surface mass densities $Σ$(R$_{edge}$)$\sim$0.5-1 M$_{\odot}$/pc$^2$. These surface brightnesses were outside the range of typical SDSS images and, therefore, unstudied. The present data suggest (although the statistics are inconclusive) that the main difference between the previous profile Types is the presence of either a more or a less intense bulge. Half of the Type II-CT galaxies are Sc, whereas half of the Type III-d galaxies are Sb. Finally, Type III-d galaxies have low mass surface density disks ($Σ$($Δ$ $R_{\mathrm{exp}}$)$\sim$0.5 M$_{\odot}$/pc$^2$) and are bluer $(g-r)_0$=0.32$\pm$0.02 than the other Types ($(g-r)_0$=0.42$\pm$0.02 for Type I and II-CT), suggesting that their disks probably formed later in time.

Figures (10)

• Figure 1: Stellar surface mass density profiles of the galaxies in our sample (grey lines), split into their different Types according to their surface brightness profiles (see Section \ref{['classification']} for details). The profiles have been normalized in the radial direction to the location of their edges Chamba_2022. In each panel, the average profile is shown in magenta. The blue and green lines correspond to exponential fits to the weighted average profiles inside and outside R$_{edge}$. The spatial range used to fit the inner exponential is dictated by the excess of stellar mass produced by the bulge region.
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