[Paper Review] The structure of galactic disks: Studying late-type spiral galaxies using SDSS

M. Pohlen, Ignacio Trujillo|ArXiv.org|Mar 24, 2006

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy59 references221 citations

TL;DR

This study analyzes the radial stellar light profiles of 90 nearby late-type spiral galaxies using SDSS imaging data, revealing that only 10% exhibit purely exponential disks down to surface brightnesses of ~27 mag/arcsec². Instead, most galaxies show broken exponential profiles: 60% have downbending breaks (steeper outer regions) at 1.5–4.5 scalelengths, while 30% show upbending breaks (shallower outer regions) at 4.0–6.0 scalelengths, with break type correlated to Hubble type.

ABSTRACT

Using imaging data from the SDSS survey, we present the g' and r' radial stellar light distribution of a complete sample of ~90 face-on to intermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. The surface brightness profiles are reliable (1sigma uncertainty less than 0.2 mag) down to mu=~27magsqarcsec. Only ~10% of all galaxies have a normal/standard purely exponential disk down to our noise limit. The surface brightness distribution of the rest of the galaxies is better described as a broken exponential. About 60% of the galaxies have a break in the exponential profile between ~1.5-4.5 times the scalelength followed by a downbending, steeper outer region. Another ~30% shows also a clear break between ~4.0-6.0 times the scalelength but followed by an upbending, shallower outer region. A few galaxies have even a more complex surface brightness distribution. The shape of the profiles correlates with Hubble type. Downbending breaks are more frequent in later Hubble types while the fraction of upbending breaks rises towards earlier types. No clear relation is found between the environment, as characterised by the number of neighbours, and the shape of the profiles of the galaxies.

Motivation & Objective

• To conduct a complete census of the outer disk structure in nearby late-type spiral galaxies using high-sensitivity imaging.
• To determine the prevalence and characteristics of disk truncations and breaks in the faint outer regions of galactic disks.
• To investigate the connection between disk profile morphology (e.g., downbending vs. upbending breaks) and galaxy properties such as Hubble type and environment.
• To test whether classical exponential profiles extend to the faintest detectable surface brightnesses or if breaks are ubiquitous.
• To classify disk profiles objectively using photometric decomposition and derive physical interpretations for different break types.

Proposed method

• Utilized g′ and r′ band imaging data from the Sloan Digital Sky Survey (SDSS) for a complete sample of ~90 face-on to intermediate-inclined, nearby late-type (Sb–Sdm) spiral galaxies.
• Measured radial surface brightness profiles with high dynamic range, achieving 1σ uncertainties <0.2 mag/arcsec² down to μ ~27 mag/arcsec².
• Applied objective profile decomposition techniques, including the use of the derivative method to quantify break radii, to classify galaxies into distinct morphological types.
• Classified galaxies into three main types: Type I (no break), Type II (downbending break), and Type III (upbending break), with subtypes for physical interpretation.
• Distinguished between classical truncations (Type II-CT) and OLR-associated breaks (Type II.o-OLR) in barred galaxies using structural and kinematic priors.
• Correlated break properties (radius, surface brightness) with absolute magnitude and Hubble type to assess structural trends.

Experimental results

Research questions

• RQ1 What is the true nature of the radial stellar light distribution in the faint outer disks of late-type spiral galaxies?
• RQ2 How common are purely exponential disks versus broken exponential profiles in nearby late-type spirals down to surface brightnesses of ~27 mag/arcsec²?
• RQ3 Is there a correlation between the type of disk break (downbending vs. upbending) and the Hubble morphological type of the galaxy?
• RQ4 What is the relationship between break radius, scalelength, and absolute magnitude in galaxies with classical truncations?
• RQ5 Does the environment, as measured by the number of nearby galaxies, influence the shape of the surface brightness profile?

Key findings

• Only 10% of the sample exhibit a purely exponential disk profile down to the noise limit of ~27 mag/arcsec², challenging the long-held assumption of universal exponential decline.
• 60% of galaxies show a downbending break (steeper outer profile) at 1.5–4.5 times the inner disk scalelength, with a typical break radius of 2.5 ± 0.6 scalelengths and a surface brightness of μ_br = 23.5 ± 0.8 r′-mag/arcsec².
• 30% of galaxies display an upbending break (shallower outer profile) at 4.0–6.0 times the scalelength, occurring at a larger radius (4.9 ± 0.6 scalelengths) and lower surface brightness (μ_br = 24.7 ± 0.8 r′-mag/arcsec²).
• Classical truncations (Type II-CT) are more frequent in later Hubble types (e.g., Sdm), while upbending breaks (Type III) become more common in earlier types (e.g., Sb), indicating a morphological dependence.
• No sharp or complete cut-offs were found; all truncated galaxies are better described as broken exponentials with a well-defined break radius, not abrupt truncations.
• For Type III galaxies, the outer upbending component is often associated with disk-like features such as spiral arms, and interactions with neighbors are a likely origin in many cases.