[Paper Review] Modelling the spectral energy distribution of galaxies. IV Correcting apparent disk scalelengths and central surface brightnesses for the effect of dust at optical and near-infrared wavelengths

C. Möllenhoff, C. C. Popescu+1 more|ArXiv.org|Jun 22, 2006

Galaxies: Formation, Evolution, Phenomena48 references58 citations

TL;DR

This paper presents a radiative transfer model to correct observed disk scalelengths and central surface brightnesses in spiral galaxies for dust extinction effects at optical and near-infrared wavelengths. Using simulated images with physically consistent star-dust geometries, it quantifies how dust increases apparent scalelengths by up to 50% and alters central surface brightnesses by up to 1.5 magnitudes, with the primary driver of observed B-band scalelengths being dust rather than intrinsic size evolution.

ABSTRACT

We present corrections for the change in the apparent scalelengths, central surface brightnesses and axis ratios due to the presence of dust in pure disk galaxies, as a function of inclination, central face-on opacity in the B-band (tau^f_B) and wavelength. The correction factors were derived from simulated images of disk galaxies created using geometries for stars and dust which can reproduce the entire spectral energy distribution from the ultraviolet (UV) to the Far-infrared (FIR)/submillimeter (submm) and can also account for the observed surface-brightness distributions in both the optical/Near-infrared and FIR/submm. We found that dust can significantly affect both the scalelength and central surface brightness, inducing variations in the apparent to intrinsic quantities of up to 50 percent in scalelength and up to 1.5 magnitudes in central surface brightness. We also identified some astrophysical effects for which, although the absolute effect of dust is non-negligible, the predicted variation over a likely range in opacity is relatively small, such that an exact knowledge of opacity is not needed. Thus, for a galaxy at a typical inclination of 37 degrees and having any tau^f_B>2, the effect of dust is to increase the scalelength in B relative to that in I by a factor of 1.12 +- 0.02 and to change the B-I central colour by 0.36 +- 0.05 magnitudes. Finally we use the model to analyse the observed scalelength ratios between B and I for a sample of disk-dominated spiral galaxies, finding that the tendency for apparent scalelength to increase with decreasing wavelength is primarily due to the effects of dust.

Motivation & Objective

• To correct observed photometric parameters (scalelength, central surface brightness, axis ratio) in disk galaxies for the effects of dust extinction.
• To quantify how dust alters apparent scalelengths and surface brightnesses as a function of inclination, B-band face-on optical depth (τ⁰_B), and wavelength.
• To determine whether observed trends in scalelength ratios (e.g., B-band vs. I-band) are primarily due to dust or intrinsic structural evolution.
• To provide a publicly available, systematic set of correction factors for use in studies of galaxy size evolution across cosmic time.
• To assess whether precise knowledge of opacity is required for certain corrections, given the sensitivity of observed trends to dust effects.

Proposed method

• Simulated face-on and inclined disk galaxy images using a physically consistent star-dust geometry derived from SED fitting across UV to submillimeter wavelengths.
• Applied radiative transfer calculations to compute the extinction and reprocessing of starlight through dust, accounting for both direct extinction and re-emission in FIR/submm.
• Fitted the simulated images with a dustless exponential disk model to extract apparent photometric parameters (scalelength, central surface brightness, axis ratio).
• Compared these apparent parameters to the intrinsic values from the simulation input to derive correction factors for dust effects.
• Systematically varied inclination (i), central face-on B-band optical depth (τ⁰_B), and wavelength (B, I, NIR) to tabulate corrections.
• Validated the model against observed scalelength ratios in a sample of disk-dominated spirals to test the dust hypothesis.

Experimental results

Research questions

• RQ1 To what extent does dust cause the observed increase in apparent disk scalelengths at shorter wavelengths (e.g., B-band) compared to longer wavelengths (e.g., I-band)?
• RQ2 How do apparent scalelengths and central surface brightnesses vary with inclination and optical depth due to dust extinction and geometry effects?
• RQ3 What are the quantitative correction factors needed to convert observed (apparent) photometric parameters into intrinsic values for disk galaxies?
• RQ4 Is the observed trend of larger B-band scalelengths than I-band scalelengths primarily driven by dust, or by intrinsic structural evolution?
• RQ5 For studies of galaxy size evolution with redshift, how significant is the bias introduced by increasing dust opacity at higher redshifts?

Key findings

• Dust can increase apparent disk scalelengths by up to 50% relative to intrinsic values, depending on inclination and optical depth.
• The apparent-to-intrinsic central surface brightness ratio can vary by up to 1.5 magnitudes due to dust, depending on the balance between extinction and increased column density from inclination.
• For a typical galaxy at 37° inclination and τ⁰_B > 2, the B-band scalelength is 12% larger than in the I-band (factor of 1.12 ± 0.02), and the B-I central colour is reddened by 0.36 ± 0.05 magnitudes.
• The observed trend of larger apparent scalelengths in B-band than in I-band is primarily due to dust effects, not intrinsic size evolution.
• The scatter in observed B-to-I scalelength ratios (σ = 0.094) is comparable to measurement errors, indicating that dust is the dominant physical driver of the observed trend.
• The model successfully reproduces observed scalelength ratios in disk-dominated spirals, unlike previous models that failed to fit the data, especially for normal galaxies without bulges.