[Paper Review] A Corona Australis cloud filament seen in NIR scattered light I. Comparison with extinction of background stars

M. Juvela, Pelkonen, V. -M.+2 more|ArXiv.org|Aug 12, 2007

Astrophysics and Star Formation Studies61 references17 citations

TL;DR

This study confirms that near-infrared (NIR) scattered light provides a reliable, high-resolution tracer of dust column density in quiescent interstellar clouds, with results agreeing closely with extinction measurements from background stars up to $A_{\rm V} \sim 15^\mathrm{m}$. The method outperforms star-based extinction mapping in spatial resolution and reveals no significant contribution from thermal dust emission in the Ks band, validating scattered light as a robust tool for mapping cloud structure at high precision.

ABSTRACT

With current near-infrared (NIR) instruments the near-infrared light scattered from interstellar clouds can be mapped over large areas. The surface brightness carries information on the line-of-sight dust column density. Therefore, scattered light could provide an important tool to study mass distribution in quiescent interstellar clouds at a high, even sub-arcsecond resolution. We wish to confirm the assumption that light scattering dominates the surface brightness in all NIR bands. Furthermore, we want to show that scattered light can be used for an accurate estimation of dust column densities in clouds with Av in the range 1-15mag. We have obtained NIR images of a quiescent filament in the Corona Australis molecular cloud. The observations provide maps of diffuse surface brightness in J, H, and Ks bands. Using the assumption that signal is caused by scattered light we convert surface brightness data into a map of dust column density. The same observations provide colour excesses for a large number of background stars. These data are used to derive an extinction map of the cloud. The two, largely independent tracers of the cloud structure are compared. Results. In regions below Av=15m both diffuse surface brightness and background stars lead to similar column density estimates. The existing differences can be explained as a result of normal observational errors and bias in the sampling of extinctions provided by the background stars. There is no indication that thermal dust emission would have a significant contribution even in the Ks band. The results show that, below Av=15mag, scattered light does provide a reliable way to map cloud structure. Compared with the use of background stars it can also in practice provide a significantly higher spatial resolution.

Motivation & Objective

• To test whether near-infrared scattered light dominates surface brightness in quiescent molecular clouds, enabling accurate dust column density estimation.
• To compare column density maps derived from NIR scattered light with those from background star extinction, assessing consistency and resolution differences.
• To evaluate the reliability of scattered light as a tracer in regions with $A_{\rm V} \leq 15^\mathrm{m}$, where extinction gradients and sparse star sampling may bias traditional methods.
• To determine whether thermal dust emission contributes significantly to observed NIR intensities, particularly in the Ks band.
• To assess the feasibility of using scattered light for high-resolution mapping in areas with low background star counts, such as cloud centers.

Proposed method

• Deep near-infrared imaging in J, H, and Ks bands was conducted using ESO telescopes at La Silla Paranal Observatory (programme ID 077.C-0338).
• Surface brightness maps were converted into dust column density maps using radiative transfer models assuming scattering dominates the signal.
• Independent extinction maps were generated using the NICER method applied to colour excesses of background stars in the same field.
• The two tracers—scattered light and star extinction—were spatially compared across the filament, with statistical analysis of discrepancies.
• Sensitivity and resolution comparisons were made between the NIR scattered light method and sub-millimeter instruments like SIMBA, accounting for integration time and beam size.
• Spectral energy distributions of scattered light were modeled using standard dust scattering laws (e.g., Mathis et al. 1983; Lehtinen et al. 1996) to verify consistency with observed intensities.

Experimental results

Research questions

• RQ1 Does near-infrared scattered light dominate the observed surface brightness in quiescent interstellar filaments, particularly in the Ks band?
• RQ2 How do column density estimates derived from NIR scattered light compare with those from background star extinction across varying $A_{\rm V}$ ranges?
• RQ3 To what extent do observational biases, such as sparse star sampling and extinction gradients, affect the accuracy of extinction maps based on background stars?
• RQ4 Is thermal dust emission a significant contributor to the observed NIR intensities in low-to-moderate extinction regions?
• RQ5 Can NIR scattered light mapping achieve higher spatial resolution than star-based extinction mapping, especially in regions with few background stars?

Key findings

• Scattered light surface brightness in the J, H, and Ks bands is consistent with a dominant contribution from dust scattering, with no significant evidence of thermal emission at low $A_{\rm V}$.
• Column density estimates from scattered light and background star extinction agree within $\sim$0.5–1.0 $A_{\rm V}$ mag in regions with $A_{\rm V} \leq 15^\mathrm{m}$, indicating strong consistency between the two tracers.
• The scattered light method provides spatial resolution a few times better than the star-based method, which is limited by stellar density and beam smoothing.
• In the $A_{\rm V} = 15-20^\mathrm{m}$ range, scattered light predicts up to 50% higher extinction than the colour excess method, likely due to bias from sparse star sampling and strong gradients.
• In the filament center, where no background stars are detected, the NICER method fails but scattered light morphology still reveals local extinction maxima, enabling qualitative mapping.
• The required observing time for a sub-millimeter instrument like SIMBA to match the accuracy of the NIR scattered light method is estimated at 13–52 hours, highlighting the efficiency of NIR scattering for high-resolution mapping.