[Paper Review] The Occulting Galaxy Pair UGC 3995; Dust properties from HST and CALIFA data

Benne W. Holwerda, William C. Keel|ThinkIR: The University of Louisville's Institutional Repository (University of Louisville)|May 23, 2013

Galaxies: Formation, Evolution, Phenomena56 references17 citations

TL;DR

This study maps dust extinction in the foreground spiral galaxy UGC 3995B using Hubble Space Telescope WFPC2 imaging and CALIFA PPAK IFU spectroscopy, revealing a non-standard extinction distribution peaking at A_V = 0.3–0.4 with an exponential decline at A_V > 0.4 and an unusually high characteristic scale length of 0.5 mag. The IFU data show structured spiral extinction but inconsistent R_V values due to spatial averaging and potential dust reprocessing in the interacting system.

ABSTRACT

UGC 3995 is an interacting and occulting galaxy pair. UGC 3995B is a foreground face-on spiral and UGC 3995A a bright background spiral with an AGN. We present analysis of the dust in the disc of UGC 3995B based on archival Hubble Space Telescope WFPC2 and PPAK IFU data from the CALIFA survey's first data release. From the HST F606W image, we construct an extinction map by modeling the isophotes of the background galaxy UGC 3995A and the resulting transmission through UGC 3995B. This extinction map of UGC 3995B shows several distinct spiral extinction features. The radial distribution of $A_V$ values declines slowly with peaks corresponding to the spiral structures. The distribution of $A_V$ values in the HST extinction map peaks near $A_V = 0.3-0.4$. Beyond this point, the distribution of $A_V$ values drops like an exponential: $N(A_V) = N_0 imes e^{(-A_V/0.5)}$. The 0.5 value is higher than typical for a spiral galaxy. The outer arms may be tidally distended; the extinction in the corresponding interarm regions is small to an unusually small radius. To analyse the PPAK IFU data, we take the ratio of a fibre spectrum in the overlap region and the corresponding background fiber spectrum to construct an extinction curve. We fit the Cardelli, Clayton and Mathis (CCM) curve to the extinction curve of each fiber element in the overlap region. A map of the extinction constructed from PPEX IFU data-cubes shows the same spiral structure of the HST extinction map but the some differences in the distribution of the normalization of the CCM fits ($A_V$). The inferred extinction slopes ($R_V$) maps do not display any structure and a range of values partly due to the sampling effects of the disc by fibers, sometimes due to bad fits, and possibly partly due to some reprocessing of dust grains in the interacting disc. (abridged)

Motivation & Objective

• To map the spatial distribution of dust extinction in the foreground spiral galaxy UGC 3995B using high-resolution HST imaging and IFU spectroscopy.
• To investigate the geometry and properties of dust in a spiral disc by analyzing the extinction of a background AGN-containing spiral galaxy (UGC 3995A) through the foreground disc.
• To assess the reliability of standard extinction laws (e.g., R_V = 3.1) in resolved regions of interacting spiral discs using IFU data.
• To determine whether dust properties—especially the extinction law slope R_V—vary across the disc due to structural asymmetry, grain processing, or spatial sampling effects.
• To compare the results with previous occulting pair studies to assess consistency in dust distribution and extinction law behavior.

Proposed method

• Constructed an extinction map of UGC 3995B using HST F606W images by modeling the isophotes of the background galaxy UGC 3995A and inferring transmission through the foreground disc.
• Fitted the Cardelli, Clayton & Mathis (CCM) extinction law to the ratio of fibre spectra in the overlap region (PPAK IFU data) to derive A_V and R_V per spatial element.
• Produced radial and azimuthal profiles of A_V to analyze the distribution of extinction across the disc and in spiral arms vs. interarm regions.
• Compared the HST-derived extinction map with the IFU-derived extinction map to validate spatial structure and detect discrepancies in A_V and R_V distributions.
• Used spectral averaging across all fibres to demonstrate how spatial mixing leads to a false recovery of a grey extinction curve (R_V ≈ 3.1) despite underlying variability.
• Assessed the impact of spatial sampling (≈0.9 kpc per fibre) and disc asymmetry on the reliability of extinction law fits, particularly R_V.

Experimental results

Research questions

• RQ1 What is the radial and azimuthal distribution of dust extinction (A_V) in the foreground spiral UGC 3995B, and how does it compare to typical spiral galaxies?
• RQ2 Why is the A_V distribution in UGC 3995B shifted toward higher values (peaking at 0.3–0.4 mag) compared to the typical A_V = 0 distribution in other galaxies?
• RQ3 To what extent do the inferred R_V values from IFU data reflect true dust grain properties or are they biased by spatial averaging and fibre sampling?
• RQ4 How does the interaction-induced asymmetry in UGC 3995B affect the reliability of extinction law measurements in resolved regions?
• RQ5 Can the observed deviation from the canonical R_V = 3.1 extinction law be attributed to dust reprocessing, grain size evolution, or instrumental/sampling limitations?

Key findings

• The A_V distribution in the HST extinction map peaks at 0.3–0.4 mag, significantly higher than typical spiral galaxies, indicating a non-uniform dust distribution or enhanced column density in the disc.
• The A_V distribution follows an exponential decline with N(A_V) = N₀ × e^(-A_V/0.5), with a characteristic scale length of 0.5 mag, which is higher than the typical value of ~0.3–0.4 mag in other spirals.
• The radial distribution of A_V in UGC 3995B declines slowly, with peaks corresponding to spiral structures, suggesting that spiral arms are the primary sites of dust concentration.
• The IFU-derived extinction map reproduces the spiral structure seen in the HST map, confirming consistency between the two datasets despite different spatial resolutions.
• The inferred R_V values from IFU data show a broad range of values, with no spatial structure, likely due to fiber sampling of mixed opaque and transparent regions, poor fits, and potential dust reprocessing in the interacting disc.
• Spectral averaging across all fibres recovers a grey extinction curve (R_V ≈ 3.1), demonstrating that unresolved spatial mixing can mask true extinction law variations, even when individual fibres show significant scatter.