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An overlap galaxy pair close to NGC 253
A couple of weeks ago, while observing on Jimi's 48-inch, Howard Banich, Jimi and I took a look at several galaxies around the periphery of NGC 253. One of these is 2MASX J00482185-2507365 = LEDA 198197, which is located 15' NE of the center of NGC 253, roughly 3' beyond the northeast end of the galaxy. It was an easy target in the big scope, noted as "fairly faint, elongated 3:2 SW-NE, ~21"x14", small brighter core. Within a N-S string of stars with a mag 14 star 1' N and a similar star 1' SE." I was surprised to find this object has a redshift of z = .064, which implies a light-travel time of ~860 million years. Furthermore, although we saw only a single glow, it's actually an overlap pair!
The galaxy is located at the very left edge of this image from ESO's 1.5-metre Danish telescope
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2MASX J00482185-2507365 has been the focus of studies with both the HST and the Very Large Telescope (VLT) at Paranal. First was a 2008 study "An Extended Dust Disk in a Spiral Galaxy: An Occulting Galaxy Pair in the ACS Nearby Galaxy Survey Treasury" (AJ, 137, 3000).
The abstract caught my attention -- "The foreground disk system (at z ≤ 0.06) shows a dusty disk much more extended than the starlight, with spiral lanes seen in extinction out to 1.5*radius, approximately 6 half-light radii. This pair is the first where extinction can be mapped reliably out to this distance from the center." Here's the HST image!
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... and here are a couple of quotes from the study. "In this paper, we report on an occulting pair with nearly ideal geometry. The background galaxy (z = 0.06) is a spiral with a prominent, regular, and smooth bulge, which is partially occulted by a foreground spiral. The pair was serendipitously discovered in HST images from the Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST) survey. We explore the radial and spatial distribution and the inferred extinction law of the dust in a smaller foreground spiral, which displays an extent of dust hitherto unseen in backlit galaxies."
"The radial extent of the dust in the foreground disk is the most striking feature of the pair. In a part of the disk where there is barely any light from the foreground galaxy, there is significant structured extinction evident in the spiral arms, readily visible in the color image (Figure 2). At the corresponding radius on the other side of the disk, even these long exposures do not detect starlight, indicating that the cold dense ISM extends well past the optical limit of the disk. The presence of significant amounts of dust at these radii also suggests that substantial quantities of metals have been transported well outside the starforming regions, either through radial mixing or a galactic fountain."
The discovery was followed up by a 2013 paper "VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve" in MNRAS, 433, 47-59.
"We present VLT/VIMOS IFU observations of an occulting galaxy pair previously discovered in HST observations. The foreground galaxy is a low-inclination spiral disk, which causes clear attenuation features seen agains the bright bulge and disk of the background galaxy. We find redshifts of z = 0.064 ± 0.003 and z=0.065 for the foreground and background galaxy respectively. This relatively small difference does not rule out gravitational interaction between the two galaxies. Emission line ratios point to a star-forming, not AGN-dominated foreground galaxy."
2MASX J00482185-2507365 is plotted on Megastar as MAC 0048-2507. Larry Mitchell assigned it an approximate magnitude of 15.0 and HyperLeda calls it B = 16.03 +/- 0.47. Is it in the range of an 18-inch scope? Someone needs to take a look!
