LIGHTS. The Thin Encircling Stellar Stream of NGC 3938

Abstract

We present a stellar stream found in images of the nearby, nearly face-on, late-type galaxy, NGC 3938 obtained for the LBT Imaging of Galactic Halos and Tidal Structures (LIGHTS) survey that is thin, has very low mean surface brightness ($\langleμ_g\rangle \approx$ 28.7 mag arcsec$^{-2}$ and $\langleμ_r\rangle \approx$ 28.1 mag arcsec$^{-2}$), appears to lie nearly on the plane of the sky, and wraps more than half way around a host galaxy that is otherwise apparently isolated. We estimate that the progenitor had a stellar mass of $\sim 3.7\times 10^7$ M$_\odot$. Despite an intriguing apparent offset between the centroid of the host galaxy and the apparent center of the stream orbit, we find that we can reproduce the morphology, including this apparent off-centering, with simple models and standard assumptions about the host (thin disk centered within a canonical spherical dark matter halo) and the progenitor satellite orbit. We identify a number of detailed features of the stream, such as changes in curvature and density, that will require more complex models to reproduce. Even this rather simple system provides a rich set of constraints with which to explore the accretion history and gravitational potential of an otherwise unremarkable late-type galaxy. Given the depth of the LIGHTS images, this system is an example of the types of stellar stream that could be found in a majority of nearby giant galaxies with the 10-year stack of Rubin/LSST data.

Figures (7)

• Figure 1: Two views, with negative image contrast, of NGC 3938 in the $g-$band at the same angular scale but differing contrast levels. The left panel highlights the central galaxy, a classic late type disk with no clear evidence of abnormalities except for some level of asymmetry. The surface brightness apparent sensitivity in this panel is similar to that of the digitized Palomar Sky Survey image of this source. The right panel, focusing on low surface brightness features, shows the presence of a thin ($\sim 2$ kpc) tidal stream that stretches nearly 230$^\circ$ around the central galaxy. The average $g-$band surface brightness of the stellar stream is 28.7 mag arcsec$^{-2}$.
• Figure 2: Stellar stream with key reference points labeled. The two panels contain the same $g$-band image, which has had some masking and smoothing applied to enhance the stream appearance. A discussion of various salient features is presented in the text. In the left panel we present the labeled locations that are referenced in the text, while in the right panel we add a reference circle that matches the majority of the stream for discussion. In orange we mark two features near location H that are discussed in the text. The location of the galaxy nucleus is marked $+$. The center of the drawn circle is marked X. Within the blue dashed line we enclose the allowed locations of the center of mass as determined from our stream curvature analysis (see §\ref{['sec:discussion']}). The white scale bar at the bottom right represents 5 kpc at the distance of NGC 3938.
• Figure 3: Example particle spray stream orbits in orange superposed on the LIGHTS image. In the left panel we present one possible configuration where the progenitor is located at the top of the observed stream, near the highest surface density region (between locations G and F). In the right panel we present a second possible configuration where the progenitor is located near the midpoint of the observed stream (location D).
• Figure 4: Magnitude 8.7 star simulated with the g-band PSF from sedighe in a single LBT CCD. This simulation corresponds to the bright star HD 103082 in the field of NGC 3938, which has a Gaia G-mag of 8.73 mag.
• Figure 6: A close-up of one individual $g$-band frame before (left) and after (right) our subtraction of the scattered light from the star HD 103082. In neither case is the mean background subtracted.