Tracing satellite planes in the Sculptor group: II. Discovery of five faint dwarf galaxies in the DESI Legacy Survey

TL;DR

This study reports the discovery of five faint dwarf spheroidal candidates (Do V–Do IX) in the NGC 253 region of the Sculptor group using DESI Legacy Survey imaging and visual inspection. Through GALFIT photometric modeling and ePSF-based processing, the authors derive structural and photometric properties consistent with Local Group dwarfs at the distance of NGC 253. The expanded census (15 members within 1 Mpc) reveals a three-dimensional distribution that is less flattened and broadly compatible with Lambda-CDM expectations from IllustrisTNG-100, though notable lopsidedness persists and complete conclusions await distance confirmations and spectroscopic velocities. The work highlights the importance of distance and velocity information for robust tests of planar satellite configurations and underscores the need for follow-up observations to confirm membership and dynamics.

Abstract

Although substantial progress has been made in reconciling LCDM simulations with the observed abundance and distribution of satellite galaxies, important tensions persist. Studying satellite systems around spiral galaxies thus remains key in addressing these tensions. In this series of papers we report the first results of an on-going systematic survey of faint dwarf spheroidal galaxies in the vicinity of the bright late-type spiral NGC 253 galaxy, the brightest member of the Sculptor filament located at a distance of 3.7 Mpc. We performed a new NGC 253 satellite search by means of visual inspection using co-added image cutouts reprocessed in the DESI Legacy image surveys, reaching a very low surface brightness regime (28.0--29.0 mag arcsec-2). Five new dwarf galaxy candidates have been discovered in the vicinity of NGC 253, that we named them Do V, Do VI, Do VII, Do VIII and Do IX. Assuming they are associated to NGC 253, their total absolute V-magnitudes fall in the -7 to -9 mag range, which is typical for dwarf satellites in the local Universe. The central surface brightness tend to be extremely low for all the discovered dwarfs and fall roughly in the range of 25--26 mag arcsec-2 in g-band. We present a new list of galaxies located around the giant spiral NGC 253. With the inclusion of these additional satellite candidates, the overall spatial distribution of the system becomes less flattened and is now broadly consistent with analogs drawn from Lambda-CDM expectations. Interestingly, the distribution appears to be rather lopsided. Yet, firm conclusions on the presence of absence of a correlated satellite structure are hampered since distance information is lacking, the census of observed dwarfs in the system remains far from complete, and spectroscopic velocities are not even available for most known satellites.

Figures (6)

• Figure 1: Left panel: Position of the six dwarf galaxy candidates (solid red circles) reported in this study with respect to the spiral NGC 253. The purple circular line corresponds to the area explored by the PISCes survey 2016ApJ...816L...5T extending up to $\sim$ 150 kpc from the centre of NGC 253. The total field of view of this image is 15$\fdg$$\times$ 15$\fdg$. Top and right panels: Full colour version of the image cutouts obtained with legacypipe for Do V, VI, VII, VIII and IX. North is up and east is left. The field of view of all these image cutouts is $2.5\arcmin \times 2.5\arcmin$.
• Figure 2: Light profile GALFIT modeling results Top Row:$g$-band for Do VI; Bottom Row:$r$-band for Do VII.
• Figure 3: Comparison of the satellite dwarf galaxy candidates against confirmed Local Group dwarf galaxies.
• Figure 4: Distribution of galaxies around NGC 253 in the supergalactic coordinates. Left-hand panel shows the projection on the plane of the Local Supercluster, while right-hand panel presents its edge-on view. The sizes and colors of the dots reflect the $B$-band absolute magnitude and morphology of the galaxies, according to the legends above. The line segments correspond to the distance errors. The dotted circles mark the virial zone of $R_{200}=190$ and the turn-around radius of $R_\mathrm{ta}=710$ kpc around NGC 253. The expected positions of the discovered galaxies are indicated by magenta symbols.
• Figure 5: On-sky distribution of the galaxies around NGC 253 in equatorial coordinates relative to the position of NGC 253 (black cross). Galaxies with available velocity measurements are color-coded by their line-of-sight velocity component relative to that of NGC 253. Galaxies with available distance measurements are plotted as open white circles, while our new candidates are plotted as filled black stars. Yellow boxes mark objects within 600 kpc of NGC 253, either measured in three dimensions if distances have been measured, or in projection for those objects for which such data is not available. The on-sky orientation of the major axis of the spatial distribution of these highlighted objects is shown as a dashed black line.