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

Abstract: The problem of missing satellites still remains one of the well-known problems of the Lambda-CDM cosmological theory. Despite significant progress in cosmological modeling achieved in recent years, the search for new dwarf galaxies-satellites of nearby giant galaxies remains extremely important. 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 galaxies 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 satellites, 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.

• The paper reports the discovery of five faint dwarf galaxies that help resolve the missing satellites problem.
• The study uses visual inspection of deep DESI Legacy Survey images and galfit software to derive key photometric and structural properties.
• The findings expand NGC 253’s satellite count from nine to fifteen, offering new insights into anisotropic galaxy distributions in ΛCDM models.

Discovery of Five Faint Dwarf Galaxies in the Sculptor Group

The presented research investigates the discovery of five new faint dwarf galaxies in the vicinity of the NGC 253 galaxy within the Sculptor group using data from the DESI Legacy Survey. This work addresses pivotal issues related to the small-scale problems in the context of the $\Lambda$CDM cosmology, specifically focusing on the so-called "missing satellites" problem and the phenomenon of satellite planes which have thus far posed challenges to the current theoretical frameworks.

Research Context and Objectives

In light of the persistent discrepancies between observed and predicted satellite galaxy numbers, the study ushers in a systematic survey targeted at faint and low surface brightness dwarf galaxies to expand the known census around NGC 253. This is crucial for understanding both cosmological predictions and the specific dynamics of galaxy groups.

Methodology

The methodology employs visual inspection of co-added image cutouts from the DESI Legacy Surveys, designed to reach very low surface brightness thresholds (28.0--29.0 mag arcsec$^{-2}$). Photometric and structural properties of the detected dwarf galaxy candidates, designated as Do V, Do VI, Do VII, Do VIII, and Do IX, were derived using galfit software. Their calculated absolute magnitudes range from $-7$ to $-9$ mag, consistent with typical dwarf satellites.

Results

The discovery of these five additional dwarf galaxies increases the number of NGC 253 satellites from nine to fifteen, suggesting a broader distribution that aligns reasonably with the $\Lambda$CDM expectations upon the inclusion of this updated census. Interestingly, the spatial distribution is observed to be lopsided, which holds particular interest in the study of galaxy formations and structures.

Implications

The implications of these findings are profound. Firstly, the detection of these additional satellites helps bridge the gap in the mismatch known as the "missing satellites" problem. Secondly, the inclusion of these satellites reduces the previously considered planar distribution of satellites into a direction more consistent with anisotropic patterns predicted by hierarchical structure formation models.

Future Directions

These discoveries not only enhance the current understanding of satellite distributions but also suggest future avenues for research, such as comprehensive kinematic studies that require spectroscopic follow-up to determine the velocities of these satellites. The potential existence of such velocity-coherent structures could provide further insight into mechanisms of satellite coherency, akin to what is observed in other well-studied planes of satellite systems like the Milky Way and Andromeda.

Conclusion

This research significantly enriches the known environment of NGC 253 within the Sculptor group by adding potential faint dwarf satellites to its hierarchy. These discoveries enhance the mapping of satellite distributions and provide new grounds to challenge and possibly validate the predictions made by $\Lambda$CDM models, offering vital contributions to this enduring field of study within astrophysics. As such, the work heralds further exploration into the dynamic complexities of galaxy formation and distribution in our universe.