Science

The Atlas of Peculiar Galaxies was produced by Halton Arp in 1966. The catalogue contains 338 galaxies with distorted morphologies and/or interacting systems, spanning an optical magnitude from 6 to 17 mag, with a peak of the distribution around 13 - 14 mag. The Atlas images were acquired with the Hale and Schmidt telescopes of the Carnegie Observatories, providing the best photographic plates of the time, which revealed the faint galactic details with very high resolution.

As stated in the preface of Arp’s catalogue, “the peculiarities of the galaxies pictured in this Atlas represent perturbations, deformations, and interactions which should enable us to analyse the nature of the real galaxies which we observe and which are too remote to experiment on directly.”

This sentence summarises Arp’s main goal for this catalogue. At that time, the physical processes that shaped the Hubble sequence galaxies were not fully understood. Therefore, the Arp catalogue aimed to provide examples of the different kinds of peculiar structures found in galaxies to be considered "laboratories" where astronomers can study the mechanisms acting on main-sequence galaxies, i.e. spiral or elliptical galaxies. 

The atlas does not include all kinds of peculiar galaxies known to date. It collects several examples of gravitational interactions between galaxies, grouped as follows:

• individual peculiar spiral galaxies or spiral galaxies that apparently have small companions

• elliptical-like galaxies

• individual or groups of galaxies with distorted morphology

• double galaxies.

  Some examples are shown in Fig.1 and Fig.2.

The main science deliverables that this project can offer are presented below

• Interaction and merging: combining the deep images acquired in the g and r bands and the large covered area of the VST data proposed here, the tidal tails or stellar streams can be detected down to the faintest levels of μg  ~ 28 - 31 mag/arcsec2 and out to unexplored regions, at larger radii from the target centre. The structure of these features can be directly compared with the predictions of all phases of the gravitational merging between galaxies with different mass ratios. The main outcome of these studies is to provide more stringent constraints on the formation of massive early-type galaxies and the mass assembly process.

• Detection of small satellites:  to date, the confirmation of the LCDM cosmological model relies on our ability to find the baryonic counterparts of the predicted low-mass DM halos, which means deriving a complete census of the faintest, less massive stellar systems such as dwarf galaxies. In this context, the UDGs have a special role, being the extreme LSB tail of the size-luminosity distribution of dwarf galaxies. The depth and the large FOV of the acquired images can offer a unique opportunity to improve the detection of satellite dwarf galaxies, in particular the most low surface brightness and diffuse systems.

• Detection and formation of the tidal dwarfs and tidal UDGs: as a follow-up to the detection of the dwarf galaxies, cited above, this project provides an excellent sample of interacting galaxies, where a statistically significant sample of tidal-dwarf galaxies can be detected. These kinds of systems, formed in the tidal tails of the merging spiral galaxies, are particularly interesting since they are dark matter free (Duc et al. 2015). The structure and stability of these galaxies are still an open issue to be investigated.

• Distribution of dust in the galaxy structure: the g - i colour maps can be used to trace the regions of dust absorption inside each galaxy. 

• Star formation regions: the H𝛼 images acquired with this project can be used (i) to identify and study the star-forming regions and determine whether they might have been affected by environmental interactions: any asymmetry in the star formation rate map derived from H𝛼 can indicate whether (and in which part) a galaxy is experiencing a star formation rate enhancement or a possible quenching of star formation; (ii) to identify displacement of ionized gas with respect to the stellar component, which might reveal on-going (and possibly subtle) hydrodynamical interactions. This offers the exquisite opportunity to identify interesting regions for further follow-ups to understand how gas is being removed.  

• Spectral Energy Distribution: combining the multi-band images released with this proposal, which cover the optical wavelength range, with available data in other bands, from UV to NIR, the spectral energy distribution can be addressed for each target of the sample. An example of such a kind of analysis, by combining the UV data with the optical VST images, was performed in Rampazzo et al. (2022), who derived the star formation history of the galaxies in the Dorado group.

• Dissemination of knowledge: Arp galaxies are truly fascinating objects: with their curious shapes and intricate features, they not only attract the attention of astronomers but may also trigger the curiosity of the general public. Therefore,  the multi-band images obtained from this project can be used to produce spectacular colour-composite images, to be widely distributed via the INAF online media channels. Astronomical images are widely featured by national and international media and tend to be extensively appreciated and shared on image-based social media platforms that are popular with the young public. In combination with existing Hubble space telescope images, which depict ARP galaxies and their immediate surroundings, VST images help paint a broader picture of the “lives” of galaxies, by showing tidal tails and other effects of their interactions on a much broader scale, thus providing a more complete story of galaxy evolution and demonstrating the complementarity of multiple telescopes with different characteristics. Once the program is completed, the collection of images can form the core of a coffee-table book showcasing beautiful views of the cosmos as observed by the VST.

• Finally, all targets in the catalogue can be explored more in detail with follow-up studies, using different observing facilities and instruments.