The Arp Catalog is still relevant for studying the evolution of galaxies with unusual shapes, says expert.

▲ On the left, a rare example of an interacting galaxy pair in the southern hemisphere. On the right, a large galaxy with unusual extended spiral arms. NASA photo

Eirinet Gómez

La Jornada Newspaper, Thursday, September 4, 2025, p. 6

The catalogue of peculiar galaxies – that is, those with unusual or out-of-the-ordinary shapes – compiled in 1966 by astronomer Halton Arp remains a fundamental tool for studying how new groups of galaxies evolve and can emerge in the universe, said Natalia Sofía Roa Gil, a researcher at the Higher School of Physics and Mathematics of the National Polytechnic Institute (IPN), when speaking about the challenge of looking into space and distinguishing between an optical illusion and a genuine formation.

"We must remember that we see galaxies in two dimensions, but they are three-dimensional structures. The position from which we observe can obscure or distort the structures, making them difficult to detect. Something that looks like a clump when viewed head-on could be just part of a tail," he warned.

Therefore, the Arp catalog allows us to study galaxies that are interacting with each other and, therefore, are in the process of transformation, added Roa Gil during a conference where he presented his research dedicated to exploring the peculiar shapes of galaxies and their significance for understanding the dynamics of the cosmos.

Arp included 338 galactic systems with unusual morphologies in his registry: matter tails, bridges between galaxies, multiple nuclei, or distorted structures. This work has now gained new impetus thanks to contemporary research that examines it with modern technology and new questions.

“Arp's approach to compiling his catalog was based on a critique of the traditional morphological classification of galaxies. He believed that this excluded many systems that, because they did not fit into established categories, were generically classified as 'irregular.' But for Arp, these galaxies deserved special attention precisely because of their peculiarities.”

The researcher added that, using data from space telescopes such as GALEX and SDSS, she undertook a visual analysis of the galaxies in the Arp catalog. Much of her work focused on analyzing tidal structures (filaments of gas, dust, and stars that are released from galaxies when they interact gravitationally).

"These structures are classified as tails, bridges, and clumps, and are direct evidence of encounters between galaxies, which can lead to mergers or even phenomena like galactic cannibalism, when a more massive galaxy absorbs a smaller one," explained Roa Gil. "The clumps we observe in tails or bridges could become tidal dwarf galaxies," he noted.

He emphasized the need to combine observations with computer simulations. “Since the 1970s, astronomers have used multi-body models—that is, simulations that calculate how many stars or galaxies interact with each other under the influence of gravity—to mimic how they behave when interacting. Today, these models are more sophisticated and include the behavior of gas and dust, not just stars.

“The Arp catalog, far from being a vestige of last century's astronomy, remains a source of questions today. What do we really see when we look at the sky? What is hidden behind each distortion? How many new galaxies could be emerging in the chaos?” concluded the IPN researcher.

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