Halton Arp and his peculiar Galaxies

The interacting galaxy system Arp 273 is not only a visually striking example of gravitational interaction, but also a direct entry point into the work of the astronomer Halton Arp (1927 – 2013), whose systematic study of such peculiar and interacting galaxies fundamentally shaped our understanding of galactic morphology and dynamics. In this context, I revisited Arp’s ideas today by watching his lecture “Intrinsic Redshifts”, presented at the Kronia Group Conference on Sept 23, 2000 in Portland, Oregon. The one-hour-lecture is available on YouTube – in its course he outlines his observational arguments questioning the exclusively cosmological interpretation of redshift, particularly in relation to quasars and active galaxies.

Based on observational correlations Halton Arp found out that some high-redshift objects, particularly quasars, appear physically associated with nearby low-redshift galaxies. He interpreted this as evidence for a possible intrinsic redshift component, challenging the idea that redshift is always a reliable distance indicator. While these views are not accepted within the current cosmological consensus, Arp’s work exemplifies a rigorously observational approach and underscores the importance of confronting theoretical frameworks with empirical anomalies.

Arp 273 (PGC 8961, UGC 1810 & UGC 183) – The Rose

His Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced in 1966 and originally published by the California Institute of Technology. A total of 338 galaxies are presented in the atlas. The primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies.

The pair of galaxies known as Arp273 is some 300 million light-years away and is to be found in the constellation Andromeda:

3 hours of L-RGB-data (Dec 27, 2025)

As described in my post from Dec 21, 2025 („One Night with N.I.N.A.“) I did (for the very first time) plan a complete photography session using N.I.N.A.`s Advanced Sequencer tool. The object of desire was Arp 273, just as a test project in order to find out about the parameters used for the session, as well as to test the guiding setup and also to check out the collimation results. Of course such a small target is not the best choice for a f/3-setup with 750mm of focal length only (Starizona Nexus 0.75 reducer was mounted already).

The results of 3 hours of exposure time with this widefield-setup will at least be used as comparison data for something new to come – something that was on my wishlist since starting with astrophotography … further details to follow in the next post 🙂

The sequence of Arp273 originally considered 2 hours of Luminance data and each of the R, G, B-channels with one hour. Due to the late start of the session I decided to collect 1.5 hours of Luminance and only 30min of the colour channels – so only 3 hours in total.

The Automatic Stretch of the ScreenTransferFunction showed already quite good Luminance data. In the further stages of processing, it will be particularly interesting to see whether the relatively short exposure times of the color channels already reveal the first color structures within the galaxies themselves. In any case, the stellar colors of the two bright foreground stars next to the system (with apparent magnitudes of 8.8 and 10.3,) should already be clearly visible in blue and red.

(W)BPP – no weighting

During my Standard PixInsight preprocessing, the standard Weighted Batch Preprocessing (WBPP) workflow unexpectedly failed. Although calibration, registration, and local normalization completed successfully, WBPP repeatedly aborted during ImageIntegration and no master frames were generated.

In the course of debugging, potential causes such as dithering amplitude (10 px), image overlap, registration settings, autocrop, Local Normalization, and ImageIntegration parameters were excluded. Only after examining the JavaScript runtime log was the actual cause identified: an invalid CSV weights file generated by Subframe Weighting (Weighting Formula) in WBPP v2.8.1. Disabling Subframe Weighting resolved the issue immediately and allowed successful generation of all master frames. Lesson learned: no weighting when only a small number of frames are to be processed!

L-RGB workflow

As the RAW-stacks showed a massive gradient in all channels I first of all cropped the frames. After Gradient Removal (AutoDBE by SetiAstro) and BlurXTerminator a LinearFit was applied to the red and blue channel using the green channel as reference. After RGB-ChannelCombination and running the Image Solver, the colors were calibrated with SPCC (Spectometric ColorCalibration). Luminance was finally added with L-RGB Combination, slightly reduced in its weight because of the star centers. Finally HistogramTransformation, a slight increase of hues with the Curves Tool, NoiseXTerminator and there it is:

At the end of the post, a cropped overview is shown covering a field of view of 51 × 34 arcminutes. Within this field, the two interacting galaxies themselves appear extremely small, with angular sizes of only 1.2 and 1.8 arcminutes: