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SN 2025adpq: A Type Ia supernova in a collisional ring formed during a major galaxy merger

Brendan O'Connor, Xander J. Hall, Tomas Cabrera, Lei Hu, Antonella Palmese, Louis-Gregory Strolger, Ariel J. Amsellem, Akash Anumarlapudi, Igor Andreoni, Saul Baltasar, Jonathan Carney, David A. Coulter, James Freeburn, Julius Gassert, Xiaosheng Huang, Keerthi Kunnumkai, Justin D. R. Pierel, Mathew R. Siebert, Christopher J. Storfer

Abstract

Galaxy mergers can both trigger star formation and rearrange where stars live, producing long-lived tidal structures and collisionally driven density waves (known as collisional rings) that can extend for tens of kpc from their host galaxy centers. Here we report the discovery of SN 2025adpq, a Type Ia supernova at $z=0.1540$, found within a collisional ring, which we call Pika's Halo, with circumference $\sim$70 kpc that was produced by a major merger between two comparable mass galaxies ($\log(M_*/M_\odot)\approx10.5)$. The supernova lies along the ring at a projected offset of $\sim$11.4 kpc from the nucleus of the primary galaxy (hereafter G1). Optical spectroscopy obtained with the Southern African Large Telescope (SALT) and Gemini South reveal signatures consistent with merger induced ongoing star formation, while prominent Calcium H and K absorption indicates a substantial old stellar population within the ring. Given the long delay times expected for most SN Ia progenitors, we argue that SN 2025adpq most likely arose from an old progenitor system that was displaced from G1 during the head-on encounter. The progenitor was likely stripped from its parent galaxy by the collisionally induced pressure wave and exploded far from its birthplace. This event highlights collisional rings as a pathway for producing large offset SNe Ia, and it motivates targeted searches for faint, dynamically displaced old populations in seemingly hostless SN Ia environments. We additionally identify other supernovae, including supernova siblings, in the low redshift sample of collisional ring galaxies, and find that SN 2025adpq is one of only a handful of classified supernova identified in the expanding ring of a collisional ring complex.

SN 2025adpq: A Type Ia supernova in a collisional ring formed during a major galaxy merger

Abstract

Galaxy mergers can both trigger star formation and rearrange where stars live, producing long-lived tidal structures and collisionally driven density waves (known as collisional rings) that can extend for tens of kpc from their host galaxy centers. Here we report the discovery of SN 2025adpq, a Type Ia supernova at , found within a collisional ring, which we call Pika's Halo, with circumference 70 kpc that was produced by a major merger between two comparable mass galaxies (. The supernova lies along the ring at a projected offset of 11.4 kpc from the nucleus of the primary galaxy (hereafter G1). Optical spectroscopy obtained with the Southern African Large Telescope (SALT) and Gemini South reveal signatures consistent with merger induced ongoing star formation, while prominent Calcium H and K absorption indicates a substantial old stellar population within the ring. Given the long delay times expected for most SN Ia progenitors, we argue that SN 2025adpq most likely arose from an old progenitor system that was displaced from G1 during the head-on encounter. The progenitor was likely stripped from its parent galaxy by the collisionally induced pressure wave and exploded far from its birthplace. This event highlights collisional rings as a pathway for producing large offset SNe Ia, and it motivates targeted searches for faint, dynamically displaced old populations in seemingly hostless SN Ia environments. We additionally identify other supernovae, including supernova siblings, in the low redshift sample of collisional ring galaxies, and find that SN 2025adpq is one of only a handful of classified supernova identified in the expanding ring of a collisional ring complex.
Paper Structure (24 sections, 2 equations, 15 figures, 3 tables)

This paper contains 24 sections, 2 equations, 15 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Discovery
  4. Photometry
  5. DECam
  6. Spectroscopic Follow-up
  7. SALT
  8. Gemini
  9. Archival Imaging
  10. Results and Analysis
  11. A Collisional Ring from a Major Galaxy Merger
  12. Spectral Analysis
  13. Galaxy SED Modeling
  14. Supernova Classification
  15. Supernova Lightcurve Modeling
  16. ...and 9 more sections

Figures (15)

  • Figure 1: The DECam $g$-band discovery image of SN 2025adpq. The archival template is shown in the left panel. The middle panel shows the science image obtained on 2025-11-14 and the right panel is the difference image. North is up and East is to the left.
  • Figure 2: Left: Archival RGB image from the KiDS survey using the $gri$ bands. The transient position is shown by a magenta circle. The collisional ring and merging galaxies G1 and G2 are labeled. The black rectangle shows the placement of the Gemini GMOS-S slit and the blue rectangle shows the SALT RSS slit. Right: RGB image using VIKING near-infrared images in the $YJK_s$ bands. The scale and labels are the same. North is up and East is to the left.
  • Figure 3: DECam optical ($griz$) lightcurve of SN 2025adpq in apparent (left side) and absolute (right side) magnitude. The best fit SALT2 models are also shown in each band (see § \ref{['sec:lcfit']}). The black dashed line marks the epoch of our Gemini classification spectrum (see § \ref{['sec:class']}).
  • Figure 4: Optical spectra obtained with SALT/RSS of the galaxies G1 (black) and G2 (red) and the ring (blue). Narrow emission and absorption features at a common redshift $z$$=$$0.1537$ are shown by vertical lines. The right panels show the individual spectra for an easier comparison of the features. The chip gaps of SALT/RSS are clearly visible around $\sim$$4900\AA$ and $6200\AA$.
  • Figure 5: Visualization of the 2D Gemini GMOS-S spectrum obtained on 2025-11-24 ($\sim$ 10 days from discovery). Three different panels are shown that zoom in on relevant absorption features. In the bottom panel the location of the transient, central arc, and eastern portion of the ring are labeled in red, black, and blue, respectively. The central region refers to a region of low surface brightness between the transient location and brightest portion of the ring as visible in Figure \ref{['fig:DeepImage']}. The spectral direction is the X-axis which goes from blue to red wavelengths from left to right. The spatial direction along the slit is the Y-axis. Each panel covers 250 Å in width and is 8.5$^{\prime\prime}$ in height. The scale is chosen to maximize the visibility of the emission lines. In the bottom panel the [NII] line is impacted by telluric absorption. A few cosmic rays can be seen throughout the figure, but they do not impact the emission features.
  • ...and 10 more figures