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Characterizing the host galaxies and delay times of Ca-rich gap transients vs 91bg-like SNe and normal Type Ia SNe

Using Zwicky Transient Facility data, this study reveals that Ca-rich gap transients and 91bg-like SNe share similar host galaxy properties and long delay time distributions, distinguishing them from normal Type Ia and Type II supernovae and suggesting a potential physical connection between the two transient classes.

Original authors: Peter Scherbak, Abigail Polin, Mansi Kasliwal, Kishalay De, Peter Behroozi, Dave Cook, Wynn Jacobson-Galán

Published 2026-05-19

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CC BY 4.0

Original authors: Peter Scherbak, Abigail Polin, Mansi Kasliwal, Kishalay De, Peter Behroozi, Dave Cook, Wynn Jacobson-Galán

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, bustling city where stars are born, live, and eventually die in spectacular explosions called supernovae. For a long time, astronomers have been trying to figure out the "family trees" of these explosions: Who are the parents? How old are they when they explode? And where do they live?

This paper focuses on a mysterious, shy, and fast-moving type of explosion called a Calcium-rich gap transient. Think of these as the "ghosts" of the supernova world. They are faint, they fade away quickly, and they are packed with calcium (the stuff in your bones). We don't know exactly what causes them, but we do know they usually hang out in old, quiet neighborhoods of the galaxy, far away from where new stars are being born.

The researchers wanted to solve a mystery: Are these "ghosts" related to a specific type of known explosion called a "91bg-like" supernova?

Here is how they investigated, using simple analogies:

1. The Detective Work: Gathering the Clues

The team used a powerful telescope survey called the Zwicky Transient Facility (ZTF), which acts like a security camera scanning the night sky. They looked for four different types of "explosions" (transients):

Calcium-rich gap transients (The mystery ghosts).
91bg-like SNe (A dim, fast version of a standard Type Ia supernova).
Normal Type Ia SNe (The standard, bright "candles" used to measure the universe).
Type II SNe (The massive explosions of huge, young stars).

They didn't just look at the explosions; they looked at the host galaxies (the "neighborhoods") where these explosions happened. They asked: Is this neighborhood a busy construction site full of new buildings (star-forming), or is it a quiet retirement community (quiescent)?

2. The Neighborhood Check: Who Lives Where?

To figure out the "personality" of each neighborhood, the team used two methods:

The Quick Census (CLU Catalog): A pre-made list of galaxy data, like looking up an address in a phone book.
The Deep Dive (PROSPECTOR): A sophisticated computer model that analyzes the light from the galaxy in extreme detail, like a forensic scientist examining the dust and bricks of a house to guess its age and history.

The Findings:

The "Retirement Communities": The Calcium-rich gap transients and the 91bg-like SNe almost exclusively lived in old, quiet, massive galaxies. They were like retirees who had moved far away from the city center.
The "Construction Sites": The normal Type Ia and Type II SNe lived in younger, active galaxies where new stars were being born. They were like families living in a bustling, growing suburb.

The study found that the Calcium-rich "ghosts" and the 91bg-like SNe are practically neighbors. They live in the same type of quiet, massive neighborhoods and have very similar "lifestyles."

3. The Timeline: How Long Did They Wait?

The researchers also calculated the Delay Time Distribution (DTD). Imagine this as a "waiting room" timer. It measures how long a star system waits after it forms before it finally explodes.

Type II SNe (The Impatient): These explode very quickly, like teenagers leaving home immediately after high school (within about 10 million years).
Normal Type Ia SNe (The Middle Agers): These take a bit longer, exploding after about 1 billion years.
Calcium-rich & 91bg-like SNe (The Long Waiters): These are the ultimate procrastinators. They wait a very long time—about 10 billion years—before exploding. This is almost as long as the age of the universe itself!

4. The Big Conclusion: A Family Connection?

Because the Calcium-rich gap transients and the 91bg-like SNe share the same "retirement neighborhood" and the same "long waiting time," the authors suggest they might be cousins.

We know 91bg-like SNe are caused by white dwarf stars (the dead cores of sun-like stars) in a binary system (a pair of stars).
Since the Calcium-rich transients hang out in the same old neighborhoods and wait just as long to explode, the paper suggests they might also involve white dwarfs in binary systems.

Summary

In short, this paper is like a cosmic real estate study. It found that two very different-looking types of stellar explosions (the Calcium-rich ghosts and the 91bg-like dim stars) actually live in the exact same kind of old, quiet neighborhoods and have the exact same long lifespans. This strongly suggests they are related, possibly both being the result of white dwarf stars in binary systems waiting billions of years to go out with a bang.

The paper does not claim to know the exact mechanism of how they explode yet, but it provides strong evidence that they belong to the same family tree.

Problem and Motivation
Calcium-rich gap transients are a class of faint, fast-evolving, hydrogen-poor supernovae characterized by strong nebular [Ca II] emission. Their progenitor systems remain uncertain, though they are frequently observed in old, quiescent host galaxies and often at large offsets from their host centers. While several progenitor models exist (e.g., helium detonation on white dwarfs, double detonations, or mergers), none have been fully validated.

There are noted similarities between Ca-rich gap transients and 91bg-like SNe (a subluminous, fast-declining subclass of Type Ia SNe). Both classes prefer older/quiescent stellar populations, exhibit larger offsets from host centers compared to normal Type Ia SNe, and share spectroscopic features. However, a systematic comparison of their host environments and delay time distributions (DTDs) has been lacking. Since DTDs (the rate of transients as a function of time following a star formation burst) can constrain progenitor scenarios, characterizing the DTDs of Ca-rich gap transients relative to normal Type Ia, 91bg-like, and Type II SNe is essential for understanding their origins.

Methodology
The authors utilize data from the Zwicky Transient Facility (ZTF) Census of the Local Universe (CLU) experiment to construct four distinct transient samples:

Ca-rich gap transients: An 8-object "Gold sample" identified in the ZTF-CLU experiment (2018–2019) and a "Silver sample" of 19 objects from Dong et al. (2022), with careful filtering to exclude hydrogen-rich events.
91bg-like SNe: 30 objects.
Normal Type Ia SNe: ~290 objects.
Type II SNe: ~270 objects.

Host galaxy properties (stellar mass $M_*$ , star formation rate SFR, and specific star formation rate sSFR) were determined using two complementary methods:

CLU Catalog: Direct retrieval of precomputed values, where SFR is derived from FUV flux (GALEX) and dust-corrected using W4 flux (WISE).
PROSPECTOR Modeling: For the Ca-rich gap and 91bg-like samples, the authors performed spectral energy distribution (SED) fitting using the PROSPECTOR code with a CONTINUITY_SFH_MODEL. This non-parametric approach utilizes multi-band photometry (SDSS, WISE, 2MASS, GALEX) to infer stellar population properties, including metallicity, dust extinction, and star formation history.

The authors define quiescent galaxies as those with sSFR $< 10^{-11}$ yr $^{-1}$ . To construct Delay Time Distributions (DTDs), they employed PYROMETER, which forward-models expected transient rates by convolving candidate DTDs (assumed to be power laws) with the star formation histories (SFHs) derived from the UNIVERSEMACHINE empirical model. This approach allows for the estimation of DTDs based on the host galaxy mass and star-forming status across the survey volume.

Key Results

Host Environment Similarity: The hosts of Ca-rich gap transients and 91bg-like SNe occupy a similar parameter space in terms of stellar mass and sSFR. Both classes preferentially occur in massive, quiescent galaxies. In contrast, normal Type Ia and Type II SNe hosts span a wider range of masses, including lower-mass galaxies, and are predominantly star-forming.
Methodological Discrepancies: The authors found significant discrepancies between stellar mass and SFR estimates derived from the CLU catalog versus PROSPECTOR modeling. PROSPECTOR generally yielded higher stellar masses and lower SFRs (and thus lower sSFRs) for quiescent hosts compared to the catalog values. This is attributed to the mass-to-light ratio dependence on star formation activity; the catalog may underestimate masses for quiescent systems.
Delay Time Distributions (DTDs): The constructed DTDs reveal distinct peak delay times for the different classes:
- Ca-rich gap transients and 91bg-like SNe: Both exhibit the longest peak delay times, approximately $10^4$ Myr (10 Gyr).
- Normal Type Ia SNe: Peak delay time is approximately $10^3$ Myr (1 Gyr).
- Type II SNe: Peak delay time is approximately 10 Myr.
Subclass Analysis: Within the small Gold sample of Ca-rich gap transients, the authors noted that the single "Ca-Ia" object (SNe Ia-like features) had the highest sSFR among the sample, though still quiescent. Literature suggests Ca-Ia objects may be associated with more star-forming hosts compared to the "Ca-Ib/c" subclasses, but the sample size is too small for definitive conclusions.

Significance and Claims
The paper claims to be the first to calculate DTDs specifically for Ca-rich gap transients. The primary significance of the work lies in the demonstrated similarity between Ca-rich gap transients and 91bg-like SNe regarding their host environments (massive, quiescent) and their delay times (peaking at $\sim 10^4$ Myr).

The authors state that this similarity is a "necessary ingredient" to motivate further analysis of the relationship between these two transient classes. While not a conclusive proof of a shared progenitor, the results support the hypothesis that Ca-rich gap transients, like 91bg-like SNe, may involve white dwarfs in binary systems (potentially mergers of CO and He white dwarfs). The authors caution that the large offsets of these transients from their hosts suggest local environment properties may also play a role, and that larger samples and spectroscopic follow-up are required to validate specific progenitor channels.

1. The Detective Work: Gathering the Clues

2. The Neighborhood Check: Who Lives Where?

3. The Timeline: How Long Did They Wait?

4. The Big Conclusion: A Family Connection?

Summary

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