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starxiv.bsky.social
The StarXiv ✨ podcast
@starxiv.bsky.social
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A podcast where Payel Das (@payeldas.bsky.social) and Michelle Collins (@runningastronomer.bsky.social) discuss papers from astroph

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starxiv.bsky.social
Owing to a technical hiccup, the StarXiv will be arriving a little later this week. Expect to hear Michelle and Payel chat about the latest from astro-ph this Wednesday instead! It'll be worth the wait ☄️ 🔭
November 10, 2025 at 8:00 AM
starxiv.bsky.social
Michelle's final paper discussed the 'failed' galaxies. Thought to be living in massive dark matter halos that have formed fewer stars than expected, this population is poorly undertsood. This paper looks for their progenitors in the MAGNETICUM simulations 🔭 ☄️https://arxiv.org/abs/2510.04416
These 4 plots show how the simulated failed galaxy progenitors (UDGs) compare to more normal galaxies in the same simulation. They tend to form stars more rapidly (panel 1), have lower metallicities (panel 2), have higher star formation rates (panel 3) and later halo assembly (panel 4) than typical galaxies.
October 27, 2025 at 5:20 PM
starxiv.bsky.social
Nicole’s third paper reported two RR Lyrae stars enriched in heavy s-process elements, a rare class previously represented only by TY Gruis. Their chemistry could reveal ancient mass transfer from AGB companions. arxiv.org/abs/2510.15723 🔭☄️
[Eu/Fe] vs. [La/Fe] ratios for CEMP-r and CEMP-s stars by Masseron et al. (2010), CEMP-s stars by Roederer et al. (2014), Ba stars by Roriz et al. (2021), and the s-rich RRLs DQ Hya and TY Gruis. The dashed line is the 1:1 relationship.
October 27, 2025 at 12:36 PM
starxiv.bsky.social
In Michelle's second paper, she updates us all on the offensively faint Ursa Major III/UNIONS 1 stellar association. Is it a galaxy wtih dark matter or a star cluster? This new paper searches for signs of dark matter and/or binary stars to answer this question! arxiv.org/abs/2510.02431 🔭 ☄️
These two plots show the previous work on measuring the velocity dispersion in Ursa Major III/ UNIONS 1 on the left. Including all stars they measure a clear result that would indicate a dark matter halo (orange Gaussian at 3.7 km/s). But these authors noted if one star (a possible binary) was removed, they can no longer resolve this value (dot-dashed curve), making it consistent with no dark matter. Using follow up data, this paper confirms that star is a binary and revises the velocity dispersion (right panel). Now we see it is entirely consistent with no dark matter (gray vertical band). So, *probably* a star cluster!
October 27, 2025 at 11:35 AM
starxiv.bsky.social
Nicole’s second paper showed our Galaxy’s oldest stars mark a dense proto-Galaxy at its center, mirroring compaction-driven starbursts seen in Milky Way–like Auriga simulations that shaped the early, central galaxy. arxiv.org/abs/2510.17693 🔭☄️
Panel (a): The universal SFMS of star-forming Au18 as a function of M⋆ and redshift z. The black-solid line indicates the SFMS ridge, while the gray-shaded region denotes a ±0.3 dex scatter of the SFMS (S. Tacchella et al., 2016). The labeled look-back times tlb1, tlb2, tlb3, and tlb4, mark the approximate durations of three compaction events identified in the Au18 evolution, using the same colors as the associated redshifts. Panel (b): The edge-on stellar number-density map of stars formed during the earliest episode of compaction and quenching events, corresponding to ages between tlb2 and tlb1 at z=0. Panels (c) and (d): The same as panel (b), but for the second and third compaction episodes, respectively. The border colors of panels (b)-(d) match the colors of the relevant redshifts. Panel (e): The edge-on stellar number-density distribution at z=0 for all stars formed during the three compaction episodes shown in panel (a). This figure indicates the possible connection between proto-Galaxy formation and multiple high-z (z≳3) compaction episodes.
October 27, 2025 at 10:42 AM
starxiv.bsky.social
Michelle's first paper discussed MEGATRON! No, it's not some sort of transformer, but a neat suite of simulations that can help us understand what sets the metallicity of the faintest galaxies. Tune in for more! 🔭 ☄️ arxiv.org/abs/2510.05232
Two plots from the paper, the one on the left shows the average iron abundance of their simulated galaxies using different Population II star formation and feedback models. Then, on the right they show the averages of these per magnitude bin and show that not only do they capture the mass metallicity relation, but they reproduce the metallicity plateau!
October 27, 2025 at 9:36 AM
starxiv.bsky.social
Episode 23 has dropped! Tune in to hear about faint galaxies, failed galaxies and how binaries are always causing trouble. 🔭☄️ starxiv.com/2025/10/27/e...
Episode 23 – Failed galaxies, faint galaxies, and the proto-Galaxy
In this episode, Michelle and Nicole explore the chemistry of faint galaxies and RR-Lyrae stars, tracing the proto-Galaxy, discussing the failure of some galaxies, examining quasi-stars in relation…
starxiv.com
October 27, 2025 at 9:21 AM
starxiv.bsky.social
Nicole’s first paper showed that quasi-stars, black holes cocooned in stellar envelopes, can reproduce JWST’s “Little Red Dots,” matching their red colours and Balmer breaks. These could be SMBHs in formation. arxiv.org/abs/2510.17952 🔭☄️
Predicted SEDs of a 10^6 M⊙ total mass, late-stage quasi-star both on its own (black dashed line) and embedded in a host galaxy (solid blue line) compared with JWST observations of LRDs (red) that includes the emission from the host stellar component with masses ∼10^9.5 M⊙. The top row shows comparisons with UNCOVER-45924 (I. Labbe et al., 2024) and the bottom row with MoM-BH*-1 (R. P. Naidu et al., 2025). We show models with (left) and without (right) dust. Semi-synthetic models of quasi-stars embedded in a host galaxy are shown in blue, using hosts properties inferred for UNCOVER-24996 and MoM-UDS-948311 respectively. The faded colors surrounding the lines represent 1​σ uncertainties in JWST measurements. Gray dashed lines indicate key wavelengths: the Balmer break (H∞), Hγ, Hβ, [OIII], and both CaII lines. Discrepancies in emission lines likely arise from the use of LTE atmospheres at nonzero metallicity, particularly in the Hβ emission and CaII absorption lines - these are discussed in Sec. 2.2. Shaded regions indicate the far-UV and NIR approach, highlighting a few of the well fit continuum regions discussed in Sec. 3. The stellar parameters of the quasi-star model at this point are Teff=6241 K, log​(g)=−0.003, and Z=0.
October 27, 2025 at 9:09 AM
starxiv.bsky.social
Episode 23 – Failed galaxies, faint galaxies, and the proto-Galaxy

In this episode, Michelle and Nicole explore the chemistry of faint galaxies and RR-Lyrae stars, tracing the proto-Galaxy, discussing the failure of some galaxies, examining quasi-stars in relation to Little Red Dots, and…
Episode 23 – Failed galaxies, faint galaxies, and the proto-Galaxy
In this episode, Michelle and Nicole explore the chemistry of faint galaxies and RR-Lyrae stars, tracing the proto-Galaxy, discussing the failure of some galaxies, examining quasi-stars in relation to Little Red Dots, and discussing whether UNIONS 1 is a galaxy or a star cluster. Listen on major podcast platforms.
starxiv.com
October 27, 2025 at 6:12 AM
starxiv.bsky.social
Nicole’s last paper traced the inner Milky Way’s past using super-metal-rich stars in the solar neighbourhood. Their age distribution reveals six starburst episodes, a bursty formation history likely driven by bars, mergers, and interactions. arxiv.org/abs/2510.02238 🔭☄️
Integrated stellar age distribution of the stars within 0.6 kpc of the plane of the disc (|z|<0.6 kpc). We divide it into total (including all stars, shaded black area); metal-rich (considering only stars with [M/H] ≥0.3, red); and rest (stars with [M/H] < 0.3, blue).
October 13, 2025 at 2:11 PM
starxiv.bsky.social
In Payel's second paper, she discusses a new approach for finding Einstein rings using spectra and images. The authors find 2164 new lens candidates in DESI DR1, and a new category of 318 “dimple-lens” candidates that may represent dwarf galaxy lensing.
arxiv.org/pdf/2509.16033
October 13, 2025 at 12:45 PM
starxiv.bsky.social
Nicole’s next paper looked at rare radio objects found by the RAD@home citizen-science collaboratory. One of these is an Odd Radio Circle (ORC) with intersecting ~300 kpc rings, pointing to a relic synchrotron origin. arxiv.org/abs/2510.01999 🔭☄️
An optical radio map with LoTSS 6′′ resolution contours overlaid on optical at is shown, where green circles have been drawn to guide the eye to the double-ring structure. Mild brightness enhancements are visible at the ring intersections.
October 13, 2025 at 11:01 AM
starxiv.bsky.social
In Payel's first paper, she discusses a paper that claims a bona fide discovery of an intermediate-age RR Lyra! Always thought to have been a safe bet indicator of a metal-poor and old stellar population. Found out more here:
arxiv.org/abs/2509.22336
October 13, 2025 at 10:13 AM
starxiv.bsky.social
Episode 22 – ORCs, rings, and Galactic histories

In this episode, Payel and Nicole discuss what happens to the metallicity gradient of a satellite galaxy after merging, the existence of intermediate-age RR lyrae and radio rings, and much more! Have a listen at starxiv.com/2025/10/13/e....
Episode 22 – ORCs, rings, and Galactic histories
In this episode, Payel and Nicole discuss what happens to the metallicity gradient of a satellite galaxy after it merges with its host, the existence of intermediate-age RR lyrae and radio rings, t…
starxiv.com
October 13, 2025 at 10:13 AM
starxiv.bsky.social
Nicole’s first paper looked at how the Gaia-Enceladus/Sausage merger erased its progenitor’s radial [Fe/H] gradient. Simulations show 78–98% of that gradient is lost to tidal mixing, but average trends survive, offering clues to past mergers. arxiv.org/abs/2509.24705 🔭☄️
The pre-merger and post-merger picture of GES in Auriga. The sub-panels contain the XY projection of the GES progenitor coloured by the average [Fe/H] along the z axis in the first column where redder means higher [Fe/H], the 2D histogram of [Fe/H] vs RGES in the second column, and the 2D histogram of [Fe/H] vs orbital energy in the third column. The mean values of [Fe/H] are denoted with red circles for the trends against RGES and orbital energy, with the best-fit linear (blue) and exponential (orange) relations over-plotted. The [Fe/H] vs Rhost panel is coloured by RGES while the [Fe/H] vs present-day orbital energy panel is coloured by the orbital energy in the GES progenitor.
October 13, 2025 at 9:00 AM
starxiv.bsky.social
Episode 21 – Mercury, chemistry and quasi periodic eruptions 

In this episode, Michelle and Nicole explore various astronomical topics, including merger debris around a galaxy, metal-mixing in early stars, quasi-periodic eruptions in Ansky, relic galaxies, Mercury's magnetosphere, and…
Episode 21 – Mercury, chemistry and quasi periodic eruptions 
In this episode, Michelle and Nicole explore various astronomical topics, including merger debris around a galaxy, metal-mixing in early stars, quasi-periodic eruptions in Ansky, relic galaxies, Mercury's magnetosphere, and second-generation stars in globular clusters. Tune in on Spotify, Apple Podcasts, or other platforms for more insights.
starxiv.com
October 12, 2025 at 9:36 AM
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Nicole’s final paper discussed this new take on second-gen (G2) stars in globular clusters: maybe they weren’t born polluted… maybe they merged into it. Mergers of MS + HeWD binaries can produce G2-like stars with He and N enrichment — even in the field.
arxiv.org/abs/2509.15337 🔭☄️
Left panel: optical CMD, namely F555W-(F439W-F555W), of the globular cluster NGC 362 based on HST photometry from Piotto et al. (2002); three red filled squires show the expected consecutive locations of an initial MS binary in its evolution toward RGB-like merger remnant, schematically shown in the right panel; the blue points show the stars of the RGB in its total range beginning from the RGB base. Right panel: schematic illustration of the main consecutive stages of the evolution of a hard binary initially composed of two MS stars (1), then converted into a (MSS+HeWD) binary (2), which finally merges forming RGB-like merger product (3) evolving along the cluster RGB.
September 29, 2025 at 4:30 PM
starxiv.bsky.social
Michelle's final paper takes us through BepiColombo's latest results on the magnetosphere of Mercury with data taken form 3 fly-bys over the nightside of the planet. Listen to learn more about the complex snap-backs and plasma flows of this little planet. 🔭 ☄️ arxiv.org/abs/2509.17872
The BepiColombo trajectories for MFB1 (marked, blue), MFB2 (marked, green) and MFB3 (marked, purple) in MSM coordinates: (a) X-Z plane, (b) X-Y plane, and (c) X-ρ plane. Each trajectory is shaded as a function of time and arrows indicate the MIPA boresight for each flyby, spaced 10 minutes apart. Panel (a) includes example field lines from the KT17 model. Panel (c) includes the location of the magnetospheric boundaries observed by MIPA (bow shock = X, magnetopause = +) and closest approach (star). All panels include the Shue magnetopause model (dark blue, Shue et al., 1997) and average bow shock (red) positions using parameters from Winslow et al. (2013).
September 29, 2025 at 4:20 PM
starxiv.bsky.social
Nicole’s second paper looked at the fossil galaxy Mrk1216 — compact and untouched since z ≈ 2. Its hot gas is still α-enhanced, with [Mg/Fe] ≈ 0.37 and a flat Type Ia SN contribution out to large radii. All signs point to early, rapid star formation.
arxiv.org/abs/2509.14613 🔭☄️
Adaptively smoothed and combined image of Mrk1216. The solid white annuli mark the regions selected for analysis, extending out to R500. The central region is coloured black for visual clarity. The green dashed annulus indicates the background region.
September 29, 2025 at 11:40 AM
starxiv.bsky.social
In Michelle's second paper, we learned about quasi-periodic eruptions from Ansky and what might generate them. In this case, new monitoring shows that the period and intensity of Ansky's QPEs are increasing, but why? Tune in to find out ☄️ 🔭 !https://arxiv.org/abs/2509.16304
Data showing the flare profiles measured in 2024 and 2025. The luminosity and period have dramatically increased!
September 29, 2025 at 11:15 AM
starxiv.bsky.social
Nicole talked about early metal mixing with Aeos, a sim of Pop III enrichment. Gas is only chemically coherent within 100 pc and 7 Myr after a SN. Beyond that, stars form with distinct abundance patterns. This Mahalanobis distance plot shows this divergence.
arxiv.org/abs/2509.13580 🔭☄️
Top: Number of stars per time bin. Bottom: dM along with 16th to 84th percentile spread of dM over TSF assuming instantaneous and homogeneous mixing across spheres of radius R. (The zero-point of TSF is the time at which each star forms.) Prior to ∼7Myr after a star forms, smaller spheres have smaller dM. The dM for each sphere intersects at ∼7Myr, suggesting τchem=7Myr. After 7Myr, the dM of spheres R≤0.1kpc continues to increase dramatically, consistent with rapidly changing local gas, whereas the dM of spheres R≥0.5kpc flattens, consistent with expectations for an IMF-averaged gas abundance.
September 29, 2025 at 9:13 AM
starxiv.bsky.social
Michelle's first paper discusses the Pearl in the Shell, a study of an ultra compact dwarf and a debris cloud around a nearby spiral galaxy. The study combines data from professional and amatCheck it out in our latest episode! arxiv.org/abs/2509.14038 🔭 ☄️
Amateur image of NGC 7531 (left). DESI Legacy Imaging Survey image of NGC 7531 (right). Sky-subtracted image of NGC 7531 as processed by Gnuastro’s NoiseChisel program was used as a basis for photometry measurements. Features are labelled: (a): main shell; (b): faint outer shell; c: counter plume.
September 29, 2025 at 8:36 AM
starxiv.bsky.social
Episode 21 is live! Tune in to here the latest results on Mercury's magnetosphere, population III stars, relic galaxies, quasi-periodic eruptions and more, with your hosts, Michelle and Nicole. Available on our website, Spotify, Apple, or wherever you get podcasts 🔭 ☄️
starxiv.com
September 29, 2025 at 7:00 AM
starxiv.bsky.social
Episode 21 – Mercury, chemistry and quasi periodic eruptions 

In this episode, Michelle and Nicole explore various astronomical topics, including merger debris around a galaxy, metal-mixing in early stars, quasi-periodic eruptions in Ansky, relic galaxies, Mercury's magnetosphere, and…
Episode 21 – Mercury, chemistry and quasi periodic eruptions 
In this episode, Michelle and Nicole explore various astronomical topics, including merger debris around a galaxy, metal-mixing in early stars, quasi-periodic eruptions in Ansky, relic galaxies, Mercury's magnetosphere, and second-generation stars in globular clusters. Tune in on Spotify, Apple Podcasts, or other platforms for more insights.
starxiv.com
September 29, 2025 at 5:05 AM
starxiv.bsky.social
Payel's final paper claims a remarkable agreement between theory and observations with regards to gravitational wave mission from stellar mass black hole binaries! 🔭 ☄️
arxiv.org/pdf/2508.20787
September 15, 2025 at 6:43 PM