arXiv subjects

Jens Hjorth

Publications and source records attributed to Jens Hjorth.

At least 19 recordsLinked to original sources

arXiv2026

Leviathan: a very massive, extended sub-millimetre galaxy spectroscopically confirmed at $\rm z = 4.05$

We present Leviathan, a sub-millimetre galaxy (SMG) spectroscopically confirmed at $z=4.051$ with JWST/NIRSpec. Modelling 31 bands of photometry from the rest-frame ultraviolet to the millimetre, we find a stellar mass of $\log(M_\star/M_\odot)=11.34^{+0.15}_{-0.22}$, making Leviathan one of the most massive spectroscopically confirmed galaxies at $z>4$, yet consistent with $Λ$CDM for a star-formation efficiency of approximately 20%. Although SMGs are often thought to be building the dense cores of compact quiescent galaxies, Leviathan is extended, with a rest-frame $0.9\,μ\mathrm{m}$ half-light radius of 4.3 kpc. Spatially resolved SED fitting gives a half-mass radius of $3.1\pm0.4$ kpc, consistent with its size in rest-frame $1.5\,μ\mathrm{m}$ imaging. This is three times larger than quiescent galaxies of the same mass, but consistent with the size-mass relation of star-forming galaxies at $4<z<5$ extrapolated to $M_\star=10^{11.3}M_\odot$. Leviathan forms stars on the main sequence (approximately $400\,M_\odot\,\mathrm{yr}^{-1}$) with dust spread across its disc. Both its stellar mass and dust are more extended than those of $z\approx3$ SMGs. Alongside GN20.2b, which has a similar mass and star-formation rate but is three times smaller, these show that massive dusty galaxies at $z\approx4$ can have very different stellar structures, and that not all have their stellar mass concentrated in a compact core.

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arXiv2026

Inferring the delay time distribution of type Ia supernovae with CAALAM I: SED modelling choices and their effect on the delay time distribution

A delay time distribution (DTD) quantifies the lag between progenitor formation and the subsequent explosive event. The DTD for a sample of Type Ia supernovae (SNe Ia) from a survey can be modelled through relative rates and the star formation histories (SFH) of all individual galaxies. Modern wide-field surveys require computationally efficient methods to model the DTD without modelling large samples of non-host galaxies. We model the DTD for a volume limited sample from SNe Ia from the Zwicky Transient Facility. The aim is to study the effect of varying SED modelling assumptions, including stellar templates, dust attenuation, SFH models, and photometric coverage. We present a new modelling framework, the Conditional Astral Ancestry Latency Analysis Machine (CAALAM), to infer the DTD of a sample of SNe Ia from the posterior of their host galaxy SFH. A model consisting of a standard stellar template, a non-parametric SFH and a flexible dust attenuation law, is favoured by the data and predicts a non-parametric DTD peaking at 100-330 Myr. When modelled as a power law, the DTD has a characteristic exponent of $-1.30 \pm 0.04$. The exponent is sensitive to dust attenuation, photometry, or SFH. When the DTD is convolved with the SFHs of the host galaxies, we find a bimodal progenitor age distribution, with $\sim60\%$ of progenitors with ages around 1.1-3.6 Gyr and the remaining progenitors older than 3.6 Gyr. We show a correspondence between the modes of the distributions of SN Ia light curve stretch and the progenitor ages. Our inferred DTD implies that most SNe Ia ($\sim 60\%$) occur promptly within 330 Myr of progenitor formation. However, the implied progenitor ages from the DTD convolved with the SFHs from our sample are significantly older. This suggests that the DTD might be a mixture of a prompt and an extended progenitor channel.

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arXiv2026

Extreme Stellar Death and Galaxy Feedback at z = 2: A Rest-frame Ultraviolet Characterization of the Strongly Lensed Superluminous Supernova 2025wny

We present photometric and spectroscopic follow-up observations of the strongly-lensed H-poor superluminous supernova (SLSN) 2025wny at $z = 2.0155$. We use integral-field Keck-II/KCWI spectroscopy to obtain rest-frame ultraviolet spectra spanning $11$ to $61$ days after maximum light. We use Pan-STARRS photometry to constrain the brightest image's magnification, finding $μ_A=19.4\pm1.6$, a factor of $\sim$2-4 higher than mass-modeling estimates. We fit the Pan-STARRS photometry with a magnetar energy-injection model including $^{56}$Ni decay using the Modular Open Source Fitter for Transients (MOSFiT); the model can reproduce the near-maximum light curve with parameters typical of local universe SLSNe-I, including a magnetar spin period $P_{\rm spin}=4.1^{+0.7}_{-1.0}\,\mathrm{ms}$ and magnetic field $B_{\perp}=2.4^{+1.1}_{-0.8}\times10^{14}\,\mathrm{G}$; however, the late-time plateau suggests an additional power source. Spectroscopically, iron-group element line blanketing is virtually absent in the rest-frame UV spectra, and the absorption features match previously identified SLSN-I features from abundance tomography. Based on narrow ISM line metallicity estimates, SN 2025wny exploded in a sub-solar-metallicity galaxy. From host-galaxy lines in our rest-frame UV-to-optical spectrum we estimate high-velocity outflows of $\sim$575 km s$^{-1}$ and an exceptionally high Ly$α$ escape fraction of $f_{esc}^{Lyα}=0.23\pm0.03$. SN 2025wny matches lower-redshift SLSNe reasonably well. Upcoming surveys like LSST and Roman will discover additional high-redshift SLSNe, further testing whether SLSNe-I explosions remain consistent at the population level in the early universe.

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arXiv2026

SN 2022riv in RX J2129: Discovery, Spectroscopic Classification, and Microlensing of a Strongly Lensed Type Ia Supernova from JWST and HST Observations

The multiply imaged SN 2022riv was discovered through a search of galaxy cluster fields as part of a Hubble Space Telescope (HST) SNAP program to find highly magnified stars. The supernova (SN) was detected in the image corresponding to the longest time delay of a galaxy at redshift $z=1.522$ strongly lensed by the foreground galaxy cluster RX J2129.7+0005. Follow up James Webb Space Telescope (JWST) NIRSpec G140M and PRISM spectroscopy yields a Type Ia SN classification. Using the SALT3-NIR light-curve fitter, we obtain a cosmology-independent measurement of the magnification of $5.35\pm1.01$ for the last-to-arrive image of the SN, with multiple SALT SN spectral time-series models yielding consistent constraints. The last-to-arrive image of SN 2022riv we detect appeared adjacent to the brightest cluster galaxy (BCG) at a location with an exceptionally high stellar mass density ($\sim 1-2$ dex higher than that of SN Refsdal), where microlensing is expected to introduce a 20-50% modulation of the magnification. Analyzing six independent lens models of the cluster, we find that four predict the magnification with much greater precision ($p < 0.05$) than would be expected by random chance, given the large effect anticipated from microlensing. Five models yield magnifications of roughly $4-7$ (within $1σ$) prior to accounting for microlensing, whereas HoliGRALE favors a significantly higher value of $15.39 \pm 0.85$. After incorporating nominal microlensing, the HoliGRALE prediction is within $1σ$ tension with our measurement. A companion paper (Dalrymple et al.) will present constraints on the relative time delay of the image that arrived earlier.

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arXiv2026

Probabilistic kilonova prediction from gravitational wave inferred binary neutron star parameters

Kilonovae provide a key electromagnetic window into binary neutron star mergers, revealing the properties of the merging system while probing r-process nucleosynthesis of the Universe. However, only a few kilonova candidates have been detected to date, with AT2017gfo being the only one associated with a gravitational wave event, GW170817. In this work, we present \textsc{Genova}, a probabilistic framework for predicting kilonova spectra and light curves directly from gravitational wave posterior samples of binary neutron star mergers. This method uses a conditional normalising flow to learn the distribution of rest-frame spectra conditioned on the source-frame component masses, tidal deformabilities, viewing angle and time since merger. Other kilonova model parameters, such as ejecta opacities, are marginalised over during training, so that their effects are propagated into the predicted spectra as predictive uncertainty. In the self-consistency test, the flow model reproduces the median light curves with residuals typically below $\sim 0.1$ mag, and the ratio of the predicted central 68\% interval widths remains predominantly between $0.8$ and $1.4$ over $\sim 0.4$--$8.0$ days. Comparisons with a physically distinct kilonova model show that the probabilistic prediction can remain informative beyond the model used for training. We apply \textsc{Genova} to GW170817/AT2017gfo using multi-band observations, including newly re-reduced $Y$-, $J$-, $K_s$-band photometry from the Visible and Infrared Survey Telescope for Astronomy, which we present in this work. The resulting predictive intervals broadly encompass the observations while capturing both gravitational wave posterior uncertainty and the variation induced by marginalised kilonova model parameters.

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arXiv2026

Two-population model of type Ia supernovae and their associations with host galaxies in ZTF DR2

We constrain type Ia supernova intrinsic properties, extinction, and probabilistic supernova-host associations using the volume-limited sample from the Zwicky Transient Factory DR2, the largest selection-free data set of type Ia supernovae to date. We employ Bayesian hierarchical modelling to jointly analyse the distribution of SALT2 light-curve parameters and global host-galaxy properties (stellar mass and rest-frame g-z colour). The adopted model is a mixture of distributions representing two supernova populations corresponding to two distinct modes of the stretch-parameter distribution, and two host-galaxy populations corresponding to two modes in the host-galaxy parameter space (red/massive and blue/less massive). Motivated by observations, high-stretch supernovae are allowed to populate both host-galaxy populations, whereas low-stretch supernovae are assumed to be exclusively associated with the red/massive host-galaxy population. The apparent non-linearity of the supernova magnitude-stretch relation implies a luminosity gap between the supernova populations, with the low-stretch population being Delta MB=0.14+/-0.03 mag brighter at x1=0, and different slopes (Delta alpha=0.064+/-0.023, steeper for the low-stretch population). The mean extinction coefficient is RB=3.89+/-0.29 (consistent with typical Milky Way values) in the blue/less massive host-galaxy population, which contains 68 per cent of high-stretch supernovae, and RB=3.08+/-0.08 in the red/massive host-galaxy population. Host-galaxy step corrections, both in stellar mass and colour, naturally emerge from the way the two supernova populations, characterised by different intrinsic luminosities and exctinctions, are distributed across host-galaxy populations. The adopted framework for modelling supernova populations and supernova-host associations makes host-galaxy step corrections redundant.

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arXiv2026

Spatially resolved metallicity and ionization in the merging system Gz9p3 at z=9.3

Studying the interstellar medium (ISM) in merging high-redshift galaxies is crucial for understanding early galaxy assembly, star formation, and black hole growth, predicted by hierarchical $Λ$CDM models. Deep imaging and spatially resolved spectroscopy with JWST enable unprecedented insight into these processes, even for galaxies in the Epoch of Reionization. We present NIRSpec and MIRI integral field spectroscopy and MIRI imaging of the merging galaxy Gz9p3 at z=9.3 of the UV and optical rest-frame showing a clumpy morphology in the continuum as well as line emission covering the entire galaxy over a range of 5 kpc from the central clump to the tail region. We analyze the integrated spectrum as well as different apertures in the galaxy allowing a spatially resolved characterization of the ionized ISM of this galaxy. We compare our measurements with archival NIRCam imaging and ALMA data. We measure a total star formation rate of 13.4 $\pm$ 1.8 Msun yr$^{-1}$, a metallicity of 12+log(O/H) = 7.84 $\pm$ 0.05 and $ξ_{ion}$= 25.4 $\pm$ 0.1 erg$^{-1}$ Hz and a burstiness parameter of 0.9 $\pm$ 0.1 for the integrated spectrum. We find large spatial differences in these parameters between the central clump and the tail region. The optical [OIII] emission peaks in the main galaxy, the far-infrared [OIII] emission peaks towards the tail, indicating different physical conditions in the ISM of the tail and main galaxy. This study presents the spatially resolved ISM analyses of a galaxy at z>9, revealing nebular line emission and strong spatial variations in star formation, metallicity, physical conditions, and ionizing efficiency. The results indicate a recent, metal-poor starburst in a tail alongside a more evolved, enriched central clump with evidence for extreme excitation. This demonstrates the power of spatially resolved JWST spectroscopy of galaxies in the Epoch of Reionization.

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arXiv2026

MIDIS: Strong H$β$+[OIII] Line Emitters at $z \geq 9$

We present a search for strong H$β$+[O III] emitters at $z=9.4-11.3$ in the HUDF using ultra-deep JWST/MIRI F560W imaging from the MIDIS survey. Three galaxies are identified via pronounced F560W flux excesses, consistent with strong rest-frame optical line emission. SED modelling yields rest-frame H$β$+[O III] equivalent widths of $\sim 600-1300$AA (median $\simeq 1260$AA), placing these sources among the most extreme known at these epochs. Combining these with a literature sample of 16 spectroscopically confirmed galaxies at $z\geq 9$, we find a median ${\rm EW}^{\rm Hβ+[O III]}_{\rm rest}\simeq 1300$AA, similar to values at $z\sim6-9$. We find no evidence for either a strong increase or decline in EW beyond $z\sim9$. A tentative trend of higher EW with increasing UV luminosity is observed, while no statistically significant anti-correlation with stellar mass is found. We place a first constraint on the H$β$+[O III] luminosity function at $z\simeq9-11$ ($Φ\sim10^{-3.4}\,{\rm Mpc^{-3}\,dex^{-1}}$ at $\log( L_{\rm Hβ+[OIII]}/{\rm erg\,s^{-1}})=42.5$), consistent with a decline relative to $z\sim7-8$. The MIDIS sources have $\log(ξ_{\rm ion}/{\rm Hz\,erg^{-1}})=25.1-25.4$. We find significant correlations between $ξ_{\rm ion}$ and EW and $β$, but not with UV luminosity, consistent with trends at lower redshift. These results suggest that the physical conditions governing nebular emission and ionising efficiency are already in place at $z\sim9-11$, extending trends established at $z\sim6-9$.

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arXiv2026

The Hubble Constant from Blue Type Ia Supernovae

There is a persistent tension of about $5σ-6σ$ between the value of the Hubble constant, as derived from the local distance ladder vs. the cosmic microwave background, signaling either unaccounted for systematics in the measurements or `new physics', such as early dark energy. Determining the Hubble constant using Type Ia supernovae requires nontrivial and accurate corrections for dust extinction. To circumvent this obstacle, we here determine the Hubble constant from blue, and hence presumably unextinguished, supernovae. For two different sets of Type Ia supernova data and lightcurve fitting methods, we find that when using blue supernovae only, the derived Hubble constant is consistently lower by $\sim$ 3 km s$^{-1}$ Mpc$^{-1}$ ($70 \pm 2.1$ and $70.3 \pm 3.0$ km s$^{-1}$ Mpc$^{-1}$), and within 1$σ$ of the cosmic microwave background measurement, compared to when using all supernovae. This is consistent with the hypothesis that systematic effects in dust corrections may affect standard supernova cosmology. However, the number of blue calibrating Type Ia supernovae is small (about six), and values of the Hubble constant for a range of different supernova colors are consistent at the 1.2$σ$ level. Upcoming major transient surveys will discover numerous unextinguished SNe~Ia, and thus be able to increase the precision of the Hubble constant measured from blue SNe~Ia, heralding a promising path toward resolving the Hubble constant tension.

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arXiv2026

AT 2024ahzi: A Type IIP Supernova Discovered by the LSST Commissioning Camera

As part of its commissioning, the Vera C. Rubin Observatory observed several fields repeatedly for a month with ComCam, an instrument that uses the same hardware as the LSST camera but covers a smaller field of view. We photometrically classify AT 2024ahzi, a transient discovered by ComCam, as a Type IIP supernova (SN IIP) using both ComCam and DECam photometry. We find that the duration, luminosity, and color of AT 2024ahzi's photometric plateau are all consistent with those from a large sample of SNe II. By comparing its multi-band light curves to SN II models and analytic relations, we place constraints on the SN progenitor, explosion dynamics, and circumstellar environment. We argue that the progenitor has an extended density profile indistinguishable from a slowly accelerating CSM. We discuss how a similar workflow can identify and characterize future Rubin SNe II.

astro-ph.HE↗

arXiv2026

Genesis-Metallicity: Universal Non-Parametric Gas-Phase Metallicity Estimation

We introduce genesis-metallicity, a gas-phase metallicity measurement python software employing the direct and strong-line methods depending on the available oxygen lines. The non-parametric strong-line estimator is calibrated based on a kernel density estimate in the 4-dimensional space of O2 = [O II]$λλ3727,29$/H$β$; O3 = [O III]$λ5007$/H$β$; H$β$ equivalent width EW(H$β$); and gas-phase metallicity $12 + \log$(O/H). We use a calibration sample of 1510 galaxies at $0 < z < 10$ with direct-method metallicity measurements, compiled from the JWST/NIRSpec and ground-based observations. In particular, we report 122 new NIRSpec direct-method metallicity measurements at $z > 1$. We show that the O2, O3, and EW(H$β$) measurements are sufficient for a gas-phase metallicity estimate that is more accurate than 0.09 dex. Our calibration is universal, meaning that its accuracy does not depend on the target redshift. Furthermore, the direct-method module employs a non-parametric ${\rm T}_{\rm e}$(O II) electron temperature estimator based on a kernel density estimate in the 5-dimensional space of O2, O3, EW(H$β$), ${\rm T}_{\rm e}$(O III), and ${\rm T}_{\rm e}$(O II). This ${\rm T}_{\rm e}$(O II) estimator is calibrated based on 1004 spectra with detections of both [O III]$λ4363$ and [O II]$λλ7320,30$, notably reporting 20 new NIRSpec detections of the [O II]$λλ7320,30$ doublet. We make genesis-metallicity and its calibration data publicly available and commit to keeping both up-to-date in light of the incoming data.

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arXiv2025

JWST and HST observations of the host galaxy and supernova, SN 2024aihh in EP240801a at z=1.67

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the counterpart of EP240801a, at z=1.67, the first fast X-ray transient (FXT) identified as an X-ray flash (XRF) by the Einstein Probe (EP) and Fermi-GBM. Our observations reveal strong photometric and spectroscopic evidence for an associated broad-lined Type Ic supernova (SN) SN 2024aihh, the most distant spectroscopically identified gamma ray-burst (GRB)-SN to date. The SN exhibits similar luminosity and light curve evolution to the proto-type GRB-SN 1998bw with an absolute magnitude of the SN at $\sim$23 days rest-frame of $M_{F140W} \approx$-19 mag. The SN is located in a host galaxy with complex morphology at a large ($\sim$6 kpc) offset in a region of relatively low surface brightness. The region around the SN has a modest star formation rate and is dominated by an intermediate mass-weighted age (1.4$\pm$0.3 Gyr) population, despite the apparent presence of a young, massive broad-lined Type Ic SN progenitor. These observations demonstrate that observations with HST and JWST can greatly extend the redshift range over which the GRB/FXT-SN connection can be studied, including in relatively low luminosity, X-ray rich events. They demonstrate little apparent evolution in the SN properties from local examples despite EP240801a originating from an epoch 10 Gyr ago.

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arXiv2025

Double Trouble: Two spectroscopically confirmed low-mass quiescent galaxies at z>5 in overdensities

We present the discovery of two low-mass, high-redshift, quiescent galaxies, GS-z5-Q1 and COS-z5-Q1, using JWST NIRSpec spectroscopy alongside NIRCam and MIRI photometry. Observed at a redshift of z=5.39 and z=5.11 respectively, and with stellar masses of $\rm 10^{9.6}M_\odot$ and $\rm 10^{9.5}M_\odot$, GS-z5-Q1 and COS-z5-Q1 are two of the most distant quiescent galaxies spectroscopically confirmed to-date, and are by far the least massive ($\sim10\times$ lower mass). Full spectrophotometric modelling reveals that COS-z5-Q1 appears to have quenched more than 300Myr prior to observation ($z\sim 7$) and has a formation redshift of around z$\sim$11, whilst GS-z5-Q1 formed and quenched in a single burst around 150Myr prior to observation ($z\sim6$). GS-z5-Q1 is found to lie near the centre of a known high-z overdensity in GOODS-S, as would be expected by galaxy formation models, while COS-z5-Q1 lies towards the outskirts of an overdense region. This highlights the role that environment could play in accelerating galaxy evolutionary processes and could possibly be linked to the galaxies' quiescent nature. By modelling their stellar populations, we show that these types of low-mass quiescent galaxies could potentially be descendants of the higher-z "mini-quenched" galaxies. The discovery of these two low-mass $z>5$ quiescent galaxies illuminates a previously undiscovered galaxy population and motivates dedicated follow-up surveys to investigate the overall population.

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arXiv2025

Diverse reddening distributions in sight lines to type Ia supernovae

Precise cosmological constraints from type Ia supernovae require adequately accurate corrections for host-galaxy extinction. Modelling these corrections is challenged by the problem of disentangling supernova intrinsic colours from host-galaxy interstellar reddening. The latter is commonly modelled in a probabilistic way assuming an exponential distribution exp(-E(B-V)/τ) as a universal prior which is applied across all types of supernova host galaxies. We test the robustness of the exponential model and its universality against predictions based on simulating dust and type Ia supernova distributions in host galaxies of different morphological types. We find substantial differences between predicted interstellar reddening in late- and early-type host galaxies, primarily driven by the stellar-to-dust mass ratios. The mean simulated reddening in late-type galaxies matches well those derived from type Ia supernova observations, but it is significantly lower for early-type host galaxies. The reddening distributions exhibit an excess of sight lines with vanishing reddening with respect to the exponential model, although the difference is quite mild for late-type galaxies. On the other hand, the distribution may peak at E(B-V)>0 when considering a population of young type Ia supernovae originating from lower heights within the dust disc. We demonstrate that assuming a universal reddening prior distribution for modeling peak magnitude-colour relation, which is currently a common practice, gives rise to a spurious scatter in the derived extinction properties. It may also bias relative distances between supernovae originating from different host-galaxy populations. The discrepancy between the simulated reddening in average early-type host galaxies and the observed occurrence of reddened supernovae suggests that reddening does not originate from interstellar dust expected in these galaxies.

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arXiv2025

A close look at the black hole masses and hot dusty toruses of the first quasars with MIRI-MRS

The presence of supermassive black holes (SMBHs, $M_\text{BH}\sim10^9 M_\odot$) at $z>7$ remains a puzzle. While their existence appears to require exotic formation or growth processes, it is possible that BH mass estimates are incorrect due to differences from the low-$z$ quasars where BH mass scaling relations are calibrated. In this work, we employ JWST MIRI-MRS spectroscopy to measure the rest-frame optical/IR properties of the four highest-redshift known luminous type-1 quasars at $7.08\leq z<7.64$. We use three new broad lines to measure updated BH masses, H$α$, Pa$α$ and Pa$β$, finding them to be in the range $(4-15)\cdot10^8 M_\odot$. Our black hole mass estimates from all tracers agree with each other and with previous, less accurate, ground-based measurements based on MgII. The flux ratios of the H lines deviate from expectations for case A and B recombination in the same way as in $z<3$ quasars, indicating similar physical conditions in the Broad Line Region. Rest-frame near-IR continuum emission from a hot dusty torus surrounding the accretion disc is unambiguously detected in all four objects. We model the emission with SKIRTOR and constrain the inclination (face-on) and the opening angle ($θ=40-60^\circ$) of the tori. These constraints are consistent for the four objects and with expectations from luminous quasars. We estimate a total dust mass $(1-4)\cdot10^6 M_\odot$ in the tori, corresponding to $(0.2-7)\%$ of the total dust in the quasar host galaxies. Given observed accretion rates, these SMBHs will deplete their tori in only $\sim5$ Myr. Overall, we confirm that $z>7$ SMBHs in quasars could not have grown from stellar-remnant BHs if the radiative efficiency of accretion is $10\%$. We also find no evidence that inferred BH masses and accretion processes in $z>7$ quasars differ significantly from their near-identical counterparts at $z<3$.

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arXiv2025

The rise of the galactic empire: luminosity functions at $z\sim17$ and $z\sim25$ estimated with the MIDIS$+$NGDEEP ultra-deep JWST/NIRCam dataset

We present a sample of six F200W and three F277W dropout sources identified as $16 16$ candidates present mass-weighted ages around 30 Myr, and attenuations $\mathrm{A(V)}<0.1$ mag. Their average stellar mass is $\mathrm{M}_\bigstar\sim10^{7}\,\mathrm{M}_\odot$, implying a stellar-to-baryon mass fraction around 10% if the emissivity increases with redshift, or significantly higher otherwise. Three candidates present very blue UV spectral slopes ($β\sim-3$) compatible with Pop III young ($\lesssim10$ Myr) stars and/or high escape fractions of ionizing photons; the rest have $β\sim-2.5$ similar to $z=10-12$ samples.

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arXiv2025

Stretch to stretch, dust to dust: lower-value local $H_{0}$ measurement from two-population modelling of type Ia supernovae

We revisit the local Hubble constant measurement from type Ia supernovae calibrated with Cepheids (SH0ES) by remodelling the supernova data using two supernova populations emerging from the observed bimodal distribution of the SALT2 stretch parameter. Our analysis accounts for population differences in both intrinsic properties (related to possible initial conditions, including supernova progenitor channels) and host-galaxy extinction (expected from well-known environmental differences associated observationally with the two populations). Based on a two-population Bayesian hierarchical modelling of the SALT2 light-curve parameters from the Pantheon+ compilation, we simultaneously constrain intrinsic and extrinsic properties of both supernova populations, match probabilistically the calibration supernovae with the corresponding population in the Hubble flow, and derive the Hubble constant. The difference between the two supernova populations is primarily driven by their mean absolute magnitudes and total-to-selective extinction coefficients. This is related but not equivalent to the traditional mass-step correction (including its broadening for reddened supernovae). The mean extinction coefficient of the supernova population used to propagate distances from the calibration galaxies to the Hubble flow is found to be consistent with the Milky Way-like interstellar dust model with R_B~4 and substantially higher than the extinction model assumed in the SH0ES measurement. Allowing for possible differences between reddening in the calibration galaxies and the corresponding population in the Hubble flow, we obtain H_0=70.59+/-1.15 km/s/Mpc. For the most conservative choice assuming equal prior distributions, we find H_0=71.45+/-1.03 km/s/Mpc. Our reanalysis of type Ia supernovae results in a reduction of the discrepancy with the Planck H_0 by at least 30 and up to 50 per cent (3.5-2.2sigma).

astro-ph.CO↗

arXiv2025

Exploring over 700 massive quiescent galaxies at z = 2-7: Demographics and stellar mass functions

High-redshift ($z>2$) massive quiescent galaxies are crucial tests of early galaxy formation and evolutionary mechanisms through their cosmic number densities and stellar mass functions (SMFs). We explore a sample of 743 massive ($\rm M_*> 10^{9.5}M_\odot$) quiescent galaxies from $z=2-7$ in over 800 arcmin$^2$ of NIRCam imaging from a compilation of public JWST fields (with a total area $>$ 5 $\times$ previous JWST studies). We compute and report their cosmic number densities, stellar mass functions, and cosmic stellar mass density. We confirm a significant overabundance of massive quiescent galaxies relative to a range of cosmological hydrodynamical simulations and semi-analytic models (SAMs). We find that no simulations or SAMs accurately reproduce the SMF for massive quiescent galaxies at any redshift within the interval $z=2-5$. This shows that none of these models' feedback prescriptions are fully capturing high-z galaxy quenching, challenging the standard formation scenarios. We find a greater abundance of lower-mass ($\rm M_*<10^{10}M_\odot$) quiescent galaxies than previously found, highlighting the importance of sSFR cuts rather than simple colour selection. We show the importance of this selection bias, alongside individual field-to-field variations caused by cosmic variance, in varying the observed quiescent galaxy SMF, especially at higher-z. We also find a steeper increase in the cosmic stellar mass density for massive quiescent galaxies than has been seen previously, with $ρ_*\propto (1+z)^{-7.2\pm0.3}$, indicating the dramatic increase in the importance of galaxy quenching within these epochs.

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