arXiv subjects

Josephine F. W. Baggen

Publications and source records attributed to Josephine F. W. Baggen.

12 recordsLinked to original sources

arXiv2026

Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots

Balmer absorption is common among little red dots (LRDs), but absorbers at or redward of systemic are rare, occurring in only $\sim10-15$\% of H$α$ absorbers. In this paper, we study two such exceptional cases with deep JWST/NIRSpec spectroscopy: 15 hr of high-resolution (G395H) observations of RUBIES-EGS-49140 (z=6.68), resolving the absorption in all four transitions from H$α$ through H$δ$, and medium-resolution spectroscopy (10 hr of G235M, 2 hr of G395M) of UNCOVER-A2744-45924 (z=4.46; 1.7x magnification). Both sources are among the optically reddest and most luminous LRDs known, and both show deep, near-systemic Balmer absorption troughs. We find two systematic trends along the Balmer series: the absorption centroids become more redshifted toward higher-order transitions, while the absorbed equivalent widths decline only weakly with increasing order, far less than expected from the atomic optical-depth ratios for a single attenuating screen. Ca\,{\sc{ii}}\,K is detected in absorption in both sources, whose offset follows the H$α$ trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,$λ$3870, along with broad [O\,{\sc{iii}}]\,$λ$4364, [O\,{\sc{iii}}]\,$\lambda5008$, and He\,{\sc{i}}\,$\lambda5877,\lambda7067$, while He\,{\sc{ii}}\,$λ$4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,$\lambda7067/\lambda5877$ ratio. A possible explanation for the relative strengths of the Balmer absorption lines could be a dense, optically thick medium whose re-emission modifies their apparent absorption strengths, while the velocity progression may arise from stratification in the absorbing gas.

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arXiv2026

(Re)solving the Complex Multiscale Morphology and V-shaped Spectral Energy Distribution of a Newly Discovered Strongly Lensed Little Red Dot in A383

We present a luminous "little red dot" (LRD) at $z$ = 6.027, doubly imaged by the galaxy cluster A383 and observed with the James Webb Space Telescope (JWST) NIRCam. Owing to its large magnifications, $μ$ $\sim$ 11 for image A383-LRD1A and $μ$ $\sim$ 7 for A383-LRD1B, the system is exceptionally bright and highly stretched, providing a rare, spatially resolved view of an LRD. The images reveal a complex morphology with a compact red dot, a spatially offset blue dot, and faint emission bridging and surrounding the two. After correcting for lensing, the blue and red dots have rest-frame UV and optical sizes of $\sim 60$ pc and $\lesssim 150$ pc, respectively, while extended emission traced most clearly in F356W ([O III]+H$β$) reaches scales of order $\sim 1$ kpc. Spatially resolved spectral energy distribution (SED) analysis reveals that the characteristic V-shaped SED arises from the superposition of a flat UV continuum from the blue dot, consistent with a young stellar population, and a steep red SED from the red dot, which, based on Hubble Space Telescope+JWST photometry alone, admits a straightforward stellar interpretation as a massive, heavily dust-attenuated component. However, this interpretation is challenged by Atacama Large Millimeter/submillimeter Array dust continuum upper limits, suggesting a nonstellar origin such as dense gas configurations. Separated by only $\sim$ 300 pc in the source plane, these components would blend into a single compact source in unlensed observations with the canonical LRD colors. This system therefore provides a rare opportunity to resolve the internal structure of an LRD and to begin unraveling the physical nature of this population.

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arXiv2026

Connecting the Dots: UV-Bright Companions of Little Red Dots as Lyman-Werner Sources Enabling Direct Collapse Black Hole Formation

We compile a sample of 83 Little Red Dots (LRDs) with JWST imaging and find that a substantial fraction ($\sim$43%, rising to $\gtrsim$85% for the most luminous LRDs) host one or more spatially offset, UV-bright companions at projected separations of $0.5\rm \, kpc \lesssim d\lesssim 5 \rm \,kpc$, with median of $\langle d \rangle = 1.0\,\mathrm{kpc}$. This fraction is even higher when smaller spatial scales are probed at high S/N ratio: we show that the two most strongly lensed LRDs known to date, A383-LRD and the newly discovered A68-LRD, both have UV-bright companions at separations of only $d\sim0.3$ kpc, below the resolution limit of most unlensed JWST samples. We explore whether these ubiquitous red/blue configurations may be physically linked to the formation of LRDs, in analogy with the "synchronized pair" scenario originally proposed for direct-collapse black hole formation. In this picture, ultraviolet radiation from the companions, which typically have modest stellar masses ($M_\ast \sim 10^{8-9}M_\odot$), suppresses molecular hydrogen cooling in nearby gas, allowing nearly isothermal collapse and the formation of extremely compact objects, such as massive black holes or quasi-stars. Using component-resolved photometry and SED modeling, we infer Lyman-Werner radiation fields of $J_{21,LW} \sim 10^{2.5}$-$10^{5}$ at the locations of the red components, comparable to those required in direct-collapse models, suggesting that the necessary photodissociation conditions are realized in many LRD systems. This framework provides a simple and self-consistent explanation for the extreme compactness and distinctive spectral properties of LRDs, and links long-standing theoretical models for early compact object formation directly to a population now observed with JWST in the early universe.

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arXiv2026

The Impact of Baryonic Effects on the Dynamical Masses Inferred Using Satellite Kinematics

Satellite kinematics offers a powerful method to infer dynamical halo masses and has been demonstrated to yield tight constraints on the galaxy-halo connection. However, previous studies have assumed that the halos in which the satellites orbit are composed solely of dark matter, neglecting the role of baryons. Here, we develop an analytical model incorporating stars, gas, and the adiabatic response of dark matter to assess the impact of baryonic effects on the inference from satellite kinematics. The model covers halos in the mass range $10^{12}-10^{15}M_\odot$ and is tuned to agree with well-established observational scaling relations. In addition, the model uses simple functional forms for the mass fractions of ejected baryons and diffuse halo stars, calibrated to the median trends in the EAGLE hydrodynamical simulations. We find that baryonic effects mainly result in a reduction of the satellite line-of-sight velocity dispersion due to the ejection of baryons and the resulting response of the dark matter halo. The effect is minimal (less than 1%) for the most massive halos, but reaches ~5-6% for halos in the mass range $10^{12}-10^{13}M_\odot$, and up to 8% in extreme cases. We propose a simple formalism for correcting the satellite line-of-sight velocity dispersion for baryonic effects, and for marginalizing over the uncertainties. We integrate this correction function into BASILISK, a Bayesian hierarchical inference method applied to satellite kinematics data extracted from large redshift surveys, and find that this shifts central galaxies to higher inferred halo masses at fixed luminosity by up to ~0.3 dex. In a forthcoming work, we demonstrate that these few-percent level baryonic effects can have a non-negligible impact on the inference of cosmological parameters, motivating a novel approach to constraining the efficiency of feedback processes associated with galaxy formation.

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arXiv2025

Further Evidence for a Direct-Collapse Origin of the Supermassive Black Hole at the Center of the Infinity Galaxy

The z=1.14 $\infty$ galaxy consists of two ringed nuclei with an active supermassive black hole (SMBH) in between them. The system is likely the result of a nearly face-on collision between two disk galaxies with massive bulges. In van Dokkum et al. (2025) we suggested that the SMBH may have formed from shocked and compressed gas at the collision site, in a runaway gravitational collapse. Here we test this hypothesis using newly obtained JWST NIRSpec IFU observations. We first confirm that the system has a cloud of gas in between the nuclei that is photo-ionized by an AGN-like object near its center. Next, we constrain the origin of the SMBH from its radial velocity. If it formed in the cloud its velocity should be similar to the surrounding gas, whereas it would be offset if the SMBH had escaped from one of the nuclei or were associated with a faint galaxy. We find that the radial velocity of the SMBH is within $\sim 50$ km/s of that of the surrounding gas, as expected if the SMBH formed within the cloud. Unexpectedly, we find that both nuclei have active SMBHs as well, as inferred from very broad H$α$ emission with FWHM $\sim 3000$ km/s. This rules out scenarios where the central SMBH was ejected from one of the nuclei in a gravitational recoil. Taken together, these results strengthen the hypothesis that the object at the center of the $\infty$ galaxy is a newly formed SMBH.

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arXiv2025

The Infinity Galaxy: a Candidate Direct-Collapse Supermassive Black Hole Between Two Massive, Ringed Nuclei

We report the discovery of an unusual z=1.14 object, dubbed the $\infty$ galaxy, in JWST imaging of the COSMOS field. Its rest-frame near-IR light is dominated by two compact nuclei with stellar masses of $\sim 10^{11}$ Msun and a projected separation of 10 kpc. Both nuclei have a prominent ring or shell around them, giving the galaxy the appearance of a figure eight or an $\infty$ symbol. The morphology resembles that of the nearby system II Hz 4, where the head-on collision of two galaxies with parallel disks led to the formation of collisional rings around both of their bulges. Keck spectroscopy, VLA radio data, and Chandra X-ray data show that the $\infty$ galaxy hosts an actively accreting supermassive black hole (SMBH), with quasar-like radio and X-ray luminosity. Remarkably, the SMBH is not associated with either of the two nuclei, but is in between them in both position and radial velocity. Furthermore, from excess emission in the NIRCAM F150W filter we infer that the SMBH is embedded in an extended distribution of H$α$-emitting gas, with a rest-frame equivalent width ranging from 400 - 2000 Angstrom. The gas spans the entire width of the system and was likely shocked and compressed at the collision site, in a galaxy-scale equivalent of what happened in the bullet cluster. We suggest that the SMBH formed within this gas in the immediate aftermath of the collision, when it was dense and highly turbulent. If corroborated with simulations and follow-up JWST spectroscopy, this would demonstrate that `direct' SMBH formation by a runaway gravitational collapse is possible in extreme conditions.

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arXiv2025

A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among the largest Hydrogen Balmer breaks reported at any redshift, broad multi-peaked H$β$ emission, and Balmer line absorption in multiple transitions. We model this source as a "black hole star" (BH*) where the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free "atmosphere" around a supermassive black hole. This source may provide evidence of an early black hole embedded in dense gas -- a theoretical configuration proposed to rapidly grow black holes via super-Eddington accretion. Radiation from the BH* appears to dominate almost all observed light, leaving limited room for contribution from its host galaxy. We demonstrate that the recently discovered "Little Red Dots" (LRDs) with perplexing spectral energy distributions can be explained as BH*s embedded in relatively brighter host galaxies. This source provides evidence that black hole masses in the LRDs may be over-estimated by orders of magnitude -- the BH* is effectively dust-free contrary to the steep dust corrections applied while modeling LRDs, and the physics that gives rise to the complex line shapes and luminosities may deviate from assumptions underlying standard scaling relations.

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arXiv2025

RUBIES Reveals a Massive Quiescent Galaxy at z=7.3

We report the spectroscopic discovery of a massive quiescent galaxy at $z_{\rm spec}=7.29\pm0.01$, just $\sim700\,$Myr after the Big Bang. RUBIES-UDS-QG-z7 was selected from public JWST/NIRCam and MIRI imaging from the PRIMER survey and observed with JWST/NIRSpec as part of RUBIES. The NIRSpec/PRISM spectrum reveals one of the strongest Balmer breaks observed thus far at $z>6$, no emission lines, but tentative Balmer and Ca absorption features, as well as a Lyman break. Simultaneous modeling of the NIRSpec/PRISM spectrum and NIRCam and MIRI photometry (spanning $0.9-18\,μm$) shows that the galaxy formed a stellar mass of log$(M_*/M_\odot)=10.23^{+0.04}_{-0.04}$ before $z\sim8$, and ceased forming stars $50-100\,$Myr prior to the time of observation, resulting in $\log(\rm{sSFR/Gyr}^{-1})<-1$. We measure a small physical size of $209_{-24}^{+33}\,{\rm pc}$, which implies a high stellar mass surface density within the effective radius of $\log(Σ_{*,\rm e}/M_\odot\,kpc^{-2})=10.85_{-0.12}^{+0.11}$ comparable to the highest densities measured in quiescent galaxies at $z\sim2-5$. The 3D stellar mass density profile of RUBIES-UDS-QG-z7 is remarkably similar to the central densities of local massive ellipticals, suggesting that at least some of their cores may have already been in place at $z>7$. The discovery of RUBIES-UDS-QG-z7 has strong implications for galaxy formation models: the estimated number density of quiescent galaxies at $z\sim7$ is $>100\times$ larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.

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arXiv2024

The Small Sizes and High Implied Densities of `Little Red Dots' with Balmer Breaks Could Explain Their Broad Emission Lines Without an AGN

Early JWST studies found an apparent population of massive, compact galaxies at redshifts $z\gtrsim7$. Recently three of these galaxies were shown to have prominent Balmer breaks, demonstrating that their light at $λ_{\rm rest} \sim 3500$ $Å$ is dominated by a stellar population that is relatively old ($\sim$200 Myr). All three also have broad H$β$ emission with $σ> 1000 \,\rm km s^{-1}$, a common feature of such `little red dots'. From Sérsic profile fits to the NIRCam images in F200W we find that the stellar light of galaxies is extremely compact: the galaxies have half-light radii of $r_{\rm e}\sim$ 100 pc, in the regime of ultra compact dwarfs in the nearby Universe. Their masses are uncertain, as they depend on the contribution of possible light from an AGN to the flux at $λ_{\rm rest}>5000$ $Å$. If the AGN contribution is low beyond the Balmer break region, the masses are $M_* \sim 10^{10}-10^{11}\mathrm{M}_{\odot}$, and the central densities are higher than those of any other known galaxy population by an order of magnitude. Interestingly, the implied velocity dispersions of $\sim$1500 kms$^{-1}$ are in very good agreement with the measured H$β$ line widths. We suggest that some of the broad lines in `little red dots' are not due to AGNs but simply reflect the kinematics of the galaxies, and speculate that the galaxies are observed in a short-lived phase where the central densities are much higher than at later times. We stress, however, that the canonical interpretation of AGNs causing the broad H$β$ lines also remains viable.

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arXiv2024

RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z \sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec

The identification of red, apparently massive galaxies at $z>7$ in early JWST photometry suggests a strongly accelerated timeline compared to standard models of galaxy growth. A major uncertainty in the interpretation is whether the red colors are caused by evolved stellar populations, dust, or other effects such as emission lines or AGN. Here we show that three of the massive galaxy candidates at $z=6.7-8.4$ have prominent Balmer breaks in JWST/NIRSpec spectroscopy from the RUBIES program. The Balmer breaks demonstrate unambiguously that stellar emission dominates at $λ_{\rm rest} = 0.4\,μ$m, and require formation histories extending hundreds of Myr into the past in galaxies only 600--800 Myr after the Big Bang. Two of the three galaxies also show broad Balmer lines, with H$β$ FWHM $>2500~{\rm km\,s^{-1}}$, suggesting that dust-reddened AGN contribute to, or even dominate, the SEDs of these galaxies at $λ_{\rm rest}\gtrsim 0.6\,μ$m. All three galaxies have relatively narrow [O III] lines, seemingly ruling out a high-mass interpretation if the lines arise in dynamically-relaxed, inclined disks. Yet, the inferred masses also remain highly uncertain. We model the high-quality spectra using Prospector to decompose the continuum into stellar and AGN components, and explore limiting cases in stellar/AGN contribution. This produces a wide range of possible stellar masses, spanning $M_\star \sim 10^9 - 10^{11}\,{\rm M_{\odot}}$. Nevertheless, all fits suggest a very early and rapid formation, most of which follow with a truncation in star formation. Potential origins and evolutionary tracks for these objects are discussed, from the cores of massive galaxies to low-mass galaxies with over-massive black holes. Intriguingly, we find all of these explanations to be incomplete; deeper and redder data are needed to understand the physics of these systems.

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arXiv2023

Sizes and mass profiles of candidate massive galaxies discovered by JWST at 7<z<9: evidence for very early formation of the central ~100 pc of present-day ellipticals

The first JWST data revealed an unexpected population of red galaxies that appear to have redshifts of $z\sim 7-9$ and high masses of $M_*$ $\sim$ 10$^{10}$ M$_{\odot}$ (Labbé et al. 2023). Here we fit Sérsic profiles to the F200W NIRCam images of the 13 massive galaxy candidates of Labbé et al., to determine their structural parameters. Satisfactory fits were obtained for nine galaxies. We find that their effective radii are extremely small, ranging from $r_{\rm e}\sim 80$ pc to $r_{\rm e} \sim 300$ pc, with a mean of $\langle r_{\rm e}\rangle \approx 150$ pc. For their apparent stellar masses, the galaxies are smaller than any other galaxy population that has been observed at any other redshift. We use the fits to derive circularized three-dimensional stellar mass profiles of the galaxies, and compare these to the mass profiles of massive quiescent galaxies at $z\sim$2.3 and nearby elliptical galaxies. We find that, despite the high redshift galaxies having $10-20$ times smaller half-light radii, the central stellar densities are comparable to those of their putative descendants at later times. The most straightforward interpretation is that the dense compact inner regions of the most massive ellipticals today were already in place $\sim 600$ Myr after the Big Bang. We caution that the redshifts and masses of the galaxies remain to be confirmed, and that the complex NIRCam point spread function is not yet fully characterized.

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arXiv2021

Comparing hypervelocity star populations from the Large Magellanic Cloud and the Milky Way

We predict and compare the distributions and properties of hyper-velocity stars (HVSs) ejected from the centres of the Milky Way (MW) and the Large Magellanic Cloud (LMC). In our model, HVSs are ejected at a constant rate -- equal in both galaxies -- via the Hills mechanism and are propagated in a combined potential, where the LMC orbits the MW on its first infall. By selecting $m>2\, \mathrm{M_\odot}$ HVSs well-separated from the Magellanic Clouds and Galactic midplane, we identify mock HVSs which would stand out from ordinary stars in the stellar halo in future data releases from the Gaia satellite and the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). We find that in these deep surveys, LMC HVSs will outnumber MW ones by a factor $\sim 2.5$, as HVSs can more easily escape from the shallower potential of the LMC. At an assumed HVS ejection rate of $10^{-4} \, \mathrm{yr^{-1}}$, HVSs detectable in the final Gaia data release and LSST from the LMC (MW) will number $125_{-12}^{+11}$ ($50_{-8}^{+7}$) and $140_{-11}^{+10}$ ($42_{-7}^{+6}$), respectively. The MW and LMC HVS populations show different kinematics and spatial distributions. While LMC HVSs have more modest total velocities and larger Galactocentric distances clustered around those of the LMC itself, HVSs from the MW show broader distributions, including a prominent high-velocity tail above $500 \, \mathrm{km \ s^{-1}}$ that contains at least half of the stars. These predictions are robust against reasonable variation of the Galactic potential and of the LMC central black hole mass.

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