arXiv subjects

Peter Senchyna

Publications and source records attributed to Peter Senchyna.

At least 19 recordsLinked to original sources

arXiv2026

SPURS: An Ultra-deep View Inside the Compact, Nitrogen-Enriched Nuclei of Little Red Dots

We present the first ultra-deep rest-UV spectroscopy of four UV-bright Little Red Dots (LRDs), obtained from the SPURS Cycle 4 Large Program. The spectra reveal broad CIV (FWHM $\approx2700-2800$ km s$^{-1}$) in two LRDs, alongside narrow-line densities elevated above star-forming galaxies ($n_e\sim10^4-10^5$ cm$^{-3}$, reaching $10^6$ cm$^{-3}$ in the most extreme source) and nitrogen-enhancements in all four LRDs. We detect broad HeII emission (FWHM $\approx930$ km s$^{-1}$) in one LRD, and two others with fast P-Cygni absorption ($\gtrsim2200$ km s$^{-1}$). Strong interstellar absorption lines and Ly$\alpha$ damping wings reveal the UV continuum is deeply embedded in neutral gas ($N_{\rm HI}\gtrsim10^{22}$ cm$^{-2}$) in all four LRDs. Detections of fluorescent FeII and OI emission and fine-structure absorption indicate this gas lies close to the UV-emitting region. In archival $z>4$ samples, we find nitrogen and strong CIII] emission are significantly more common in LRDs than in the galaxy population. The transmission of broad CIV, tracing the broad-line region or cocoon, depends on rest-optical color within our sample, consistent with an orientation-dependent picture in which bluer, less obscured sightlines offer a more direct, polar view of the central engine and its outflows. We find several potential signatures of very massive stars, whose winds may contribute to nitrogen enhancement. We investigate other abundance patterns expected from supermassive stars but our results are inconclusive. Our results place the UV-emitting region within $\lesssim8$ pc of the LRD nucleus, consistent with an actively assembling nuclear star cluster. Dynamical interactions in this extremely dense environment, including tidal disruption of stars, may explain the high incidence of nitrogen enhancements in LRDs.

astro-ph.GA↗

arXiv2026

SPURS: Massive Stars, Dense Gas, and Ly$\alpha$ Escape in GN-z11 at $z = 10.6$

We present ultra-deep {\it JWST} spectroscopy of GN-z11 ($z=10.6$) obtained through the SPURS Cycle 4 Large Program, providing the deepest rest-UV view yet obtained of a galaxy at $z>10$. GN-z11 was previously found to be nitrogen-enhanced with detectable Ly$\alpha$. The SPURS spectrum reveals P-Cygni stellar wind features and broad He II emission that are jointly reproduced by stellar population models incorporating very massive stars (VMS; $>100\,M_\odot$) at low metallicity and young ages ($\lesssim3$ Myr). We also detect a broad ($\rm FWHM=1670$ km s$^{-1}$) component to N IV] $\lambda1486$, now seen in several nitrogen emitters, potentially arising from dense WN-like winds or LBV-like outbursts associated with a population of VMS in a dense environment, though an AGN-driven wind cannot be excluded. In either scenario, this broad component may trace the gas producing GN-z11's nitrogen enhancement. Rest-UV absorption lines reveal a fast ($\sim500$~km~s$^{-1}$), highly ionized outflow and a negligible neutral gas covering fraction. We resolve the weak Ly$\alpha$ emission (EW=5.6 \AA, $f_{\rm esc,Ly\alpha}=2.7$\%), finding a broad red wing (44\% of flux at $>500$ km s$^{-1}$) that should experience reduced IGM damping wing suppression and help explain Ly$\alpha$ visibility at $z>10$. Fine-structure O I* $\lambda1304$ emission indicates dense neutral gas near a subset of the ionizing sources, which may also scatter Ly$\alpha$ to the large observed velocities. The weak low-ionization absorption favors a picture in which this dense neutral gas is confined to a compact nuclear region. Together, these results are consistent with a rapid burst of star formation building up the dense nuclear regions and surrounding clusters in GN-z11.

astro-ph.GA↗

arXiv2026

JWST absorption line spectroscopy with SPURS: ISM covering fractions and kinematics in individual galaxies at $z=5-9$

We present deep rest-ultraviolet (UV) spectra of six luminous $z=5$ -- 9 galaxies in the Abell-2744 field taken as part of the JWST Cycle 4 Large Program SPURS. The individual galaxy spectra show unambiguous detections of interstellar medium (ISM) metal absorption lines from low- and high-ionization states of enriched gas, which we use to probe the ISM gas porosity and kinematics. We find a striking diversity in the absorption profiles. We find low-ionization gas covering fractions ranging from 0.2 to 0.9, indicating a heterogeneous and patchy neutral ISM. The low-ionization kinematics also show a large diversity, with velocity centroid values ranging from $+$70 to a significantly blueshifted $-140$ km$\,$s$^{-1}$, while the high-ion gas shows mostly blueshifted absorption, indicating the presence of multiphase outflows. While all sources show outflow signatures in blueshifted wings, we also find that half of our sample, in particular those with the lowest stellar masses and highest sSFRs, have low-ionization velocity centroids close to systemic velocities. This is in contrast to near-ubiquitous bulk low-ionization gas outflows at lower redshifts. We suggest that this diversity of kinematics may be due to the bulk of the cold gas having low outflow velocities in the lowest mass and highest sSFR systems, potentially due to inefficient entrainment and/or an unresolved infalling component. These spectra reveal a metal-enriched ISM with complex gas geometry and kinematics, and highlight the potential of deep JWST grating spectroscopy to reveal the properties of the ISM during the reionization era.

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arXiv2026

SPURS: Bursty Star Formation in an Extremely Luminous Weak Emission Line Galaxy at $z=9.3$

JWST has revealed a population of super-luminous early galaxies with a volume density in excess of most expectations. The spectra reveal diverse properties: while some reveal strong emission lines characteristic of galaxies in the midst of strong bursts, others show weak emission lines that could reflect old stellar populations, large escape fractions, or post-burst star formation histories. Through the JWST Cycle 4 large program SPURS, we have obtained ultra-deep (29 hr) rest-frame UV spectroscopy of a z=9.3 super-luminous ($M_{\rm UV}=-21.66$) galaxy with large assembled stellar mass (1.6$\times$10$^9$ $M_\odot$) and extremely weak emission lines (H$\beta$ EW $\approx25$~\AA). The strong stellar wind features and rest-optical line ratios suggest the galaxy is already significantly enriched, with a metallicity of 0.4--0.7~Z$_\odot$. The interstellar absorption lines reveal outflows ($v\simeq -161$~km~s$^{-1}$) with a large neutral gas covering fraction, suggesting that the weak emission lines are not due to large escape fractions. The combination of the Balmer break, weak emission lines, and stellar wind features constrains the star formation history, indicating a recent burst of star formation lasting 10--20 Myr followed by a downturn over the last 10~Myr. The observations suggest that $z\gtrsim 9$ weak emission line galaxies such as this source can be explained by stochastic star formation, provided that the downturns in star formation are recent (i.e., <10 Myr prior to observation). The ultra-deep grating spectrum enables the IGM damping wing to be characterized, decoupling the effects of local absorption. The smooth Ly$\alpha$ break indicates that this source, one of the most massive galaxies known at z>9, is likely situated in a small ionized bubble ($0.29_{-0.09}^{+0.11}$~pMpc), as is common at large neutral hydrogen fractions ($\bar{x}_{\rm HI}=0.81_{-0.21}^{+0.14}$).

astro-ph.GA↗

arXiv2026

SPURS: Evidence for Clumpy Neutral Envelopes and Ionized IGM Surrounding Little Red Dots in Abell 2744 from Ultra-Deep Rest-UV Spectroscopy

Rest-frame ultraviolet (UV) spectra of Little Red Dots (LRDs) often show Ly$\alpha$ emission. Along with broad Balmer emission, LRDs are expected to produce broad Ly$\alpha$ emission. However, the large column density of neutral gas invoked to explain the Balmer break should significantly redshift and further broaden the Ly$\alpha$ line, making it challenging to detect without sensitive, moderate-resolution spectra. We present ultra-deep (29 hours) G140M JWST/NIRSpec observations covering the rest-UV of two LRDs in Abell2744 from the SPURS Cycle 4 Large Program. One of our targets is Abell2744-QSO1, a gravitationally-lensed LRD at $z=7.04$ with faint UV emission (M$_{\rm UV}=-16.9$), and the other source (UNCOVER-2476) is newly-confirmed at $z=4.02$ with a very bright UV continuum (M$_{\rm UV}=-19.6$). We find that Abell2744-QSO1 has a broad Ly$\alpha$ profile, along with narrow CIV, FeII$\lambda1786$, and OI$\lambda1302$ emission. The Ly$\alpha$ profile suggests an origin similar to the broad H$\alpha$, but the line is considerably less redshifted than expected from existing dense gas models. We show that the line profile can be explained if the dense neutral gas is clumpy, allowing Ly$\alpha$ to escape by scattering off of the clump surfaces. We find that UNCOVER-2476 has narrow [NeIV] emission, indicating either a hard radiation field or shocks. We confirm two close neighbors with Ly$\alpha$ emission around Abell2744-QSO1, indicating it traces a dense environment that may have ionized its surrounding IGM. We suggest that LRDs may preferentially trace bubbles carved by their dense environments, contributing to the prevalence of Ly$\alpha$ in the population.

astro-ph.GA↗

arXiv2026

Consistent Gas-Phase Temperatures and Metallicities from UV and Optical Nebular Emission: A Reliable Foundation from z=0 to Cosmic Dawn

The rest-frame UV spectra of star-forming galaxies are increasingly important as they become one of the primary windows to probe the physical properties of cosmic dawn (z>8) galaxies with the James Webb Space Telescope. However, the systematic discrepancies between UV and optical gas-phase metallicity measurements remain poorly understood in the local universe, partly due to challenges in achieving precise comparisons between UV and optical spectra for the same objects. In this work, we introduce a novel method that leverages the HeII 1640 and HeII 4686 nebular emission lines to achieve accurate aperture and reddening corrections between UV and optical spectra. Here we apply this method to three nearby Blue Compact Dwarf (BCD) galaxies. Our results demonstrate that this approach enables precise measurements, with electron temperatures ($T_e$) derived from UV and optical spectra exhibiting closer agreement compared to previous studies, and O/H abundance agreeing within 0.1 dex. However, two BCDs appear to have lower UV-based electron temperatures $T_{e~1666} < T_{e~4363}$, in contrast to expectations from the temperature fluctuation model. We consider a variety of possible explanations for these unphysical temperatures - differential dust attenuation, aperture differences, and spatial extent of emission lines - but no suitable cause is identified. These findings suggest a complex gaseous environment associated with star formation, and underscore the need for additional observations to further investigate the nature of HeII nebular emission and address the systematic issues between UV and optical nebular properties. Nonetheless, the close empirical agreement of these results indicates that UV- and optical-based nebular temperature and abundance measurements can be reliably compared within 0.1 dex, providing a solid foundation for evolutionary studies from the local Universe to cosmic dawn.

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arXiv2025

CLASSY XIII. Cutting through the Clouds - Comparing Indirect Tracers of Ionizing Photon Escape

The Epoch of Reionization (EoR) provides critical insights into the role of early galaxies in shaping the ionization state of the universe. However, because of the opacity of the intergalactic medium, it is often not possible to make direct measurements of the ionizing photon escape fraction ($f_{\mathrm{esc}}^{\: \mathrm{LyC}}$) of high-redshift ($z \gtrsim 4$) galaxies. To explore the agreement and systematics of common indirect approaches, we applied six empirically calibrated diagnostics to predict $f_{\mathrm{esc}}^{\: \mathrm{LyC}}$ for the 45 nearby star-forming galaxies from the COS Legacy Spectroscopic SurveY (CLASSY). These methods- based on ultraviolet (UV) absorption lines, the UV continuum slope, Ly$\alpha$ kinematics, a multivariate model, radiation-hydrodynamic simulations, and nebular emission line ratios- enable us to explore systematic differences between predictions and assess how galactic properties influence inferred LyC escape. Despite significant variations in method predictions, there is broad consistency in the resulting weak and strong LyC leaker classifications, with approximately half exhibiting predicted escape fractions $>$1%. We find evidence for two different pathways of LyC escape in nearby star-forming galaxies: (1) an early escape model driven by very young stellar populations, and (2) a delayed escape model that is consistent with supernova-driven outflows and time-dependent ISM clearing. The early escape model is favored among galaxies with a single, intense burst of recent star formation. In contrast, the delayed escape model is common among galaxies with more extended starburst histories. To interpret ionizing photon escape during the EoR, it will be necessary to recognize and understand this diversity in LyC escape mechanisms.

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arXiv2025

Echoes of the First Stars: Massive Star Evolution in Extremely Metal-Poor Environments with the Habitable Worlds Observatory

A remarkable span of frontier astrophysics, from gravitational-wave archaeology to the origin of the elements to interpreting snapshots of the earliest galaxies, depends sensitively on our understanding of massive star formation and evolution in near-pristine, relatively enriched gas. From the surprisingly massive black holes detected by LIGO/Virgo to highly ionized nebulae with peculiar enrichment patterns observed in galaxies at Cosmic Dawn, evidence is mounting that our understanding of massive-star populations at very low metallicity remains critically incomplete. The fundamental limitation is the hand nature has dealt us: only a few star-forming galaxies within $\lesssim$1 Mpc can currently be resolved into individual stars, and none reach the extreme metallicities and star-formation intensities that characterized the early Universe. With an ultraviolet integral-field spectrograph aboard the Habitable Worlds Observatory (HWO), this barrier will finally be broken. HWO will bring rare, actively star-forming, extremely metal-poor dwarf galaxies at $\sim$10-20 Mpc such as I Zw 18 within reach of resolved UV-optical spectroscopy, providing our first direct, statistical view of individual massive stars and the feedback they drive at $>$30 $M_\odot$ and $<$10% $Z_\odot$. This science is deeply synergistic with many next-generation facilities, yet requires the unique combination of spatial resolution and UV/optical sensitivity that only HWO can provide. The massive star science enabled by HWO within the Local Volume represents a transformational advance in our ability to probe the earliest stellar populations - those that seeded the Milky Way and other galaxies with the first heavy elements, and paved the way for life in the transparent, reionized Universe we inhabit today.

astro-ph.SR↗

arXiv2025

Consistent gas-phase C/O abundances from UV and optical emission lines: a robust scale for chemical evolution across cosmic time

The carbon to oxygen (C/O) abundance ratio is a valuable tracer of star formation history, as C and O enrichment occurs on different timescales. However, measurements based on ultraviolet (UV) collisionally excited lines and those based on optical recombination lines may be subject to biases from the abundance discrepancy factor (ADF), which is well established for oxygen but uncertain for carbon. We present precise UV-based measurements of gas-phase C$^{2+}$/O$^{2+}$ ionic abundance in four H II regions which have prior optical-based measurements, combined with archival UV data for two additional H II regions, in order to establish a reliable abundance scale and to investigate biases between the two methods. We find a clear ADF for the C$^{2+}$ ion which is consistent with that of O$^{2+}$, assuming a similar temperature structure in the zones of the nebula which these ions occupy. The C/O abundance derived from UV collisional lines and optical recombination lines is therefore also consistent to within $<0.1$ dex, with an offset of $0.05\pm0.03$ dex in C$^{2+}$/O$^{2+}$ for the standard T$_e$ method. While the absolute C/H and O/H abundances are subject to large uncertainty from the ADF, our results establish that C/O abundances measured from these different methods can be reliably compared. Thus we confirm the robustness of gas-phase C/O measurements for studying galaxy evolution and star formation timescales, including from rest-UV observations of high redshift galaxies with JWST.

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arXiv2025

CLASSY XII: Nitrogen Enrichment Shaped by Gas Density and Feedback

We investigate the chemical evolution of N/O using a sample of 45 local star-forming galaxies (SFGs) from the CLASSY survey. This sample spans a wide range of galaxy properties, with robust determinations of nitrogen and oxygen abundances via the direct-$T_{\rm e}$ method. We explore how N/O relates to density structure, stellar mass, star formation rate (SFR), stellar age, compactness, and gas kinematics. In addition, we compare our results with those of galaxies at $z =2-10$ where N/O ratios were derived from optical or UV nitrogen lines, aiming to identify chemical enrichment pathways across cosmic time. Our analysis shows that the N/O-O/H relation in CLASSY galaxies aligns with the trends seen in local galaxies and extragalactic HII regions, and that galaxies at $z = 2-6$ exhibit similar N/O values, indicating no significant redshift evolution in N/O for a fixed metallicity. We identify a significant correlation between electron density $n_{\rm e}$([S II]) and N/O, suggesting that density structure contributes to the scatter in the N/O-O/H relation. The CLASSY galaxies with high SFRs or compact star formation show elevated N/O, though no strong correlation with stellar mass is found. We also find that high-velocity outflows (v$_{out}$ > 350 km/s) and low mass-loading factors are linked to elevated N/O, indicating that feedback plays a significant role. These results highlight the importance of density, star formation, and feedback from young stellar populations in shaping N/O enrichment and provide key insights for interpreting high-$z$ galaxies observed with JWST.

astro-ph.GA↗

arXiv2025

Very Massive Stars with the Habitable Worlds Observatory

Very massive stars (VMS) are defined as stars with an initial mass in excess of 100 Msun. Because of their short lifetime and the shape of the stellar mass function, they are rare objects. Only about twenty of them are known in the Galaxy and the Large Magellanic Cloud. However VMS are important in several ways. They efficiently spread nucleosynthesis products through their boosted stellar winds, they are predicted to explode as pair-instability supernovae or to form heavy black-holes from direct collapse, and they outshine all other types of stars in the ultraviolet light, thus dominating the integrated light of starbursts. Their presence is indirectly suspected across all redshifts, all the way to cosmic dawn where they may have played a key role in the formation of the first galaxies. Their search and identification is currently hampered by instrumental limitation, especially spatial resolution. An integral field spectrograph working at the diffraction limit of HWO (5mas) and with a spectral resolution of about 2000 would revolutionize the understanding of VMS. We make the case for such an instrument in this contribution.

astro-ph.IM↗

arXiv2025

JWST/NIRSpec Observations of High Ionization Emission Lines in Galaxies at High Redshift

JWST spectroscopy has built large emission line samples at $z\gtrsim4$, but it has yet to confidently reveal many galaxies with the hard radiation fields commonly associated with AGN photoionization. While this may indicate a weaker UV ionizing spectrum in many $z>4$ AGNs or obscuration from dense neutral gas and dust, the complete picture remains unclear owing to the small number of deep rest-UV spectra. Here we characterize the strength of high ionization lines in $53$ new galaxies observed with NIRSpec $R=2700$ grating spectroscopy. We present new detections of narrow NV$\lambda1240$ in two galaxies. One is a previously-confirmed $z=6.98$ Little Red Dot (LRD) with broad H$\beta$, and the other is a $z=8.72$ galaxy with a narrow line spectrum. Neither source exhibits CIV or HeII emission, indicating large NV/CIV and NV/HeII ratios that may reflect a combination of nitrogen-enhancement and resonant scattering effects. We investigate the incidence of narrow high ionization lines in a large database of $851$ NIRSpec grating spectra, and we separately quantify the fraction of LRDs with narrow high ionization UV emission lines. Our results likely suggest that hard radiation fields are indeed present in a small subset of LRDs ($12.5^{+23.7}_{-10.4}\%$) and UV-selected galaxies ($2.2^{+1.7}_{-1.0}\%$) at $z>4$. The identification of narrow high ionization lines in the population of LRDs with strong Balmer absorption suggests the dense neutral hydrogen gas may not uniformly cover the nucleus. The strong NV (coupled with weak CIV and HeII) suggests that efforts to identify high ionization lines should extend down in wavelength to the NV doublet.

astro-ph.GA↗

arXiv2024

Early Bright Galaxies from Helium Enhancements in High-Redshift Star Clusters

The first few cycles of JWST have identified an overabundance of UV-bright galaxies and a general excess of UV luminosity density at $z\gtrsim10$ compared to expectations from most (pre-JWST) theoretical models. Moreover, some of the brightest high-redshift spectroscopically confirmed galaxies exhibit peculiar chemical abundance patterns, most notably extremely high N/O ratios. Since N/O has been empirically shown to scale strongly with He/H, as expected for hot hydrogen burning, these same bright high-redshift galaxies are likely also helium-enhanced. Under simplistic assumptions for stellar evolution, the bolometric luminosity of a star scales as $L\propto(2-\frac{5}{4}Y)^{-4}(2-Y)^{-1}$ -- hence a higher He/H leads to brighter stars. In this Letter, we evolve a series of MESA models to the zero-age main-sequence and highlight that the helium enhancements at the levels measured and inferred for high-redshift galaxies can boost the 1500 $\mathring{\rm A}$ UV luminosity by up to $\sim50\%$, while simultaneously increasing the stellar effective temperature. The combination of helium enhancements with nebular continuum emission expected for intense bursts of star formation have the potential to help reduce the tension between JWST observations and certain galaxy formation models.

astro-ph.GA↗

arXiv2024

Shining a Light on the Connections between Galactic Outflows Seen in Absorption and Emission Lines

Galactic outflows provide important feedback effects to regulate the evolution of host galaxies. Two primary diagnostics of outflows are broad and/or blueshifted emission and absorption lines. Even though well-established methods exist to analyze these outflow signatures, connections between them are rarely studied and largely unknown. In this paper, we conduct such a study in a sample of 33 low-redshift starburst galaxies. Their UV absorption lines are detected by Hubble Space Telescope, and optical emission lines are observed by Keck or Very Large Telescope. We find outflow properties derived from emission and absorption lines are tightly correlated. These include outflow maximum velocity, line width, and radial extent. On average, in the same galaxy, the maximum velocity and line width of outflows measured from emission lines reach only 60 -- 70% of those from the absorption lines. We also find outflow rates derived from emission lines are consistently lower than those from absorption lines by 0.2 -- 0.5 dex. These findings can be explained by a radial decline in density and a corresponding increase in outflow velocity, combined with the fact that emission line luminosity scales with the square of the density while absorption line depth scales linearly. We test both spherical and bi-conical outflow models, and find the same radial outflow velocity and density distributions can explain the observed correlations. These results provide novel calibration between galactic outflow properties measured from the two diagnostics and underscore the need for high-fidelity UV and optical spectra to accurately assess galactic feedback effects in high-z galaxies.

astro-ph.GA↗

arXiv2024

Deep rest-UV JWST/NIRSpec spectroscopy of early galaxies: the demographics of CIV and N-emitters in the reionization era

JWST has recently discovered a subset of reionization era galaxies with ionized gas that is metal poor in oxygen and carbon but heavily-enriched in nitrogen. This abundance pattern is almost never seen in lower redshift galaxies but is commonly observed in globular cluster stars. We have recently demonstrated that this peculiar abundance pattern appears in a compact ($\simeq 20$ pc) metal-poor galaxy undergoing a strong burst of star formation. This galaxy was originally selected based on strong CIV emission, indicating a hard radiation field rarely seen locally. In this paper, we present JWST/NIRSpec observations of another reionization-era galaxy known to power strong CIV emission, the $z=7.04$ gravitationally-lensed galaxy A1703-zd6. The emission line spectrum reveals this is a metal poor galaxy ($12+\log(\rm O/H) = 7.47\pm0.19$) dominated by a young stellar population ($1.6^{+0.5}_{-0.4}$ Myr) that powers a very hard ionizing spectrum (CIV EW = 19.4 $\unicode{x212B}$, He II EW = 2.2 $\unicode{x212B}$). The ISM is highly-enriched in nitrogen ($\log(\rm N/O)=-0.6$) with very high electron densities ($8-19\times10^4$ cm$^{-3}$) and extreme ionization conditions rarely seen at lower redshift. We also find intense CIV emission (EW$\gtrsim20$ $\unicode{x212B}$) in two new $z\gtrsim 6$ metal poor galaxies. To put these results in context, we search for UV line emission in a sample of 737 $z\gtrsim 4$ galaxies with NIRSpec spectra, establishing that 40(30)% of systems with [OIII]+H$\beta$ EW $>2000\unicode{x212B}$ have NIV] (CIV) detections with EW$>5$ $\unicode{x212B}$ ($>10$ $\unicode{x212B}$). These results suggest high N/O ratios and hard ionizing sources appear in a brief phase following a burst of star formation in compact high density stellar complexes.

astro-ph.GA↗

arXiv2024

Joint Optical and Infrared Observations of N and O Reveal the Dust-Obscured Gas in Haro 3

Accurate chemical compositions of star-forming regions are a critical diagnostic tool to characterize the star formation history and gas flows which regulate galaxy formation. However, the abundance discrepancy factor (ADF) between measurements from the "direct" optical electron temperature ($T_e$) method and from the recombination lines (RL) represents $\sim0.2$ dex systematic uncertainty in oxygen abundance. The degree of uncertainty for other elements is unknown. We conduct a comprehensive analysis of O$^{++}$ and N$^+$ ion abundances using optical and far-infrared spectra of a star-forming region within the nearby dwarf galaxy Haro 3, which exhibits a typical ADF. Assuming homogeneous conditions, the far-IR emission indicates an O abundance which is higher than the $T_e$ method and consistent with the RL value, as would be expected from temperature fluctuations, whereas the far-IR N abundance is too large to be explained by temperature fluctuations. A two-phase analytical model reveals that differential dust obscuration associated with temperature inhomogeneity is likely required to explain all the emission line ratios, and that the total oxygen metallicity of two phases is consistent with the RL metallicity. Our findings underscore the critical importance of resolving the cause of abundance discrepancies and understanding the biases between different metallicity methods. This work represents a promising methodology, and we identify further approaches to address the current dominant uncertainties.

astro-ph.GA↗

arXiv2024

Reply to: Effects of density and temperature variations on the metallicity of Mrk 71

In Chen et al., 2023 (C23; arXiv:2304.09898), we introduced a new method to directly measure temperature fluctuations and applied it to a nearby dwarf galaxy, Mrk 71, finding a temperature fluctuation parameter $t^2 = 0.008\pm 0.043$. This result is lower by $\sim 2\sigma$ than the value required to explain the abundance discrepancy (AD) in this object. In the Matters Arising article submitted by Mendez-Delgado et al. (arXiv:2310.01197), the authors claim that using the same data presented in C23 in a different way, it is possible to conclude that the measurements are consistent with a larger $t^2 \simeq 0.1$ inferred indirectly from recombination lines (RLs). However, this requires a higher density such that the infrared [O III] 52 $\mu$m and [O III] 88 $\mu$m lines -- which form the basis of the direct measurement method -- are mutually inconsistent. Moreover, to reach agreement between the direct $t^2$ measurement and the larger $t^2$ value inferred from RLs requires systematically varying four parameters by $\sim 1\sigma$ from their best-determined values, which collectively amount to a $\sim2\sigma$ difference, consistent with the significance ($\sim 2 \sigma$) originally reported in C23. Therefore, we conclude that the results of C23 hold, and that the combined optical and infrared [O III] data disfavour $t^2 \simeq 0.1$ at the $\approx2\sigma$ level in Mrk 71. Future work is nonetheless warranted to better understand the AD associated with both optical and infrared emission line analysis.

astro-ph.GA↗

arXiv2024

Metal-poor star formation at $z>6$ with JWST: new insight into hard radiation fields and nitrogen enrichment on 20 pc scales

Nearly a decade ago, we began to see indications that reionization-era galaxies power hard radiation fields rarely seen at lower redshift. Most striking were detections of nebular CIV emission in what appeared to be typical low mass galaxies, requiring an ample supply of 48 eV photons to triply ionize carbon. The nature of this population has long remained unclear owing to limitations of ground-based spectroscopy. We have obtained deep JWST/NIRSpec R=1000 spectroscopy of the two z>6 CIV-emitting galaxies known prior to JWST. Here we present a rest-UV to optical spectrum of one of these two systems, the multiply-imaged z=6.1 lensed galaxy RXCJ2248-ID. NIRCam imaging reveals two compact (<22pc) clumps separated by 220pc, with one comprising a dense concentration of massive stars ($>10,400M_{\odot}$/yr/kpc$^2$) formed in a recent burst. We stack spectra of 3 images of the galaxy (J=24.8-25.9), yielding a very deep spectrum providing a high S/N template of strong emission line sources at z>6. The spectrum reveals narrow high ionization lines (HeII, CIV, NIV]) with line ratios consistent with powering by massive stars. The rest-optical spectrum is dominated by very strong emission lines ([OIII] EW=2800\AA), albeit with weak emission from low-ionization transitions ([OIII]/[OII]=184). The electron density is found to be very high($6.4-31\times10^4$cm$^{-3}$) based on three UV transitions. The ionized gas is metal poor ($12+\log(\rm O/H)=7.43^{+0.17}_{-0.09}$), yet highly enriched in nitrogen ($\log(\rm N/O)=-0.39^{+0.11}_{-0.10}$). The spectrum appears broadly similar to that of GNz11 at z=10.6, without showing the same AGN signatures. We suggest that the hard radiation field and rapid nitrogen enrichment may be a short-lived phase that many z>6 galaxies go through as they undergo strong bursts of star formation. We comment on the potential link of such spectra to globular cluster formation.

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