Searcharxiv⌕ Search

arXiv subjects

Lisa Kewley

Publications and source records attributed to Lisa Kewley.

At least 19 recordsLinked to original sources
arXiv2026

First Very Long Baseline Interferometry Fringe Detection at 690GHz

We report the first very long baseline interferometry (VLBI) experiment conducted in the 690 GHz atmospheric window. On 2024 November 21, observations with the Atacama Large Millimeter/submillimeter Array, the Atacama Pathfinder EXperiment (APEX), and the James Clerk Maxwell Telescope (JCMT) were carried out using ALMA's newly developed Band 9 phasing capability. Fringes were detected on the ALMA-APEX baseline during a scan of the quasar J0423-0120, with a signal-to-noise ratio of ~12 and useful fringe recovery over solution intervals of order tens of seconds, representing the highest-frequency ground-based VLBI fringe detection reported to date. No fringes were found on the ALMA-JCMT baseline, despite excellent weather conditions, consistent with sensitivity predictions and supporting baseline performance models. The ALMA Phasing System maintained stable phasing at Band 9 for ~1-2 minutes before gradually degrading, indicating limitations under these observing conditions. Our analysis shows that, under excellent weather conditions, 690 GHz VLBI can still support fringe recovery over short solution intervals, despite rapid atmospheric phase fluctuations at these frequencies. This work validated key elements of near-terahertz VLBI operation and establishes a technical foundation for routine observations in the 690 GHz atmospheric window.

astro-ph.IM↗
arXiv2025

CEERS: Forging the First Dust Grains in the Universe? A Population of Galaxies with spectroscopically-derived Extremely Low Dust Attenuation (GELDA) at 4.0<z<11.4

We investigate the coevolution of metals and dust in 173 galaxies at $4.0 8.8$ (83.3%) than at $z<8.8$ (26.3%) suggesting they dominate the early Universe. Assuming a prior FIR dust spectrum (from ALPINE), we examine dust-stellar mass trends. The $M_{dust}$ vs. $M_{star}$ diagram shows upper and lower sequences linked by possible transitional galaxies. A transition at $M_{star}=10^8.5 M_\odot$ ($Z_{crit}=12+\log_{10}(O/H)=7.60$ or $Z/Z_\odot=0.1$) may mark the shift from stellar dust production to ISM grain growth, in agreement with theoretical predictions. Our full sample has a high mean gas fraction ($f_{gas}>0.9$), indicating retained gas across all galaxies. Their small sizes and large gas masses imply high gas surface densities but relatively low star formation efficiency. High-redshift GELDAs may naturally explain the observed excess of bright galaxies at $z>9$ compared to theoretical expectations.

astro-ph.GA↗
arXiv2023

The MAGPI Survey: Effects of Spiral Arms on Different Tracers of the Interstellar Medium and Stellar Populations at z~0.3

Spiral structures are important drivers of the secular evolution of disc galaxies, however, the origin of spiral arms and their effects on the development of galaxies remain mysterious. In this work, we present two three-armed spiral galaxies at z~0.3 in the Middle Age Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey. Taking advantage of the high spatial resolution (~0.6'') of the Multi-Unit Spectroscopic Unit (MUSE), we investigate the two-dimensional distributions of different spectral parameters: Halpha, gas-phase metallicity, and D4000. We notice significant offsets in Halpha (~0.2 dex) as well as gas-phase metallicities (~0.05 dex) among the spiral arms, downstream and upstream of MAGPI1202197197 (SG1202). This observational signature suggests the spiral structure in SG1202 is consistent with arising from density wave theory. No azimuthal variation in Halpha or gas-phase metallicities is observed in MAGPI1204198199 (SG1204), which can be attributed to the tighter spiral arms in SG1204 than SG1202, coming with stronger mixing effects in the disc. The absence of azimuthal D4000 variation in both galaxies suggests the stars at different ages are well-mixed between the spiral arms and distributed around the disc regions. The different azimuthal distributions in Halpha and D4000 highlight the importance of time scales traced by various spectral parameters when studying 2D distributions in spiral galaxies. This work demonstrates the feasibility of constraining spiral structures by tracing interstellar medium (ISM) and stellar population at z~0.3, with a plan to expand the study to the full MAGPI survey.

astro-ph.GA↗
arXiv2023

Spatially-Resolved Temperature and Density Structures of Nearby HII regions

Photoionization models frequently assume constant temperature or density within HII regions. We investigate this assumption by measuring the detailed temperature and density structures of four HII regions in the Large Magellanic Cloud and the Small Magellanic Cloud, using integral-field spectroscopic data from the Wide-Field Spectrograph on the ANU 2.3m telescope. We analyse the distribution of emission-lines of low-ionization species, intermediate-ionization species and high-ionization species. We present the complex electron temperature and density structures within HII regions. All four nebulae present a negative gradient in the electron density profile. Both positive and negative temperature gradients are observed in the nebulae. We create a series of nebula models with a constant ISM pressure and varying temperature and density distributions. Comparison of the line ratios between our HII regions and models suggests that none of the simple nebula models can reproduce the observed temperature and density structures. Comparison between the models and the data suggests that the ISM pressure of nebulae in LMC and SMC is between log(P/k)=6-7.5. Complex internal structures of the nebulae highlight the importance of future Monte-Carlo photoionization codes for accurate nebula modeling, which include a comprehensive consideration of arbitrary geometries of HII regions.

astro-ph.GA↗
arXiv2023

CEERS MIRI Imaging: Data Reduction and Quality Assessment

The Cosmic Evolution Early Release Science Survey (CEERS), targeting the Extended Groth Strip extragalactic field, is one of the JWST Director's Discretionary Early Release Science programs. To date, all observations have been executed and include NIRCam/MIRI imaging and NIRSpec/NIRCam spectroscopic exposures. Here, we discuss the MIRI imaging, which includes eight pointings, four of which provide deep imaging with the bluer bands (F560W, F770W) and four with contiguous wavelength coverage in F1000W, F1280W, F1500W, and F1800W, where two of these also include coverage in F770W and F2100W. We present a summary of the data, the data quality, and data reduction. The data reduction is based on the JWST Calibration Pipeline combined with custom modifications and additional steps designed to enhance the output quality, including improvements in astrometry and the removal of detector artifacts. We estimate the image depth of the reduced mosaics, and show that these generally agree with expectations from the Exposure Time Calculator. We compare the MIRI F560W and F770W flux densities for bright sources to measurements from Spitzer/IRAC Ch3 (5.8 $μ$m) and Ch4 (8.0 $μ$m), and we find that they agree with systematic differences of $<0.1$ mag. For the redder MIRI bands, we assess their quality by studying the spectral energy distributions (SEDs) of Galactic stars. The SEDs are consistent with the expected Rayleigh-Jeans law with a deviation $\sim 0.03$ mag, indicating that the MIRI colors are reliable. We also discuss all publicly released data products (images and source catalogs), which are available on the CEERS website (https://ceers.github.io/).

astro-ph.GA↗
arXiv2023

Beyond BPT: A New Multi-Dimensional Diagnostic Diagram for Classifying Power Sources Tested Using the SAMI Galaxy Survey

Current methods of identifying the ionizing source of nebular emission in galaxies are well defined for the era of single fiber spectroscopy, but still struggle to differentiate the complex and overlapping ionization sources in some galaxies. With the advent of integral field spectroscopy, the limits of these previous classification schemes are more apparent. We propose a new method for distinguishing the ionizing source in resolved galaxy spectra by use of a multi-dimensional diagnostic diagram that compares emission line ratios with velocity dispersion on a spaxel by spaxel basis within a galaxy. This new method is tested using the SAMI Galaxy Survey Data Release 3, which contains 3068 galaxies at z $<$ 0.12. Our results are released as ionization maps available alongside the SAMI DR3 public data. Our method accounts for a more diverse range of ionization sources than the standard suite of emission line diagnostics; we find 1433 galaxies with significant contribution from non-star-forming ionization using our improved method as compared to 316 galaxies identified using only emission line ratio diagnostics. Within these galaxies, we further identify 886 galaxies hosting unique signatures inconsistent with standard ionization by H2 regions, AGN, or shocks. These galaxies span a wide range of masses and morphological types and comprise a sizable portion of the galaxies used in our sample. With our revised method, we show that emission line diagnostics alone do not adequately differentiate the multiple ways to ionize gas within a galaxy.

astro-ph.GA↗
arXiv2023

JWST reveals a possible $z \sim 11$ galaxy merger in triply-lensed MACS0647$-$JD

MACS0647$-$JD is a triply-lensed $z\sim11$ galaxy originally discovered with the Hubble Space Telescope. Here we report new JWST imaging, which clearly resolves MACS0647$-$JD as having two components that are either merging galaxies or stellar complexes within a single galaxy. Both are very small, with stellar masses $\sim10^8\,M_\odot$ and radii $r<100\,\rm pc$. The brighter larger component "A" is intrinsically very blue ($β\sim-2.6$), likely due to very recent star formation and no dust, and is spatially extended with an effective radius $\sim70\,\rm pc$. The smaller component "B" appears redder ($β\sim-2$), likely because it is older ($100-200\,\rm Myr$) with mild dust extinction ($A_V\sim0.1\,\rm mag$), and a smaller radius $\sim20\,\rm pc$. We identify galaxies with similar colors in a high-redshift simulation, finding their star formation histories to be out of phase. With an estimated stellar mass ratio of roughly 2:1 and physical projected separation $\sim400\,\rm pc$, we may be witnessing a galaxy merger 400 million years after the Big Bang. We also identify a candidate companion galaxy C $\sim3\,{\rm kpc}$ away, likely destined to merge with galaxies A and B. The combined light from galaxies A+B is magnified by factors of $\sim$8, 5, and 2 in three lensed images JD1, 2, and 3 with F356W fluxes $\sim322$, $203$, $86\,\rm nJy$ (AB mag 25.1, 25.6, 26.6). MACS0647$-$JD is significantly brighter than other galaxies recently discovered at similar redshifts with JWST. Without magnification, it would have AB mag 27.3 ($M_{UV}=-20.4$). With a high confidence level, we obtain a photometric redshift of $z=10.6\pm0.3$ based on photometry measured in 6 NIRCam filters spanning $1-5\rmμm$, out to $4300\,Å$ rest-frame. JWST NIRSpec observations planned for January 2023 will deliver a spectroscopic redshift and a more detailed study of the physical properties of MACS0647$-$JD.

astro-ph.GA↗
arXiv2022

CLEAR: The Ionization and Chemical-Enrichment Properties of Galaxies at 1.1 < z < 2.3

We use deep spectroscopy from the Hubble Space Telescope (HST) Wide-Field-Camera 3 (WFC3) IR grisms combined with broad-band photometry to study the stellar populations, gas ionization and chemical abundances in star-forming galaxies at $z\sim 1.1-2.3$. The data stem from the CANDELS Lyman-$α$ Emission At Reionization (CLEAR) survey. At these redshifts the grism spectroscopy measure the [OII] 3727, 3729, [OIII] 4959, 5008, H-$β$ strong emission features, which constrain the ionization parameter and oxygen abundance of the nebular gas. We compare the line flux measurements to predictions from updated photoionization models (MAPPINGS (Kewley et al. 2019), which include an updated treatment of nebular gas pressure, log P/k = $n_e T_e$. Compared to low-redshift samples ($z\sim 0.2$) at fixed stellar mass, llog M / M$_\odot$ = 9.4-9.8, the CLEAR galaxies at z=1.35 (z=1.90) have lower gas-phase metallicity, $Δ$(log Z) = 0.25 (0.35) dex, and higher ionization parameters, $Δ$(log q) = 0.25 (0.35) dex, where U = q/c. We provide updated analytic calibrations between the [OIII], [OII], and H-$β$ emission line ratios, metallicity, and ionization parameter. The CLEAR galaxies show that at fixed stellar mass, the gas ionization parameter is correlated with the galaxy specific star-formation rates (sSFRs), where $Δ$ log q = 0.4 $Δ$(log sSFR), derived from changes in the strength of galaxy H-$β$ equivalent width. We interpret this as a consequence of higher gas densities, lower gas covering fractions, combined with higher escape fraction of H-ionizing photons. We discuss both tests to confirm these assertions and implications this has for future observations of galaxies at higher redshifts.

astro-ph.GA↗
arXiv2022

Messenger Monte-Carlo MAPPINGS V (M^3) -- A self-consistent three-dimensional photoionization code

The Messenger Interface Monte-Carlo Mappings V (M^3) is a photoionization code adopting the fully self-consistent Monte-Carlo radiative transfer technique, which presents a major advance over previous photoionization models with simple geometries. M^3 is designed for modeling nebulae in arbitrary three-dimensional geometries. In this paper, we describe the Monte-Carlo radiative transfer technique and the microphysics implemented in M^3, including the photoionization, collisional ionization, the free-free and free-bound recombination, and two-photon radiation. We put M^3 through the Lexington/Meudon benchmarks to test the reliability of the new code. We apply M^3 to three HII region models with fiducial geometries, demonstrating that M^3 is capable of dealing with nebulae with complex geometries. M^3 is a promising tool for understanding emission-line behavior in the era of SDSS-V/LVM and JWST, which will provide high-quality data of spatially-resolved nearby HII regions and highly turbulent local and high-redshift HII regions.

astro-ph.GA↗
arXiv2021

Resolving star-forming clumps in a z $\sim$ 2 lensed galaxy: a pixelated Bayesian approach

We present a pixelized source reconstruction method applied on Integral Field Spectroscopic (IFS) observations of gravitationally lensed galaxies. We demonstrate the effectiveness of this method in a case study on the clumpy morphology of a $z \sim 2$ lensed galaxy behind a group-scale lens. We use a Bayesian forward source modelling approach to reconstruct the surface brightness distribution of the source galaxy on a uniformly pixelized grid while accounting for the image point spread function (PSF). The pixelated approach is sensitive to clump sizes down to 100 pc and resolves smaller clump sizes with an improvement in the signal to noise ratio (SNR) by almost a factor of ten compared with more traditional ray-tracing approaches.

astro-ph.GA↗
arXiv2020

A Comparison of Rest-frame Ultraviolet and Optical Emission-Line Diagnostics in the Lensed Galaxy SDSS J1723+3411 at Redshift z=1.3293

For the extremely bright lensed galaxy SDSS J1723+3411 at z=1.3293 , we analyze spatially integrated MMT, Keck, and Hubble Space Telescope spectra that fully cover the rest-frame wavelength range of 1400 to 7200 Angstroms. We also analyze near-IR spectra from Gemini that cover H alpha for a portion of the lensed arc. We report fluxes for 42 detected emission lines, and upper limits for an additional 22. This galaxy has extreme emission line ratios and high equivalent widths that are characteristic of extreme emission-line galaxies. We compute strong emission line diagnostics from both the rest-frame optical and rest-frame ultraviolet (UV), to constrain physical conditions and test the spectral diagnostics themselves. We tightly determine the nebular physical conditions using the most reliable diagnostics, and then compare to results from other diagnostics. We find disappointing performance from the UV--only diagnostics: they either are unable to measure the metallicity or dramatically under-estimate it; they over-estimate the pressure; and the UV diagnostic of ionization parameter has a strong metallicity dependence in this regime. Based on these results, we suggest that upcoming James Webb Space Telescope spectroscopic surveys of galaxies in the reionization epoch should invest the additional integration time to capture the optical [O II] and [O III] emission lines, and not rely solely on the rest-frame UV emission lines. We make available the spectra; they represent one of the highest-quality emission line spectral atlases of star-forming galaxy available beyond the local universe, and will aid planning observations with JWST.

astro-ph.GA↗
arXiv2020

Unravelling the enigmatic ISM conditions in Minkowski's Object

Local examples of jet-induced star formation lend valuable insight into its significance in galaxy evolution and can provide important observational constraints for theoretical models of positive feedback. Using optical integral field spectroscopy, we present an analysis of the ISM conditions in Minkowski's Object ($z = 0.0189$), a peculiar star-forming dwarf galaxy located in the path of a radio jet from the galaxy NGC 541. Full spectral fitting with PPXF indicates that Minkowski's Object primarily consists of a young stellar population $\sim 10$ Myr old, confirming that the bulk of the object's stellar mass formed during a recent jet interaction. Minkowski's Object exhibits line ratios largely consistent with star formation, although there is evidence for a low level ($\lesssim 15$ per cent) of contamination from a non-stellar ionising source. Strong-line diagnostics reveal a significant variation in the gas-phase metallicity within the object, with $\log\left( \rm O / H \right) + 12$ varying by $\sim 0.5$ dex, which cannot be explained by in-situ star formation, an enriched outflow from the jet, or enrichment of gas in the stellar bridge between NGC 541 and NGC 545/547. We hypothesise that Minkowski's Object either (a) was formed as a result of jet-induced star formation in pre-existing gas clumps in the stellar bridge, or (b) is a gas-rich dwarf galaxy that is experiencing an elevation in its star formation rate due to a jet interaction, and will eventually redden and fade, becoming an ultra-diffuse galaxy as it is processed by the cluster.

astro-ph.GA↗
arXiv2020

Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross-section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The mm/submm recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultra-compact HII regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using mm recombination lines of HI, HeI and HeII. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: L_ν~ ν^{-4.5 +/- 0.4}. We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in HII regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star forming regions providing critical insight into photon production rates at λ< 912 A and can serve as calibration to starburst synthesis models, improving our understanding of star formation in distant universe and the properties of ionizing flux during reionization.

astro-ph.GA↗
arXiv2020

Spatial Variation in Strong Line Ratios and Physical Conditions in Two Strongly-Lensed Galaxies at z~1.4

For studies of galaxy formation and evolution, one of the major benefits of the James Webb Space Telescope is that space-based IFUs like those on its NIRSpec and MIRI instruments will enable spatially resolved spectroscopy of distant galaxies, including spectroscopy at the scale of individual star-forming regions in galaxies that have been gravitationally lensed. In the meantime, there is only a very small subset of lensed sources where work like this is possible even with the Hubble Space Telescope's Wide Field Camera 3 infrared channel grisms. We examine two of these sources, SDSS J1723+3411 and SDSS J2340+2947, using HST WFC3/IR grism data and supporting spatially-unresolved spectroscopy from several ground-based instruments to explore the size of spatial variations in observed strong emission line ratios like O32, R23, which are sensitive to ionization parameter and metallicity, and the Balmer decrement as an indicator of reddening. We find significant spatial variation in the reddening and the reddening-corrected O32 and R23 values which correspond to spreads of a few tenths of a dex in ionization parameter and metallicity. We also find clear evidence of a negative radial gradient in star formation in SDSS J2340+2947 and tentative evidence of one in SDSS J1723+3411, though its star formation is quite asymmetric. Finally, we find that reddening can vary enough spatially to make spatially-resolved reddening corrections necessary in order to characterize gradients in line ratios and the physical conditions inferred from them, necessitating the use of space-based IFUs for future work on larger, more statistically robust samples.

astro-ph.GA↗
arXiv2020

MOSEL: Strong [OIII]5007 Å Emitting Galaxies at (3<z<4) from the ZFOURGE Survey

To understand how strong emission line galaxies (ELGs) contribute to the overall growth of galaxies and star formation history of the universe, we target Strong ELGs (SELGs) from the ZFOURGE imaging survey that have blended (Hb+[OIII]) rest-frame equivalent widths of >230A and 2.5 60%, i.e. the inferred gas masses can easily fuel a starburst to double stellar masses within ~10-100 Myr. Combined with recent results using ZFOURGE, our analysis indicates that 1) strong [OIII]5007A emission signals an early episode of intense stellar growth in low mass (Mstar<0.1M*) galaxies and 2) many, if not most, galaxies at z>3 go through this starburst phase. If true, low-mass galaxies with strong [OIII]5007A emission (EW_rest>200A) may be an increasingly important source of ionizing UV radiation at z>3.

astro-ph.GA↗
arXiv2020

Reconstructing the observed ionizing photon production efficiency at z~2 using stellar population models

The ionizing photon production efficiency, $ξ_{ion}$, is a critical parameter that provides a number of physical constraints to the nature of the early Universe, including the contribution of galaxies to the timely completion of the reionization of the Universe. Here we use KECK/MOSFIRE and ZFOURGE multi-band photometric data to explore the $ξ_{ion}$ of a population of galaxies at $z\sim2$ with $log_{10}(M_*/M_\odot)\sim9.0-11.5$. Our 130 \Halpha\ detections show a median $log_{10}(ξ_{ion}[Hz/erg])$ of $24.8\pm0.5$ when dust corrected using a Calzetti et al. (2000) dust prescription. Our values are typical of mass/magnitude selected $ξ_{ion}$ values observed in the $z\sim2$ Universe. Using BPASSv2.2.1 and Starburst99 stellar population models with simple parametric star-formation-histories (SFH), we find that even with models that account for effects of stellar evolution with binaries/stellar rotation, model galaxies at $log_{10}(ξ_{ion}[Hz/erg])\lesssim25.0$ have low H$α$ equivalent widths (EW) and redder colors compared to our $z\sim2$ observed sample. We find that introducing star-bursts to the SFHs resolve the tension with the models, however, due to the rapid time evolution of $ξ_{ion}$, H$α$ EWs, and rest-frame optical colors, our Monte Carlo simulations of star-bursts show that random distribution of star-bursts in evolutionary time of galaxies are unlikely to explain the observed distribution. Thus, either our observed sample is specially selected based on their past SFH or stellar models require additional mechanisms to reproduce the observed high UV luminosity of galaxies for a given production rate of hydrogen ionizing photons.

astro-ph.GA↗
arXiv2019

The Effects of Diffuse Ionized Gas and Spatial Resolution on Metallicity Gradients: TYPHOON Two-Dimensional Spectrophotometry of M83

We present a systematic study of the diffuse ionized gas (DIG) in M83 and its effects on the measurement of metallicity gradients at varying resolution scales. Using spectrophotometric data cubes of M83 obtained at the 2.5m duPont telescope at Las Campanas Observatory as part of the TYPHOON program, we separate the HII regions from the DIG using the [SII]/H$α$ ratio, HIIphot (HII finding algorithm) and the H$α$ surface brightness. We find that the contribution to the overall H$α$ luminosity is approximately equal for the HII and DIG regions. The data is then rebinned to simulate low-resolution observations at varying resolution scales from 41 pc up to 1005 pc. Metallicity gradients are measured using five different metallicity diagnostics at each resolution. We find that all metallicity diagnostics used are affected by the inclusion of DIG to varying degrees. We discuss the reasons of why the metallicity gradients are significantly affected by DIG using the HII dominance and emission line ratio radial profiles. We find that applying the [SII]/H$α$ cut will provide a closer estimate of the true metallicity gradient up to a resolution of 1005 pc for all metallicity diagnostics used in this study.

astro-ph.GA↗