arXiv subjects

Lisa J. Kewley

Publications and source records attributed to Lisa J. Kewley.

At least 73 records · Page 4Linked to original sources

arXiv2021

"Direct" Gas-Phase Metallicity in Local Analogs of High-Redshift Galaxies: Empirical Metallicity Calibrations for High-Redshift Star-Forming Galaxies

We study the direct gas-phase oxygen abundance using the well-detected auroral [OIII]4363 line in the stacked spectra of a sample of local analogs of high-redshift galaxies. These local analogs share the same location as $z\sim2$ star-forming galaxies on the Baldwin-Phillips-Terlevich diagram. This type of analog has the same ionized interstellar medium (ISM) properties as high-redshift galaxies. We establish empirical metallicity calibrations between the direct gas-phase oxygen abundances ($7.8<12+\log(\rm{O/H})<8.4$) and the N2/O3N2 indices in our local analogs. We find significant systematic offsets between the metallicity calibrations for our local analogs of high-redshift galaxies and those derived from the local HII regions and a sample of local reference galaxies selected from the Sloan Digital Sky Survey (SDSS). The N2 and O3N2 metallicities will be underestimated by 0.05-0.1~dex relative to our calibration if one simply applies the local metallicity calibration in previous studies to high-redshift galaxies. Local metallicity calibrations also cause discrepancies of metallicity measurements in high-redshift galaxies using the N2 and O3N2 indicators. In contrast, our new calibrations produce consistent metallicities between these two indicators. We also derive metallicity calibrations for R23, O32, log([OIII]/Hβ), and log([NeIII]/[OII]) indices in our local analogs, which show significant offset compared to those in the SDSS reference galaxies. By comparing with MAPPINGS photoionization models, the different empirical metallicity calibration relations in the local analogs and the SDSS reference galaxies can be shown to be primarily due to the change of ionized ISM conditions.

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arXiv2021

Revisiting the Giant Radio Galaxy ESO 422-G028: Part I. Discovery of a neutral inflow and recent star formation in a restarted giant

Giant radio galaxies provide important clues into the life cycles and triggering mechanisms of radio jets. With large-scale jets spanning 1.8 Mpc, ESO 422-G028 ($z = 0.038$) is a giant radio galaxy that also exhibits signs of restarted jet activity in the form of pc-scale jets. We present a study of the spatially-resolved stellar and gas properties of ESO 422-G028 using optical integral field spectroscopy from the WiFeS spectrograph. In addition to the majority $\sim 13\,\rm Gyr$ old stellar population, ESO 422-G028 exhibits a much younger ($\lesssim 10\,\rm Myr$ old) component with an estimated mass of $ 10^{7.6}\,\rm M_\odot$ which is predominantly located in the North-West region of the galaxy. Unusually, the ionised gas kinematics reveal two distinct disks traced by narrow ($σ_{\rm Hα} < 100 \,\rm km\,s^{-1}$) and broad ($σ_{\rm Hα} > 150 \,\rm km\,s^{-1}$) H$α$ emission respectively. Both ionised gas disks are misaligned with the axis of stellar rotation, suggesting an external origin. This is consistent with the prominent interstellar Na D absorption, which traces a $1 - 3 \,\rm M_\odot \, yr^{-1}$ inflow of neutral gas from the North. We posit that an inflow of gas - either from an accretion event or a gas-rich merger - has triggered both the starburst and the restarted jet activity, and that ESO 422-G028 is potentially on the brink of an epoch of powerful AGN activity.

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arXiv2021

Detection of metallicity correlations in 100 nearby galaxies

In this paper we analyse the statistics of the 2D gas-phase oxygen abundance distributions of 100 nearby galaxies drawn from the Calar Alto Legacy Integral Field spectroscopy Area survey. After removing the large-scale radial metallicity gradient, we compute the two-point correlation functions of the resulting metallicity fluctuation maps. We detect correlations in the majority of our targets, which we show are significantly in excess of what is expected due to beam-smearing, and are robust against the choice of metallicity diagnostic. We show that the correlation functions are generally well-fit by the predictions of a simple model for stochastic metal injection coupled with diffusion, and from the model we show that, after accounting for the effects of both beam smearing and noise, the galaxies in our sample have characteristic correlation lengths of $\sim1$ kpc. Correlation lengths increase with both stellar mass and star formation rate, but show no significant variation with Hubble type, barredness, or merging state. We also find no evidence for a theoretically-predicted relationship between gas velocity dispersion and correlation length, though this may be due to the small dynamic range in gas velocity dispersion across our sample. Our results suggest that measurements of 2D metallicity correlation functions can be a powerful tool for studying galaxy evolution.

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arXiv2021

On the origin of nitrogen at low metallicity

Understanding the evolution of the N/O ratio in the interstellar medium (ISM) of galaxies is essential if we are to complete our picture of the chemical evolution of galaxies at high redshift, since most observational calibrations of O/H implicitly depend upon the intrinsic N/O ratio. The observed N/O ratio, however, shows large scatter at low O/H, and is strongly dependent on galactic environment. We show that several heretofore unexplained features of the N/O distribution at low O/H can be explained by the N seen in metal-poor galaxies being mostly primary nitrogen that is returned to the ISM via pre-supernova winds from rapidly rotating massive stars ($M \gtrsim 10$ M$_\odot$, $v/v_{\rm crit} \gtrsim 0.4$). This mechanism naturally produces the observed N/O plateau at low O/H. We show that the large scatter in N/O at low O/H also arises naturally from variations in star-formation efficiency. By contrast, models in which the N and O come primarily from supernovae provide a very poor fit to the observed abundance distribution. We propose that the peculiar abundance patterns we observe at low O/H are a signature that dwarf galaxies retain little of their SN ejecta, leaving them with abundance patterns typical of winds.

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arXiv2020

Spatially resolved direct method metallicity in a high-redshift analogue local galaxy: temperature structure impact on metallicity gradients

We investigate how HII region temperature structure assumptions affect "direct-method" spatially-resolved metallicity observations using multispecies auroral lines in a galaxy from the SAMI Galaxy Survey. SAMI609396B, at redshift $z=0.018$, is a low-mass galaxy in a minor merger with intense star formation, analogous to conditions at high redshifts. We use three methods to derive direct metallicities and compare with strong-line diagnostics. The spatial metallicity trends show significant differences among the three direct methods. Our first method is based on the commonly used electron temperature $T_e$([OIII]) from the [OIII]$λ$4363 auroral line and a traditional $T_e$([OII]) -- $T_e$([OIII]) calibration. The second method applies a recent empirical correction to the O$^+$ abundance from the [OIII]/[OII] strong-line ratio. The third method infers the $T_e$([OII]) from the [SII]$λλ$4069,76 auroral lines. The first method favours a positive metallicity gradient along SAMI609396B, whereas the second and third methods yield flattened gradients. Strong-line diagnostics produce mostly flat gradients, albeit with unquantified contamination from shocked regions. We conclude that overlooked assumptions about the internal temperature structure of HII regions in the direct method can lead to large discrepancies in metallicity gradient studies. Our detailed analysis of SAMI609396B underlines that high-accuracy metallicity gradient measurements require a wide array of emission lines and improved spatial resolutions in order to properly constrain excitation sources, physical conditions, and temperature structures of the emitting gas. Integral-field spectroscopic studies with future facilities such as JWST/NIRSpec and ground-based ELTs will be crucial in minimising systematic effects on measured gradients in distant galaxies.

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arXiv2020

Gas-phase metallicity gradients of TNG50 star-forming galaxies

We present the radial gas-phase, mass-weighted metallicity profiles and gradients of the TNG50 star-forming galaxy population measured at redshifts $z=$ 0--3. We investigate the redshift evolution of gradients and examine relations between gradient steepness and galaxy properties. We find that TNG50 gradients are predominantly negative at all redshifts, although we observe significant diversity among these negative gradients. We determine that the gradient steepness of all galaxies increases approximately monotonically with redshift at a roughly constant rate. This rate does not vary significantly with galaxy mass. We observe a weak negative correlation between gradient steepness and galaxy stellar mass at redshifts $z\leq2$. However, when we normalize gradients by a characteristic radius defined by the galactic star formation distribution, we find that these normalized gradients remain invariant with both stellar mass and redshift. We place our results in the context of previous simulations and show that TNG50 high-redshift gradients are steeper than those of models featuring burstier feedback, which may further highlight high-redshift gradients as important discriminators of galaxy formation models. We also find that redshift $z=0$ and $z=0.5$ TNG50 gradients are consistent with the gradients observed in galaxies at these redshifts, although the preference for flat gradients observed in redshift $z\gtrsim1$ galaxies is not present in TNG50. If future JWST and ELT observations validate these flat gradients, it may indicate a need for simulation models to implement more powerful radial gas mixing within the ISM, possibly via turbulence and/or stronger winds

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arXiv2020

Oxygen Loss from Simulated Galaxies and the Metal Flow Main Sequence: Predicting the Dependence on Mass and Environment

We predict the mass fraction of oxygen lost from galaxies in a cosmological simulation as a function of stellar mass and environment at the present day. The distribution with stellar mass is bimodal, separating star-forming and quenched galaxies. The metallicity of gas and stars is self-consistently calculated using a chemical evolution model that includes supernovae type II and Ia, hypernovae, and asymptotic giant branch stars. The mass of oxygen lost from each galaxy is calculated by comparing the existing oxygen in gas and stars in the galaxy to the oxygen that should have been produced by the present-day population of stars. More massive galaxies are able to retain a greater fraction of their metals ($\sim 100$ per cent) than low-mass galaxies ($\sim 40 - 70$ per cent). As in the star formation main sequence, star-forming galaxies follow a tight relationship also in terms of oxygen mass lost -- a metal flow main sequence, ZFMS -- whereas massive quenched galaxies tend to have lost a greater fraction of oxygen (up to 20 per cent), due to AGN-driven winds. The amount of oxygen lost by satellite galaxies depends on the details of their interaction history, and those in richer groups tend to have lost a greater fraction of their oxygen. Observational estimates of metal retention in galaxies will provide a strong constraint on models of galaxy evolution.

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arXiv2020

Quantifying the effects of spatial resolution and noise on galaxy metallicity gradients

Metallicity gradients are important diagnostics of galaxy evolution, because they record the history of events such as mergers, gas inflow and star-formation. However, the accuracy with which gradients can be measured is limited by spatial resolution and noise, and hence measurements need to be corrected for such effects. We use high resolution (~20 pc) simulation of a face-on Milky Way mass galaxy, coupled with photoionisation models, to produce a suite of synthetic high resolution integral field spectroscopy (IFS) datacubes. We then degrade the datacubes, with a range of realistic models for spatial resolution (2 to 16 beams per galaxy scale length) and noise, to investigate and quantify how well the input metallicity gradient can be recovered as a function of resolution and signal-to-noise ratio (SNR) with the intention to compare with modern IFS surveys like MaNGA and SAMI. Given appropriate propagation of uncertainties and pruning of low SNR pixels, we show that a resolution of 3-4 telescope beams per galaxy scale length is sufficient to recover the gradient to ~10-20% uncertainty. The uncertainty escalates to ~60% for lower resolution. Inclusion of the low SNR pixels causes the uncertainty in the inferred gradient to deteriorate. Our results can potentially inform future IFS surveys regarding the resolution and SNR required to achieve a desired accuracy in metallicity gradient measurements.

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arXiv2020

Spinning Bar and a Star-formation Inefficient Repertoire: Turbulence in Hickson Compact Group NGC7674

The physics regulating star formation (SF) in Hickson Compact Groups (HCG) has thus far been difficult to describe, due to their unique kinematic properties. In this study we expand upon previous works to devise a more physically meaningful SF relation able to better encompass the physics of these unique systems. We combine CO(1--0) data from the Combined Array from Research in Millimeter Astronomy (CARMA) to trace the column density of molecular gas $Σ_\mathrm{gas}$ and deep H$α$ imaging taken on the Southern Astrophysical Research (SOAR) Telescope tracing $Σ_\mathrm{SFR}$ to investigate star formation efficiency across face-on HCG, NGC7674. We find a lack of universality in star formation, with two distinct sequences present in the $Σ_\mathrm{gas}-Σ_\mathrm{SFR}$ plane; one for inside and one for outside the nucleus. We devise a SF relation based on the multi-freefall nature of gas and the critical density, which itself is dependent on the virial parameter $α_{\mathrm{vir}}$, the ratio of turbulent to gravitational energy. We find that our modified SF relation fits the data and describes the physics of this system well with the introduction of a virial parameter of about 5--10 across the galaxy. This $α_{\mathrm{vir}}$ leads to an order-of-magnitude reduction in SFR compared to $α_{\mathrm{vir}}\approx 1$ systems.

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arXiv2020

MOSEL Survey: Tracking the Growth of Massive Galaxies at 2<z<4 using Kinematics and the IllustrisTNG Simulation

We use K-band spectroscopic data from the Multi-Object Spectroscopic Emission Line (MOSEL) survey to analyze the kinematic properties of galaxies at z>3. Our sample consists of 34 galaxies at 3.0 10.2 at z > 3 have 56 +/- 21 km/s lower integrated velocity dispersion compared to galaxies at z ~ 2 of similar stellar mass. Massive galaxies at z > 3 have either a flat or declining star formation history (SFH), whereas similar stellar mass galaxies at z~2.0 exhibit a slight peak in the past 500 Myrs. Comparing with the IllustrisTNG cosmological simulation, we find that (i) the dynamical mass of massive galaxies in simulations (log(M_star) > 10.0) increases by ~0.1 dex at a fixed stellar mass between z=2.0-3.0, and (ii) dynamical mass growth is coupled with a rapid rise in the ex-situ stellar mass fraction (stars accreted from other galaxies) for massive galaxies at z < 3.5. We speculate that the rising contribution of ex-situ stellar mass to the total stellar mass growth of massive galaxies is driving the higher integrated velocity dispersion and rising SFHs of massive galaxies at z~2.0 compared to galaxies of similar stellar masses at z > 3.

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arXiv2020

ZFIRE: Measuring Electron Density with [OII] as a function of environment at z = 1.62

The global star formation rates (SFR) of galaxies at fixed stellar masses increase with redshift and are known to vary with environment unto z~2. We explore here whether the changes in the star formation rates can also apply to the electron densities of the inter-stellar medium (ISM) by measuring the [OII] (3727A/3729A) ratio for cluster and field galaxies at z~2. We measure a median electron density of ne = 366+/-84 cm-3 for six galaxies (with 1-sigma scatter = 163 cm-3) in the UDS proton-cluster at z=1.62. We find that the median electron density of galaxies in the UDS photo-cluster environment is three times higher compared to the median electron density of field galaxies (ne = 113+/- 63 cm-3 and 1-sigma scatter = 79 cm-3) at comparable redshifts, stellar mass and SFR. However, we note that a sample of six photo-cluster galaxies is insufficient to reliably measure the electron density in the average porto-cluster environment at z~2. We conclude that the electron density increases with redshift in both cluster and field environments up to z~2 (ne = 30 +/- 1 cm-3 for z ~ 0 to ne =254+/- 76 cm-3 for z~1.5). We find tentative evidence (~2.6 sigma ) for a possible dependence of electron density on environment, but the results require confirmation with larger sample sizes.

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arXiv2020

What Drives the Redshift Evolution of Strong Emission Line Ratios?

We study the physical mechanisms that cause the offset between low-redshift and high-redshift galaxies on the [OIII]/H$β$ versus [NII]/H$α$ ``Baldwin, Phillips & Terlevich'' (BPT) diagram using a sample of local analogues of high-redshift galaxies. These high-redshift analogue galaxies are selected from the Sloan Digital Sky Survey. Located in the same region on the BPT diagram as the ultra-violet selected galaxies at $z\sim2$, these high-redshift analogue galaxies provide an ideal local benchmark to study the offset between the local and high-redshift galaxies on the BPT diagram. We compare the nitrogen-to-oxygen ratio (N/O), the shape of the ionising radiation field, and ionisation parameters between the high-redshift analogues and a sample of local reference galaxies. The higher ionisation parameter in the high-redshift analogues is the dominant physical mechanism driving the BPT offset from low- to high-redshift, particularly at high {\nii/\ha}. Furthermore, the N/O ratio enhancement also plays a minor role to cause the BPT offset. However, the shape of the ionising radiation field is unlikely to cause the BPT offset because the high-redshift analogues have a similar hard ionising radiation field as local reference galaxies. This hard radiation field cannot be produced by the current standard stellar synthesis models. The stellar rotation and binarity may help solve the discrepancy.

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arXiv2019

Understanding Galaxy Evolution through Emission Lines

We review the use of emission-lines for understanding galaxy evolution, focusing on excitation source, metallicity, ionization parameter, ISM pressure and electron density. We show that the UV, optical and infrared contain complementary diagnostics that can probe the conditions within different nebular ionization zones. In anticipation of upcoming telescope facilities, we provide new self-consistent emission-line diagnostic calibrations for complete spectral coverage from the UV to the infrared. These diagnostics can be used in concert to understand how fundamental galaxy properties have changed across cosmic time. We describe new 2D and 3D emission-line diagnostics to separate the contributions from star formation, AGN and shocks using integral field spectroscopy. We discuss the physics, benefits, and caveats of emission-line diagnostics, including the effect of theoretical model uncertainties, diffuse ionized gas, and sample selection bias. Accounting for complex density gradients and temperature profiles is critical for reliably estimating the fundamental properties of H ii regions and galaxies. Diffuse ionized gas can raise metallicity estimates, flatten metallicity gradients, and introduce scatter in ionization parameter measurements. We summarize with a discussion of the challenges and major opportunities for emission-line diagnostics in the coming years.

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arXiv2019

Mapping electron temperature variations across a spiral arm in NGC 1672

We report one of the first extragalactic observations of electron temperature variations across a spiral arm. Using MUSE mosaic observations of the nearby galaxy NGC 1672, we measure the [N II]5755 auroral line in a sample of 80 HII regions in the eastern spiral arm of NGC1672. We discover systematic temperature variations as a function of distance perpendicular to the spiral arm. The electron temperature is lowest on the spiral arm itself and highest on the downstream side. Photoionization models of different metallicity, pressure, and age of the ionizing source are explored to understand what properties of the interstellar medium drive the observed temperature variations. An azimuthally varying metallicity appears to be the most likely cause of the temperature variations. The electron temperature measurements solidify recent discoveries of azimuthal variations of oxygen abundance based on strong lines, and rule out the possibility that the abundance variations are artefacts of the strong-line calibrations.

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arXiv2019

A Tale of Two Clusters: An Analysis of Gas-Phase Metallicity and Nebular Gas Conditions in Proto-cluster Galaxies at z~2

The ZFIRE survey has spectroscopically confirmed two proto-clusters using the MOSFIRE instrument on Keck 1: one at z=2.095 in COSMOS and another at z=1.62 in UDS. Here we use an updated ZFIRE dataset to derive the properties of ionized gas regions of proto-cluster galaxies by extracting fluxes from emission lines H$β$ 4861Å, [OIII] 5007Å, H$α$ 6563Å, [NII] 6585Å, and [SII] 6716,6731Å. We measure gas-phase metallicity of members in both proto-clusters using two indicators, including a strong-line indicator relatively independent of ionization parameter and electron density. Proto-cluster and field galaxies in both UDS and COSMOS lie on the same Mass-Metallicity Relation with both metallicity indicators. We compare our results to recent IllustrisTNG results, which reports no significant gas-phase metallicity offset between proto-cluster and field galaxies until z=1.5. This is in agreement with our observed metallicities, where no offset is measured between proto-cluster and field populations. We measure tentative evidence from stacked spectra that UDS high mass proto-cluster and field galaxies have differing [OIII]/H$β$ ratios, however these results are dependent on the sample size of the high mass stacks.

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arXiv2019

Theoretical ISM pressure and electron density diagnostics for local and high-redshift galaxies

We derive new self-consistent theoretical UV, optical, and IR diagnostics for the ISM pressure and electron density in the ionized nebulae of star-forming galaxies. Our UV diagnostics utilize the inter-combination, forbidden and resonance lines of silicon, carbon, aluminum, neon, and nitrogen. We also calibrate the optical and IR forbidden lines of oxygen, argon, nitrogen and sulfur. We show that line ratios used as ISM pressure diagnostics depend on the gas-phase metallicity with a residual dependence on the ionization parameter of the gas. In addition, the traditional electron density diagnostic [S II] λ6731/[S II] λ6717 is strongly dependent on the gas-phase metallicity. We show how different emission-line ratios are produced in different ionization zones in our theoretical nebulae. The [S II] and [O II] ratios are produced in different zones, and should not be used interchangeably to measure the electron density of the gas unless the electron temperature is known to be constant. We review the temperature and density distributions observed within H II regions and discuss the implications of these distributions on measuring the electron density of the gas. Many H II regions contain radial variations in density. We suggest that the ISM pressure is a more meaningful quantity to measure in H II regions or galaxies. Specific combinations of line ratios can cover the full range of ISM pressures (4 < log(P/k) < 9). As H II regions become resolved at increasingly high redshift through the next generation telescopes, we anticipate that these diagnostics will be important for understanding the conditions around the young, hot stars from the early universe to the present day.

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arXiv2019

Rest-frame UV and optical emission line diagnostics of ionised gas properties: a test case in a star-forming knot of a lensed galaxy at z~1.7

We examine the diagnostic power of rest-frame ultraviolet (UV) nebular emission lines, and compare them to more commonly used rest-frame optical emission lines, using the test case of a single star-forming knot of the bright lensed galaxy RCSGA 032727-132609 at redshift z~1.7. This galaxy has complete coverage of all the major rest-frame UV and optical emission lines from Magellan/MagE and Keck/NIRSPEC. Using the full suite of diagnostic lines, we infer the physical properties: nebular electron temperature (T_e), electron density (n_e), oxygen abundance (log(O/H)), ionisation parameter (log(q)) and interstellar medium (ISM) pressure (log(P/k)). We examine the effectiveness of the different UV, optical and joint UV-optical spectra in constraining the physical conditions. Using UV lines alone we can reliably estimate log(q), but the same is difficult for log(O/H). UV lines yield a higher (~1.5 dex) log(P/k) than the optical lines, as the former probes a further inner nebular region than the latter. For this comparison, we extend the existing Bayesian inference code IZI, adding to it the capability to infer ISM pressure simultaneously with metallicity and ionisation parameter. This work anticipates future rest-frame UV spectral datasets from the James Webb Space Telescope (JWST) at high redshift and from the Extremely Large Telescope (ELT) at moderate redshift.

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arXiv2019

Separating Line Emission from Star Formation, Shocks, and AGN Ionisation in NGC 1068

In the optical spectra of galaxies, the separation of line emission from gas ionised by star formation and an AGN, or by star formation and shocks, are very well-understood problems. However, separating line emission between AGN and shocks has proven difficult. With the aid of a new three-dimensional diagnostic diagram, we show the simultaneous separation of line emission from star formation, shocks, and AGN in NGC 1068, and quantify the ratio of star formation, shocks, and AGN in each spaxel. The AGN, shock, and star formation luminosity distributions across the galaxy accurately align with X-ray, radio, and CO(3-2) observations, respectively. Comparisons with previous separation methods show that the shocked emission heavily mixes with the AGN emission. We also show that if the H$α$ flux is to be used as a star formation rate indicator, separating line emission from as many sources as possible should be attempted to ensure accurate results.

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