arXiv subjects

Sara L. Ellison

Publications and source records attributed to Sara L. Ellison.

At least 55 records · Page 3Linked to original sources

arXiv2024

Interacting galaxies in the IllustrisTNG simulations -- VI: Reconstructed orbits, close encounters and mergers

Cosmological simulations have been used to study interacting galaxies as a function of galaxy pair separation, enabling comparisons with observational studies of galaxy pairs. The study of interacting galaxies as a function of time (i.e. merger stage) has mostly been limited to high resolution merger simulations, due to the poor time sampling available in cosmological simulations. Building on an earlier study of galaxy pairs in the IllustrisTNG cosmological simulations, we reconstruct the orbits of galaxy pairs involving massive galaxies ($M_* > 10^{10}M_{\odot}$) at redshifts of $0 \leq z < 1$, using a novel kinematic interpolation scheme to model the orbits in between the IllustrisTNG snapshots (which are separated by 162 Myr on average). We assess the accuracy of these interpolations using a pre-existing suite of merger simulations, and find that kinematic interpolations provide a remarkable improvement in accuracy compared with interpolations that use only radial separations or 3D positions. We find that nearly 90 per cent of the closest pairs ($r < 25$ kpc) have had a pericentre encounter within the past Gyr. Many of these close pairs are found on rapidly shrinking orbits, and roughly 85 per cent of these pairs will merge within 1 Gyr. However, approximately 3 per cent of these close pairs appear to be flyby systems that will never merge. These reconstructed orbits will be used in future studies to investigate how and when galaxy properties change during close encounters and mergers between galaxies in IllustrisTNG.

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arXiv2024

The ALMaQUEST Survey XIII: Understanding radial trends in star formation quenching via the relative roles of gas availability and star formation efficiency

Star formation quenching is one of the key processes that shape the evolution of galaxies. In this study, we investigate the changes in molecular gas and star formation properties as galaxies transit from the star-forming main sequence to the passive regime. Our analysis reveals that as galaxies move away from the main sequence towards the green valley the radial profile of specific star formation rate surface density ($Σ_\mathrm{sSFR}$) is suppressed compared with main sequence galaxies out to a galactocentric radius of 1.5 $R_{e}$ ($\sim$ 7 kpc for our sample). By combining radial profiles of gas fraction ($f_\mathrm{gas}$) and star formation efficiency (SFE), we can discern the underlying mechanism that determines $Σ_\mathrm{sSFR}$ at different galactocentric radii. Analysis of relative contributions of $f_\mathrm{gas}$ and SFE to $Σ_\mathrm{sSFR}$ uncovers a diverse range of quenching modes. Star formation in approximately half of our quenching galaxies is primarily driven by a single mode (i.e. either $f_\mathrm{gas}$ or SFE), or a combination of both. A collective analysis of all galaxies reveals that the reduction in star formation within the central regions ($R$ $<$ 0.5 $R_{e}$) is primarily attributable to a decrease in SFE. Conversely, in the disk regions ($R$ $>$ 0.5 $R_{e}$), both $f_\mathrm{gas}$ and SFE contribute to the suppression of star formation. Our findings suggest that multiple quenching mechanisms may be at play in our sample galaxies, and even within a single galaxy. We also compare our observational outcomes with those from galaxy simulations and discuss the implications of our data.

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arXiv2024

Interacting galaxies in the IllustrisTNG simulations -- VII: The connection between the most luminous active galactic nuclei and galaxy interactions

We investigate the connection between the most luminous active galactic nuclei (AGN), galaxy pairs, and post-mergers in the IllustrisTNG simulation. We select galaxy pairs and post-mergers with a mass ratio between 1:10 $< μ<$ 1:1 and a redshift between $0 10^{44}$ erg/s occur in $\sim $26\% of paired galaxies with a companion within 20 kpc, compared with $\sim $14\% in controls (a fractional excess of $\sim$2). The enhancement of AGN in galaxy pairs is luminosity dependent, with the highest excess (a factor of $\sim6\pm2$ at the closest separations) for AGN with $L_{\mathrm{bol}}>10^{45}$ erg/s. Additionally, pairs exhibit a modest yet statistically significant excess of luminous AGN up to $\sim$150 kpc in separation. For pairs which merge between $0 10^{45}$ erg/s, $\sim$55\% are interacting, compared with a 30\% interaction fraction in AGN with $10^{44}<L_{\mathrm{bol}}<10^{44.5}$ erg/s. Our results support a picture in which interactions play a dominant role in (but are not the sole cause of) triggering the most luminous AGN.

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arXiv2024

The ALMaQUEST Survey XIV: do radial molecular gas flows affect the star-forming ability of barred galaxies?

We investigate whether barred galaxies are statistically more likely to harbour radial molecular gas flows and what effect those flows have on their global properties. Using 46 galaxies from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we identify galaxies hosting optical bars using a combination of the morphological classifications in Galaxy Zoo 2 and HyperLEDA. In order to detect radial molecular gas flows, we employ full 3D kinematic modelling of the ALMaQUEST CO(1-0) datacubes. By combining our bar classifications with our radial bar-driven flow detections, we find that galaxies classed as barred are statistically more likely to host large-scale radial gas motions compared to their un-barred and edge-on galaxy counterparts. Moreover, the majority of barred galaxies require multi-component surface brightness profiles in their best-fit models, indicative of the presence of resonance systems. We find that galaxies classed as barred with radial bar-driven flows ("barred + radial flow" subset) have significantly suppressed global star-formation efficiencies compared to barred galaxies without radial bar-driven flows and galaxies in the other morphological sub-samples. Our "barred + radial flow" subset galaxies also possess consistently centrally concentrated molecular gas distributions, with no indication of depleted gas mass fractions, suggesting that gas exhaustion is not the cause of their suppressed star formation. Furthermore, these objects have higher median gas mass surface densities in their central 1 kpc, implying that a central gas enhancements do not fuel central starbursts in these objects. We propose that dynamical effects, such as shear caused by large-scale inflows of gas, act to gravitationally stabilise the inner gas reservoirs.

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arXiv2024

The limitations (and potential) of non-parametric morphology statistics for post-merger identification

Non-parametric morphology statistics have been used for decades to classify galaxies into morphological types and identify mergers in an automated way. In this work, we assess how reliably we can identify galaxy post-mergers with non-parametric morphology statistics. Low-redshift (z<0.2), recent (t_post-merger < 200 Myr), and isolated (r > 100 kpc) post-merger galaxies are drawn from the IllustrisTNG100-1 cosmological simulation. Synthetic r-band images of the mergers are generated with SKIRT9 and degraded to various image qualities, adding observational effects such as sky noise and atmospheric blurring. We find that even in perfect quality imaging, the individual non-parametric morphology statistics fail to recover more than 55% of the post-mergers, and that this number decreases precipitously with worsening image qualities. The realistic distributions of galaxy properties in IllustrisTNG allow us to show that merger samples assembled using individual morphology statistics are biased towards low mass, high gas fraction, and high mass ratio. However, combining all of the morphology statistics together using either a linear discriminant analysis or random forest algorithm increases the completeness and purity of the identified merger samples and mitigates bias with various galaxy properties. For example, we show that in imaging similar to that of the 10-year depth of the Legacy Survey of Space and Time (LSST), a random forest can identify 89% of mergers with a false positive rate of 17%. Finally, we conduct a detailed study of the effect of viewing angle on merger observability and find that there may be an upper limit to merger recovery due to the orientation of merger features with respect to the observer.

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arXiv2024

The ALMaQUEST Survey XII: Dense Molecular Gas as traced by HCN and HCO$^{+}$ in Green Valley Galaxies

We present ALMA observations of two dense gas tracers, HCN(1-0) and HCO$^{+}$(1-0), for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO(1-0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFE$_{\rm mol}$) that leads to the low specific star formation rate in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction $f_{\rm dense}$) or the low star formation efficiency of dense gas (SFE$_{\rm dense}$). We find that SFE$_{\rm mol}$ as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFE$_{\rm dense}$ and depends only weakly on $f_{\rm dense}$. The specific star formation rate (sSFR) on kpc scales is primarily driven by SFE$_{\rm mol}$ and SFE$_{\rm dense}$, followed by the dependence on $f_{\rm mol}$, and is least correlated with $f_{\rm dense}$ or the dense-to-stellar mass ratio ($R_{\rm dense}$). When compared with other works in the literature, we find that our green valley sample shows lower global SFE$_{\rm mol}$ as well as lower SFE$_{\rm dense}$ while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the 3 green valley galaxies with a normal abundance of molecular gas is suppressed mainly due to the reduced SFE$_{\rm dense}$ rather than the lack of dense gas.

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arXiv2023

The ALMaQUEST Survey XI: A strong but non-linear relationship between star formation and dynamical equilibrium pressure

We present the extended ALMA MaNGA QUEnching and STar formation survey, a combination of the original 46 ALMaQUEST galaxies plus new ALMA observations for a further 20 interacting galaxies. Three well-studied scaling relations are fit to the 19,999 star-forming spaxels in the extended sample, namely the resolved Schmidt-Kennicutt (rSK) relation, the resolved star forming main sequence (rSFMS) and the resolved molecular gas main sequence (rMGMS). We additionally investigate the relationship between the dynamical equilibrium pressure (PDE) and star formation rate surface density (Sigma_SFR), which we refer to as the resolved PDE (rPDE) relation. Contrary to previous studies that have focussed on normal star-forming galaxies and found an approximately linear rPDE relation, the presence of more vigourously star-forming galaxies in the extended ALMaQUEST sample reveals a marked turnover in the relation at high pressures. Although the scatter around the linear fit to the rPDE relation is similar to the other three relations, a random forest analysis, which can extract non-linear dependences, finds that PDE is unambiguously more important than either Sigma_H2 or Sigma_star for predicting Sigma_SFR. We compare the observed rPDE relation to the prediction of the pressure-regulated feedback-modulated (PRFM) model of star formation, finding that galaxies residing on the global SFMS do indeed closely follow the rPDE relation predicted by the PRFM theory. However, galaxies above and below the global SFMS show significant deviations from the model. Galaxies with high SFR are instead consistent with models that include other contributions to turbulence in addition to the local star formation feedback.

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arXiv2023

$Σ_{\mathrm{SFR}}$-M* Diagram: A Valuable Galaxy Evolution Diagnostic to Complement (s)SFR-M* Diagrams

The specific star formation rate (sSFR) is commonly used to describe the level of galaxy star formation (SF) and to select quenched galaxies. However, being a relative measure of the young-to-old population, an ambiguity in its interpretation may arise because a small sSFR can be either because of a substantial previous mass build up, or because SF is low. We show, using large samples spanning 0 < z < 2, that the normalization of SFR by the physical extent over which SF is taking place (i.e., SFR surface density, $Σ_{\mathrm{SFR}}$) overcomes this ambiguity. $Σ_{\mathrm{SFR}}$ has a strong physical basis, being tied to the molecular gas density and the effectiveness of stellar feedback, so we propose $Σ_{\mathrm{SFR}}$-M* as an important galaxy evolution diagram to complement (s)SFR-M* diagrams. Using the $Σ_{\mathrm{SFR}}$-M* diagram we confirm the Schiminovich et al. (2007) result that the level of SF along the main sequence today is only weakly mass dependent - high-mass galaxies, despite their redder colors, are as active as blue, low-mass ones. At higher redshift, the slope of the "$Σ_{\mathrm{SFR}}$ main sequence" steepens, signaling the epoch of bulge build-up in massive galaxies. We also find that $Σ_{\mathrm{SFR}}$ based on the optical isophotal radius more cleanly selects both the starbursting and the spheroid-dominated (early-type) galaxies than sSFR. One implication of our analysis is that the assessment of the inside-out vs. outside-in quenching scenarios should consider both sSFR and $Σ_{\mathrm{SFR}}$ radial profiles, because ample SF may be present in bulges with low sSFR (red color).

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arXiv2023

VERTICO and IllustrisTNG: The spatially resolved effects of environment on galactic gas

It has been shown in previous publications that the TNG100 simulation quantitatively reproduces the observed reduction in each of the total atomic and total molecular hydrogen gas for galaxies within massive halos, i.e.~dense environments. In this Letter, we study how well TNG50 reproduces the resolved effects of a Virgo-like cluster environment on the gas surface densities of satellite galaxies with $m_* > \! 10^9\,{\rm M}_\odot$ and ${\rm SFR} \! > 0.05\,{\rm M}_\odot\,{\rm yr}^{-1}$. We select galaxies in the simulation that are analogous to those in the HERACLES and VERTICO surveys, and mock-observe them to the common specifications of the data. Although TNG50 does not quantitatively match the observed gas surface densities in the centers of galaxies, the simulation does qualitatively reproduce the trends of gas truncation and central density suppression seen in VERTICO in both HI and H$_2$. This result promises that modern cosmological hydrodynamic simulations can be used to reliably model the post-infall histories of cluster satellite galaxies.

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arXiv2023

VERTICO VII: Environmental quenching caused by suppression of molecular gas content and star formation efficiency in Virgo Cluster galaxies

We study how environment regulates the star formation cycle of 33 Virgo Cluster satellite galaxies on 720 parsec scales. We present the first resolved star-forming main sequence for cluster galaxies, dividing the sample based on their global HI properties and comparing to a control sample of field galaxies. HI-poor cluster galaxies have reduced star formation rate (SFR) surface densities with respect to both HI-normal cluster and field galaxies (0.5 dex), suggesting that mechanisms regulating the global HI content are responsible for quenching local star formation. We demonstrate that the observed quenching in HI-poor galaxies is caused by environmental processes such as ram pressure stripping (RPS) simultaneously reducing molecular gas surface density and star formation efficiency (SFE), compared to regions in HI-normal systems (by 0.38 and 0.22 dex, respectively). We observe systematically elevated SFRs that are driven by increased molecular gas surface densities at fixed stellar mass surface density in the outskirts of early-stage RPS galaxies, while SFE remains unchanged with respect to the field sample. We quantify how RPS and starvation affect the star formation cycle of inner and outer galaxy discs as they are processed by the cluster. We show both are effective quenching mechanisms with the key difference being that RPS acts upon the galaxy outskirts while starvation regulates the star formation cycle throughout disc, including within the truncation radius. For both processes, the quenching is caused by a simultaneous reduction in molecular gas surface densities and SFE at fixed stellar mass surface density.

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arXiv2023

Interacting galaxies in the IllustrisTNG simulations -- V: Comparing the influence of star-forming vs. passive companions

We study interacting galaxy pairs in the TNG100-1 and TNG300-1 cosmological simulations using previously generated closest companion samples. We study the specific star formation rates (sSFR) of massive ($10^{10} M_{\odot} < M_* < 10^{12} M_{\odot}$) galaxies at $z \leq 0.2$ as a function of separation from the closest companion galaxy. We split our sample based on whether the companion galaxy is star-forming or passive. We find that galaxies with close star-forming companions have sSFRs that are enhanced (on average) by a factor of $2.9 \pm 0.3$ in TNG100-1 and $2.27 \pm 0.06$ in TNG300-1 compared to controls, with enhancements present out to separations of $\sim 300$ kpc. Galaxies with passive companions in TNG300-1 exhibit mild sSFR suppression ($\sim12$ percent) at 100-300 kpc and small sSFR enhancements at separations below 50 kpc. sSFR suppression is strongest in pairs where the galaxy's stellar mass is more than 2 times that of its passive companion. By generating a stellar mass-matched ("twinned") sample in TNG300-1, we show that differences in sSFR trends between companion types are not a result of intrinsic stellar mass differences in star-forming vs. passive galaxies. We compare with an analogous sample of galaxy pairs from SDSS, finding consistent results between observations and simulations. Overall, we find that star-forming galaxies show enhanced sSFRs regardless of companion type, but that galaxies with close passive companions are more likely to be passive themselves.

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arXiv2023

VERTICO V: The environmentally driven evolution of the inner cold gas discs of Virgo cluster galaxies

The quenching of cluster satellite galaxies is inextricably linked to the suppression of their cold interstellar medium (ISM) by environmental mechanisms. While the removal of neutral atomic hydrogen (HI) at large radii is well studied, how the environment impacts the remaining gas in the centres of galaxies, which are dominated by molecular gas, is less clear. Using new observations from the Virgo Environment traced in CO survey (VERTICO) and archival HI data, we study the HI and molecular gas within the optical discs of Virgo cluster galaxies on 1.2-kpc scales with spatially resolved scaling relations between stellar ($Σ_{\star}$), HI ($Σ_\mathrm{HI}$), and molecular gas ($Σ_\mathrm{mol}$) surface densities. Adopting HI deficiency as a measure of environmental impact, we find evidence that, in addition to removing the HI at large radii, the cluster processes also lower the average $Σ_\mathrm{HI}$ of the remaining gas even in the central 1.2 kpc. The impact on molecular gas is comparatively weaker than on the HI, and we show that the lower $Σ_\mathrm{mol}$ gas is removed first. In the most HI-deficient galaxies, however, we find evidence that environmental processes reduce the typical $Σ_\mathrm{mol}$ of the remaining gas by nearly a factor of 3. We find no evidence for environment-driven elevation of $Σ_\mathrm{HI}$ or $Σ_\mathrm{mol}$ in HI-deficient galaxies. Using the ratio of $Σ_\mathrm{mol}$-to-$Σ_\mathrm{HI}$ in individual regions, we show that changes in the ISM physical conditions, estimated using the total gas surface density and midplane hydrostatic pressure, cannot explain the observed reduction in molecular gas content. Instead, we suggest that direct stripping of the molecular gas is required to explain our results.

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arXiv2023

VERTICO II: effects of HI-identified environmental mechanisms on molecular gas

In this VERTICO early science paper we explore in detail how environmental mechanisms, identified in HI, affect the resolved properties of molecular gas reservoirs in cluster galaxies. The molecular gas is probed using ALMA ACA (+TP) observations of 12CO(2-1) in 51 spiral galaxies in the Virgo cluster (of which 49 are detected), all of which are included in the VIVA HI survey. The sample spans a stellar mass range of 9 < log M*/Msol < 11. We study molecular gas radial profiles, isodensity radii, and surface densities as a function of galaxy HI deficiency and morphology. There is a weak correlation between global HI and H2 deficiencies, and resolved properties of molecular gas correlate with HI deficiency: galaxies that have large HI deficiencies have relatively steep and truncated molecular gas radial profiles, which is due to the removal of low-surface density molecular gas on the outskirts. Therefore, while the environmental mechanisms observed in HI also affect molecular gas reservoirs, there is only a moderate reduction of the total amount of molecular gas.

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arXiv2023

AGN in post-mergers from the Ultraviolet Near Infrared Optical Northern Survey

The kinematic disturbances associated with major galaxy mergers are known to produce gas inflows, which in turn may trigger accretion onto the supermassive black holes (SMBH) of the participant galaxies. While this effect has been studied in galaxy pairs, the frequency of active galactic nuclei (AGN) in fully coalesced post-merger systems is poorly constrained due to the limited size or impurity of extant post-merger samples. Previously, we combined convolutional neural network (CNN) predictions with visual classifications to identify a highly pure sample of 699 post-mergers in deep r-band imaging. In the work presented here, we quantify the frequency of AGN in this sample using three metrics: optical emission lines, mid-infrared (mid- IR) colour, and radio detection of low-excitation radio galaxies (LERGs). We also compare the frequency of AGN in post-mergers to that in a sample of spectroscopically identified galaxy pairs. We find that AGN identified by narrow-line optical emission and mid-IR colour have an increased incidence rate in post-mergers, with excesses of ~4 over mass- and redshift-matched controls. The optical and mid-IR AGN excesses in post-mergers exceed the values found for galaxy pairs, indicating that AGN activity in mergers peaks after coalescence. Conversely, we recover no significant excess of LERGs in post-mergers or pairs. Finally, we find that the [OIII] luminosity (a proxy for SMBH accretion rate) in post-mergers that host an optical AGN is ~0.3 dex higher on average than in non-interacting galaxies with an optical AGN, suggesting that mergers generate higher accretion rates than secular triggering mechanisms.

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arXiv2022

PHANGS-JWST First Results: A Global and Moderately Resolved View of Mid-Infrared and CO Line Emission from Galaxies at the Start of the JWST Era

We explore the relationship between mid-infrared (mid-IR) and CO rotational line emission from massive star-forming galaxies, which is one of the tightest scalings in the local universe. We assemble a large set of unresolved and moderately ($\sim 1$ kpc) spatially resolved measurements of CO (1-0) and CO (2-1) intensity, $I_{\rm CO}$, and mid-IR intensity, $I_{\rm MIR}$, at 8, 12, 22, and 24$μ$m. The $I_{\rm CO}$ vs. $I_{\rm MIR}$ relationship is reasonably described by a power law with slopes $0.7{-}1.2$ and normalization $I_{\rm CO} \sim 1$ K km s$^{-1}$ at $I_{\rm MIR} \sim 1$ MJy sr$^{-1}$. Both the slopes and intercepts vary systematically with choice of line and band. The comparison between the relations measured for CO~(1-0) and CO (2-1) allow us to infer that $R_{21} \propto I_{\rm MIR}^{0.2}$, in good agreement with other work. The $8μ$m and $12μ$m bands, with strong PAH features, show steeper CO vs. mid-IR slopes than the $22μ$m and $24μ$m, consistent with PAH emission arising not just from CO-bright gas but also from atomic or CO-dark gas. The CO-to-mid-IR ratio correlates with global galaxy stellar mass ($M_\star$) and anti-correlates with SFR/$M_\star$. At $\sim 1$ kpc resolution, the first four PHANGS-JWST targets show CO to mid-IR relationships that are quantitatively similar to our larger literature sample, including showing the steep CO-to-mid-IR slopes for the JWST PAH-tracing bands, although we caution that these initial data have a small sample size and span a limited range of intensities.

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arXiv2022

The interconnection between galaxy mergers, AGN activity and rapid quenching of star formation in simulated post-merger galaxies

We investigate the role of galaxy mergers on supermassive black hole (SMBH) accretion and star formation quenching in three state-of-the-art cosmological simulations with contrasting physics models: EAGLE, Illustris and IllustrisTNG. We find that recently coalesced 'post-mergers' in all three simulations have elevated SMBH accretion rates by factors of ~2-5. However, rapid (within 500 Myr of coalescence) quenching of star formation is rare, with incidence rates of 0.4% in Illustris, 4.5% in EAGLE and 10% in IllustrisTNG. The rarity of quenching in post-mergers results from substantial gas reservoirs that remain intact after the merger. The post-mergers that do successfully quench tend to be those that had both low pre-merger gas fractions as well as those that experience the largest gas losses. Although rare, the recently quenched fraction of post-mergers is still elevated compared to a control sample of non-mergers by factors of two in IllustrisTNG and 11 in EAGLE. Conversely, quenching is rarer in Illustris post-mergers than in their control. Recent observational results by Ellison et al. have found rapid quenching to be at least 30 times more common in post-mergers, a significantly higher excess than found in any of the simulations. Our results, therefore, indicate that whilst merger-induced SMBH accretion is a widespread prediction of the simulations, its link to quenching depends sensitively on the physics models, and that none of the subgrid models of the simulations studied here can fully capture the connection between mergers and rapid quenching seen in observations.

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arXiv2022

The metallicity's fundamental dependence on both local and global galactic quantities

We study the scaling relations between gas-phase metallicity, stellar mass surface density ($Σ_*$), star formation rate surface density ($Σ_{SFR}$), and molecular gas surface density ($Σ_{H_2}$) in local star-forming galaxies on scales of a kpc. We employ optical integral field spectroscopy from the MaNGA survey, and ALMA data for a subset of MaNGA galaxies. We use Partial Correlation Coefficients and Random Forest regression to determine the relative importance of local and global galactic properties in setting the gas-phase metallicity. We find that the local metallicity depends primarily on $Σ_*$ (the resolved mass-metallicity relation, rMZR), and has a secondary anti-correlation with $Σ_{SFR}$ (i.e. a spatially-resolved version of the 'Fundamental Metallicity Relation', rFMR). We find that $Σ_{H_2}$ is less important than $Σ_{SFR}$ in determining the local metallicity. This result indicates that gas accretion, resulting in local metallicity dilution and local boosting of star formation, is unlikely to be the primary origin of the rFMR. The local metallicity depends also on the global properties of galaxies. We find a strong dependence on the total stellar mass ($M_*$) and a weaker (inverse) dependence on the total SFR. The global metallicity scaling relations, therefore, do not simply stem out of their resolved counterparts; global properties and processes, such as the global gravitational potential well, galaxy-scale winds and global redistribution/mixing of metals, likely contribute to the local metallicity, in addition to local production and retention.

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arXiv2022

A New Physical Picture for AGNs Lacking Optical Emission Lines

In this work, we use ~500 low-redshift (z ~ 0.1) X-ray AGNs observed by XMM-Newton and SDSS to investigate the prevalence and nature of AGNs that apparently lack optical emission lines (``optically dull AGNs''). Although 1/4 of spectra appear absorption-line dominated in visual assessment, line extraction with robust continuum subtraction from the MPA/JHU catalog reveals usable [OIII] measurements in 98% of the sample, allowing us to study [OIII]-underluminous AGNs together with more typical AGNs in the context of the L$_{\mathrm{[OIII]}}$--L$_{X}$ relation. We find that ``optically dull AGNs'' do not constitute a distinct population of AGNs. Instead, they are the [OIII]-underluminous tail of a single, unimodal L$_{\mathrm{[OIII]}}$--L$_{X}$ relation that has substantial scatter (0.6 dex). We find the degree to which an AGN is underluminous in [OIII] correlates with the specific SFR or D$_{4000}$ index of the host, which are both linked to the molecular gas fraction. Thus the emerging physical picture for the large scatter seems to involve the gas content of the narrow-line region. We find no significant role for previously proposed scenarios for the presence of optically dull AGNs, such as host dilution or dust obscuration. Despite occasionally weak lines in SDSS spectra, >80% of X-ray AGNs are identified as such with the BPT diagram. >90% are classified as AGNs based only on [NII]/H$α$, providing more complete AGN samples when [OIII] or H$β$ are weak. X-ray AGNs with LINER spectra obey essentially the same \lxo\ relation as Seyfert 2s, suggesting their line emission is produced by AGN activity.

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