Rm Dex Research Articles

Survey for Distant Solar Twins (SDST) – III. Identification of new solar twin and solar analogue stars

ABSTRACT The Survey for Distant Solar Twins aims to find stars very similar to the Sun at distances of 1–$4\, {\rm kpc}$, several times more distant than any currently known solar twins and analogues. The goal is to identify the best stars with which to test whether the fine-structure constant, α, varies with dark matter density in our Galaxy. Here, we use epic, our line-by-line differential technique, to measure the stellar parameters – effective temperature Teff, surface gravity log g, and metallicity [Fe/H] – from moderate-resolution (R ≲ 32 000) spectra of 877 solar twin and analogue candidates (547 at 1–$4\, {\rm kpc}$) observed with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) on the Anglo-Australian Telescope. These are consistent with expectations for Teff and log g from photometry, and for [Fe/H] from the Besançon stellar population model. epic provides small enough uncertainties ($\sim 90\, {\rm K}$, $0.08\, {\rm dex}$, and $0.05\, {\rm dex}$, respectively), even at the low signal-to-noise ratios available (${\rm S/N}\gtrsim$ 25 per pixel), to identify 299 new solar analogues ($\ge 90~{{\ \rm per\ cent}}$ confidence) and 20 solar twins (≥50 per cent confidence), 206 and 12 of which are at 1–$4\, {\rm kpc}$. By extending epic to measure line broadening and lithium abundance from HERMES spectra, and with ages derived from isochrone fitting with our stellar parameters, we identify 174 solar analogues at 1–$4\, {\rm kpc}$ that are relatively inactive, slowly rotating, and with no evidence of spectroscopic binarity. These are the preferred targets for follow-up spectroscopy to measure α.

The resolved chemical composition of the starburst dwarf galaxy CGCG007-025: direct method versus photoionization model fitting

ABSTRACTThis work focuses on the gas chemical composition of CGCG007-025. This compact dwarf is undergoing a galaxy wide star-forming burst, whose spatial behaviour has been observed by VLT/MUSE. We present a new line measurement library to treat almost 7800 voxels. The direct method chemical analysis is limited to 484 voxels with good detection of the $[\mathrm{S}\, {\small III}]6312\, \mathring{\rm A}$ temperature diagnostic line. The recombination fluxes are corrected for stellar absorption via a population synthesis. Additionally, we discuss a new algorithm to fit photoionization models via neural networks. The eight ionic abundances analysed show a spatial normal distribution with a $\sigma \sim 0.1\, {\rm dex}$, where only half this value can be explained by the uncertainty in the measurements. The oxygen abundance distribution is 12 + log(O/H) = 7.88 ± 0.11. The ${\it T}_{e}[\mathrm{S}\, {\small III}]$ and ne[S ii] are also normally distributed. However, in the central and brightest region, the ne[S ii] is almost thrice the mean galaxy value. This is also reflected in the extinction measurements. The ionization parameter has a distribution of ${\rm log}(U) = -2.52^{0.17}_{0.19}$. The parameter spatial behaviour agrees with the S2+/S+ map. Finally, the discrepancies between the direct method and the photoionization model fitting are discussed. In the latter technique, we find that mixing lines with uneven uncertainty magnitudes can impact the accuracy of the results. In these fittings, we recommend overestimating the minimum flux uncertainty one order below the maximum line flux uncertainty. This provides a better match with the direct method.

The size–mass and other structural parameter (n, μz, Rz) relations for local bulges/spheroids from multicomponent decompositions

ABSTRACT We analyse the bulge/spheroid size–(stellar mass), Re, Sph − M*, Sph, relation and spheroid structural parameters for 202 local (predominantly $\lesssim 110~\rm Mpc$) galaxies spanning $M_*\sim 3\times 10^{9}\!-\!10^{12}~\rm M_{\odot }$ and $0.1 \lesssim R_\mathrm{e, Sph}\lesssim 32~\rm kpc$ from multicomponent decomposition. The correlations between the spheroid Sérsic index (nSph), central surface brightness (μ0, Sph), effective half-light radius (Re, Sph), absolute magnitude ($\mathfrak {M}_\mathrm{Sph}$), and stellar mass (M*, Sph) are explored. We also investigate the consequences of using different scale radii, $R_{z,\rm Sph}$, encapsulating a different fraction (z, from 0 to 1) of the total spheroid luminosity. The correlation strengths for projected mass densities, Σz and 〈Σ〉z, vary significantly with the choice of z. Spheroid size ($R_\mathrm{z, \rm Sph}$) and mass (M*, Sph) are strongly correlated for all light fractions z. We find $\log (R_\mathrm{e,Sph}/\rm kpc) = 0.88\log (M_\mathrm{*,Sph}/\rm M_{\odot })-9.15$ with a small scatter of $\Delta _{rms} = 0.24~\rm dex$ in the log (Re, Sph) direction. This result is discussed relative to the curved size–mass relation for early-type galaxies due to their discs yielding larger galaxy radii at lower masses. Moreover, the slope of our spheroid size–mass relation is a factor of ∼3, steeper than reported bulge size–mass relations, and with bulge sizes at M*, sph ∼3 × 109 M⊙ which are 2–3 times smaller. Our spheroid size–mass relation present no significant flattening in slope in the low-mass end ($M_{\rm *,sph}\sim 10^9 - 10^{10}\rm ~M_{\odot }$). Instead of treating galaxies as single entities, future theoretical and evolutionary models should also attempt to recreate the strong scaling relations of specific galactic components. Additional scaling relations, such as log (nSph) − log (M*, Sph), log (Σ0, Sph) − log (nSph), and log (nSph) − log (Re, Sph), are also presented. Finally, we show that the local spheroids align well with the size-mass distribution of quiescent galaxies at z ∼ 1.25–2.25. In essence, local spheroids and high-z quiescent galaxies appear structurally similar, likely dictated by the virial theorem.

Radial distributions of globular clusters trace their host dark matter halo: insights from the E-MOSAICS simulations

ABSTRACT Globular clusters (GCs) are bright objects that span a wide range of galactocentric distances, and can therefore probe the structure of dark matter (DM) haloes. In this work, we explore whether the projected number density radial profiles of GCs can be used to infer the structural properties of their host DM haloes. We use the simulated GC populations in a sample of 166 central galaxies from the $(34.4~\rm cMpc)^3$ periodic volume of the E-MOSAICS project. We find that more massive galaxies host stellar and GC populations with shallower density profiles that are more radially extended. In addition, metal-poor GC subpopulations tend to have shallower and more extended profiles than metal-rich subsamples, which we relate to the preferentially accreted origin of metal-poor GCs. We find strong correlations between the power-law slopes and effective radii of the radial profiles of the GC populations and the structural properties of the DM haloes, such as their power-law slopes, Navarro–Frenk–White scale radii, and concentration parameters. Accounting for a dependence on the galaxy stellar mass decreases the scatter of the two-dimensional relations. This suggests that the projected number counts of GCs, combined with their galaxy mass, trace the density profile of the DM halo of their host galaxy. When applied to extragalactic GC systems, we recover the scale radii and the extent of the DM haloes of a sample of early-type galaxies with uncertainties smaller than $0.2~\rm dex$. Thus, extragalactic GC systems provide a novel avenue to explore the structure of DM haloes beyond the Local Group.

Kinematics of the molecular interstellar medium probed by Gaia: steep velocity dispersion–size relation, isotropic turbulence, and location-dependent energy dissipation

ABSTRACT The evolution of the molecular interstellar medium is controlled by processes such as turbulence, gravity, stellar feedback, and Galactic shear. AL a part of the ISM-6D project, using Gaia astrometric measurements towards a sample of young stellar objects (YSOs), we study the morphology and kinematic structure of the associated molecular gas. We identify 150 YSO associations with distance $d \lesssim 3 \,\,\rm kpc$. The YSO associations are elongated, with a median aspect ratio of 1.97, and are oriented parallel to the disc mid-plane, with a median angle of 30°. The turbulence in the molecular clouds as probed by the YSOs is isotropic, and the velocity dispersions are related to the sizes by σv, 2D = 0.74 (r/pc)0.67 (km s−1) . The slope is on the steeper side, yet consistent with previous measurements. The energy dissipation rate of turbulence $\dot{\epsilon } = \sigma _{v,{\rm 3D}}^3 /L$ decreases with the Galactocentric distance, with a gradient of 0.2 $\rm dex \,\, kpc^{-1}$, which can be explained if turbulence is driven by cloud collisions. In this scenario, the clouds located in the inner Galaxy have higher chances to accrete smaller clouds and are more turbulent. Although the density structures of the complexes are anisotropic, the turbulence is consistent with being isotropic. If the alignment between density structures and the Galactic-disc mid-plane is due to shear, we expect $t_{\rm cloud} \gtrsim t_{\rm shear}\approx 30\,\, \rm Myr$. This cloud lifetime is longer than the turbulence crossing time, and a continuous energy injection is required to maintain the turbulence.

Survey for Distant Solar Twins (SDST) – I. epic method for stellar parameter measurement

ABSTRACT Solar twins are stars of key importance to the field of astronomy and offer a multitude of science applications. Only a small number (≲200) of solar twins are known today, all of which are relatively close to our Sun (${\lesssim}{800}\, {\rm pc}$). The goal of our Survey for Distant Solar Twins (SDST) is to identify many more solar twin and solar analogue stars out to much larger distances (${\sim}{4}\, {\rm kpc}$). In this paper, we present a new method to identify solar twins using relatively low S/N, medium resolving power ($R\sim 28\, 000$) spectra that will be typical of such distant targets observed with HERMES on the ${3.9}\, {\rm m}$ Anglo-Australian Telescope (AAT). We developed a novel approach, namely epic, to measure stellar parameters (SPs) which we use to identify stars similar to our Sun. epic determines the stellar atmospheric parameters (effective temperature Teff, surface gravity log g, and metallicity [Fe/H]) using differential equivalent width (EW) measurements of selected spectroscopic absorption features and a simple model, trained on previously analysed spectra, that connects these EWs to the SPs. The reference for the EW measurements is a high S/N solar spectrum which is used to minimize several systematic effects. epic is fast, optimized for Sun-like stars and yields SP measurements with small enough uncertainties to enable spectroscopic identification of solar twin and analogue stars up to ${\sim}{4}\, {\rm kpc}$ away using AAT/HERMES, i.e. $\sigma \left(T_{\mathrm{eff}}, \log g, \textrm {[Fe/H]}\right) = \left({50}\, {\rm K}, {0.08}\, {\rm dex}, {0.03}\, {\rm dex}\right)$ on average at S/N = 25.

The galaxy–halo size relation of low-mass galaxies in FIRE

ABSTRACT Galaxy sizes correlate closely with the sizes of their parent dark matter haloes, suggesting a link between halo formation and galaxy growth. However, the precise nature of this relation and its scatter remains to be understood fully, especially for low-mass galaxies. We analyse the galaxy–halo size relation (GHSR) for low-mass ($M_\star \sim 10^{7-9}\, {\rm M}_\odot$) central galaxies over the past 12.5 billion years with the help of cosmological volume simulations (FIREbox) from the Feedback in Realistic Environments (FIRE) project. We find a nearly linear relationship between the half-stellar mass galaxy size R1/2 and the parent dark matter halo virial radius Rvir. This relation evolves only weakly since redshift z = 5: $R_{1/2}\, [{\rm kpc}] = (0.053\pm 0.002)(R_{\rm vir}/35\, {\rm kpc})^{0.934\pm 0.054}$, with a nearly constant scatter $\langle \sigma \rangle = 0.084\, [{\rm dex}]$. While this ratio is similar to what is expected from models where galaxy disc sizes are set by halo angular momentum, the low-mass galaxies in our sample are not angular momentum supported, with stellar rotational to circular velocity ratios vrot/vcirc ∼ 0.15. Introducing redshift as another parameter to the GHSR does not decrease the scatter. Furthermore, this scatter does not correlate with any of the halo properties we investigate – including spin and concentration – suggesting that baryonic processes and feedback physics are instead critical in setting the scatter in the GHSR. Given the relatively small scatter and the weak dependence of the GHSR on redshift and halo properties for these low-mass central galaxies, we propose using galaxy sizes as an independent method from stellar masses to infer halo masses.

Unravelling the enigmatic ISM conditions in Minkowski’s object

ABSTRACT 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 $\sim \! 10\, \rm Myr$ 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 ($\lesssim \! 15 \, \rm per \, cent$) of contamination from a non-stellar ionizing 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 $\sim \! 0.5\, \rm dex$, 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 hypothesize that Minkowski’s object either (i) was formed as a result of jet-induced star formation in pre-existing gas clumps in the stellar bridge, or (ii) 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 ultradiffuse galaxy as it is processed by the cluster.

High-redshift JWST predictions from IllustrisTNG: II. Galaxy line and continuum spectral indices and dust attenuation curves

ABSTRACT We present predictions for high redshift (z = 2−10) galaxy populations based on the IllustrisTNG simulation suite and a full Monte Carlo dust radiative transfer post-processing. Specifically, we discuss the H α and H β + $[\rm O \,{\small III}]$ luminosity functions up to z = 8. The predicted H β + $[\rm O \,{\small III}]$ luminosity functions are consistent with present observations at z ≲ 3 with ${\lesssim} 0.1\, {\rm dex}$ differences in luminosities. However, the predicted H α luminosity function is ${\sim }0.3\, {\rm dex}$ dimmer than the observed one at z ≃ 2. Furthermore, we explore continuum spectral indices, the Balmer break at 4000 Å; (D4000) and the UV continuum slope β. The median D4000 versus specific star formation rate relation predicted at z = 2 is in agreement with the local calibration despite a different distribution pattern of galaxies in this plane. In addition, we reproduce the observed AUV versus β relation and explore its dependence on galaxy stellar mass, providing an explanation for the observed complexity of this relation. We also find a deficiency in heavily attenuated, UV red galaxies in the simulations. Finally, we provide predictions for the dust attenuation curves of galaxies at z = 2−6 and investigate their dependence on galaxy colours and stellar masses. The attenuation curves are steeper in galaxies at higher redshifts, with bluer colours, or with lower stellar masses. We attribute these predicted trends to dust geometry. Overall, our results are consistent with present observations of high-redshift galaxies. Future James Webb Space Telecope observations will further test these predictions.

The KLEVER Survey: spatially resolved metallicity maps and gradients in a sample of 1.2 < z < 2.5 lensed galaxies

ABSTRACT We present near-infrared observations of 42 gravitationally lensed galaxies obtained in the framework of the KMOS Lensed Emission Lines and VElocity Review (KLEVER) Survey, a programme aimed at investigating the spatially resolved properties of the ionized gas in 1.2 < z < 2.5 galaxies by means of a full coverage of the YJ, H, and K near-infrared bands. Detailed metallicity maps and gradients are derived for a subsample of 28 galaxies from reconstructed source-plane emission-line maps, exploiting the variety of different emission-line diagnostics provided by the broad wavelength coverage of the survey. About $85 {{\, per\ cent}}$ of these galaxies are characterized by metallicity gradients shallower than $0.05\ \rm dex\, kpc^{-1}$ and $89{{\ \rm per\ cent}}$ are consistent with a flat slope within 3σ ($67{{\ \rm per\ cent}}$ within 1σ), suggesting a mild evolution with cosmic time. In the context of cosmological simulations and chemical evolution models, the presence of efficient feedback mechanisms and/or extended star formation profiles on top of the classical ‘inside-out’ scenario of mass assembly is generally required to reproduce the observed flatness of the metallicity gradients beyond z ∼ 1. Three galaxies with significantly (>3σ) ‘inverted’ gradients are also found, showing an anticorrelation between metallicity and star formation rate density on local scales, possibly suggesting recent episodes of pristine gas accretion or strong radial flows in place. Nevertheless, the individual metallicity maps are characterized by a variety of different morphologies, with flat radial gradients sometimes hiding non-axisymmetric variations on kpc scales, which are washed out by azimuthal averages, especially in interacting systems or in those undergoing local episodes of recent star formation.

A3COSMOS: the dust attenuation of star-forming galaxies at z = 2.5–4.0 from the COSMOS-ALMA archive

ABSTRACT We present an analysis of the dust attenuation of star-forming galaxies at z = 2.5–4.0 through the relationship between the UV spectral slope (β), stellar mass (M*), and the infrared excess (IRX = LIR/LUV) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A3COSMOS team, which includes an unprecedented sample of ∼1500 galaxies at z ∼ 3 as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 $\rm {\mu Jy\, beam}^{-1}$ (1σ). The detection rate is highly mass dependent, decreasing drastically below log (M*/M⊙) = 10.5. The detected galaxies show that the IRX–β relationship of massive (log M*/M⊙ > 10) main-sequence galaxies at z = 2.5–4.0 is consistent with that of local galaxies, while starbursts are generally offset by $\sim 0.5\, {\rm dex}$ to larger IRX values. At the low-mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX–M* relation at $\rm {log\, ({\it M}_{\ast }/\mathrm{M}_{\odot })\gt 9}$ exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at log M*/M⊙ < 10. However, our results are consistent with earlier measurements at z ∼ 5.5, indicating a potential redshift evolution between z ∼ 2 and z ∼ 6. Deeper observations targeting low-mass galaxies will be required to confirm this finding.

On the observed diversity of star formation efficiencies in Giant Molecular Clouds

Observations find a median star formation efficiency per free-fall time in Milky Way Giant Molecular Clouds (GMCs) on the order of $\epsilon_{\rm ff}\sim 1\%$ with dispersions of $\sim0.5\,{\rm dex}$. The origin of this scatter in $\epsilon_{\rm ff}$ is still debated and difficult to reproduce with analytical models. We track the formation, evolution and destruction of GMCs in a hydrodynamical simulation of a Milky Way-like galaxy and by deriving cloud properties in an observationally motivated way, measure the distribution of star formation efficiencies which are in excellent agreement with observations. We find no significant link between $\epsilon_{\rm ff}$ and any measured global property of GMCs (e.g. gas mass, velocity dispersion). Instead, a wide range of efficiencies exist in the entire parameter space. From the cloud evolutionary tracks, we find that each cloud follow a \emph{unique} evolutionary path which gives rise to wide diversity in all properties. We argue that it is this diversity in cloud properties, above all else, that results in the dispersion of $\epsilon_{\rm ff}$.

Very metal-poor stars observed by the RAVE survey

We present a novel analysis of the metal-poor star sample in the complete Radial Velocity Experiment (RAVE) Data Release 5 catalog with the goal of identifying and characterizing all very metal-poor stars observed by the survey. Using a three-stage method, we first identified the candidate stars using only their spectra as input information. We employed an algorithm called t-SNE to construct a low-dimensional projection of the spectrum space and isolate the region containing metal-poor stars. Following this step, we measured the equivalent widths of the near-infrared CaII triplet lines with a method based on flexible Gaussian processes to model the correlated noise present in the spectra. In the last step, we constructed a calibration relation that converts the measured equivalent widths and the color information coming from the 2MASS and WISE surveys into metallicity and temperature estimates. We identified 877 stars with at least a 50% probability of being very metal-poor $(\rm [Fe/H] < -2\,\rm dex)$, out of which 43 are likely extremely metal-poor $(\rm [Fe/H] < -3\,\rm dex )$. The comparison of the derived values to a small subsample of stars with literature metallicity values shows that our method works reliably and correctly estimates the uncertainties, which typically have values $\sigma_{\rm [Fe/H]} \approx 0.2\,\mathrm{dex}$. In addition, when compared to the metallicity results derived using the RAVE DR5 pipeline, it is evident that we achieve better accuracy than the pipeline and therefore more reliably evaluate the very metal-poor subsample. Based on the repeated observations of the same stars, our method gives very consistent results. The method used in this work can also easily be extended to other large-scale data sets, including to the data from the Gaia mission and the upcoming 4MOST survey.

The role of environment on the star formation history of disc galaxies

NGC\,2403, NGC\,300 and M33 are three nearby pure-disc galaxies with similar stellar mass in different environments, they are benchmarks for understanding late-type spiral galaxies in different environments. The chemical evolution and growth of their discs are investigated by using the simple chemical evolution model, in which their discs are assumed to originate and grow through the accretion of the primordial gas, and the gas outflow process is also taken into account. Through comparative study of the best-fitting model predicted star formation histories for them, we hope to derive a picture of the local environment on the evolution and star formation histories of galaxies and whether or not the isolated galaxies follow similar evolution history. Our results show that these three galaxies accumulated more than 50 percent of their stellar mass at $z\,<\,1$. It can be also found that the metallicity gradients in isolated spiral galaxies NGC\,2403 and NGC\,300 are similar and obviously steeper than that in M33, when the metallicity gradients are expressed in ${\rm dex}\,R_{\rm 25}^{-1}$. The similar metallicity gradients in NGC\,2403 and NGC\,300 indicate that they may experience similar chemical evolutionary histories. The principal epoch of star formation on the discs of NGC\,2403 and NGC\,300 is earlier than that on the disc of M33, and the mean age of stellar populations along the discs of both NGC\,2403 and NGC\,300 is older than that of M33. Our results indicates that the evolution and star formation history of a galaxy indeed depends on its local environment, at least for galaxies with stellar mass of $10^{9.2}\,\rm M_{\odot}\sim10^{9.7}\,\rm M_{\odot}$.

Apples to applesA2– II. Cluster selection functions for next-generation surveys

We present the cluster selection function for three of the largest next-generation stage-IV surveys in the optical and infrared: Euclid-Optimistic, Euclid-Pessimistic and the Large Synoptic Survey Telescope (LSST). To simulate these surveys, we use the realistic mock catalogues introduced in the first paper of this series. We detected galaxy clusters using the Bayesian Cluster Finder (BCF) in the mock catalogues. We then modeled and calibrated the total cluster stellar mass observable-theoretical mass ($M^*_{\rm CL}-M_{\rm h}$) relation using a power law model, including a possible redshift evolution term. We find a moderate scatter of $\sigma_{M^*_{\rm CL} | M_{\rm h}}$ of 0.124, 0.135 and 0.136 $\rm dex$ for Euclid-Optimistic, Euclid-Pessimistic and LSST, respectively, comparable to other work over more limited ranges of redshift. Moreover, the three datasets are consistent with negligible evolution with redshift, in agreement with observational and simulation results in the literature. We find that Euclid-Optimistic will be able to detect clusters with $>80\%$ completeness and purity down to $8\times10^{13} h^{-1} M_{\odot}$ up to $z<1$. At higher redshifts, the same completeness and purity are obtained with the larger mass threshold of $2\times10^{14} h^{-1} M_{\odot}$ up to $z=2$. The Euclid-Pessimistic selection function has a similar shape with $\sim10\%$ higher mass limit. LSST shows $\sim 5\%$ higher mass limit than Euclid-Optimistic up to $z<0.7$ and increases afterwards, reaching values of $2\times10^{14} h^{-1} M_{\odot}$ at $z=1.4$. Similar selection functions with only $80\%$ completeness threshold have been also computed. The complementarity of these results with selection functions for surveys in other bands is discussed.

Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals

We present metallicity gradients in 49 local field star-forming galaxies. We derive gas-phase oxygen abundances using two widely adopted metallicity calibrations based on the [O iii]/Hβ, [N ii]/Hα, and [N ii]/[O ii] line ratios. The two derived metallicity gradients are usually in good agreement within |$\pm 0.14\ {\rm dex} R_{25}^{-1}$| (R25 is the B-band iso-photoal radius), but the metallicity gradients can differ significantly when the ionization parameters change systematically with radius. We investigate the metallicity gradients as a function of stellar mass (⁠|$8 {<} \log (M_\ast /M_\odot) {<} 11$|⁠) and absolute B-band luminosity (−16 > MB > −22). When the metallicity gradients are expressed in dex kpc−1, we show that galaxies with lower mass and luminosity, on average, have steeper metallicity gradients. When the metallicity gradients are expressed in |${\rm dex} R_{25}^{-1}$|⁠, we find no correlation between the metallicity gradients, and stellar mass and luminosity. We provide a local benchmark metallicity gradient of field star-forming galaxies useful for comparison with studies at high redshifts. We investigate the origin of the local benchmark gradient using simple chemical evolution models and observed gas and stellar surface density profiles in nearby field spiral galaxies. Our models suggest that the local benchmark gradient is a direct result of the coevolution of gas and stellar disc under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes high (≫0.3). These models imply low current mass accretion rates ( ≲ 0.3 × SFR), and low-mass outflow rates ( ≲ 3 × SFR) in local field star-forming galaxies.

A NEW FUNDAMENTAL PLANE FOR RADIATIVELY EFFICIENT BLACK-HOLE SOURCES

In recent years, it was found that there are several low/hard state of X-ray binaries (XRBs) follow an `outliers' track of radio--X-ray correlation ($L_{\rm R}\propto L_{\rm X}^{b}$ and $b\sim1.4$), which is much steeper than the former universal track with $b\sim0.6$. In this work, we compile a sample of bright radio-quiet active galactic nuclei (AGNs) and find that their hard X-ray photon indices and Eddington ratios are positively correlated, which is similar to that of `outliers' of XRBs, where both bright AGNs and `outliers' of XRBs have bolometric Eddington ratios $\gtrsim1%L_{\rm Edd}$ ($L_{\rm Edd}$ is Eddington luminosity). The Eddington-scaled radio--X-ray correlation of these AGNs is also similar to that of `outliers' of XRBs, which has a form of $L_{\rm 5 GHz}/L_{\rm Edd}\propto (L_{\rm 2-10 keV}/L_{\rm Edd})^{c}$ with $c\simeq1.59$ and 1.53 for AGNs and XRBs respectively. Both the positively correlated X-ray spectral evolution and the steeper radio--X-ray correlation can be regulated by a radiatively efficient accretion flow (e.g., disk-corona). Based on these similarities, we further present a new fundamental plane for `outliers' of XRBs and bright AGNs in black-hole (BH) mass, radio and X-ray luminosity space: $\log L_{\rm R}=1.59^{+0.28}_{-0.22} \log L_{\rm X}- 0.22^{+0.19}_{-0.20}\log M_{\rm BH}-28.97^{+0.45}_{-0.45}$ with a scatter of $\sigma_{\rm R}=0.51\rm dex$. This fundamental plane is suitable for radiatively efficient BH sources, while the former plane proposed by Merloni et al. and Falcke et al. may be most suitable for radiatively inefficient sources.