scholarly journals Chemical abundances in active galaxies

2020 ◽  
Vol 496 (2) ◽  
pp. 2191-2203 ◽  
Author(s):  
Sophia R Flury ◽  
Edward C Moran
Keyword(s):  
Direct Method ◽  
Flux Ratio ◽  
Single Equation ◽  
Sloan Digital Sky Survey ◽  
H Ii Regions ◽  
Chemical Abundances ◽  
Temperature Relation ◽  
Oxygen Abundance ◽  
Star Forming ◽  
Liner Galaxies

ABSTRACT The Sloan Digital Sky Survey (SDSS) has proved to be a powerful resource for understanding the physical properties and chemical composition of star-forming galaxies in the local Universe. The SDSS population of active galactic nuclei (AGNs) remains as of yet less explored in this capacity. To extend the rigorous study of H ii regions in the SDSS to AGNs, we adapt methods for computing direct-method chemical abundances for application to the narrow-line regions (NLR) of AGNs. By accounting for triply ionized oxygen, we are able to more completely estimate the total oxygen abundance. We find a strong correlation between electron temperature and oxygen abundance due to collisional cooling by metals. Furthermore, we find that nitrogen and oxygen abundances in AGNs are strongly correlated. From the metal–temperature relation and the coupling of nitrogen and oxygen abundances, we develop a new, empirically and physically motivated method for determining chemical abundances from the strong emission lines commonly employed in flux-ratio diagnostic diagrams (BPT diagrams). Our approach, which for AGNs reduces to a single equation based on the BPT line ratios, consistently recovers direct-method abundances over a 1.5 dex range in oxygen abundance with an rms uncertainty of 0.18 dex. We have determined metallicities for thousands of AGNs in the SDSS, and in the process have discovered an ionization-related discriminator for Seyfert and LINER galaxies.

scholarly journals The lifecycle of molecular clouds in nearby star-forming disc galaxies

2019 ◽  
Vol 493 (2) ◽  
pp. 2872-2909 ◽  
Author(s):  
Mélanie Chevance ◽  
J M Diederik Kruijssen ◽  
Alexander P S Hygate ◽  
Andreas Schruba ◽  
Steven N Longmore ◽  
...  
Keyword(s):  
Star Formation ◽  
Flux Ratio ◽  
Molecular Gas ◽  
H Ii Regions ◽  
Dynamical Processes ◽  
Nearby Star ◽  
Star Forming ◽  
Spatially Resolved ◽  
Stellar Feedback ◽  
Disc Galaxies

ABSTRACT It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically $10\!-\!30\,{\rm Myr}$, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $\Sigma _{\rm H_2}\ge 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $\Sigma _{\rm H_2}\le 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just $1\!-\!5\,{\rm Myr}$ once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H ii regions are the fundamental units undergoing these lifecycles, with mean separations of $100\!-\!300\,{{\rm pc}}$ in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.

scholarly journals Stellar vs. Hii region chemical abundances in nearby galaxies

2009 ◽  
Vol 5 (S265) ◽  
pp. 233-236
Author(s):  
Fabio Bresolin
Keyword(s):  
Gas Phase ◽  
Spiral Galaxy ◽  
Direct Method ◽  
Chemical Abundances ◽  
Oxygen Abundance ◽  
Hii Region ◽  
Spectrophotometric Data ◽  
Gradient Based ◽  
Nearby Galaxies ◽  
First Time

AbstractWe have obtained new spectrophotometric data for 28 Hii regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines. The gradient corresponds to −0.077±0.006 dex kpc−1, which agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy. The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data. This result provides increased confidence on the direct method to determine extragalactic nebular abundances.

scholarly journals Bar effect on gas-phase abundance gradients. I. Data sample and chemical abundances

2020 ◽  
Vol 500 (2) ◽  
pp. 2359-2379 ◽  
Author(s):  
A Zurita ◽  
E Florido ◽  
F Bresolin ◽  
E Pérez-Montero ◽  
I Pérez
Keyword(s):  
Emission Line ◽  
Gas Phase ◽  
Direct Method ◽  
Structural Parameters ◽  
Companion Paper ◽  
Strong Line ◽  
H Ii Regions ◽  
Chemical Abundances ◽  
The Galaxy ◽  
H Ii Region

ABSTRACT Studies of gas-phase radial metallicity profiles in spirals published in the last decade have diminished the importance of galactic bars as agents that mix and flatten the profiles, contradicting results obtained in the 1990s. We have collected a large sample of 2831 published H ii region emission-line fluxes in 51 nearby galaxies, including objects both with and without the presence of a bar, with the aim of revisiting the issue of whether bars affect the radial metal distribution in spirals. In this first paper of a series of two, we present the galaxy and the H ii region samples. The methodology is homogeneous for the whole data sample and includes the derivation of H ii region chemical abundances, structural parameters of bars and discs, galactocentric distances, and radial abundance profiles. We have obtained O/H and N/O abundance ratios from the Te-based (direct) method for a subsample of 610 regions, and from a variety of strong-line methods for the whole H ii region sample. The strong-line methods have been evaluated in relation to the Te-based one from both a comparison of the derived O/H and N/O abundances for individual H ii regions and a comparison of the abundance gradients derived from both methodologies. The median value and the standard deviation of the gradient distributions depend on the abundance method, and those based on the O3N2 indicator tend to flatten the steepest profiles, reducing the range of observed gradients. A detailed analysis and discussion of the derived O/H and N/O radial abundance gradients and y-intercepts for barred and unbarred galaxies is presented in the companion Paper II. The whole H ii region catalogue including emission-line fluxes, positions, and derived abundances is made publicly available on the CDS VizieR facility, together with the radial abundance gradients for all galaxies.

scholarly journals Chemical abundances of Seyfert 2 AGNs – I. Comparing oxygen abundances from distinct methods using SDSS

2019 ◽  
Vol 492 (1) ◽  
pp. 468-479 ◽  
Author(s):  
O L Dors ◽  
P Freitas-Lemes ◽  
E B Amôres ◽  
E Pérez-Montero ◽  
M V Cardaci ◽  
...  
Keyword(s):  
Sloan Digital Sky Survey ◽  
Strong Line ◽  
Host Galaxies ◽  
Chemical Abundances ◽  
Oxygen Abundance ◽  
Line Intensities ◽  
Sky Survey ◽  
Direct Measurements ◽  
Low Metallicity ◽  
Semi Empirical

ABSTRACT We compare the oxygen abundance (O/H) of the narrow-line regions (NLRs) of Seyfert 2 AGNs obtained through strong-line methods and from direct measurements of the electron temperature (Te-method). The aim of this study is to explore the effects of the use of distinct methods on the range of metallicity and on the mass–metallicity relation of active galactic nuclei (AGNs) at low redshifts (z ≲ 0.4). We used the Sloan Digital Sky Survey (SDSS) and NASA/IPAC Extragalactic Database (NED) to selected optical (3000 < λ(Å) < 7000) emission line intensities of 463 confirmed Seyfert 2 AGNs. The oxygen abundances of the NLRs were estimated using the theoretical Storchi-Bergmann et al. calibrations, the semi-empirical N2O2 calibration, the Bayesian H  ii-Chi-mistry code and the Te-method. We found that the oxygen abundance estimations via the strong-line methods differ from each other up to ∼0.8 dex, with the largest discrepancies in the low-metallicity regime ($\rm 12+\log (O/H) \: \lesssim \: 8.5$). We confirmed that the Te-method underestimates the oxygen abundance in NLRs, producing unreal subsolar values. We did not find any correlation between the stellar mass of the host galaxies and the metallicity of their AGNs. This result is independent of the method used to estimate Z.

scholarly journals Azimuthal variations of oxygen abundance profiles in star-forming regions of disc galaxies in EAGLE simulations

2019 ◽  
Vol 491 (4) ◽  
pp. 4894-4901
Author(s):  
Martín Solar ◽  
Patricia B Tissera ◽  
Jose A Hernandez-Jimenez
Keyword(s):  
Stellar Populations ◽  
Chemical Abundances ◽  
Oxygen Abundance ◽  
Star Forming ◽  
Chemical Enrichment ◽  
Numerical Resolution ◽  
Star Forming Regions ◽  
Star Formation Activity ◽  
Large Dispersion ◽  
The Mean

ABSTRACT The exploration of the spatial distribution of chemical abundances in star-forming regions of galactic discs can help us to understand the complex interplay of physical processes that regulate the star formation activity and the chemical enrichment across a galaxy. We study the azimuthal variations of the normalized oxygen abundance profiles in the highest numerical resolution run of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) Project at $z$ = 0. We use young stellar populations to trace the abundances of star-forming regions. Oxygen profiles are estimated along different line of sights from a centrally located observer. The mean azimuthal variation in the EAGLE discs are ∼0.12 ± 0.03 dex $R_{\rm eff}^{-1}$ for slopes and ∼0.12 ± 0.03 dex for the zero-points, in agreement with previous works. Metallicity gradients measured along random directions correlate with those determined by averaging over the whole discs, although with a large dispersion. We find a slight trend for higher azimuthal variations in the disc components of low star-forming and bulge-dominated galaxies. We also investigate the metallicity profiles of stellar populations with higher and lower levels of enrichment than the average metallicity profiles, and we find that high star-forming regions with high metallicity tend to have slightly shallower metallicity slopes compared with the overall metallicity gradient. The simulated azimuthal variations in the EAGLE discs are in agreement with observations, although the large variety of metallicity gradients would encourage further exploration of the metal mixing in numerical simulations.

scholarly journals The Te[N ii]–Te[O iii] temperature relation in H ii regions and the reliability of strong-line methods

2020 ◽  
Vol 497 (1) ◽  
pp. 672-686
Author(s):  
K Z Arellano-Córdova ◽  
M Rodríguez
Keyword(s):  
High Resolution ◽  
Direct Method ◽  
Strong Line ◽  
Temperature Sensitive ◽  
H Ii Regions ◽  
Temperature Relation ◽  
Degree Of Ionization ◽  
Sensitive Line ◽  
The One

ABSTRACT We use a sample of 154 observations of 124 H ii regions that have measurements of both Te[O iii] and Te[N ii], compiled from the literature, to explore the behaviour of the Te[O iii]–Te[N ii] temperature relation. We confirm that the relation depends on the degree of ionization and present a new set of relations for two different ranges of this parameter. We study the effects introduced by our temperature relations and four other available relations in the calculation of oxygen and nitrogen abundances. We find that our relations improve slightly on the results obtained with the previous ones. We also use a sample of 26 deep, high-resolution spectra to estimate the contribution of blending to the intensity of the temperature-sensitive line [O iii] λ4363, and we derive a relation to correct Te[O iii] for this effect. With our sample of 154 spectra, we analyse the reliability of the R, S, O3N2, N2, ONS, and C strong-line methods by comparing the metallicity obtained with these methods with the one implied by the direct method. We find that the strong-line methods introduce differences that reach ∼0.2 dex or more, and that these differences depend on O/H, N/O, and the degree of ionization.

Direct measures of chemical abundances from stacked spectra of star-forming galaxies: Implications for the mass–metallicity–star formation rate relation

2019 ◽  
Vol 15 (S359) ◽  
pp. 424-426
Author(s):  
Slodkowski Katia Clerici ◽  
Natalia Vale Asari
Keyword(s):  
Star Formation ◽  
Star Formation Rate ◽  
Signal To Noise Ratio ◽  
Direct Method ◽  
Formation Rate ◽  
Mass Star ◽  
Chemical Abundances ◽  
Star Forming ◽  
Auroral Emission ◽  
Spectral Signal

AbstractThe stellar mass–star formation rate–metallicity relation provides clues on the chemical evolution of galaxies. We revisit this relation by measuring the gas-phase metallicity using the direct method. For metal-rich galaxies this is not straightforward, because auroral emission lines sensitive the electron temperature are lost in spectral noise. In order to increase the spectral signal-to-noise ratio and detect faint auroral lines, we stack the spectra of similar galaxies. This allows us to use the direct method to obtain consistent metallicity measurements.

scholarly journals Detecting H II Regions in “Pure” Starburst Galaxies with SDSS Data

2015 ◽  
Vol 11 (S315) ◽  
Author(s):  
Anthony Crider ◽  
Chris Richardson ◽  
Ben Kaiser
Keyword(s):  
Mean Field ◽  
Starburst Galaxies ◽  
Sloan Digital Sky Survey ◽  
Starburst Galaxy ◽  
H Ii Regions ◽  
Star Forming ◽  
Distant Galaxies ◽  
Field Independent ◽  
Galaxy Sample ◽  
The Impact

AbstractThe relationship between active galactic nuclei (AGN) and starburst galaxies is poorly understood, partially due to galaxies exhibiting both AGN and starburst activity. To better understand the connection, we analyze a sample of “pure” AGN or starburst at redshift z = 0.1 selected using mean field independent component analysis (MFICA). Simulations of starburst galaxy emission suggests that the locally optimally-emitting cloud (LOC) model can fit observations and improve our ability to distinguish the impact of differences in metallicity, ionization parameter, and ionizing flux. To test for the existence of such clouds in our galaxy sample, we examine the Sloan Digital Sky Survey (SDSS) images of our pure galaxies. At this distance, even large star-forming H II regions (e.g. 30 Doradus) only fill part of an SDSS pixel. However, we compare the morphology of the distant galaxies to more nearby ones (i.e. NGC 4713, NGC 4038/4039) to estimate the number of larger H II regions. While the clumpiness parameter of a galaxy in theory might indicate the existence of these regions, a straightforward calculation of the clumpiness parameter is ineffective for galaxies at z = 0.1. Typically, one subtracts a smoothed version of a galaxy image from the same image. We instead test a different approach to establish a smooth image and thus better identify the clumps. We subtract the smoother infrared z-band from the sharper ultraviolet u-band. We test this procedure using NGC 4713, a nearby starburst galaxy, artificially degraded to match images of our “pure” starburst galaxies.

scholarly journals Exploring chemical homogeneity in dwarf galaxies: a VLT-MUSE study of JKB 18

2020 ◽  
Vol 495 (3) ◽  
pp. 2564-2581 ◽  
Author(s):  
Bethan L James ◽  
Nimisha Kumari ◽  
Andrew Emerick ◽  
Sergey E Koposov ◽  
Kristen B W McQuinn ◽  
...  
Keyword(s):  
Galaxy Evolution ◽  
Large Scale ◽  
Dwarf Galaxies ◽  
Dwarf Galaxy ◽  
Direct Method ◽  
Mixing Time ◽  
Strong Line ◽  
H Ii Regions ◽  
Chemical Abundances ◽  
Chemical Homogeneity

ABSTRACT Deciphering the distribution of metals throughout galaxies is fundamental in our understanding of galaxy evolution. Nearby, low-metallicity, star-forming dwarf galaxies, in particular, can offer detailed insight into the metal-dependent processes that may have occurred within galaxies in the early Universe. Here, we present VLT/MUSE observations of one such system, JKB 18, a blue diffuse dwarf galaxy with a metallicity of only 12 + log(O/H)=7.6 ± 0.2 (∼0.08 Z⊙). Using high spatial resolution integral-field spectroscopy of the entire system, we calculate chemical abundances for individual H ii regions using the direct method and derive oxygen abundance maps using strong-line metallicity diagnostics. With large-scale dispersions in O/H, N/H, and N/O of ∼0.5–0.6 dex and regions harbouring chemical abundances outside this 1σ distribution, we deem JKB 18 to be chemically inhomogeneous. We explore this finding in the context of other chemically inhomogeneous dwarf galaxies and conclude that neither the accretion of metal-poor gas, short mixing time-scales or self-enrichment from Wolf–Rayet stars are accountable. Using a galaxy-scale, multiphase, hydrodynamical simulation of a low-mass dwarf galaxy, we find that chemical inhomogeneities of this level may be attributable to the removal of gas via supernovae and the specific timing of the observations with respect to star formation activity. This study not only draws attention to the fact that dwarf galaxies can be chemically inhomogeneous, but also that the methods used in the assessment of this characteristic can be subject to bias.

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