Chemical abundances in active galaxies Flury, Sophia R; Moran, Edward C
Monthly notices of the Royal Astronomical Society,
08/2020, Volume:
496, Issue:
2
Journal Article
Peer reviewed
Open access
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.
ABSTRACT
Over the past half century, gas outflows and winds have been observed as asymmetric emission lines in a wide range of astrophysical contexts, including galaxies and early-type stars. While P ...Cygni lines are modeled and understood with physically motivated profiles under the Sobolev approximation, asymmetric nebular lines are not. Previous studies of galactic outflows using nebular emission lines have made physically unjustified assumptions about the shape of the line profile. These approaches limit assessment of outflow properties and do not connect observations to the underlying physics. The physical complexity of galactic outflows requires a more robust approach. In response to this need, we present a novel profile for modeling nebular emission lines which is generalized yet physically motivated and provides insight into the underlying mechanisms of galactic outflows. To demonstrate the usefulness of this profile, we fit it to the asymmetric nebular lines observed in the nuclear region of Mrk 462, a starburst-active galactic nucleus composite galaxy. From analysis of the best-fitting profile, we conclude that the observed profile arises from a dusty radiation-pressure-driven outflow with a terminal velocity of $750\rm ~km \, s^{-1}$. This outflow, while weak by some standards, is still sufficiently strong to regulate star formation and black hole growth in the host galaxy by removing gas from the inner few kiloparsecs. Outflows like the one we observe and characterize in Mrk 462 are crucial to our understanding of episodic gas-fueled activity in galactic nuclei, which undoubtedly plays a pivotal role in galaxy evolution.
Abstract
The Lyman continuum (LyC) cannot be observed at the epoch of reionization (
z
≳ 6) owing to intergalactic H
i
absorption. To identify LyC emitters (LCEs) and infer the fraction of escaping ...LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Ly
α
, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the O
iii
/O
ii
flux ratio and star formation rate surface density, predict whether a galaxy is an LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H
i
column density, ionization parameter, and stellar feedback. We find that LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.
Abstract
The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization ...is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a
z
= 0.2–0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.
Abstract
Star-forming galaxies are considered the likeliest source of the H
i
ionizing Lyman continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above
z
≳ 6, ...the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of
indirect
indicators for LyC that can be calibrated at lower redshifts and applied in the epoch of reionization. Emission from the Mg
ii
λλ
2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for eight LyC emitter candidates, selected to have strong Mg
ii
emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2
σ
significance. This high detection rate suggests that strong Mg
ii
emitters might be more likely to leak LyC than similar galaxies without strong Mg
ii
. Using photoionization models, we constrain the escape fraction of Mg
ii
as ∼15%–60%. We confirm that the escape fraction of Mg
ii
correlates tightly with that of Ly
α
, which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg
ii
emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg
ii
emission can be adopted to estimate the escape fraction of Ly
α
and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.
The geometry of the neutral gas in and around galaxies is a key regulator of the escape of ionizing photons. We present the first statistical study aimed at linking the neutral and ionized gas ...distributions to the Lyman continuum (LyC) escape fraction ( f esc LyC ) in a sample of 22 confirmed LyC leakers and non-leakers at z ≈ 0.35 using the Keck Cosmic Web Imager (Keck/KCWI) and the Low Resolution Spectrograph 2 (HET/LRS2). Our integral field unit data enable the detection of neutral and low-ionization gas, as traced by Mg II , and ionized gas, as traced by O II , extending beyond the stellar continuum for seven and ten objects, respectively. All but one object with extended Mg II emission also show extended O II emission; in this case, Mg II emission is always more extended than O II by a factor 1.2 on average. Most of the galaxies with extended emission are non or weak LyC leakers ( f esc LyC < 5%), but we find a large diversity of neutral and low-ionization gas configurations around these weakly LyC-emitting galaxies. Conversely, the strongest leakers ( f esc LyC > 5%) appear uniformly compact in both Mg II and O II with exponential scale lengths ≲1 kpc. Most are unresolved at the resolution of our data. We also find a trend between f esc LyC and the spatial offsets of the nebular gas and the stellar continuum emission. Moreover, we find significant anticorrelations between the spatial extent of the neutral and/or low-ionization gas and the O III /O II ratio, and H β equivalent width, as well as positive correlations with metallicity and UV size, suggesting that galaxies with more compact neutral and/or low-ionization gas sizes are more highly ionized. The observations suggest that strong LyC emitters do not have extended neutral and/or low-ionization gas halos and ionizing photons may be emitted in many directions. Combined with high ionization diagnostics, we propose that the Mg II , and potentially O II , spatial compactness are indirect indicators of LyC emitting galaxies at high redshift.
The Low-Redshift Lyman Continuum Survey Saldana-Lopez, Alberto; Schaerer, Daniel; Chisholm, John ...
Astronomy and astrophysics (Berlin),
07/2022, Volume:
663
Journal Article
Peer reviewed
Open access
Aims.
Combining 66 ultraviolet (UV) spectra and ancillary data from the recent Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of ...star-forming galaxies at
z
∼ 0.2 − 0.4 with which we study the role of cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We also aim to establish empirical relations between the H
I
neutral and low-ionization state (LIS) absorption lines with different galaxy properties.
Methods.
We first constrain the massive star content (stellar ages and metallicities) and UV attenuation by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow us to determine the absolute LyC photon escape fraction for each galaxy (
f
esc
abs
). We then measure the equivalent widths and residual fluxes of multiple H
I
and LIS lines, and the geometrical covering fraction of the UV emission, adopting the picket-fence model.
Results.
The LyC escape fraction spans a wide range, with a median
f
esc
abs
(0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1
σ
upper limits of
f
esc
abs
≲ 0.01 for non-detections, typically). The H
I
and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of these lines. Additionally, Ly
α
equivalent widths scale with both the H
I
and LIS residual fluxes, but anti-correlate with the corresponding H
I
or LIS equivalent widths. The H
I
and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the low-ionization transitions. We find that the observed trends of the absorption lines and the UV attenuation are primarily driven by the geometric covering fraction of the gas. The observed nonuniform gas coverage also demonstrates that LyC photons escape through low-column-density channels in the ISM. The equivalent widths and residual fluxes of both the H
I
and LIS lines strongly correlate with
f
esc
abs
: strong LyC leakers (highest
f
esc
abs
) show weak absorption lines, low UV attenuation, and large Ly
α
equivalent widths. We provide several empirical calibrations to estimate
f
esc
abs
from UV absorption lines. Finally, we show that simultaneous UV absorption line and dust attenuation measurements can, in general, predict the escape fraction of galaxies. We apply our method to available measurements of UV LIS lines of 15 star-forming galaxies at
z
∼ 4 − 6 (plus 3 high-
z
galaxy composites), finding that these high-redshift, UV-bright galaxies (
M
UV
≲ −21) may have low escape fractions,
f
esc
abs
≲ 0.1.
Conclusions.
UV absorption lines trace the cold ISM gas of galaxies, which governs the physics of the LyC escape. We show that, with some assumptions, the absolute LyC escape can be statistically predicted using UV absorption lines, and the method can be applied to study galaxies across a wide redshift range, including in the epoch of cosmic reionization.
Sources that leak Lyman continuum (LyC) photons and lead to the reionisation of the universe are an object of intense study using multiple observing facilities. Recently the Low-redshift LyC Survey ...(LzLCS) has presented the first large sample of LyC emitting galaxies at low redshift ($z 0.3$) with the Hubble Space Telescope Cosmic Origins Spectrograph . The LzLCS sample contains a robust estimate of the LyC escape fraction ( ) for 66 galaxies, spanning a wide range of values. Here, we aim to study the dependence of on the radio continuum (RC) properties of LzLCS sources. Overall, RC emission can provide unique insights into the role of supernova feedback, cosmic rays (CRs), and magnetic fields from its non-thermal emission component. RC emission is also a dust-free tracer of the star formation rate (SFR) in galaxies. In this study, we present Karl G. Jansky Very Large Array (VLA) RC observations of the LzLCS sources at gigahertz (GHz) frequencies. We performed VLA C (4-8 GHz) and S (2-4 GHz) band observations for a sample of 53 LzLCS sources. We also observed a sub-sample of 17 LzLCS sources in the L (1-2 GHz) band. We detected RC from both C- and S-bands in 24 sources for which we are able to estimate their radio spectral index across 3-6 GHz, denoted as . We also used the RC luminosity to estimate their SFRs. The radio spectral index of LzLCS sources spans a wide range, from flat ( $ -0.1$) to very steep ($ -1.0$). They have a steeper mean -0.92$) compared to that expected for normal star-forming galaxies ( -0.64$). They also show a larger scatter in (sim 0.71) compared to that of normal star-forming galaxies (sim 0.15). The strongest leakers in our sample show flat weak leakers have close to normal star-forming galaxies and non-leakers are characterized by steep . We argue that a combination of young ages, free-free absorption, and a flat cosmic-ray energy spectrum can altogether lead to a flat for strong leakers. Non-leakers are characterized by steep spectra which can arise due to break or cutoff at high frequencies. Such a cutoff in the spectrum can arise in a single injection model of CRs characteristic of galaxies which have recently stopped star-formation. The dependence of on (which is orientation-independent) suggests that the escape of LyC photons is not highly direction-dependent at least to the first order. The radio-based SFRs ( ) of LzLCS sources show a large offset ($ dex) from the standard calibration. We find that adding as a second parameter helps us to calibrate the with and within a scatter of $ dex. For the first time, we have found a relation between and . This hints at the interesting role of supernovae feedback, CRs, and magnetic fields in facilitating the escape (alternatively, and/or the lack) of LyC photons.
Over the past half century, gas outflows and winds have been observed as asymmetric emission lines in a wide range of astrophysical contexts, including galaxies and early-type stars. While P Cygni ...lines are modeled and understood with physically-motivated profiles under the Sobolev approximation, asymmetric nebular lines are not. Previous studies of galactic outflows using nebular emission lines have made physically unjustified assumptions about the shape of the line profile. These approaches limit assessment of outflow properties and do not connect observations to the underlying physics. The physical complexity of galactic outflows requires a more robust approach. In response to this need, we present a novel profile for modeling nebular emission lines which is generalized yet physically motivated and provides insight into the underlying mechanisms of galactic outflows. To demonstrate the usefulness of this profile, we fit it to the asymmetric nebular lines observed in the nuclear region of Mrk 462, a starburst-AGN composite galaxy. From analysis of the best-fit profile, we conclude that the observed profile arises from a dusty radiation-pressure-driven outflow with a terminal velocity of 750 km s-1. This outflow, while weak by some standards, is still sufficiently strong to regulate star formation and black hole growth in the host galaxy by removing gas from the inner few kiloparsecs. Outflows like the one we observe and characterize in Mrk 462 are crucial to our understanding of episodic gas-fueled activity in galactic nuclei, which undoubtedly plays a pivotal role in galaxy evolution.
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 (AGN) remains as of yet less explored in this capacity. To extend the rigorous study of \ion{H}{ii} regions in the SDSS to AGN, we adapt methods for computing direct-method chemical abundances for application to the narrow-line regions (NLR) of AGN. 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 AGN 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 AGN 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 AGN in the SDSS, and in the process have discovered an ionization-related discriminator for Seyfert and LINER galaxies.
PDF
![Linking Mg II and [O II] sp...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-6361/small "Linking Mg II and [O II] spatial distribution to ionizing photon escape in confirmed LyC leakers and non-leakers")
