In this paper we treat the preionization problem in shocks over the velocity range 10 < vs < 1500 km s−1 in a self-consistent manner. We identify four distinct classes of solutions controlled by the ...value of the shock-precursor parameter, , where is the ionization parameter of the UV photons escaping upstream. This parameter determines both the temperature and the degree of ionization of the gas entering the shock. In increasing velocity, the shock solution regimes are cold neutral precursors (vs 40 km s−1), warm neutral precursors (40 vs 75 km s−1), warm partly ionized precursors (75 vs 120 km s−1), and fast shocks in which the preshock gas is in photoionization equilibrium and is fully ionized. The main effect of a magnetic field is to push these velocity ranges to higher values and to limit the postshock compression. In order to facilitate comparison with observations of shocks, we provide a number of convenient scaling relationships for parameters, such as postshock temperature, compression factors, cooling lengths, and Hβ and X-ray luminosity.
We study the direct gas-phase oxygen abundance using the well-detected auroral line O iiiλ4363 in the stacked spectra of a sample of local analogs of high-redshift galaxies. These local analogs share ...the same location as z ∼ 2 star-forming galaxies on the O iiiλ5007/Hβ versus N iiλ6584/H Baldwin-Phillips-Terlevich diagram. This type of analog has the same ionized interstellar medium (ISM) properties as high-redshift galaxies. We establish empirical metallicity calibrations between the direct gas-phase oxygen abundances ( ) and the N2 (log(N iiλ6584/H ))/O3N2 (log((O iiiλ5007/Hβ)/(N iiλ6584/H ))) indices in our local analogs. We find significant systematic offsets between the metallicity calibrations for our local analogs of high-redshift galaxies and those derived from the local H ii regions and a sample of local reference galaxies selected from the Sloan Digital Sky Survey (SDSS). The N2 and O3N2 metallicities will be underestimated by 0.05-0.1 dex relative to our calibration, if one simply applies the local metallicity calibration in previous studies to high-redshift galaxies. Local metallicity calibrations also cause discrepancies of metallicity measurements in high-redshift galaxies using the N2 and O3N2 indicators. In contrast, our new calibrations produce consistent metallicities between these two indicators. We also derive metallicity calibrations for R23 (log((O iiiλλ4959,5007+O iiλλ3726,3729)/Hβ)), O32(log(O iiiλλ4959,5007/O iiλλ3726,3729)), O iiiλ5007/Hβ), and log(Ne iiiλ3869/O iiλ3727) indices in our local analogs, which show significant offset compared to those in the SDSS reference galaxies. By comparing with MAPPINGS photoionization models, the different empirical metallicity calibration relations in the local analogs and the SDSS reference galaxies can be shown to be primarily due to the change of ionized ISM conditions. Assuming that temperature structure variations are minimal and ISM conditions do not change dramatically from z ∼ 2 to z ∼ 5, these empirical calibrations can be used to measure relative metallicities in galaxies with redshifts up to z ∼ 5.0 in ground-based observations.
We present a new method for inferring the metallicity (Z) and ionization parameter (q) of H II regions and star-forming galaxies using strong nebular emission lines (SELs). We use Bayesian inference ...to derive the joint and marginalized posterior probability density functions for Z and q given a set of observed line fluxes and an input photoionization model. Our approach allows the use of arbitrary sets of SELs and the inclusion of flux upper limits. The method provides a self-consistent way of determining the physical conditions of ionized nebulae that is not tied to the arbitrary choice of a particular SEL diagnostic and uses all the available information. Unlike theoretically calibrated SEL diagnostics, the method is flexible and not tied to a particular photoionization model. We describe our algorithm, validate it against other methods, and present a tool that implements it called IZI. Using a sample of nearby extragalactic H II regions, we assess the performance of commonly used SEL abundance diagnostics. We also use a sample of 22 local H II regions having both direct and recombination line (RL) oxygen abundance measurements in the literature to study discrepancies in the abundance scale between different methods. We find that oxygen abundances derived through Bayesian inference using currently available photoionization models in the literature can be in good (~30%) agreement with RL abundances, although some models perform significantly better than others. We also confirm that abundances measured using the direct method are typically ~0.2 dex lower than both RL and photoionization-model-based abundances.
We derive new self-consistent theoretical UV, optical, and IR diagnostics for the interstellar medium (ISM) pressure and electron density in the ionized nebulae of star-forming galaxies. Our UV ...diagnostics utilize the intercombination, forbidden, and resonance lines of silicon, carbon, aluminum, neon, and nitrogen. We also calibrate the optical and IR forbidden lines of oxygen, argon, nitrogen, and sulfur. We show that line ratios used as ISM pressure diagnostics depend on the gas-phase metallicity with a residual dependence on the ionization parameter of the gas. In addition, the traditional electron density diagnostic S ii λ6731/S ii λ6717 is strongly dependent on the gas-phase metallicity. We show how different emission-line ratios are produced in different ionization zones in our theoretical nebulae. The S ii and O ii ratios are produced in different zones and should not be used interchangeably to measure the electron density of the gas unless the electron temperature is known to be constant. We review the temperature and density distributions observed within H ii regions and discuss the implications of these distributions on measuring the electron density of the gas. Many H ii regions contain radial variations in density. We suggest that the ISM pressure is a more meaningful quantity to measure in H ii regions or galaxies. Specific combinations of line ratios can cover the full range of ISM pressures (4 < log(P/k) < 9). As H ii regions become resolved at increasingly high redshift through the next generation of telescopes, we anticipate that these diagnostics will be important for understanding the conditions around the young, hot stars from the early universe to the present day.
ABSTRACT
We study the physical mechanisms that cause the offset between low-redshift and high-redshift galaxies on the O iii λ5007/H β versus N ii λ6584/H α ‘Baldwin, Phillips & Terlevich’ (BPT) ...diagram using a sample of local analogues of high-redshift galaxies. These high-redshift analogue galaxies are selected from the Sloan Digital Sky Survey. Located in the same region on the BPT diagram as the ultraviolet selected galaxies at z ∼ 2, these high-redshift analogue galaxies provide an ideal local benchmark to study the offset between the local and high-redshift galaxies on the BPT diagram. We compare the nitrogen-to-oxygen ratio (N/O), the shape of the ionizing radiation field, and ionization parameters between the high-redshift analogues and a sample of local reference galaxies. The higher ionization parameter in the high-redshift analogues is the dominant physical mechanism driving the BPT offset from low- to high-redshift, particularly at high N ii λ6584/H α. Furthermore, the N/O ratio enhancement also plays a minor role to cause the BPT offset. However, the shape of the ionizing radiation field is unlikely to cause the BPT offset because the high-redshift analogues have a similar hard ionizing radiation field as local reference galaxies. This hard radiation field cannot be produced by the current standard stellar synthesis models. The stellar rotation and binarity may help solve the discrepancy.
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
$\pm 0.14\ {\rm dex} R_{25}^{-1}$
(
R
25 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 > M
B
> −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.
This
Letter
presents a new, remarkably simple diagnostic specifically designed to derive chemical abundances for high redshift galaxies. It uses only the
H
α
, N
ii
and S
ii
emission lines, which ...can usually be observed in a single grating setting, and is almost linear up to an abundance of
12
+
log
(
O
/
H
)
=
9.05
. It can be used over the full abundance range encountered in high redshift galaxies. By its use of emission lines located close together in wavelength, it is also independent of reddening. Our diagnostic depends critically on the calibration of the N/O ratio. However, by using realistic stellar atmospheres combined with the N/O vs. O/H abundance calibration derived locally from stars and H
ii
regions, and allowing for the fact that high-redshift H
ii
regions have both high ionisation parameters
and
high gas pressures, we find that the observations of high-redshift galaxies can be simply explained by the models without having to invoke arbitrary changes in N/O ratio, or the presence of unusual quantities of Wolf-Rayet stars in these galaxies.
ABSTRACT We present a sample of local analogs for high-redshift galaxies selected in the Sloan Digital Sky Survey (SDSS). The physical conditions of the interstellar medium (ISM) in these local ...analogs resemble those in high-redshift galaxies. These galaxies are selected based on their positions in the O iii/Hβ versus N ii/H nebular emission-line diagnostic diagram. We show that these local analogs share similar physical properties with high-redshift galaxies, including high specific star formation rates (sSFRs), flat UV continuums, and compact galaxy sizes. In particular, the ionization parameters and electron densities in these analogs are comparable to those in z 2-3 galaxies, but higher than those in normal SDSS galaxies by 0.6 dex and 0.9 dex, respectively. The mass-metallicity relation (MZR) in these local analogs shows −0.2 dex offset from that in SDSS star-forming galaxies at the low-mass end, which is consistent with the MZR of the galaxies. We compare the local analogs in this study with those in other studies, including Lyman break analogs (LBA) and green pea (GP) galaxies. The analogs in this study share a similar star formation surface density with LBAs, but the ionization parameters and electron density in our analogs are higher than those in LBAs by factors of 1.5 and 3, respectively. The analogs in this study have comparable ionization parameters and electron densities to the GP galaxies, but our method can select galaxies in a wider redshift range. We find the high sSFR and SFR surface density can increase the electron density and ionization parameters, but still cannot fully explain the difference in ISM condition between nearby galaxies and the local analogs/high-redshift galaxies.
The optical spectra of Seyfert galaxies are often dominated by emission lines excited by both star formation and active galactic nucleus (AGN) activity. Standard calibrations (such as for the star ...formation rate) are not applicable to such composite (mixed) spectra. In this paper, we describe how integral field data can be used to spectrally and spatially separate emission associated with star formation from emission associated with accretion on to an AGN. We demonstrate our method using integral field data for two AGN host galaxies (NGC 5728 and NGC 7679) from the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7). The spectra of NGC 5728 and NGC 7679 form clear sequences of AGN fraction on standard emission line ratio diagnostic diagrams. We show that the emission line luminosities of the majority (>85 per cent) of spectra along each AGN fraction sequence can be reproduced by linear superpositions of the emission line luminosities of one AGN dominated spectrum and one star formation dominated spectrum. We separate the Hα, Hβ, N iiλ6583, S iiλλ6716, 6731, O iiiλ5007 and O iiλλ3726, 3729 luminosities of every spaxel into contributions from star formation and AGN activity. The decomposed emission line images are used to derive the star formation rates and AGN bolometric luminosities for NGC 5728 and NGC 7679. Our calculated values are mostly consistent with independent estimates from data at other wavelengths. The recovered star-forming and AGN components also have distinct spatial distributions which trace structures seen in high-resolution imaging of the galaxies, providing independent confirmation that our decomposition has been successful.
In an earlier paper we treated the preionization problem in shocks over the velocity range 20 km s−1 km s−1 in a fully self-consistent manner. Here we investigate in detail the effect of the upstream ...UV photon field generated in the radiative zone of shocks in the range in which hydrogen is only partly ionized 20 km s−1 ( km s−1). We show that, as a result of superheating in the nonequilibrium preshock plasma, both the magnetic parameter and the Mach number of the shock are strongly affected by the preionization state of the gas, which controls to a large extent the radiative spectrum of the shock. We use these models to provide specific line diagnostics for Herbig-Haro objects, which allow us to solve for both the preshock density and shock velocity, and we present detailed models of the HH 34 jet, which allows us to derive the shock conditions, mass-loss rate, momentum flux, and chemical abundances in the jet. We show that the refractory elements Mg, Ca, Fe, and Ni are enhanced by 0.22 dex over the solar values, which provides interesting clues about the jet-launching mechanism in pre-main-sequence evolution.
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