Balmer Line Ratios Research Articles

Testing He ii Emission from Wolf–Rayet Stars as a Dust Attenuation Measure in Eight Nearby Star-forming Galaxies

The ability to determine galaxy properties such as masses, ages, and star formation rates robustly is critically limited by the ability to measure dust attenuation accurately. Dust reddening is often characterized by comparing observations to models of either nebular recombination lines or the UV continuum. Here, we use a new technique to measure dust reddening by exploiting the He ii λ1640 and λ4686 emission lines originating from the stellar winds of Wolf–Rayet stars. The intrinsic line ratio is determined by atomic physics, enabling an estimate of the stellar reddening similar to how the Balmer lines probe gas-emission reddening. The He ii line ratio is measured from UV and optical spectroscopy using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope for eight nearby galaxies hosting young massive star clusters. We compare our results to dust reddening values estimated from UV spectral slopes and from Balmer line ratios and find tentative evidence for systematic differences. The reddening derived from the He ii lines tends to be higher, whereas that from the UV continuum tends to be lower. A larger sample size is needed to confirm this trend. If confirmed, this may indicate an age sequence probing different stages of dust clearing. Broad He ii lines have also been detected in galaxies more distant than in our sample, providing the opportunity to estimate the dust reddening of the youngest stellar populations out to distances of ∼100 Mpc.

Paschen-line Constraints on Dust Attenuation and Star Formation at z ∼ 1–3 with JWST/NIRSpec

We use medium-resolution JWST/NIRSpec observations from the Cosmic Evolution Early Release Science Survey to place the first constraints on dust attenuation and star formation based on Paschen lines for a sizable sample of 63 galaxies at redshifts z = 1.0–3.1. Our analysis indicates strong correlations between the Balmer decrement, Hα/Hβ, and line ratios that include Paschen lines (i.e., Paα/Hβ, Paβ/Hβ, and the Paschen decrement, Paα/Paβ), suggesting that the former is sensitive to the overall dust obscuration toward H ii regions in high-redshift galaxies. The line ratios are used to derive nebular reddening, E(B − V)neb, and star formation rates (SFRs). There is marginal evidence that the SFRs deduced from Paschen lines may exceed by ≈25% those derived from Balmer lines alone, suggesting the presence of star formation that is optically thick in Balmer lines, though deeper observations are needed to confirm this result. Using the Paschen-line constraints on the bolometric SFRs, we reevaluate the relationship between dust obscuration and UV spectral slope, and find a reddening of the UV continuum that, on average, follows the SMC extinction curve. This analysis highlights the need for deeper spectroscopy of more representative samples to evaluate nebular dust attenuation and bolometric SFRs in high-redshift galaxies, and their relationship to the reddening of the UV continuum.

Characterisation of detachment in the MAST-U Super-X divertor using multi-wavelength imaging of 2D atomic and molecular emission processes

In this work, we provide the first 2D spatially resolved description of radiative detachment in MAST-U Super-X L-mode divertor plasmas. The Super-X magnetic configuration was designed to achieve reduced heat- and particle loads at the divertor target compared to conventional exhaust solutions. We use filtered camera imaging to reconstruct 2D emissivity profiles in the poloidal plane for multiple atomic and molecular emission lines and bands. A set of deuterium fuelling scans is discussed that, together, span attached to deeply detached divertor states observed in MAST-U. Emissivity profiles facilitate separate analysis of locked-mode induced split branches of the scrape-off layer. Molecular deuterium Fulcher band emission front tracking reveals that the deuterium electron-impact ionisation front, for which it serves a proxy, detaches at different upstream electron densities in the split branches. Upon detachment of this ionisation front, Balmer emission attributed to molecular activated recombination appears near-target. We report a simultaneous radial broadening of the emission leg, consistent with previous SOLPS-ITER modelling. With increased fuelling this emission region detaches, implying electron temperatures below eV. In this phase, 2D Balmer line ratio reconstruction indicates an onset of volumetric direct electron-ion recombination near-target. At the highest fuelling rates this emission region moves off-target, suggesting a drop in near-wall electron density accompanying the low temperatures.

Radiation characteristics of detached divertor plasmas in W7-X

• Spatial distributions of line emissions from hydrocarbon and carbon have been measured in detached divertor plasma in W7-X. • Spatial distributions of line emissions from hydrogen molecules and atoms have been measured in detached divertor plasma in W7-X. • Hydrogen Balmer line ratios indicate that the detached plasma is in excitation or partial recombination state. • Electron pressure along flux tube in SOL drops significantly but not deeper due to the lack of volume recombination in detached divertor plasma. Detached divertor plasmas has been achieved by density ramp in W7-X. During the transition into detachment, the main divertor radiation region moves from the region close to the divertor targets to the region around LCFS. The typical spatial distributions of carbon and hydrogen Balmer lines in detached divertor plasma are presented. They show that the line emissions of CH produced by chemical sputtering is still strong close to the targets. The C I fills up the whole detached divertor plasma and its intensity increases when approaching the targets due to the dissociation from hydrocarbon and self-sputtering. The C II and C III are mainly located at around LCFS region. Meanwhile, the distribution of hydrogen Balmer lines is considerably flatter in detached divertor plasma. The Balmer line ratios indicate that the detached divertor plasma seems to be still in excitation or partial recombination state. This is consistent with the divertor Langmuir probe measurements which shows that the electron temperature near strike line is around 6 eV. Finally, the electron pressure at upstream and downstream position is measured and its ratio shows that the electron pressure drops significantly along the magnetic field lines. However, the pressure can not drop further due to the lack of the volume recombination in detached divertor plasma.

First insights into the ISM at z > 8 with JWST: possible physical implications of a high [O iii] λ4363/[O iii] λ5007

ABSTRACT We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at z > 7.5. The galaxies were identified in the James Webb Space Telescope (JWST) Early Release Observations field SMACS J0723.3 − 7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift ‘analogue’ populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at z > 7.5. We find that S06355 at z = 7.67 and S10612 at z = 7.66 are similar to local Green Peas and Blueberries. In contrast, S04590 at z = 8.50 appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O iii] λ4363/[O iii] λ5007 ratio (RO3) of 0.048 (or 0.055 when dust-corrected), implying either extremely high electron temperatures, >3 × 104 K, or gas densities >104 cm−3. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterizing the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.

Optical emission lines in the most massive galaxies: Morphology, kinematics, and ionisation properties

In order to better characterise the upper end of the galaxy stellar mass range, the MUSE Most Massive Galaxies (M3G) Survey targeted the most massive galaxies (M > 1012 M⊙) found in the densest known clusters of galaxies at z ∼ 0.046. The sample is composed of 25 early-type galaxies: 14 brightest cluster galaxies (BCGs), of which 3 are in the densest region of the Shapley super cluster (SSC), and 11 massive satellites in the SSC. In the present work we use MUSE data to derive the spatial distribution and kinematics of the gas and discuss its ionisation mechanism and origin in the optical wavelength range. We fit the continuum of the spectra using an extensive library of single stellar population models and model the emission lines employing up to three Gaussian functions. In the M3G sample, ionised gas was detected in five BCGs, of which one is in the densest region of the SSC, and six massive satellites in the SSC. Among these objects, [O I] and [N I] were detected in three BCGs and one satellite. The gas is centrally concentrated in almost all objects, except for two BCGs that show filaments and two massive satellites with extended emission. Moreover, the emission line profiles of three BCGs present redshifted and/or blueshifted components. The presence of dust was revealed by analysing Balmer line ratios, obtaining a mean E(B − V) of 0.2–0.3. The emission-line diagnostic diagrams show predominately low-ionisation nuclear emission-line regions line ratios with little contamination from star formation. In the M3G sample, the gas was detected in 80% of fast rotators and 35% of slow rotators. The orientations of stellar and gaseous rotations are aligned with respect to each other for 60% of satellites and 25% of BCGs. The presence of misalignments points to an external origin of the gas for three BCGs and two satellites. On the other hand, some of these systems are characterised by triaxial and prolate-like stellar rotation, which could support an internal origin of the gas even in the case of misalignments.

Estimation of 2D distributions of electron density and temperature in the JET divertor from tomographic reconstructions of deuterium Balmer line emission

• 2D n e and T e estimates obtained from deuterium Balmer line ratios in JET divertor. • High n e observed to shift from strike point to above X-point in L-mode detachment. • Divertor T e observed to fall from 3—4 eV to 0.5—1.0 eV in L-mode detachment. • 2D estimates within 25% agreement with other spectroscopic n e and T e measurements. Estimates for 2D distributions of electron density, n e , and temperature, T e , have been obtained using reflection-corrected tomographic reconstructions of deuterium Balmer line emission in the JET divertor. The solutions are obtained by a two-dimensional Monte Carlo optimization model, based on intensity ratios of the Balmer D α , D γ and D ε emission and the ADAS photon emission coefficients with additional constraints provided by line-integrated spectroscopic measurements of n e and T e . Synthetic benchmarks utilizing EDGE2D-EIRENE simulation outputs yield reproduction of reference n e and T e distributions within 25% with statistical error estimates of 5–15%. Experimental analysis of an L-mode n e ramp shows extension and movement of the outer divertor high-n e region with n e,div up to 1.5 × 10 20 m −3 from the outer strike point to above the X-point and decrease of the divertor T e to the range of 0.5–1.0 eV within 25% agreement with the line-integrated measurements. Comparison between the local estimates and their corresponding line integrals suggests underestimation of n e and T e by up to 75% and 50%, respectively, due to the geometric effects of line integration.

What makes red quasars red?

Red quasars are very red in the optical through near-infrared (NIR) wavelengths, which is possibly due to dust extinction in their host galaxies as expected in a scenario in which red quasars are an intermediate population between merger-driven star-forming galaxies and unobscured type 1 quasars. However, alternative mechanisms also exist to explain their red colors: (i) an intrinsically red continuum; (ii) an unusual high covering factor of the hot dust component, that is, CFHD=LHD∕Lbol, where theLHDis the luminosity from the hot dust component and theLbolis the bolometric luminosity; and (iii) a moderate viewing angle. In order to investigate why red quasars are red, we studied optical and NIR spectra of 20 red quasars atz~ 0.3 and 0.7, where the usage of the NIR spectra allowed us to look into red quasar properties in ways that are little affected by dust extinction. The Paschen to Balmer line ratios were derived for 13 red quasars and the values were found to be ~10 times higher than unobscured type 1 quasars, suggesting a heavy dust extinction withAV> 2.5 mag. Furthermore, the Paschen to Balmer line ratios of red quasars are difficult to explain with plausible physical conditions without adopting the concept of the dust extinction. The CFHDof red quasars are similar to, or marginally higher than, those of unobscured type 1 quasars. The Eddington ratios, computed for 19 out of 20 red quasars, are higher than those of unobscured type 1 quasars (by factors of 3−5), and hence the moderate viewing angle scenario is disfavored. Consequently, these results strongly suggest the dust extinction that is connected to an enhanced nuclear activity as the origin of the red color of red quasars, which is consistent with the merger-driven quasar evolution scenario.

Development of a novel surface assisted volume negative hydrogen ion source

H− ion based neutral beam injector is a critical heating and current drive system in a fusion reactor. However, the present H− ion source configuration has limitations in terms of production, extraction, cesium (Cs) inventory and management. To overcome these limitations, a proof-of-principle experiment based on a novel concept regarding surface assisted volume H− ions production by sprinkling Cs coated tungsten (W) dust grains (low work function surface) into a hydrogen plasma is carried out. Four different diagnostics have been used to validate the concept. The H− ion fraction is estimated from (a) Langmuir probe diagnostic, (b) phase velocity of ion acoustic waves, (c) dust current and confirmed by the measurement of (d) Balmer line ratio. The measured H− ion fraction with respect to the plasma density for different discharge conditions varies from ~0.2 to 0.3 in presence of Cs coated W dust particles. The experimental results show good agreement with the theoretical estimation.

EXTINCTION AND NEBULAR LINE PROPERTIES OF AHERSCHEL-SELECTED LENSED DUSTY STARBURST ATz= 1.027

We present Hubble Space Telescope (HST) WFC3 imaging and grism spectroscopy observations of the {\it Herschel}-selected gravitationally-lensed starburst galaxy HATLASJ1429-0028. The lensing system consists of an edge-on foreground disk galaxy at $z=0.218$ with a nearly complete Einstein ring of the infrared luminous galaxy at $z=1.027$. The WFC3 spectroscopy with G102 and G141 grisms, covering the wavelength range of 0.8 to 1.7 $\mu$m, resulted in detections of H$\alpha$+[NII], H$\beta$, [SII], and [OIII] for the background galaxy from which we measure line fluxes and ratios. The Balmer line ratio H$\alpha$/H$\beta$ of $7.5 \pm 4.4$, when corrected for [NII], results in an extinction for the starburst galaxy of $E(B-V)=0.8 \pm 0.5$. The $H\alpha$ based star-formation rate, when corrected for extinction, is $60 \pm 50$ M$_{\odot}$ yr$^{-1}$, lower than the instantaneous star-formation rate of 390 $\pm$ 90 M$_{\odot}$ yr$^{-1}$ from the total IR luminosity. We also compare the nebular line ratios of HATLASJ1429-0028 with other star-forming and sub-mm bright galaxies. The nebular line ratios are consistent with an intrinsic ultra-luminous infrared galaxy with no evidence for excitation by an active galactic nucleus (AGN). We estimate the metallicity, $12 + log(O/H)$, of HATLASJ1429-0028 to be 8.49 $\pm$ 0.16. Such a low value is below the average relations for stellar mass vs. metallicity of galaxies at $z \sim 1$ for a galaxy with stellar mass of $\sim 2 \times 10^{11}$ M$_{\odot}$. The combination of high stellar mass, lack of AGN indicators, low metallicity, and the high star-formation rate of HATLASJ1429-0028 suggest that this galaxy is currently undergoing a rapid formation.

Using the Radiation of Hydrogen Atoms and Molecules to Determine Electron Density and Temperature in the Linear Plasma Device PSI-2

In this work we investigate the applicability of several optical emission spectroscopy methods to measure the electron density and temperature in deuterium plasma in the linear plasma generator PSI-2. The spectroscopy measurements are realized by an imaging spectrometer which delivers radial profiles of the emission lines. With the application of an inverse Abel transformation, spatially resolved measurements are obtained.The spectroscopy methods divide into two groups: The measurement of ne by Balmer line ratios and by the rotational temperature of molecules is only suitable for ionizing plasmas; the measurement of ne by the Stark broadening of Paschen lines and of Te by Paschen line ratios is only applicable for recombining plasmas.For the evaluation of these methods, different plasma conditions are produced in PSI-2. The plasma generator is capable of producing deuterium plasmas with electron densities of up to 1013 cm-3 and electron temperatures of up to 20 eV. Additional measurements with a Langmuir double probe are conducted for comparison with the spectroscopy measurements.A collisional-radiative model in the Yacora code is used to compare measured Balmer line emissions with the calculation and to investigate which reaction channels influence the recombination in PSI-2.

The analysis of the broad hydrogen Balmer line ratios: Possible implications for the physical properties of the broad line region of AGNs

We analyze the ratios of the broad hydrogen Balmer emission lines (from H\alpha to H\epsilon) in the context of estimating the physical conditions in the broad line region (BLR) of active galactic nuclei (AGN). The Balmer emission lines are obtained in three ways: i) using photoionization models obtained by a spectral synthesis code CLOUDY; ii) calculated using the recombination theory for hydrogenic ions; iii) measured from the sample of observed spectra taken from the Sloan Digital Sky Survey database. We investigate the Balmer line ratios in the frame of the so called Boltzmann-plot (BP), analyzing physical conditions of the emitting plasma for which we could use the BP method. The BP considers the ratio of Balmer lines normalized to the atomic data of the corresponding line transition, and is in that way different from the Balmer decrement. We found that for a certain range of thermodynamic parameters, there are objects that follow the BP. These AGN may have a BLR consisting of mostly high density plasma.

SPECTROPHOTOMETRY OF THE HUYGENS REGION OF THE ORION NEBULA, THE EXTENDED ORION NEBULA, AND M 43: SCATTERED LIGHT SYSTEMATICALLY DISTORTS CONDITIONS DERIVED FROM EMISSION LINES

We report on medium resolution spectrophotometry of the Orion Nebula region, including for the first time the Extended Orion Nebula and the nearby M~43. The 49 long slit observations were divided into 99 smaller samples, which have allowed determinations of the amount of extinction, extinction corrected \Hbeta\ surface brightness, electron temperatures (from [S~II], [N~II], and [O~III]), and electron densities (from [S~II] and [Cl~III]) throughout much of this complex region. We verify an earlier conclusion from a radio/optical study that beyond about 5\arcmin\ from \ori\ local emission begins to be contaminated by scattering of light from the much brighter central Huygens Region of M~42 and this scattered light component becomes dominant at large distances. This contamination means that the derived properties for the outer regions are not accurate. From comparison of the light from the dominant star in M~43 with the continuum of that nebula (which is almost entirely scattered star light) it is determined that scattered light is enhanced in the blue, which can lead to observed Balmer line ratios that are theoretically impossible and erroneous electron temperatures. This blue scattering of emission-lines is important even in the Huygens Region because it means that at anything except very high spectroscopic resolution the observed lines are a blend of the original and scattered light, with shorter wavelength lines being artificially enhanced. This can lead to over-estimates of the electron temperatures derived from the nebular and auroral line ratios of forbidden lines. This phenomenon is probably applicable to many other H~II regions.

X-shooter observations of the gravitational lens system CASSOWARY 5★

We confirm an eighth gravitational lens system in the CASSOWARY catalogue. Exploratory observations with the X-shooter spectrograph on the VLT show the system CSWA5 to consist of at least three images of a blue star-forming galaxy at z = 1.0686, lensed by an apparent foreground group of red galaxies one of which is at z = 0.3877. The lensed galaxy exhibits a rich spectrum with broad interstellar absorption lines and a wealth of nebular emission lines. Preliminary analysis of these features shows the galaxy to be young, with an age of 25-50 Myr. With a star-formation rate of approximately 20 solar masses/yr, the galaxy has already assembled a stellar mass of 3 x 10^9 solar masses and reached half-solar metallicity. Its blue spectral energy distribution and Balmer line ratios suggest negligible internal dust extinction. A more in-depth analysis of the properties of this system is currently hampered by the lack of a viable lensing model. However, it is already clear that CSWA5 shares many of its physical characteristics with the general population of UV-selected galaxies at redshifts z = 1-3, motivating further study of both the source and the foreground mass concentration responsible for the gravitational lensing.

LOW-REDSHIFT Lyα SELECTED GALAXIES FROMGALEXSPECTROSCOPY: A COMPARISON WITH BOTH UV-CONTINUUM SELECTED GALAXIES AND HIGH-REDSHIFT Lyα EMITTERS,,

We construct a sample of low-redshift Ly-alpha emission-line selected sources from GALEX grism spectroscopy of nine deep fields to study the role of Ly-alpha emission in galaxy populations with cosmic time. Our final sample consists of 119 (141) sources selected in the redshift interval z=0.195-0.44 (z=0.65-1.25) from the FUV (NUV) channel. We classify the Ly-alpha sources as AGNs if high-ionization emission lines are present in their UV spectra and as possible star-forming galaxies otherwise. We classify additional sources as AGNs using line widths for our Ly-alpha emitter (LAE) analysis. These classifications are broadly supported by comparisons with X-ray and optical spectroscopic observations. Defining the GALEX LAE sample in the same way as high-redshift LAE samples, we show that LAEs constitute only about 5% of NUV-continuum selected galaxies at z~0.3. We also show that they are less common at z~0.3 than they are at z~3. We find that the z~0.3 optically-confirmed Ly-alpha galaxies lie below the metallicity-luminosity relation of the z~0.3 NUV-continuum selected galaxies but have similar H-alpha velocity widths at similar luminosities, suggesting that they also lie below the metallicity-mass relation of the NUV-continuum selected galaxies. We show that, on average, the Ly-alpha galaxies have bluer colors, lower extinctions as measured from the Balmer line ratios, and more compact morphologies than the NUV-continuum selected galaxies. Finally, we confirm that the z~2 Lyman break galaxies (LBGs) have relatively low metallicities for their luminosities, and we find that they lie in the same metallicity range as the z~0.3 Ly-alpha galaxies.

PLANETARY NEBULAE IN FACE-ON SPIRAL GALAXIES. II. PLANETARY NEBULA SPECTROSCOPY

As the second step in our investigation of the mass-to-light ratio of spiral disks, we present the results of a spectroscopic survey of planetary nebulae (PNe) in five nearby, low-inclination galaxies: IC 342, M74 (NGC 628), M83 (NGC 5236), M94 (NGC 4736), and M101 (NGC 5457). Using 50 setups of the WIYN/Hydra and Blanco/Hydra spectrographs, and 25 observations with the Hobby-Eberly Telescope's Medium Resolution Spectrograph, we determine the radial velocities of 99, 102, 162, 127, and 48 PNe, respectively, to a precision better than 15 km/s. Although the main purpose of this data set is to facilitate dynamical mass measurements throughout the inner and outer disks of large spiral galaxies, our spectroscopy has other uses as well. Here, we co-add these spectra to show that to first order, the [O III] and Balmer line ratios of planetary nebulae vary little over the top ~1.5 mag of the planetary nebula luminosity function. The only obvious spectral change occurs with [N II], which increases in strength as one proceeds down the luminosity function. We also show that typical [O III]-bright planetaries have E(B-V) ~ 0.2 of circumstellar extinction, and that this value is virtually independent of [O III] luminosity. We discuss the implications this has for understanding the population of PN progenitors.

The direct oxygen abundances of metal-rich galaxies derived from electron temperature

Context. Direct measurement of oxygen abundance for metal-rich galaxies from electron temperature is difficult or impossible since temperature-sensitive auroral lines generally become too weak to be measured. Aims. We aim to derive the electron temperature (Te) in the gas of metal-rich star-forming galaxies, which can be obtained from their ratios of auroral lines [O ii]λλ7320, 7330 to nebular lines [O ii]λ3727, in order to establish a more robust massmetallicity relationship, and compare the Te-based (O/H) abundances with those from empirical strong-line calibrations, such as R23 (=([O ii]λ3727+[O iii]λλ4959, 5007)/Hβ). Methods. We obtained 27 spectra by stacking the spectra of several hundred (even several thousand) star-forming galaxies selected from the SDSS-DR4 in each of the 27 stellar mass bins from log(M∗) ∼ 8.0 to 10.6 (in units of log(M� )). This “stack” method sufficiently improves the signal-to-noise ratio of the auroral lines [O ii]λλ7320, 7330. Using a two-zone model for the temperature structure, we derive the electron temperature t2 in the low ionization region from the [O ii]λλ7320, 7330/[O ii]λ3727 ratio, and then use a relation derived by fitting H ii region models to estimate the electron temperature t3 in the high ionization region from t2. Then, the direct (O/H) abundances are obtained from t2, t3 and the related line-ratios. The emission lines have been carefully corrected for dust extinction using the Balmer line ratio after correcting for the underlying stellar absorption. Results. Combining our results with those from the literature with lower metallicities, we are able to provide a new relationship between the direct measurements of (O/H) and R23, which still shows an upper and a lower branch with the transition around 12 + log(O/H) ∼ 8.0−8.2. It also shows that the empirical R23 method will overestimate log(O/H) by 0.2 to 0.6 dex. The new metal-mass relationship of the galaxies with moderate metallicities is fitted by a linear fit (12 + log(O/H) = 6.223 + 0.231 × log(M∗)) confirming that empirical methods significantly overestimate (O/H). We also derived their (N/O) abundance ratios on the basis of the Te method, which are consistent with the combination of the primary and secondary components of nitrogen. Conclusions. This study provides for the first time a method to calibrate direct O/H abundances (from Te) for a large range of galaxies

Dust in the Extremely Metal‐Poor Blue Compact Dwarf Galaxy I Zw 18: TheSpitzerMid‐infrared View

I Zw 18, a blue compact dwarf (BCD) galaxy with the second lowest metallicity measured in a star-forming object, has been observed with all three instruments on board the Spitzer Space Telescope. We present the deepest 5-36 μm mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6-70 μm imaging results. As with SBS 0335-052E, another BCD with similar metallicity, I Zw 18 shows no detectable emission from polycyclic aromatic hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 μm, of I Zw 18 has a much steeper slope, more characteristic of a typical starburst galaxy of solar abundance. The neon abundance as measured from the infrared fine-structure lines is ~1/23 Z☉, and the sulfur abundance is ~1/35 Z☉, generally consistent with the nebular oxygen abundance of 1/30 Z☉ derived from optical lines. This suggests that the extinction to the infrared-emitting regions of this galaxy is low, also in agreement with the optical Balmer line ratios.

De-reddening of optical spectra – Which extinction curve?

Spectral lines are used to determine a broad range of physical properties within HII regions and planetary nebulae (PNe). It is therefore important that we possess accurate intensities for the transitions, and have the means by which observed results may be accurately de-reddened. We point out that there are serious differences between the “standard” extinction curves, and that these may lead to errors in line ratios of as high as ∼80%. It is noted that the variation in Balmer line ratios in planetary nebulae is most consistent with the extinction curves of Whitworth [Whitworth, A.E., 1958. AJ 63, 201] and Ardeberg and Virdefors [Ardeberg, A., Virdefors, B., 1982. A&A 115, 347], and that these are likely to represent the most reliable functions for spectral de-reddening.

A novel diagnostic technique for H−(D−) densities in negative hydrogen ion sources

A new diagnostic method for the determination of negative hydrogen ion densities H− (D−), namely the Hα/Hβ line ratio method, is presented and applied to high power (P≈100 kW) negative ion sources operating at low pressure (p < 1 Pa). The basis of the method is the mutual neutralization process which enhances Balmer line radiation selectively. Detailed parameter studies carried out with a collisional radiative model show that the Balmer line ratio Hα/Hβ is very well suited to obtain negative ion densities in low temperature hydrogen plasmas with typical plasma parameters of Te ≈ 1–5 eV and ne ≈ 1016–5 × 1018 m−3. The Hα/Hβ line ratio method has the great advantage that it can be accomplished easily with the non-invasive and in situ plasma diagnostic method of optical emission spectroscopy. The method is applied to the RF ion sources of IPP Garching, currently developed for the neutral beam injection system of ITER. Line-of-sight averaged negative hydrogen densities in the range of 1016–1017 m−3 are measured close to the extraction system in hydrogen and deuterium discharge. Absolute values depend on pressure, power and caesium conditioning of the source. Negative ion densities measured with this novel technique in front of the grid correlate very well with the extracted negative ion current densities.