Hβ Ratio Research Articles

Emission-line galaxies at z ∼ 1 from near-IR HST Slitless Spectroscopy: Metallicities, star formation rates and redshift confirmations from VLT/FORS2 spectroscopy

Abstract We follow up emission line galaxies identified through the near-infrared slitless HST/WFC3 WISP survey with VLT/FORS2 optical spectroscopy. Over 4 WISP fields, we targetted 85 of 138 line emission objects at 0.4 < z < 2 identified in WFC3 spectroscopy. Half the galaxies are fainter than HAB = 24 mag, and would not have been included in many well-known surveys based on broad-band magnitude selection. We confirm 95% of the initial WFC3 grism redshifts in the 38 cases where we detect lines in FORS2 spectroscopy. However, for targets which exhibited a single emission line in WFC3, up to 65% at z < 1.28 did not have expected emission lines detected in FORS2 and hence may be spurious (although this false-detection rate improves to 33% using the latest public WISP emission line catalogue). From the Balmer decrement the extinction of the WISP galaxies is consistent with A(Hα) = 1 mag. From SED fits to multi-band photometry including Spitzer 3.6 μm, we find a median stellar mass of log10(M⋆/M⊙) = 8.94. Our emission-line-selected galaxies tend to lie above the star-forming main sequence (i.e. higher specific star formation rates). Using [O iii], [O ii] and Hβ lines to derive gas-phase metallicities, we find typically sub-solar metallicities, decreasing with redshift. Our WISP galaxies lie below the z = 0 mass-metallicity relation, and galaxies with higher star formation rates tend to have lower metallicity. Finally, we find a strong increase with redshift of the Hα rest-frame equivalent width in this emission-line selected sample, with higher EW0 galaxies having larger [O iii]/Hβ and O32 ratios on average, suggesting lower metallicity or higher ionisation parameter in these extreme emission line galaxies.

Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS)

We present the Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS) project, designed for the spatial and spectral study of the diffuse ionised gas (DIG) in a selection of nearby spiral galaxies observed with the MUSE integral-field spectrograph. Our primary objective is to investigate the various ionisation mechanisms at play within the DIG. We analysed the distribution of high- and low-ionisation species in the optical spectra of the sample on a spatially resolved basis. We introduced a new methodology for spectroscopically defining the DIG, optimised for galaxies of different resolutions. Firstly, we employed an innovative adaptive binning technique on the observed datacube based on the spectroscopic signal-to-noise ratio (S/N) of the collisional [S II] line to increase the S/N of the rest of the lines including [O III], [O I], and He I. Subsequently, we created a DIG mask by eliminating the emissions associated with both bright and faint H II regions. We also examined the suitability of using Hα equivalent width (EWHα) as a proxy for defining the DIG and its associated ionisation regime. Notably, for EWHα < 3 Å – the expected emission from hot low-mass evolved stars (HOLMES) – the measured value is contingent on the chosen population synthesis technique performed. Our analysis of the showcase sample reveals a consistent cumulative DIG fraction across all galaxies in the sample, averaging around 40%–70%. The average radial distribution of the [N II]/Hα, [S II]/Hα, [O I]/Hα, and [O III]/Hβ ratios are enhanced in the DIG regimes (up to 0.2 dex). It follows similar trends between the DIG regime and the H II regions, as well as the Hα surface brightness (ΣHα), indicating a correlation between the ionisation of these species in both the DIG and the H II regions. The DIG loci in typical diagnostic diagrams are found, in general, within the line ratios that correspond to photoionisation due to the star formation. There is a noticeable offset correspondent to ionisation due to fast shocks. However, an individual diagnosis performed for each galaxy reveals that all the DIG in these galaxies can be attributed to photoionisation from star formation. The offset is primarily due to the contribution of Seyfert galaxies in our sample, which is closely aligned with models of ionisation from fast shocks and galactic outflows, thus mimicking the DIG emission. Our results indicate that galaxies exhibiting active galactic nucleus (AGN) activity should be considered separately when conducting a general analysis of the DIG ionisation mechanisms, since this emission is indistinguishable from high-excitation DIG.

CEERS Key Paper. VIII. Emission-line Ratios from NIRSpec and NIRCam Wide-Field Slitless Spectroscopy at z > 2

We use James Webb Space Telescope Near-Infrared Camera Wide Field Slitless Spectroscopy (NIRCam WFSS) and the Near-Infrared spectrograph (NIRSpec) in the Cosmic Evolution Early Release survey to measure rest-frame optical emission-line ratios of 155 galaxies at z > 2. The blind NIRCam grism observations include a sample of galaxies with bright emission lines that were not observed on the NIRSpec masks. We study the changes of the Hα, [O III]/Hβ, and [Ne III]/[O II] emission lines in terms of redshift by comparing to lower-redshift SDSS, CLEAR, and MOSDEF samples. We find a significant (>3σ) correlation between [O III]/Hβ with redshift, while [Ne III]/[O II] has a marginal (2σ) correlation with redshift. We compare [O III]/Hβ and [Ne III]/[O II] to stellar mass and Hβ SFR. We find that both emission-line ratios have a correlation with Hβ SFR and an anticorrelation with stellar mass across the redshifts 0 < z < 9. Comparison with MAPPINGS V models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-SFR galaxies. We additionally compare to IllustrisTNG predictions and find that they effectively describe the highest [O III]/Hβ ratios observed in our sample, without the need to invoke MAPPINGS models with significant shock ionization components.

CAHA/PPAK Integral-field Spectroscopic Observations of M81. II. Testing Photoionization Models in a Spatially Resolved LINER

The origin of the low-ionization nuclear emission-line region (LINER) prevalent in local galaxies and its relationship with supermassive black holes have been debated for decades. We perform a comprehensive evaluation of traditional photoionization models against the circumnuclear ionized gas in M81, for which recent CAHA/PPAK integral-field spectroscopic observations reveal a LINER characteristic out to a galactocentric radius of ∼1 kpc. Constructed with the photoionization code cloudy, the models have the novel aspect of their primary parameters being well constrained by extensive observations of a prototypical low-luminosity active galactic nucleus (LLAGN) and an old stellar bulge in M81. Additionally, these models incorporate a reasonably broad range of uncertain nebular properties. It is found that the integrated photoionization by the LLAGN and hot, low-mass stars distributed in the bulge can roughly reproduce the observed radial intensity distributions of the Hα, Hβ, and [N ii] lines, with the bulge stars dominating the ionizing flux at radii ≳200 pc. However, the models generally fail to reproduce a similarly declining profile of the [O iii] line or an accordingly flat profile of the [O iii]/Hβ ratio. This clearly points to a deficiency of ionizing photons in the outer regions despite an extended photoionization source. The discrepancy might be alleviated if much of the observed [O iii] line arose from a bulge-filling, low-density gas surrounding a denser, Hα-emitting disk, or by a higher AGN luminosity in the recent past. The case of M81 has important implications for the ionization mechanism of LINERs and low-ionization emission-line regions in general.

JADES: Discovery of extremely high equivalent width Lyman-α emission from a faint galaxy within an ionized bubble at z = 7.3

We report the discovery of a remarkable Lyα emitting galaxy at z = 7.2782, JADES-GS+53.16746−27.7720 (shortened to JADES-GS-z7-LA), with rest-frame equivalent width, EW0(Lyα) = 388.0 ± 88.8 Å and UV magnitude −17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], Hβ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Lyα line is detected in both lower resolution (R ∼ 100) PRISM as well as medium resolution (R ∼ 1000) G140M grating spectra. The line spread function-deconvolved Lyα full width at half maximum in the grating is 383.9 ± 56.2 km s−1 and the Lyα velocity offset compared to the systemic redshift is 113.3 ± 80.0 km s−1, indicative of very little neutral gas or dust within the galaxy. We estimate the Lyα escape fraction to be &gt; 70%. JADES-GS-z7-LA has a [O III]/[O II] ratio (O32) of 11.1 ± 2.2 and a ([O III] + [O II])/Hβ ratio (R23) of 11.2 ± 2.6, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by 0.23″), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III] + Hβ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a “bursty” star formation history, with a low stellar mass of ≈107 M⊙. Assuming that the Lyα transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size &gt; 1.5 pMpc is needed to explain the high Lyα EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Lyα emission. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at z &gt; 7.

JWST/NIRSpec Measurements of Extremely Low Metallicities in High Equivalent Width Lyα Emitters

Deep Very Large Telescope/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies (M UV ≈ −15; 0.01 L ⋆) at z = 2.9−6.7 due to their strong Lyα emission with no detectable continuum. The implied Lyα equivalent widths can be in excess of 100–200 Å, challenging existing models of normal star formation and indicating extremely young ages, small stellar masses, and a very low amount of metal enrichment. We use JWST/NIRSpec’s microshutter array to follow up 45 of these galaxies (11 hr in G235M/F170LP and 7 hr in G395M/F290LP), as well as 45 lower-equivalent width Lyα emitters. Our spectroscopy covers the range 1.7−5.1 micron in order to target strong optical emission lines: Hα, [O iii], Hβ, and [N II]. Individual measurements as well as stacks reveal line ratios consistent with a metal-poor nature (2%−40% Z ⊙, depending on the calibration). The galaxies with the highest equivalent widths of Lyα, in excess of 90 Å, have lower [N II]/Hα (1.9σ) and [O iii]/Hβ (2.2σ) ratios than those with lower equivalent widths, implying lower gas-phase metallicities at a combined significance of 2.4σ. This implies a selection based on Lyα equivalent width is an efficient technique for identifying younger, less chemically enriched systems.

A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake

The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in Hubble Space Telescope (HST) Advanced Camera for Surveys images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z = 0.964. Keck Low-resolution Imaging Spectrometer spectra show that the [O iii]/Hβ ratio varies from ∼1 to ∼10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [O iii] knot with a luminosity of ≈1.9 × 1041 erg s−1. The stellar continuum colors vary along the feature and are well fit by a simple model that has a monotonically increasing age with the distance from the tip. The line ratios, colors, and overall morphology are consistent with an ejected SMBH moving through the CGM at a high speed while triggering star formation. The best-fit time since ejection is ∼39 Myr, and the implied velocity is v BH ∼ 1600 km s−1. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected only in [O iii] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.

CLEAR: Spatially Resolved Emission Lines and Active Galactic Nuclei at 0.6 < z < 1.3

We investigate spatially resolved emission-line ratios in a sample of 219 galaxies (0.6 < z < 1.3) detected using the G102 grism on the Hubble Space Telescope Wide Field Camera 3 taken as part of the CANDELS Lyα Emission at Reionization survey to measure ionization profiles and search for low-luminosity active galactic nuclei (AGN). We analyze [O iii] and Hβ emission-line maps, enabling us to spatially resolve the [O iii]/Hβ emission-line ratio across the galaxies in the sample. We compare the [O iii]/Hβ ratio in galaxy centers and outer annular regions to measure ionization differences and investigate the potential of sources with nuclear ionization to host AGN. We investigate some of the individual galaxies that are candidates to host strong nuclear ionization and find that they often have low stellar mass and are undetected in X-rays, as expected for low-luminosity AGN in low-mass galaxies. We do not find evidence for a significant population of off-nuclear AGN or other clumps of off-nuclear ionization. We model the observed distribution of [O iii]/Hβ spatial profiles and find that most galaxies are consistent with a small or zero difference between their nuclear and off-nuclear line ratios, but 6%–16% of galaxies in the sample are likely to host nuclear [O iii]/Hβ that is ∼0.5 dex higher than in their outer regions. This study is limited by large uncertainties in most of the measured [O iii]/Hβ spatial profiles; therefore, deeper data, e.g., from deeper HST/WFC3 programs or from JWST/NIRISS, are needed to more reliably measure the spatially resolved emission-line conditions of individual high-redshift galaxies.

Quantifying Feedback from Narrow Line Region Outflows in Nearby Active Galaxies. IV. The Effects of Different Density Estimates on the Ionized Gas Masses and Outflow Rates

Active galactic nuclei (AGN) can launch outflows of ionized gas that may influence galaxy evolution, and quantifying their full impact requires spatially resolved measurements of the gas masses, velocities, and radial extents. We previously reported these quantities for the ionized narrow-line region outflows in six low-redshift AGN, where the gas velocities and extents were determined from Hubble Space Telescope long-slit spectroscopy. However, calculating the gas masses required multicomponent photoionization models to account for radial variations in the gas densities, which span ∼6 orders of magnitude. To simplify this method for larger samples with less spectral coverage, we compare these gas masses with those calculated from techniques in the literature. First, we use a recombination equation with three different estimates for the radial density profiles. These include constant densities, those derived from [S ii], and power-law profiles based on constant values of the ionization parameter (U). Second, we use single-component photoionization models with power-law density profiles based on constant U, and allow U to vary with radius based on the [O iii]/Hβ ratios. We find that assuming a constant density of n H = 102 cm−3 overestimates the gas masses for all six outflows, particularly at small radii where the outflow rates peak. The use of [S ii] marginally matches the total gas masses, but also overestimates at small radii. Overall, single-component photoionization models where U varies with radius are able to best match the gas mass and outflow rate profiles when there are insufficient emission lines to construct detailed models.

The HETDEX Survey: The Lyα Escape Fraction from 3D-HST Emission-Line Galaxies at z ∼ 2

Abstract In this work, we measure the Lyα escape fraction of 935 [O iii]-emitting galaxies between 1.9 &lt; z &lt; 2.35 by comparing stacked spectra from the Hubble Space Telescope/WFC3's near-IR grism to corresponding stacks from the Hobby–Eberly Telescope Dark Energy Experiment’s Internal Data Release 2. By measuring the stacks’ Hβ to Lyα ratios, we determine the Lyα escape fraction as a function of stellar mass, star-formation rate, internal reddening, size, and [O iii]/Hβ ratio. We show that the escape fraction of Lyα correlates with a number of parameters, such as galaxy size, star-formation rate, and nebular excitation. However, we also demonstrate that most of these relations are indirect, and that the primary variables controlling the escape of Lyα are likely to be stellar mass and internal extinction. Overall, the escape of Lyα declines from ≳16% in galaxies with to ≲1% for systems with , with the sample’s mean escape fraction being .

J-PAS: Measuring emission lines with artificial neural networks

In the years to come, the Javalambre-Physics of the Accelerated Universe Astrophysical Survey (J-PAS) will observe 8000 deg2 of the northern sky with 56 photometric bands. J-PAS is ideal for the detection of nebular emission objects. This paper presents a new method based on artificial neural networks (ANNs) that is aimed at measuring and detecting emission lines in galaxies up to z = 0.35. These lines are essential diagnostics for understanding the evolution of galaxies through cosmic time. We trained and tested ANNs with synthetic J-PAS photometry from CALIFA, MaNGA, and SDSS spectra. To this aim, we carried out two tasks. First, we clustered galaxies in two groups according to the values of the equivalent width (EW) of Hα, Hβ, [N II], and [O III] lines measured in the spectra. Then we trained an ANN to assign a group to each galaxy. We were able to classify them with the uncertainties typical of the photometric redshift measurable in J-PAS. Second, we utilized another ANN to determine the values of those EWs. Subsequently, we obtained the [N II]/Hα, [O III]/Hβ, and O 3N 2 ratios, recovering the BPT diagram ([O III]/Hβ versus [N II]/Hα). We studied the performance of the ANN in two training samples: one is only composed of synthetic J-PAS photo-spectra (J-spectra) from MaNGA and CALIFA (CALMa set) and the other one is composed of SDSS galaxies. We were able to fully reproduce the main sequence of star-forming galaxies from the determination of the EWs. With the CALMa training set, we reached a precision of 0.092 and 0.078 dex for the [N II]/Hα and [O III]/Hβ ratios in the SDSS testing sample. Nevertheless, we find an underestimation of those ratios at high values in galaxies hosting an active galactic nuclei. We also show the importance of the dataset used for both training and testing the model. Such ANNs are extremely useful for overcoming the limitations previously expected concerning the detection and measurements of the emission lines in such surveys as J-PAS. Furthermore, we show the capability of the method to measure a EW of 10 Å in Hα, Hβ, [N II] and [O III] lines with a signal-to-noise ratio (S/N) of 5, 1.5, 3.5, and 10, respectively, in the photometry. Finally, we compare the properties of emission lines in galaxies observed with miniJPAS and SDSS. Despite the limitation of such a comparison, we find a remarkable correlation in their EWs.

A Tale of Two Clusters: An Analysis of Gas-phase Metallicity and Nebular Gas Conditions in Proto-cluster Galaxies at z ∼ 2

Abstract The ZFIRE survey has spectroscopically confirmed two proto-clusters using the MOSFIRE instrument on Keck I: one at z = 2.095 in COSMOS and another at z = 1.62 in UKIRT Infrared Deep Sky Survey (UDS). Here, we use an updated ZFIRE data set to derive the properties of ionized gas regions of proto-cluster galaxies by extracting fluxes from emission lines Hβ 4861 Å, [O iii] 5007 Å, Hα 6563 Å, [N ii] 6585 Å, and [S ii] 6716,6731 Å. We measure gas-phase metallicity of members in both proto-clusters using two indicators, including a strong-line indicator relatively independent of the ionization parameter and electron density. Proto-cluster and field galaxies in both UDS and COSMOS lie on the same Mass–Metallicity Relation with both metallicity indicators. We compare our results to recent IllustrisTNG results, which report no significant gas-phase metallicity offset between proto-cluster and field galaxies until z = 1.5. This is in agreement with our observed metallicities, where no offset is measured between proto-cluster and field populations. We measure tentative evidence from stacked spectra that indicate UDS high-mass proto-cluster and field galaxies have differing [O iii]/Hβ ratios; however, these results are dependent on the sample size of the high-mass stacks.

Synthetic nebular emission lines of simulated galaxies over cosmic time

AbstractThis article presents an up-dated analysis of synthetic optical and UV emission lines of simulated galaxies over cosmic time. The strong emission lines are derived from self-consistently coupling novel spectral models accounting for nebular emission from young stars, AGN and Post-AGB stars to cosmological zoom-in as well as large-scale simulations. Investigating the evolution of optical line-ratios in the BPT diagrams, the simulations can successfully reproduce the observed trend of [OIII]/Hβ ratio increasing from low to high redshifts, due to evolving star formation rate and gas metallicity. Standard selection criteria in the BPT diagrams can appropriately distinguish the main ionising source(s) of galaxies at low redshifts, but they are less reliable for metal-poor galaxies, dominating the early Universe. To robustly classify the ionising radiation of such metal-poor galaxies, diagnostic diagrams based on luminosity ratios of UV lines are discussed. The novel interface between simulations and observations is potentially important for the interpretation of high-quality spectra of very distant galaxies to be gathered by next-generation telescopes, such as the James Webb Space Telescope.

1H NMR and 13C NMR characterization of n-heptane extraction of low-temperature coal tar reacted with formaldehyde

ABSTRACTThe optimum conditions of toluene insoluble-quinoline soluble fraction (TI-QS) obtained by n-heptane Soxhlet extraction residue of low-temperature coal tar (H-CT) reacted with formaldehyde were determined. And 1H NMR and 13C NMR were employed to analyze the structure of H-CT and its reaction products (P-H-CT). The results show that the yield of TI-QS is the largest at the reaction conditions as follows: reaction temperature 90°C, reaction time 3 h and with 3 wt% addition of catalyst. The contents of aliphatic carbon and aromatic carbon bonded to oxygen are higher in H-CT. The contents of aliphatic CH3 carbon and aromatic CH3 carbon are higher in P-H-CT. The content of methylene decreases and the content of methane increases, but the decrease of methylene is larger than the increase of methane; therefore, CH/CH2 carbon decreases slightly. The content of Hγ is 0.10 wt%, and the contents of Hα and Hβ are all 0.25 wt% in P-H-CT. The Hγ, Hα and Hβ ratios of P-H-CT to H-CT are small, displaying a lower branched degree of P-H-CT.

Environmental dependence of galaxy formation explored by near-infrared spectroscopy of two protoclusters at z&gt;2

AbstractProtoclusters at high redshifts are the ideal laboratories to study how the environmental dependences of galaxy properties seen in local Universe were initially set up when the progenitors of present-day early-type galaxies were in their early formation phases. We have conducted a deep near-infrared spectroscopy of Hα emitters (HAEs) associated with two protoclusters (PKS 1138–262 at z = 2.16 and USS 1558–003 at z = 2.53) with the Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope.As a result, the cluster membership of 27 and 36 HAEs are newly confirmed in these two protoclusters, respectively. The inferred dynamical masses of the protocluster cores are consistent with being the typical progenitors of present-day most massive clusters (Shimakawa et al.2014a). Also, those HAEs in the protoclusters show much higher [OIII]/Hβ ratios than local star forming galaxies. It is probably caused by the combination of their much higher specific star formation rates, lower gaseous metallicities and redshift evolution of inter-stellar medium. We also find that the mass-metallicity relation in the protocluster galaxies is offset to higher metallicity compared to those of field galaxies at a given stellar mass at M∗&lt;1011M⊙ (Shimakawa et al.2014b). This trend is compatible with the recent work (Kulas et al.2013). The mass-metallicity relation is regulated not only by star formation history hence metal production history, but also by inflow and outflow processes that are known to be very active at z &gt; 2 (Steide et al.2010). It suggests that the higher gaseous metallicities of protocluster galaxies may be caused by those gas transfer processes that are dependent on surrounding environments.

ESO 089−G018 and ESO 089−G019: long-slit spectroscopy of emission-line galaxies

We present the first spectroscopic observations for the galaxies ESO 089-G018 (hereafter G18, an Sb(?)-type galaxy seen nearly edge-on) and ES0089-G019 (hereafter G19, an SA(s): a peculiar galaxy), extracted from the sample of ring-shaped galaxies compiled in Faundez-Abans & de Oliveira-Abans. The main goal of this work is to investigate the spectral classification using the three line-ratio diagrams, called diagnostic diagrams, of Veilleux & Osterbrock. However, in order to separate the different types of galaxies [H II galaxies, Seyfert 2 galaxies and low-ionization nuclear emission-line region galaxies (LINERs)] we have to used empirical boundaries between them. Based on the observed spectra, we suggest G18 is a 'weak-[O I] LINER' or even a 'transition object' or LINER/H II. In the case of G19, we see Hβ in absorption and no [O III] lines, impeding the [O III]/Hβ ratio to be estimated. However, other lines ratios have been evaluated for the discussion. We classify the nature of G19 as ambiguous, because of the difficulty in determining its ionizing source (narrow-line active galactic nuclei or H II galaxies) in different diagnostic diagrams. The errors in the fluxes were mostly caused by uncertainties in the placement of the continuum level. We have estimated nuclear redshift of z = 0.034 (G18) and z = 0.039 (G19), corresponding to a heliocentric velocity of 10 246 and 11 734 km s -1 , respectively. Some other physical parameters have been derived whenever possible. All spectra were reduced and analysed in a homogeneous way with the standard IRAF procedures.

An Evaluation of the Excitation-Class Parameter for the Central Stars of Planetary Nebulae

AbstractThe three main methods currently in use for estimating the excitation class of planetary nebulae (PNe) central stars are compared and evaluated using 586 newly discovered and previously known PNe in the Large Magellanic Cloud (LMC). In order to achieve this we ran a series of evaluation tests using line ratios derived from de-reddened, flux-calibrated spectra. Pronounced differences between the methods are exposed after comparing the distribution of objects to their derived excitation. Line ratio comparisons show that each method's input parameters have a strong effect on the estimated excitation of a central star. Diagrams were created by comparing excitation classes with Hβ line fluxes. The best methods are then compared to published temperatures using the Zanstra method and assessed for their ability to reflect central star effective temperatures and evolution. As a result we call for a clarification of the term ‘excitation class’ according to the different input parameters used. The first method, which we refer to as Exneb relies purely on the ratios of certain key emission lines. The second method, which we refer to as Ex* includes modeling to create a continuous variable and, for optically thick PNe in the Magellanic Clouds, is designed to relate more closely to intrinsic stellar parameters. The third method, we refer to as Ex[Oiii]/Hβ since the [Oiii]/Hβ ratio is used in isolation to other temperature diagnostics. Each of these methods is shown to have serious drawbacks when used as an indicator for central star temperature. Finally, we suggest a new method (Exρ) for estimating excitation class incorporating both the [Oiii]/Hβ and the HeII λ4686/Hβ ratios. Although any attempt to provide accurate central star temperatures using the excitation class derived from nebula lines will always be limited, we show that this new method provides a substantial improvement over previous methods with better agreement to temperatures derived through the Zanstra method.

EMISSION-LINE GALAXIES FROM THEHUBBLE SPACE TELESCOPEPROBING EVOLUTION AND REIONIZATION SPECTROSCOPICALLY (PEARS) GRISM SURVEY. I. THE SOUTH FIELDS

We present results of a search for emission-line galaxies (ELGs) in the southern fields of the Hubble Space Telescope Probing Evolution And Reionization Spectroscopically (PEARS) grism survey. The PEARS South Fields consist of five Advanced Camera for Surveys pointings (including the Hubble Ultra Deep Field) with the G800L grism for a total of 120 orbits, revealing thousands of faint object spectra in the GOODS-South region of the sky. ELGs are one subset of objects that are prevalent among the grism spectra. Using a two-dimensional detection and extraction procedure, we find 320 emission lines originating from 226 galaxy "knots" within 192 individual galaxies. Line identification results in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field. We measure emission-line fluxes using standard Gaussian fitting techniques. At the resolution of the grism data, the Hβ and [O iii] doublet are blended. However, by fitting two Gaussian components to the Hβ and [O iii] features, we find that many of the PEARS ELGs have high [O iii]/Hβ ratios compared to other galaxy samples of comparable luminosities. The star formation rates of the ELGs are presented, as well as a sample of distinct giant star-forming regions at z ∼ 0.1–0.5 across individual galaxies. We find that the radial distances of these H ii regions in general reside near the galaxies' optical continuum half-light radii, similar to those of giant H ii regions in local galaxies.

Diffuse Ionized Gas in Irregular Galaxies. I. GR 8 and ESO 245-G05

We have studied the spectral characteristics of the diffuse ionized gas (DIG) in two irregular galaxies with low metallicities and intermediate star formation rates: ESO 245-G05 and GR 8. The [O III]/Hβ ratio in these galaxies is higher than in the DIG of spiral galaxies but not as high as in other irregular galaxies previously studied, such as IC 10 and NGC 6822. The [N II]/Hα and [S II]/Hα ratios have very small values, indicating the absence of shocks as the ionization source for this gas. This ionization can be explained in both galaxies with photon leakage from the H II regions as the only source. The percentage of photons that have escaped from the H II regions is small in ESO 245-G05, only 35%, but varies from 35% up to 60% in GR 8. We also investigated whether the differences found between spiral and irregular galaxies in the [O III]/Hβ and the [N II]/Hα ratios are due to differences in the metal content between these types of galaxies. Although the number of galaxies studied is not very large, it can be concluded that the [O III]/Hβ ratio is not related to the oxygen content, while the situation is more ambiguous for the [N II]/Hα ratio.

Deep optical observations of the supernova remnants G 126.2+1.6, G 59.8+1.2 and G 54.4-0.3

Optical CCD imaging and spectroscopic observations of three supernova remnants are presented. Optical emission from G 54.4−0.3 and G 59.8+1.2 is detected for the first time, while the first flux calibrated CCD images of the supernova remnant G 126.2+1.6 were performed in the optical emission lines of Hα + [N ], [O  ]a nd [S]. A mixture of filamentary and diffuse structures is observed in G 54.4−0.3 and G 59.8+1.2, mainly in Hα + [N ], while the deep optical images of G 126.2+1.6 reveal several new filamentary and diffuse structures inside the extent of the remnant as defined by its known radio emission. In all cases, the radio emission is found to be well correlated with the optical filaments. [O ] emission was not detected at G 54.4−0.3 and G 59.8+1.2 while in G 126.2+1.6, significant morphological differences between the low and medium ionization images are present suggesting incomplete shock structures. Deep long-slit spectra were taken at different positions of the remnants. Both the flux calibrated images and the long-slit spectra clearly show that the emission originates from shock-heated gas, while some spectra of G 126.2+1.6 are characterized by large [O ]/Hβ ratios. This remnant’s [O ] flux suggests shock velocities into the interstellar “clouds” between 100 and 120 km s −1 , while the [O ] absence in the other two remnants indicates slower shock velocities. For all remnants, the [S ]λλ 6716/6731 ratio indicates electron densities below 600 cm −3 with particularly low densities for G 54.4−0.3 (below 50 cm −3 ). Finally, the Hα emission has been measured to be between 3.0 to 15.2 × 10 −17 erg s −1 cm −2 arcsec −2 ,3 .2× 10 −17 erg s −1 cm −2 arcsec −2 and between 6.5 to 16.8 × 10 −17 erg s −1 cm −2 arcsec −2 for G 54.4−0.3, G 59.8+1.2 and G 126.2+1.6, respectively.