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Register a new userCharacterizing Extreme Emission Line Galaxies II: A Self-Consistent Model of Their Ionizing Spectrum
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Observations of high-redshift galaxies ($z >$ 5) have shown that these galaxies have extreme emission lines with equivalent widths much larger than their local star-forming counterparts. Extreme emission line galaxies (EELGs) in the nearby universe are likely analogues to galaxies during the Epoch of Reionization and provide nearby laboratories to understand the physical processes important to the early universe. We use HST/COS and LBT/MODS spectra to study two nearby EELGs, J104457 and J141851. The FUV spectra indicate that these two galaxies contain stellar populations with ages $< sim$ 10 Myr and metallicities $leq$ 0.15 Z$_odot$. We use photoionization modeling to compare emission lines from models of single-age bursts of star-formation to observed emission lines and find that the single-age bursts do not reproduce high-ionization lines including [O III] or very-high ionization lines like He II or [O IV]. Photoionization modeling using the stellar populations fit from the UV continuum similarly are not capable of reproducing the emission lines from the very-high ionization zone. We add a blackbody to the stellar populations fit from the UV continuum to model the necessary high-energy photons to reproduce the very-high ionization lines of He II and [O IV]. We find that we need a blackbody of 80,000 K and $sim$60-70% of the luminosity from the young stellar population to reproduce the very-high ionization lines while simultaneously reproducing the low- intermediate-, and high-ionization emission lines. Our self-consistent model of the ionizing spectra of two nearby EELGs indicates the presence of a previously unaccounted-for source of hard ionizing photons in reionization analogues.
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Stellar population models produce radiation fields that ionize oxygen up to O+2, defining the limit of standard HII region models (<54.9 eV). Yet, some extreme emission line galaxies, or EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV-HST/COS and optical-LBT/MODS spectra of two nearby EELGs that have very-high-ionization emission lines (e.g., HeII 1640,4686 CIV 1548,1550, [FeV] 4227, [ArIV] 4711,4740). We define a 4-zone ionization model that is augmented by a very-high-ionization zone, as characterized by He+2 (>54.4 eV). The 4-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients, higher volume-averaged ionization parameters, and higher central T_e, n_e, and logU values. Traditional 3-zone estimates of the ionization parameter can under-estimate the average log U by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the alpha/H-abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are alpha/Fe-enriched by >3 times. However, there still is a high-energy ionizing photon production problem (HEIP^3). Even for such alpha/Fe-enrichment and very-high log Us, photoionization models cannot reproduce the very-high-ionization emission lines observed in EELGs.
We utilize the CLASH (Cluster Lensing And Supernova survey with Hubble) observations of 25 clusters to search for extreme emission-line galaxies (EELGs). The selections are carried out in two central bands: F105W (Y105) and F125W (J125), as the flux of the central bands could be enhanced by the presence of [O III] 4959, 5007 at redshift of about 0.93-1.14 and 1.57-1.79, respectively. The multi-band observations help to constrain the equivalent widths of emission lines. Thanks to cluster lensing, we are able to identify 52 candidates down to an intrinsic limiting magnitude of 28.5 and to a rest-frame [O III] 4959,5007 equivalent width of about 3737 angstrom. Our samples include a number of EELGs at lower luminosities that are missed in other surveys, and the extremely high equivalent width can be only found in such faint galaxies. These EELGs can mimic the dropout feature similar to that of high redshift galaxies and contaminate the color-color selection of high redshift galaxies when the S/N ratio is limited or the band coverage is incomplete. We predict that the fraction of EELGs in the future high redshift galaxy selections cannot be neglected.
We compute synthetic optical and ultraviolet (UV) emission-line properties of galaxies in a full cosmological framework by coupling, in post-processing, new-generation nebular-emission models with high-resolution, cosmological zoom-in simulations of massive galaxies. Our self-consistent modelling accounts for nebular emission from young stars and accreting black holes (BHs). We investigate which optical- and UV-line diagnostic diagrams can best help to discern between the main ionizing sources, as traced by the ratio of BH accretion to star formation rates in model galaxies, over a wide range of redshifts. At low redshift, simulated star-forming galaxies, galaxies dominated by active galactic nuclei and composite galaxies are appropriately differentiated by standard selection criteria in the classical [OIII]$lambda$5007/H$beta$ versus [NII]$lambda$6584/H$alpha$ diagram. At redshifts $z gt 1$, however, this optical diagram fails to discriminate between active and inactive galaxies at metallicities below $0.5 Z_odot$. To robustly classify the ionizing radiation of such metal-poor galaxies, which dominate in the early Universe, we confirm 3 previous, and propose 11 novel diagnostic diagrams based on equivalent widths and luminosity ratios of UV emission lines, such as EW(OIII]$lambda$1663) versus OIII]$lambda$1663/HeII$lambda$1640, CIII]$lambda$1908/HeII$lambda$1640 versus OIII]$lambda$1663/HeII$lambda$1640, and CIV$lambda$1550/CIII]$lambda$1908 versus CIII]$lambda$1908/CII$lambda$2326. We formulate associated UV selection criteria and discuss some caveats of our results (e.g., uncertainties in the modelling of the HeII$lambda$1640 line). These UV diagnostic diagrams are potentially important for the interpretation of high-quality spectra of very distant galaxies to be gathered by next-generation telescopes, such as the James Webb Space Telescope.
Because the 157.74 micron [C II] line is the dominant coolant of star-forming regions, it is often used to infer the global star-formation rates of galaxies. By characterizing the [C II] and far-infrared emission from nearby Galactic star-forming molecular clumps, it is possible to determine whether extragalactic [C II] emission arises from a large ensemble of such clumps, and whether [C II] is indeed a robust indicator of global star formation. We describe [C II] and far-infrared observations using the FIFI-LS instrument on the SOFIA airborne observatory toward four dense, high-mass, Milky Way clumps. Despite similar far-infrared luminosities, the [C II] to far-infrared luminosity ratio, L([C II])/L(FIR) varies by a factor of at least 140 among these four clumps. In particular, for AGAL313.576+0.324, no [C II] line emission is detected despite a FIR luminosity of 24,000 L_sun. AGAL313.576+0.324 lies a factor of more than 100 below the empirical correlation curve between L([C II])/L(FIR) and S_ u (63 micron)/S_ u (158 micron) found for galaxies. AGAL313.576+0.324 may be in an early evolutionary protostellar phase with insufficient ultraviolet flux to ionize carbon, or in a deeply embedded ``hypercompact H II region phase where dust attenuation of UV flux limits the region of ionized carbon to undetectably small volumes. Alternatively, its apparent lack of cii, emission may arise from deep absorption of the cii, line against the 158 micron continuum, or self-absorption of brighter line emission by foreground material, which might cancel or diminish any emission within the FIFI-LS instruments broad spectral resolution element (~250 km/s)
A tight relation between the [CII]158$mu$m line luminosity and star formation rate is measured in local galaxies. At high redshift ($z>5$), though, a much larger scatter is observed, with a considerable (15-20%) fraction of the outliers being [CII]-deficient. Moreover, the [CII] surface brightness ($Sigma_{rm CII}$) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [CII]-deficiency we have developed an analytical model that fits local [CII] data, and has been validated against radiative transfer simulations performed with CLOUDY. The model predicts an overall increase of $Sigma_{rm CII}$ with the surface star formation rate ($Sigma_*$). However, for $Sigma_* > 1 M_odot~{rm yr}^{-1}~{rm kpc}^{-2}$, $Sigma_{rm CII}$ saturates. We conclude that underluminous [CII] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt-Schmidt relation ($kappa_s gg 1$), parameterized by the burstiness parameter $kappa_s$; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [CII] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [OIII]88$mu$m, CIII]1909A, CO lines). Simple formulae are given to interpret available data for low and high-$z$ galaxies.
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