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Register a new userSynthetic nebular emission from massive galaxies II: ultraviolet-line diagnostics of dominant ionizing sources
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Michaela Hirschmann Ph.D.
Publication date
2018
fields
Physics
and research's language is
English
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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$1640 line). These UV diagnostic diagrams are potentially important for the interpretation of high-quality spectra of very distant galaxies to be gathered by next-generation telescopes, such as the James Webb Space Telescope.
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Galaxies occupy different regions of the [OIII]$lambda5007$/H$beta$-versus-[NII]$lambda6584$/H$alpha$ emission-line ratio diagram in the distant and local Universe. We investigate the origin of this intriguing result by modelling self-consistently, for the first time, nebular emission from young stars, accreting black holes (BHs) and older, post-asymptotic-giant-branch (post-AGB) stellar populations in galaxy formation simulations in a full cosmological context. In post-processing, we couple new-generation nebular-emission models with high-resolution, cosmological zoom-in simulations of massive galaxies to explore which galaxy physical properties drive the cosmic evolution of the optical-line ratios [OIII]$lambda5007$/H$beta$, [NII]$lambda6584$/H$alpha$, [SII]$lambdalambda6717,6731$/H$alpha$ and [OI]$lambda6300$/H$alpha$. The line ratios of simulated galaxies agree well with observations of both star-forming and active local SDSS galaxies. Towards higher redshifts, at fixed galaxy stellar mass, the average [OIII]/H$beta$ increases and [NII]/H$alpha$, [SII]/H$alpha$ and [OI]/H$alpha$ decrease -- widely consistent with observations. At fixed stellar mass, we identify star formation history, which controls nebular emission from young stars via the ionization parameter, as the primary driver of the cosmic evolution of [OIII]/H$beta$ and [NII]/H$alpha$. For [SII]/H$alpha$ and [OI]/H$alpha$, this applies only to redshifts above $z=1.5$, the evolution at lower redshift being driven in roughly equal parts by nebular emission from AGN and post-AGB stars. Instead, changes in the hardness of ionizing radiation, ionized-gas density, the prevalence of BH accretion relative to star formation and the dust-to-metal mass ratio (whose impact on the gas-phase N/O ratio we model at fixed O/H) play at most a minor role in the cosmic evolution of simulated galaxy line ratios.
Motivated by the observed differences in the nebular emission of nearby and high-redshift galaxies, we carry out a set of direct numerical simulations of turbulent astrophysical media exposed to a UV background. The simulations assume a metallicity of $Z/Z_{odot}$=0.5 and explicitly track ionization, recombination, charge transfer, and ion-by-ion radiative cooling for several astrophysically important elements. Each model is run to a global steady state that depends on the ionization parameter $U$, and the one-dimensional turbulent velocity dispersion, $sigma_{rm 1D}$, and the turbulent driving scale. We carry out a suite of models with a T=42,000K blackbody spectrum, $n_e$ = 100 cm$^{-3}$ and $sigma_{rm 1D}$ ranging between 0.7 to 42 km s$^{-1},$ corresponding to turbulent Mach numbers varying between 0.05 and 2.6. We report our results as several nebular diagnostic diagrams and compare them to observations of star-forming galaxies at a redshift of $zapprox$2.5, whose higher surface densities may also lead to more turbulent interstellar media. We find that subsonic, transsonic turbulence, and turbulence driven on scales of 1 parsec or greater, have little or no effect on the line ratios. Supersonic, small-scale turbulence, on the other hand, generally increases the computed line emission. In fact with a driving scale $approx 0.1$ pc, a moderate amount of turbulence, $sigma_{rm 1D}$=21-28 km s$^{-1},$ can reproduce many of the differences between high and low redshift observations without resorting to harder spectral shapes.
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.
We present basic properties of $sim$3,300 emission line galaxies detected by the FastSound survey, which are mostly H$alpha$ emitters at $z sim$ 1.2-1.5 in the total area of about 20 deg$^2$, with the H$alpha$ flux sensitivity limit of $sim 1.6 times 10^{-16} rm erg cm^{-2} s^{-1}$ at 4.5 sigma. This paper presents the catalogs of the FastSound emission lines and galaxies, which will be open to the public in the near future. We also present basic properties of typical FastSound H$alpha$ emitters, which have H$alpha$ luminosities of $10^{41.8}$-$10^{43.3}$ erg/s, SFRs of 20--500 $M_odot$/yr, and stellar masses of $10^{10.0}$--$10^{11.3}$ $M_odot$. The 3D distribution maps for the four fields of CFHTLS W1--4 are presented, clearly showing large scale clustering of galaxies at the scale of $sim$ 100--600 comoving Mpc. Based on 1,105 galaxies with detections of multiple emission lines, we estimate that contamination of non-H$alpha$ lines is about 4% in the single-line emission galaxies, which are mostly [OIII]$lambda$5007. This contamination fraction is also confirmed by the stacked spectrum of all the FastSound spectra, in which H$alpha$, [NII]$lambda lambda$6548,6583, [SII]$lambda lambda$6717, 6731, and [OI]$lambda lambda$6300,6364 are seen.
We study the kinematical properties of galaxies in the Epoch of Reionization via the [CII] 158$mu$m line emission. The line profile provides information on the kinematics as well as structural properties such as the presence of a disk and satellites. To understand how these properties are encoded in the line profile, first we develop analytical models from which we identify disk inclination and gas turbulent motions as the key parameters affecting the line profile. To gain further insights, we use Althaea, a highly-resolved ($30, rm pc$) simulated prototypical Lyman Break Galaxy, in the redshift range $z = 6-7$, when the galaxy is in a very active assembling phase. Based on morphology, we select three main dynamical stages: I) Merger , II) Spiral Disk, and III) Disturbed Disk. We identify spectral signatures of merger events, spiral arms, and extra-planar flows in I), II), and III), respectively. We derive a generalised dynamical mass vs. [CII]-line FWHM relation. If precise information on the galaxy inclination is (not) available, the returned mass estimate is accurate within a factor $2$ ($4$). A Tully-Fisher relation is found for the observed high-$z$ galaxies, i.e. $L_{rm[CII]}propto (FWHM)^{1.80pm 0.35}$ for which we provide a simple, physically-based interpretation. Finally, we perform mock ALMA simulations to check the detectability of [CII]. When seen face-on, Althaea is always detected at $> 5sigma$; in the edge-on case it remains undetected because the larger intrinsic FWHM pushes the line peak flux below detection limit. This suggests that some of the reported non-detections might be due to inclination effects.
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