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تسجيل مستخدم جديدTheoretical predictions for the effect of nebular emission on the broad band photometry of high-redshift galaxies
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By combining optical and near-IR observations from the Hubble Space Telescope with NIR photometry from the Spitzer Space Telescope it is possible to measure the rest-frame UV-optical colours of galaxies at z=4-8. The UV-optical spectral energy distribution of star formation dominated galaxies is the result of several different factors. These include the joint distribution of stellar masses, ages, and metallicities, and the subsequent reprocessing by dust and gas in the ISM. Using a large cosmological hydrodynamical simulation we investigate the predicted spectral energy distributions of galaxies at high-redshift with a particular emphasis on assessing the potential contribution of nebular emission. We find that the average pure stellar UV-optical colour correlates with both luminosity and redshift such that galaxies at lower-redshift and higher-luminosity are typically redder. Assuming the escape fraction of ionising photons is close to zero, the effect of nebular emission is to redden the UV-optical 1500-V_w colour by, on average, 0.4 mag at z=8 declining to 0.25 mag at z=4. Young and low-metallicity stellar populations, which typically have bluer pure stellar UV-optical colours, produce larger ionising luminosities and are thus more strongly affected by the reddening effects of nebular emission. This causes the distribution of 1500-V_w colours to narrow and the trends with luminosity and redshift to weaken. The strong effect of nebular emission leaves observed-frame colours critically sensitive to the source redshift. For example, increasing the redshift by 0.1 can result in observed frame colours changing by up to ~0.6. These predictions reinforce the need to include nebular emission when modelling the spectral energy distributions of galaxies at high-redshift and also highlight the difficultly in interpreting the observed colours of individual galaxies without precise redshifts.
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A substantial fraction of the light emitted from young or star-forming galaxies at ultraviolet to near-infrared wavelengths comes from the ionized interstellar medium in the form of emission lines and a nebular continuum. At high redshifts, star form
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High signal-to-noise, representative spectra of star-forming galaxies at z~2, obtained via stacking, reveal a high-velocity component underneath the narrow H-alpha and [NII] emission lines. When modeled as a single Gaussian, this broad component has
FWHM > 1500 km/s; when modeled as broad wings on the H-alpha and [NII] features, it has FWHM > 500 km/s. This feature is preferentially found in the more massive and more rapidly star-forming systems, which also tend to be older and larger galaxies. We interpret this emission as evidence of either powerful starburst-driven galactic winds or active supermassive black holes. If galactic winds are responsible for the broad emission, the observed luminosity and velocity of this gas imply mass outflow rates comparable to the star formation rate. On the other hand, if the broad line regions of active black holes account for the broad feature, the corresponding black holes masses are estimated to be an order of magnitude lower than those predicted by local scaling relations, suggesting a delayed assembly of supermassive black holes with respect to their host bulges.
We present a combined analysis of rest-frame far-UV (1000-2000 A) and rest-frame optical (3600-7000 A) composite spectra formed from very deep observations of a sample of 30 star-forming galaxies with z=2.4+/-0.1, selected to be representative of the
full KBSS-MOSFIRE spectroscopic survey. Since the same massive stars are responsible for the observed FUV continuum and the excitation of the observed nebular emission, a self-consistent stellar population synthesis model must simultaneously match the details of the far-UV stellar+nebular continuum and-- when inserted as the excitation source in photoionization models-- account for all observed nebular emission line ratios. We find that only models including massive star binaries, having low stellar metallicity (Z_*/Z_{sun} ~ 0.1) but relatively high ionized gas-phase oxygen abundances (Z_{neb}/Z_{sun} ~ 0.5), can successfully match all of the observational constraints. We argue that this apparent discrepancy is naturally explained by highly super-solar O/Fe [4-5 times (O/Fe)_{sun}], expected for gas whose enrichment is dominated by the products of core-collapse supernovae. Once the correct ionizing spectrum is identified, photoionization models reproduce all of the observed strong emission line ratios, the direct T_e measurement of O/H, and allow accurate measurement of the gas-phase abundance ratios of N/O and C/O -- both of which are significantly sub-solar but, as for O/Fe, are in remarkable agreement with abundance patterns observed in Galactic thick disk, bulge, and halo stars with similar O/H. High nebular excitation is the rule at high-z (and rare at low-z) because of systematically shorter enrichment timescales (<<1 Gyr): low Fe/O environments produce harder (and longer-lived) stellar EUV spectra at a given O/H, enhanced by dramatic effects on the evolution of massive star binaries.
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 o
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