Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies

TL;DR

We integrate advanced stellar-population synthesis with a physically motivated ISM transfer model to diagnose ultraviolet signatures of stars, the ionized ISM, and the neutral ISM in star-forming galaxies. By validating SPS predictions against LMC cluster data and developing a cloud-based treatment that couples nebular emission with interstellar absorption, we show most UV indices in ISM-rich environments are contaminated, while a subset remains robust as tracers of specific ISM phases. The approach yields age, metallicity, and stellar-mass estimates consistent with literature and highlights how stochastic IMF sampling chiefly biases mass estimates in low-mass clusters. Overall, the framework enables quantitative interpretation of rest-frame UV spectra over a range of metallicities and SF histories, with implications for studying star formation and chemical evolution in the early universe.

Abstract

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young ($\sim10$-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.

Figures (21)

• Figure 1: Strengths of the 19 Fanelli1992 ultraviolet spectral indices defined in Table \ref{['tab:tabindices']} plotted against age, for SSPs with a smoothly sampled Chabrier2003 IMF and metallicities $Z=0.0001$, 0.0002, 0.0005, 0.001, 0.002, 0.004, 0.006, 0.008, 0.010, 0.014, 0.017, 0.02, 0.03 and 0.04, colour-coded as indicated on the right-hand scale (for clarity, the ordinate scale in each panel has been adjusted to reflect the dynamic range spanned by the strength of the corresponding index). Also shown next to the index name in each panel is the typical measurement error in the IUE spectra of the LMC clusters observed by Cassatella1987. In each panel, the dotted vertical line marks the age of 10 Myr, before which nebular emission can strongly affect index strengths (see text).
• Figure 2: Strengths of the 19 Fanelli1992 ultraviolet spectral indices defined in Table \ref{['tab:tabindices']} plotted against age, for SSPs with a stochastically sampled Chabrier2003 IMF and integrated stellar masses $\hbox{$M_{\rm{cl}}$}=10^3\hbox{M$_{\rm{\odot}}$}$ (cream), $10^4\hbox{M$_{\rm{\odot}}$}$ (teal blue) and $10^5\hbox{M$_{\rm{\odot}}$}$ (indian red), for the metallicity $Z=0.017$ (for clarity, the ordinate scale in each panel has been adjusted to reflect the dynamic range spanned by the strength of the corresponding index). In each panel, the filled area shows the range spanned by 99 per cent of 1100 SSP realisations with these three target stellar masses at each age, while crosses show the remaining 1 per cent of most distant outliers. The dotted vertical line has the same meaning as in Fig. \ref{['fig:indmet']}.
• Figure 3: Absolute magnitudes of the same models as in Fig. \ref{['fig:ind017']} [with integrated stellar masses $\hbox{$M_{\rm{cl}}$}=10^3\hbox{M$_{\rm{\odot}}$}$ (cream), $10^4\hbox{M$_{\rm{\odot}}$}$ (teal blue) and $10^5\hbox{M$_{\rm{\odot}}$}$ (indian red), for the metallicity $Z=0.017$], computed through the GALEX FUV and NUV, SDSS $ugriz$ and 2MASS $JHK_\textsc{s}$ filters ( GALEX and SDSS magnitudes are on the AB system, while 2MASS ones are on the Vega system). For comparison, black curve shows the evolution of a $10^6$ M$_{\rm{\odot}}$, $Z=0.017$ model with a smoothly sampled IMF.
• Figure 4: Distribution of the difference in index strength between best-fitting model and observed spectra, in units of the observational error, for the 10 clusters in the Cassatella1987 sample and the 19 Fanelli1992 ultraviolet spectral indices defined in Table \ref{['tab:tabindices']} (as indicated). For each index, the filled (green) and hatched (violet) histograms show the distributions obtained using models with stochastically sampled and smoothly sampled IMFs, respectively, while the black dot-dashed line shows a reference Gaussian distribution with unit standard deviation.
• Figure 5: Posterior probability distributions of age (top row), metallicity (middle row) and stellar mass (bottom row) for the 10 clusters in the Cassatella1987 sample, obtained as described in Section \ref{['sec:paramest']} using models with stochastically sampled (green filled histograms) and smoothly sampled (violet hatched histograms) IMFs.