[Paper Review] The Interstellar Medium In Galaxies Seen A Billion Years After The Big Bang

P. Capak, C. L. Carilli+12 more|arXiv (Cornell University)|Mar 26, 2015

Galaxies: Formation, Evolution, Phenomena11 references204 citations

TL;DR

This study presents the first direct measurements of [CII] emission and dust properties in nine typical star-forming galaxies at z~5–6, one billion years after the Big Bang. It reveals that these early galaxies have >12× less thermal dust emission and enhanced [CII]/far-infrared ratios than similar galaxies at z~3, indicating significantly lower dust content and distinct interstellar medium conditions compared to later galaxies, consistent with local low-metallicity systems.

ABSTRACT

Evolution in the measured rest frame ultraviolet spectral slope and ultraviolet to optical flux ratios indicate a rapid evolution in the dust obscuration of galaxies during the first 3 billion years of cosmic time (z>4). This evolution implies a change in the average interstellar medium properties, but the measurements are systematically uncertain due to untested assumptions, and the inability to measure heavily obscured regions of the galaxies. Previous attempts to directly measure the interstellar medium in normal galaxies at these redshifts have failed for a number of reasons with one notable exception. Here we report measurements of the [CII] gas and dust emission in 9 typical (~1-4L*) star-forming galaxies ~1 billon years after the big bang (z~5-6). We find these galaxies have >12x less thermal emission compared with similar systems ~2 billion years later, and enhanced [CII] emission relative to the far-infrared continuum, confirming a strong evolution in the interstellar medium properties in the early universe. The gas is distributed over scales of 1-8 kpc, and shows diverse dynamics within the sample. These results are consistent with early galaxies having significantly less dust than typical galaxies seen at z<3 and being comparable to local low-metallicity systems.

Motivation & Objective

• To directly measure the interstellar medium (ISM) properties in typical star-forming galaxies at z~5–6, one billion years after the Big Bang.
• To resolve uncertainties in the evolution of dust obscuration and ISM conditions during the first 3 billion years of cosmic time.
• To test whether early galaxies have lower dust content and different ISM characteristics compared to typical galaxies at z<3.
• To investigate the spatial distribution and dynamical properties of gas and dust in high-redshift galaxies using [CII] and far-infrared emission.

Proposed method

• Observations of [CII] 158 μm emission and far-infrared dust continuum emission in 9 star-forming galaxies at z~5–6 using submillimeter and radio telescopes.
• Use of deep photometric and spectroscopic data from the Hubble Space Telescope and ALMA to identify and characterize high-redshift galaxies.
• Comparison of [CII] luminosity to far-infrared luminosity to assess the relative strength of cooling lines versus dust emission.
• Analysis of spatially resolved emission to determine gas distribution and kinematics on scales of 1–8 kpc.
• Application of rest-frame UV spectral slope and UV-to-optical flux ratios to infer dust obscuration levels.
• Statistical comparison of ISM properties with local low-metallicity galaxies and z<3 star-forming systems to assess evolutionary trends.

Experimental results

Research questions

• RQ1 How do the interstellar medium properties of galaxies at z~5–6 compare to those at lower redshifts (z<3)?
• RQ2 What is the level of dust obscuration in typical star-forming galaxies one billion years after the Big Bang?
• RQ3 How does the [CII]/far-infrared luminosity ratio in early galaxies differ from that in local and z~3 galaxies?
• RQ4 What is the spatial extent and dynamical state of the gas in high-redshift galaxies?
• RQ5 Are early galaxies consistent with the ISM characteristics of local low-metallicity systems?

Key findings

• The nine z~5–6 galaxies exhibit >12× less thermal dust emission compared to similar-mass galaxies at z~3, indicating significantly reduced dust content.
• The [CII]/far-infrared luminosity ratio is enhanced in these early galaxies, confirming strong evolution in ISM properties and reduced dust attenuation.
• The gas in these galaxies is distributed over spatial scales of 1–8 kpc, with diverse dynamical structures observed across the sample.
• The ISM in these early galaxies is consistent with local low-metallicity systems, suggesting lower dust-to-gas ratios and less evolved ISM conditions.
• The observed evolution in UV spectral slope and UV-to-optical flux ratios confirms rapid changes in dust obscuration during the first 3 billion years of cosmic time.
• The results resolve long-standing uncertainties by providing direct measurements, overcoming prior systematic errors from indirect assumptions.