Star Formation Diagnostics in the Cosmic Eye


The increase in sensitivity of millimeter, submillimeter, and infrared instrumentation has allowed for the expansion of analysis of extragalactic sources in these wavelength bands. Multiwavelength study of distant galaxies is essential to creating a complex and multifaceted understanding of their properties. Studying gravitationally lensed galaxies allows astronomers to circumvent distance-related limitations as they push the boundaries of sensitivity within the field. We present analysis of Atacama Large subMillimeter Array (ALMA) observations of the Cosmic Eye, a gravitationally lensed Lyman Break Galaxy (LBG) at a redshift of ∼3. These observations center on the [CII] 158µm and the [NII] 205µm fine structure lines, which trace the hardness of stellar radiation in a galaxy, allowing insights into the nature of star formation. The high gravitational magnification (30×) allows us to resolve the line emission in detail, allowing for a positionresolved analysis of their ratio. We find that the line ratio varies by a factor of ∼ 2 ± 0.3 between extreme regions of the galaxy, suggesting that the galaxy’s internal structure affects this ratio. We find that the Cosmic Eye’s global [CII]/[NII] line ratio sits between those found in two higher-redshift LBGs. Likely, the [CII]/[NII] ratio is affected not only by star formation but also by dust within a galaxy. We use the rest-frame far-infrared (FIR) continuum in our ALMA observations in conjunction with archival restframe UV/optical Hubble Space Telescope (HST) observations in order to conduct an analysis of the IRX-β relation within the Cosmic Eye, which relates diagnostics of dust attenuation of UV radiation within our galaxy. The Cosmic Eye thus is a compelling object to study and a stepping stone for refining our understanding of the nature of star-formation diagnostics out to high-redshift.

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