With the increase in large surveys imaging the sky in multiple bandpasses, the number of galaxies with multi-wavelength imaging has increased dramatically over the last decade.
This information enables us to group galaxies together by the shapes of their observed spectral energy distributions (SEDs). When the galaxies in question span a range of redshifts, imaging probes different rest-frame wavelengths. By de-redshifting and combining the multiband imaging of galaxies across a redshift range, we fill in the ‘gaps’ in the photometry.
These composite SEDs allow analysis of features such as emission lines which are normally only available with spectroscopy. The details of our methodology are contained in Forrest, et al. (2018). This is based on the composite SED grouping method introduced in Kriek, et al. (2011).
Composite SEDs can be downloaded here. Users are highly advised to familiarize themselves with the details of construction before using.
The IRX-B Relation
The most massive, shortest lived stars emit large amounts of ultraviolet (UV) radiation. Due to their short lives, measuring the amount of UV light is therefore a reliable proxy for recent star formation.
However, many galaxies also have significant dust attenuation, which reduces the amount of this UV light observed- the energy is instead reemitted by the heated dust in the infrared (IR). As a result, measures of galaxy star formation use both UV and IR light when available.
The efficiency of this transformation depends on a variety of parameters, including how the dust is distributed relative to stars and what species of dust grain are present. By comparing the ratio of IR to UV light (the infrared excess, IRX) to the slope of the galaxy spectrum in the UV (β), one can infer the dust attenuation law.
In Forrest, et al. (2016), we built a set of composite SEDs from galaxies in the ZFOURGE survey at 1<z<3. Through the addition of far-IR data from the Candels-Herschel group, we plotted these composite SEDs on the IRX-β plane, finding results similar to a starburst attenuation curve.
Extreme Emission Line Galaxies
In 2009, Cardamone, et al. found a population of galaxies in SDSS with impressively strong [OIII] 5007 emission. These ‘green pea’ galaxies have since been followed up with great interest and generally have low metallicities and high (specific) star formation rates.
Similar galaxies at higher redshifts, termed extreme emission line galaxies (EELGs) have now been found and studied extensively. Through the construction of composite SEDs, we have found such galaxies at 1<z<4 (Forrest, et al., 2017; Forrest, et al., 2018). EELGs appear more commonly at higher redshifts, and may have played a significant role in cosmic reionization.
While many galaxies in the universe are actively forming new stars, there is a significant fraction that are not. While these ‘quiescent’ galaxies have become much more common over the lifetime of the cosmos, the mechanisms that cause galaxies to cease forming stars are not well understood.
Broadly speaking, if a galaxy runs out of gas to turn into stars, or if any remaining gas is prevented from collapsing gravitationally (via heating, ejection, or stabilization), star formation will shut down.
The key to unlocking the processes responsible is finding galaxies in the act of turning off their star formation. In Forrest, et al., 2018, we find a population of such galaxies, as identified by their composite SED D4000, emission lines, and dust content.
Interestingly, these galaxies have smaller sizes and larger Sérsic indices than star forming galaxies of similar mass, but larger sizes and smaller Sérsic indices than quiescent galaxies. These galaxies also become more prevalent as the universe ages, and their number densities imply quenching timescales on the order of 1-2 Gyr.