Title: blue monsters. Why are JWST very early massive galaxies so blue?
Authors: Francesco Zibarro, Andrea Ferrara, Laura Somovigo, Mahsa Quandil
First Author Foundation: Scuola Normale Superiore, Pisa, Italy
condition: Submitted to MNRAS, available on arXiv
The James Webb Space Telescope (JWST) is already reshaping our understanding of the formation and evolution of galaxies, and continues to break the record for the oldest known galaxy! It turns out that some of these early galaxies may challenge current models of galaxies and their dust. Today’s authors are looking at a group of galaxies discovered by JWST that are metal-rich, extremely bright, and massive. A galaxy with these qualities usually contains a lot of dust, and therefore will undergo significant attenuation – reddening of light due to dust. However, the residual ultraviolet frame emission of this group of galaxies (observed with the JWST infrared due to its high redshift) makes it appear completely blue, and the composition of its spectral energy distributions (SEDs) reveals that these galaxies experience minimal attenuation. Where did all the dust go? The authors suggest two explanations for how to get a bright, fertile galaxy without much visible dust: either the dust is being expelled, or the dust is not present where the emission of ultraviolet radiation occurs.
For the dust expulsion theory to work, two conditions must be met. The ejection of dust is driven by radiation pressure, the external force that comes from the emission of ultraviolet radiation in the galaxy. Radiation pressure is related to the galactic surface density star formation rate (SFR), or the number of stars a galaxy forms annually per unit area. The radiation pressure must overcome gravitational pressure, which is the pressure that keeps galactic matter together, for the outflows to occur. The ratio of radiation to gravitational pressure is known as the Eddington ratio, and if this ratio is greater than one, the radiation pressure exceeds the gravitational pressure. By looking at how the Eddington ratio depends on the surface density of the SFR, the author examined the type of galaxy that could feed the dust flow. The density of star formation depends on the parameter κs, which is a measure of a galaxy’s explosion – essentially, a higher value for this parameter indicates a higher density of star formation. As shown by the green curve in Figure 1, only with κs= 3.3 or more The Eddington ratio can be exceeded, indicating that dust ejection will occur in starburst-type galaxies.
In addition, for dust removal to be effective in removing dust, the dust expelling rate must be higher than the dust production rate. Figure 2 shows how the outflow rate depends on the density of the SFR surface—only in the dark shaded region to the left of the red line will the outflow rate exceed dust production. Although dust ejection is a plausible theory for the apparent lack of dust in bright blue galaxies observed by JWST, the authors note that some galaxies with high redshifts may not meet the physical criteria for dust ejection and could instead fall into a dust accretion regime. .
But what if the dust is not taken out? The authors suggest another idea: perhaps the ultraviolet emission comes mostly from the diffuse interstellar medium, while the dust is most concentrated in the infrared emitting giant molecular clouds. With this assumption, the authors can make predictions of a minimum flux in the far infrared, which may originate from dusty molecular clouds. Because of the large redshift of these galaxies, their far infrared emission is redshifted to millimeter ranges observable with ALMA. For the high redshift galaxy GHZ2/GL-z13 observed with both JWST and ALMA, the far-infrared flux decreased well below the expected minimum. The authors suggest that this result is more consistent with a dust-ejection scenario where the galaxy is dim in the far infrared due to its lack of dust.
Mysterious creatures have emerged with JWST that may shake current galactic formation and evolution models to the core! Dust ejection or its spatial distribution may explain why some bright, high redshift galaxies appear blue, but for a single galaxy with ALMA data, an ejection model seems more likely. Observations with facilities like ALMA can test physical theories about strange new discoveries from JWST, many of which are sure to come.
Edited by William Palmer
Image Credits: NASA, ESA, CSA, STScI, and Openclipart
About Sarah Bodansky
I am a first year graduate student at the University of Massachusetts Amherst studying galaxies. My current research focuses on using observations to better understand the evolution of dust mass in star-forming galaxies. Outside of research, I enjoy reading, cooking, and hanging out with my cat.