Probing the Magnetic Fields in Cosmic Web Filaments

We present a project to constrain the properties of magnetic fields in Cosmic Web filaments. We first perform rotation measure (RM) synthesis on 24 S-band Karl G Jansky Very Large Array (VLA) observations of moderate-redshift active galactic nuclei (AGN). These observations are combined with an existing catalog of Cosmic Web filaments in order to determine the number of filaments a given sight-line passes near or through. By tracking how observables such as rotation measure and polarization fraction change as a function of the number of intervening cosmic web filaments, we take the first steps towards providing observational constraints on the magnetic field strength within these structures. We find that the dispersion in RM increases as the distance between a line of sight and the filament closest to that line of sight decreases, suggesting that we are detecting a signal from magnetic fields within these structures

ALMA reveals a stable rotating gas disk in a paradoxical low-mass, ultra-dusty galaxy at z = 4.274

We report ALMA detections of [CII] and dust continuum in Az9, a multiply-imaged galaxy behind the Frontier Field cluster MACSJ0717.5+3745. The bright [CII] emission line provides a spectroscopic redshift of z = 4.274. This strongly lensed (mu = 7 +/- 1) galaxy has an intrinsic stellar mass of only 2e9 Msun and a total star formation rate of 26 Msun/yr (~80% of which is dust obscured). Using public magnification maps, we reconstruct the [CII] emission in the source plane to reveal a stable, rotation-dominated disk with V/sigma = 5.3, which is > 2x higher than predicted from simulations for similarly high-redshift, low-mass galaxies. In the source plane, the [CII] disk has a half-light radius of 1.8 kpc and, along with the dust, is spatially offset from the peak of the stellar light by 1.4 kpc. Az9 is not deficient in [CII]; L[CII]/LIR = 0.0027 consistent with local and high redshift normal star forming galaxies. While dust-obscured star formation is expected to dominate in higher mass galaxies, such a large reservoir of dust and gas in a lower mass disk galaxy 1.4 Gyr after the Big Bang challenges our picture of early galaxy evolution. Furthermore, the prevalence of such low-mass dusty galaxies has important implications for the selection of the highest redshift dropout galaxies with JWST. As one of the lowest stellar mass galaxies at z > 4 to be detected in dust continuum and [CII], Az9 is an excellent laboratory in which to study early dust enrichment in the interstellar medium.Comment: Accepted for publication in ApJ Letter

Delayed Stellar Mass Assembly in the Low Surface Brightness Dwarf Galaxy KDG215

We present HI spectral line and optical broadband images of the nearby low surface brightness dwarf galaxy KDG215. The HI images, acquired with the Karl G. Jansky Very Large Array (VLA), reveal a dispersion dominated ISM with only weak signatures of coherent rotation. The HI gas reaches a peak mass surface density of 6 M$_{\odot}$ pc$^{-2}$ at the location of the peak surface brightness in the optical and the UV. Although KDG215 is gas-rich, the H$\alpha$ non-detection implies a very low current massive star formation rate. In order to investigate the recent evolution of this system, we have derived the recent and lifetime star formation histories from archival Hubble Space Telescope images. The recent star formation history shows a peak star formation rate $\sim$1 Gyr ago, followed by a decreasing star formation rate to the present day quiescent state. The cumulative star formation history indicates that a significant fraction of the stellar mass assembly in KDG215 has occurred within the last 1.25 Gyr. KDG215 is one of only a few known galaxies which demonstrates such a delayed star formation history. While the ancient stellar population (predominantly red giants) is prominent, the look-back time by which 50% of the mass of all stars ever formed had been created is among the youngest of any known galaxy.Comment: Accepted for publication in the Astrophysical Journal Letter