Detection of the Diffuse H i Emission in the Circumgalactic Medium of NGC 891 and NGC 4565

Abstract

We present detections of 21 cm emission from neutral hydrogen (H I) in the circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565 using the Robert C. Byrd Green Bank Telescope (GBT). With our 5σ sensitivity of 8.2 10 16 cm −2 calculated over a 20 km s −1 channel, we achieve s >5 detections out to -90 120 kpc along the minor axes. The velocity width of the CGM emission is as large as that of the disk »500 km s −1 , indicating the existence of a diffuse component permeating the halo. We compare our GBT measurements with interferometric data from the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the H Iemission from the disk at high signal-to-noise ratio but has limited surface brightness sensitivity at the angular scales probed with the GBT. After convolving the WSRT data to the spatial resolution of the GBT (FWHM=9 1), we find that the emission detected by the WSRT accounts for -+ 48 25 15 % ( -+ 58 18 4 %) of the total flux recovered by the GBT from the CGM of NGC 891 (NGC 4565). The existence of significant GBT-only flux suggests the presence of a large amount of diffuse, low column density H I emission in the CGM. For reasonable assumptions, the extended diffuse H I could account for 5.2±0.9% and 2.0±0.8% of the total H I emission of NGC 891 and NGC 4565. Unified Astronomy Thesaurus concepts: Circumgalactic medium (1879); Extragalactic astronomy (506); H I line emission (690); Radio spectroscopy (1359); Single-dish antennas (1460); Diffuse radiation (383); Extended radiation sources (504)

A Census of the Extended Neutral Hydrogen around 18 MHONGOOSE Galaxies

“…This may seem like an intuitive result if H I permeates the CGM of most galaxies, and yet it has evaded most observational studies of H I emission in the CGM of galaxies. This result is consistent with Das et al (2020), where H I emission was detected in each measurement at increasing distances along the minor axis of the two galaxies, NGC 891 and NGC 4565. These detections could be the outcome of ubiquitous CGM H I, which, in mapping observations such as this current work, would result in a larger amount of H I detected over larger physical areas.…”

“…A dip in the profile is likely indicative of an unphysical negative feature in the image, causing the cumulative flux to decrease at that radius. If indeed we see rising cumulative flux levels at increasing radii, these would be consistent with the detection of H I at large impact parameters as is seen in Das et al (2020), where H I is detected at each point along the minor axis out to ∼120 kpc.…”

A Census of the Extended Neutral Hydrogen Around 18 MHONGOOSE Galaxies

“…A dip in the profile is likely indicative of an unphysical negative feature in the image, causing the cumulative flux to decrease at that radius. If indeed we see rising cumulative flux levels at increasing radii, these would be consistent with the detection of Hi at large impact parameters as is seen in Das et al (2020) where Hi is detected at each point along the minor axis out to ∼ 120 kpc.…”

GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption

“…Simulations, like that of Nuza et al (2019), put this process along with accretion by vertical infall in context over cosmological time scales. The CGM seen in faint 21-cm emission with the Green Bank Telescope around many nearby spirals (Pisano 2014;Pingel et al 2018;Martin et al 2019;Das et al 2020;Sardone et al 2021) is the reservoir from which the cold-mode accretion flows. The structure, dynamics, thermodynamics, and ionization of the CGM are not yet fully understood; they present many compelling astrophysical questions that may ultimately tie together the physics of the interstellar medium in spiral disks with the process of galaxy formation studied through cosmological simulations and high redshift absorption line surveys.…”