The Search for HI Emission at z $approx0.4$ in Gravitationally Lensed Galaxies with the Green Bank Telescope

Neutral Hydrogen (HI) provides a very important fuel for star formation, but is difficult to detect at high redshift due to weak emission, limited sensitivity of modern instruments, and terrestrial radio frequency interference (RFI) at low frequencies. We the first attempt to use gravitational lensing to detect HI line emission from three gravitationally lensed galaxies behind the cluster Abell 773, two at redshift of 0.398 and one at z=0.487, using the Green Bank Telescope. We find a 3 sigma upper limit for a galaxy with a rotation velocity of 200 km/s is M_HI=6.58x10^9 and 1.5x10^10 M_solar at z=0.398 and z=0.487. The estimated HI masses of the sources at z=0.398 and z=0.487 are a factor of 3.7 and ~30 times lower than our detection limits at the respective redshifts. To facilitate these observations we have used sigma clipping to remove both narrow- and wide-band RFI but retain the signal from the source. We are able to reduce the noise of the spectrum by ~25% using our routine instead of discarding observations with too much RFI. The routine is most effective when ~10 of the integrations or fewer contains RFI. These techniques can be used to study HI in highly magnified distant galaxies that are otherwise too faint to detect.

Green Bank Telescope observations of low column density HI around NGC 2997 and NGC 6946

Observations of ongoing HI accretion in nearby galaxies have only identified about 10% of the needed fuel to sustain star formation in these galaxies. Most of these observations have been conducted using interferometers and may have missed lower column density, diffuse, HI gas that may trace the missing 90% of gas. Such gas may represent the so-called cold flows predicted by current theories of galaxy formation to have never been heated above the virial temperature of the dark matter halo. As a first attempt to identify such cold flows around nearby galaxies and complete the census of HI down to N(HI)~10^18 cm^-2, I used the Robert C. Byrd Green Bank Telescope (GBT) to map the circumgalactic (r < 100-200 kpc) HI environment around NGC 2997 and NGC 6946. The resulting GBT observations cover a four square degree area around each galaxy with a 5-sigma detection limit of N(HI)~10^18 cm^-2 over a 20 km/s linewidth. This project complements absorption line studies, which are well-suited to the regime of lower N(HI). Around NGC 2997, the GBT HI data reveal an extended HI disk and all of its surrounding gas-rich satellite galaxies, but no filamentary features. Furthermore, the HI mass as measured with the GBT is only 7% higher than past interferometric measurements. After correcting for resolution differences, the HI extent of the galaxy is 23% larger at the N(HI)~1.2x10^18 cm^-2 level as measured by the GBT. On the other hand, the HI observations of NGC 6946 reveal a filamentary feature apparently connecting NGC 6946 with its nearest companions. This HI filament has N(HI)~10^18 cm^-2 and a FWHM of 55+-5 km/s and was invisible in past interferometer observations. The properties of this filament are broadly consistent with being a cold flow or debris from a past tidal interaction between NGC 6946 and its satellites.

GHIGLS: HI mapping at intermediate Galactic latitude using the Green Bank Telescope

This paper introduces the data cubes from GHIGLS, deep Green Bank Telescope surveys of the 21-cm line emission of HI in 37 targeted fields at intermediate Galactic latitude. The GHIGLS fields together cover over 1000 square degrees at 9.55 spatial resolution. The HI spectra have an effective velocity resolution about 1.0 km/s and cover at least -450 < v < +250 km/s. GHIGLS highlights that even at intermediate Galactic latitude the interstellar medium is very complex. Spatial structure of the HI is quantified through power spectra of maps of the column density, NHI. For our featured representative field, centered on the North Ecliptic Pole, the scaling exponents in power-law representations of the power spectra of NHI maps for low, intermediate, and high velocity gas components (LVC, IVC, and HVC) are -2.86 +/- 0.04, -2.69 +/- 0.04, and -2.59 +/- 0.07, respectively. After Gaussian decomposition of the line profiles, NHI maps were also made corresponding to the narrow-line and broad-line components in the LVC range; for the narrow-line map the exponent is -1.9 +/- 0.1, reflecting more small scale structure in the cold neutral medium (CNM). There is evidence that filamentary structure in the HI CNM is oriented parallel to the Galactic magnetic field. The power spectrum analysis also offers insight into the various contributions to uncertainty in the data. The effect of 21-cm line opacity on the GHIGLS NHI maps is estimated.

The 2X-HI disks of spiral galaxies

The outskirts of galaxies - especially the very extended HI disks of galaxies - are strongly affected by their local environment. I highlight the giant 2X-HI disks of nearby galaxies (M 83, NGC 3621, and NGC 1512), studied as part of the Local Volume HI Survey (LVHIS), their kinematics and relation to XUV disks, signatures of tidal interactions and accretion events, the MHI - DHI relation as well as the formation of tidal dwarf galaxies. - Using multi-wavelength data, I create 3D visualisations of the gas and stars in galaxies, with the shape of their warped disks obtained through kinematic modelling of their HI velocity fields.

HI constraints from the cross-correlation of eBOSS galaxies and Green Bank Telescope intensity maps

We present the joint analysis of Neutral Hydrogen (HI) Intensity Mapping observations with three galaxy samples: the Luminous Red Galaxy (LRG) and Emission Line Galaxy (ELG) samples from the eBOSS survey, and the WiggleZ Dark Energy Survey sample. The HI intensity maps are Green Bank Telescope observations of the redshifted 21cm emission on 100deg2 covering the redshift range $0.6<z<1.0$. We process the data by separating and removing the foregrounds with FastICA, and construct a transfer function to correct for the effects of foreground removal on the HI signal. We cross-correlate the cleaned HI data with the galaxy samples and study the overall amplitude as well as the scale-dependence of the power spectrum. We also qualitatively compare our findings with the predictions by a semi-analytic galaxy evolution simulation. The cross-correlations constrain the quantity $Omega_{{HI}} b_{{HI}} r_{{HI},{opt}}$ at an effective scale $k_{eff}$, where $Omega_{HI}$ is the HI density fraction, $b_{HI}$ is the HI bias, and $r_{{HI},{opt}}$ the galaxy-hydrogen correlation coefficient, which is dependent on the HI content of the optical galaxy sample. At $k_{eff}=0.31 , h/{Mpc}$ we find $Omega_{{HI}} b_{{HI}} r_{{HI},{Wig}} = [0.58 pm 0.09 , {(stat) pm 0.05 , {(sys)}}] times 10^{-3}$ for GBT-WiggleZ, $Omega_{{HI}} b_{{HI}} r_{{HI,{ELG}}} = [0.40 pm 0.09 , {(stat) pm 0.04 , {(sys)}}] times 10^{-3}$ for GBT-ELG, and $Omega_{{HI}} b_{{HI}} r_{{HI},{LRG}} = [0.35 pm 0.08 , {(stat) pm 0.03 , {(sys)}}] times 10^{-3}$ for GBT-LRG, at $zsimeq 0.8$. We also report results at $k_{eff}=0.24 , h/{Mpc}$ and $k_{eff}=0.48 , h/{Mpc}$. With little information on HI parameters beyond our local Universe, these are amongst the most precise constraints on neutral hydrogen density fluctuations in an underexplored redshift range.