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Register a new userGHIGLS: HI mapping at intermediate Galactic latitude using the Green Bank Telescope
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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.
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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.
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.
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.
Observations of Galactic H I gas for seven targeted regions at intermediate Galactic latitude are presented at 1 angular resolution using data from the DRAO Synthesis Telescope (ST) and the Green Bank Telescope (GBT). The DHIGLS data are the most extensive arcminute resolution measurements of the diffuse atomic interstellar medium beyond those in the Galactic plane. The acquisition, reduction, calibration, and mosaicking of the DRAO ST data and the cross calibration and incorporation of the short-spacing information from the GBT are described. The high quality of the resulting DHIGLS products enables a variety of new studies in directions of low Galactic column density. We analyze the angular power spectra of maps of the integrated H I emission (column density) from the data cubes for several distinct velocity ranges. Fitting power spectrum models based on a power law, but including the effects of the synthesized beam and noise at high spatial frequencies, we find exponents ranging from -2.5 to -3.0.. Power spectra of maps of the centroid velocity for these components give similar results. These exponents are interpreted as being representative of the 3D density and 3D velocity fields of the atomic gas, respectively. We find evidence for dramatic changes in the H I structures in channel maps over even small changes in velocity. This narrow line emission has counterparts in absorption spectra against bright background radio sources, quantifying that the gas is cold and dense and can be identified as the cold neutral medium phase. Fully reduced DHIGLS H I data cubes and other data products are available at www.cita.utoronto.ca/DHIGLS.
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