A Random Search for Galactic Neutral Hydrogen High-Velocity Clouds: Preliminary Results

Sara M. Petty-Powell

The Evergreen State College, 2700 Evergreen Parkway N, Olympia, WA, 98505

Vince Urick2

Department of Physics, Bloomsburg University, 400 East 2nd Street, Bloomsburg, PA 17815

Felix J. Lockman

National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944


Edward M. Murphy

Department of Physics and Astronomy, Johns Hopkins University, 300 North Charles Street, Baltimore, MD 21218


This paper is the continuation of the Murphy, Lockman & Savage (1995) analysis. A sensitive search was conducted in the direction of approximately 275 quasars to detect neutral hydrogen high-velocity clouds above and below the galactic plane using the 43 m telescope at the Green Bank National Radio Astronomy Observatory. Preliminary results indicate a 14% high-velocity cloud detection. A table of the objects and their analysis is included.

1. Introduction

Galactic H I high-velocity clouds (HVCs) are redshifting and blueshifting clouds that defy galactic rotation and represent a phenomenon currently open to conjecture. The Magellanic Stream has been described as the tidal debris of the Magellanic Clouds (Wakker 1991; Murai & Fujimoto 1980; Lin & Lynden-Bell 1982), but other HVC regions cannot be explained by this model. With the exception of the Magellanic Stream, the origin of high-velocity clouds can be explained in many different models, but none are conclusive. Because most distances to the HVC phenomenon are unknown, little can be resolved concerning their physical properties. This survey, the second part of the Murphy, Lockman, and Savage (1995) (hereafter MLS) velocity cloud search, defines a high-velocity cloud as having |VLSR| 9B 100km s-1 for positions above and below the galactic plane, and the radial velocity must be inconsistent with galactic rotation.

In the Hulsbosch and Wakker (1988) Dwingeloo survey, HVC phenomenon is found to be evenly distributed in galactic longitude and concentrated along the galactic plane (Wakker 1991).

Several ideas exist concerning the origin of the HVC phenomenon. The Galactic origin (Oort 1970) suggests the velocities were initiated through accretion and exists within the galactic halo. These velocities should be consistent with a gravitational potential velocity due to the galaxy.

The Local Group HVC hypothesis (Blitz et al. 1999) proposes that the clouds can be described as members of the Local Group, which can enhance the current understanding of the galaxy formation processes. The cloud velocities should have velocities conducive to motions within the gravitational potential of the Local Group. Blitz's (1999) model of the Local Group infall resulted in describing two complexes elicited in the Wakker (1991) Aitoff projection of the Dwingeloo HVC survey.

Another model for the HVC phenomena, called the "galactic fountain" (see Blitz 1999 and references therein), suggests that the clouds were initiated by supernovae explosions which heated gas to temperatures of ~106 K. Through convection, the gas would expand out to the Galactic corona, cool and fall back toward the galactic plane. A high metallicity and |VLSR| 8B 200 km s-1 would need to be observed to substantiate this model.

In the MLS sensitive 21-cm Galactic HVC search, a 5s detection level is stated giving a complete detection limit of NH3D 7x 1017 cm-2. The survey calculations revealed the number of clouds in a given column density interval were peaked at roughly 1 x 1018 cm-2. The boundaries for clouds detected in an interval are at 1 x 1017 cm-2 and 1 x 1020 cm-2. The validity of this decrease in cloud detection may be resolved in this second part of the analysis, due to the completeness limit of NH3D 3 x 1017 cm-2 for Urick and I. The first part of this survey resulted in a 37% detection (Murphy, Lockman, & Savage 1995). The Hulsbosch & Wakker (1988) and Bajaja et al. (1985) combined all-sky 21-cm data indicate an 18% coverage at a completeness limit of 2 x 1018 cm-2 (see MLS and references therein).

This paper reports preliminary results of the remaining quasar positions as observed by MLS.

2. Data & Analysis

The MLS survey was conducted by using the NRAO 43 m telescope to observe in the directions of approximately 275 distant quasars in a random search for 21-cm H I high-velocity clouds. The spectra cover the VLSR range of -1000 km s-1 to +800 km s-1, have a typical rms of 5.4 mK, and are hanning smoothed where needed (such as when "ringing" occurs). Position switching was used for reduction, and for situations of unresolvable spectra, frequency switched data was obtained. The observed positions are toward the quasar and 1.BA71 cosd directly east and west of the quasar. The resolution of the spectra is 8 km s-1 for hanning smoothed spectra, 4 km s-1 for position switched, and 1km s-1 for frequency switched spectra.

For any spectra the intensity, Ion, is

Ion3D G[Ton(u) + Tsys(u) +Tsr(u)], (1)

where G is the gain, and Ton(u), Tsys(u), and Tsr(u) are the brightness temperature functions of the hydrogen, system, and stray radiation, respectively. Stray radiation enters the receiver and comes from wide angles. For a reference spectra the intensity, Ioff, is defined as

Ioff3D G[Toff(u) + Tsys(u) +Tsr(u)]. (2)

Where Toff(u) is the detected hydrogen of the "off" spectra.

The convention adopted for reduction of the position switch is

(Ion - Ioff ) / Ioff . (3)

Using equations (1) and (2), and multiplying through by Tsys

THI3D Tsys (Ton 96 Toff) / (Toff + Tsys + Tsr). (4)

Because Tsr 8B8B Tsys and Tsr 8B Toff,, the stray radiation is effectively removed.

The frequency switched spectra is not referenced and therefor cannot have a stray radiation elimination using the convention of equation (3). A program was written by Murphy to correct for this when necessary.

A problem with position switching occurs when emission is detected in all positions at the same peak velocities. As equation (3) shows, the "on" will be subtracted from by the "off", and frequency switched data is needed to resolve the spectra.

Gaussian fits were used to determine the column densities of the clouds. The definition of the column density of a high-velocity cloud is (Lockman 1990)

NH(t8B8B1)3D 1.823 x 1018 ∫ Tb dv cm-2 . (5)

Where Tb , in Kelvin, is the peak temperature brightness and dv, in km s-1, is the Gaussian half-width.

3. Preliminary Results

A table of spectra that have been completed is included at the end of this paper. It contains the object, heliocentric and galactic positions, the peak temperature, half-width, velocity, and rms noise for the Gaussian and baseline. The errors are based on the deviation from a Gaussian curve fit. Galaxy detections have been omitted. This is a preliminary table and will be changed upon completion of the full data reduction.

The average s of the baseline for the sample is 5.8 mK. An estimated 14% detection of HVCs was calculated. This is closer to the Hulsbosch & Wakker (1988) and Bajaja et al. (1985) result, than the MLS result. The current data being corrected are also detections, and would change the actual number to ~20%. The highest blue-shifted (negative velocity) object has VLSR 3D -866 km s-1, while the most extreme red-shifted object has VLSR 3D 740 km s-1.

Figure 1. displays the percent of the sample versus the log of the column density. There appears to be a similar drop off at NH3D 1 x1018 cm-2 as with the MLS survey. Assuming the sensitivity limits are accurate, this could imply a genuine drop in the number of clouds at this column density. The maximum column density lies in the 1 x 1020 cm-2 range before no clouds are detected, which is consistent with MLS. The non-detection at the lower end of the column density could be from sensitivity limits, while the higher end is probably the actual drop in the existence of HVCs.

The Aitoff projection in figure 2. shows the part of sky observed. Because of its bright emission at the 21-cm line, the galactic plane is excluded. The region of latitude 9630 to 9690 degrees at longitude 60 is below the telescope horizon. The two histograms in figure 2. indicate the number of directions covered versus the longitude and latitude. The positions suggest a random distribution throughout the visible sky outside of the galactic plane.

3.2 Neutral Hydrogen Galaxy Detections

Four galaxy detections were verified (using the NED search by position) for spectra containing fits comparable to that of a neutral hydrogen galaxy detection. The four galaxies include M51, UGC5340, NGC4242, and VCC0381. Because of the velocity range of the spectra, more galaxies were not detected. Table 2. displays the velocities with respect to the LSR and GSR.




Table 2. Galaxies detected with their positions and velocities

with respect to LSR and GSR.


























4. Concluding Remarks & Future Work

The results of this part of the survey are preliminary. The method of analysis appears reliable and will be continued to finish the remainder of the survey. The percent of HVCs detected resembles the calculations of the Hulsbosch & Wakker (1988) and Bajaja et al. (1985) survey. There exists an apparent decrease in the amount of HVCs at 1 x 1018 cm-2 in both the MLS and this survey. This could be real unless, once completed, the trouble data reveal smaller column densities. If the column densities drop below 7 x 1017 they might not be detected in this survey because of covering a small portion of the sky, or having been ionized (Murphy, Lockman, & Savage 1995). A more quantitative and careful study of the detections will be constructed upon completion of the trouble data. The finished results should extend and improve the current understandings of the HVC phenomenon.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.



Blitz, L., Spergel, D.N., Teuben, P.J., Hartmann, D., Burton, W.B. 1999, APJ, 514, 818

Hulsbosch, A.N.M., & Wakker, B.P. 1988, A&AS, 75, 191

Lin, D.N.C., & Lynden-Bell, D. 1982, MNRAS, 198, 707

Lockman, F.J. 1990, Ann. Rev. A&A, 28, 215

Murai, T., & Fujimoto, M. 1980, PASJ, 32, 581

Murphy, E.M., Lockman, F.J., & Savage, B.D. 1995, APJ, 447, 642

Oort, J.H. 1970, A&A, 7, 381

Wakker, B.P., & van Woerden, H. 1991, A&A, 250, 509