Abundance Of Neutral Hydrogen Research Articles

The evolution of cold neutral gas and the star formation history

There is a well known disparity between the evolution the star formation rate density, {\psi}*, and the abundance of neutral hydrogen (HI), the raw material for star formation. Recently, however, we have shown that {\psi}* may be correlated with the fraction of cool atomic gas, as traced through the 21-cm absorption of HI. This is expected since star formation requires cold (T ~ 10 K) gas and so this could address the issue of why the star formation rate density does not trace the bulk atomic gas. The data are, however, limited to redshifts of z < 2, where both {\psi}* and the cold gas fraction exhibit a similar steep climb from the present day (z = 0), and so it is unknown whether the cold gas fraction follows the same decline as {\psi}* at higher redshift. In order to address this, we have used unpublished archival observations of 21-cm absorption in high redshift damped Lyman-{\alpha} absorption systems to increase the sample at z > 2. The data suggest that the cold gas fraction does exhibit a decrease, although this is significantly steeper than {\psi}* at z ~ 3. This is, however, degenerate with the extents of the absorbing galaxy and the background continuum emission and upon removing these, via canonical evolution models, we find the mean spin temperature of the gas to be <T> ~ 3000 K, compared to the ~2000 K expected from the fit at z < 2. These temperatures are consistent with the observed high neutral hydrogen column densities, which require T < 4000 K in order for the gas not to be highly ionised.

Probing satellite galaxies in the Local Group by using FAST

The abundance of neutral hydrogen (HI) in satellite galaxies in the local group is important for studying the formation history of our local group. In this work, we generated mock HI satellite galaxies in the Local Group using the high mass-resolution hydrodynamic APOSTLE simulation. The simulated HI mass function agrees with the ALFALFA survey very well above 106 M⊙, although there is a discrepancy below this scale because of the observed flux limit. After carefully checking various systematic elements in the observations, including fitting of line width, sky coverage, integration time and frequency drift due to uncertainty in a galaxy’s distance, we predicted the abundance of HI in galaxies in a future survey that will be conducted by FAST. FAST has a larger aperture and higher sensitivity than the Arecibo telescope. We found that the HI mass function could be estimated well around 105 M⊙ if the integration time is 40 minutes. Our results indicate that there are 61 HI satellites in the Local Group and 36 in the FAST field above 105 M⊙. This estimation is one order of magnitude better than the current data, and will put a strong constraint on the formation history of the Local Group. Also more high resolution simulated samples are needed to achieve this target.

Early preheating and galaxy formation

Winds from pregalactic starbursts and 'miniquasars' may pollute the intergalactic medium (IGM) with metals and raise its temperature to a much higher adiabat than expected from photoionization, and so inhibit the formation of early galaxies by increasing the cosmological Jeans mass. We compute the thermal history of the IGM when it experiences a period of rapid, homogeneous 'preheating' at high redshifts, and the impact of such a global feedback mechanism on the IGM ionization state and the subsequent galaxy formation and evolution. Measurements of the temperature of the Lyα forest at redshift z ∼ 3 constrain the redshift and energy of preheating, and rule out models that preheat too late or to too high a temperature, i.e. to T I G M ? 10 6 K at z? 10. The IGM thermal history is used to estimate the effects of preheating on the formation of galaxies at later epochs, allowing us to predict galaxy luminosity functions in preheated universes. The results depend crucially on whether the baryonic smoothing scale in the IGM is computed globally, or in a local, density-dependent fashion (since the IGM temperature can become highly inhomogeneous in the post-preheating epoch). Using a globally averaged smoothing scale, we find that models with excessive preheating produce too few L * and fainter galaxies, and are therefore inconsistent with observational data. More moderate preheating scenarios, with T I G M ? 10 5 K at z ∼ 10, are able to flatten the faint-end slope of the luminosity function, producing excellent agreement with observations, without the need for any local feedback mechanism within galaxies. A density-dependent smoothing scale requires more energetic preheating to achieve the same degree of suppression in the faint-end slope. All models, however, appear unable to explain the sharp cut-off in the luminosity function at bright magnitudes - a problem that is also common to more conventional local feedback prescriptions. Supernova-driven preheating scenarios tend to raise the mean metallicity of the universe well above the minimum levels observed in the Lya clouds. The high energies associated with preheating cause a sharp drop in the abundance of neutral hydrogen in the IGM and are often sufficient to double ionize helium at high redshift, well before the 'quasar epoch'. We find that ionizing photon escape fractions must be significantly higher than 10 per cent in order to explain the low inferred H I fraction at z 6, particularly when using a globally averaged smoothing scale. While early preheating causes strong suppression of dwarf galaxy formation, we show that it is not able to reproduce the observed abundance of satellite galaxies in the Local Group in detail. The detailed thermal history of the universe during the formative early stages around z = 10-15 remains one of the crucial missing links in galaxy-formation and evolution studies.

Charge transfer and X-ray emission from supernova remnants

The X-ray line emission excited by charge transfer between neutral hydrogen and the hydrogenic or fully stripped ions of heavy elements has been computed for a grid of nonequilibrium ionization models for supernova remnants. The effects of charge transfer excitation on the X-ray spectra of supernova remnants are discussed. The charge transfer process depends very sensitively upon the abundance of neutral hydrogen behind the shock. Consequently, a careful treatment of the ionization of hydrogen was required. The model line fluxes are compared with those excited by conventional collisional processes involving electrons. Results are presented in the form of contour plots for both the flux excited by charge transfer and the ratio of the charge transfer flux to that associated solely with collisional processes involving electrons. Charge transfer never contributes more than 10 percent of the flux of the strongest electron collisionally excited lines, and its contribution is typically 0.001 to 10 to the -5th for a shock moving into a medium composed of neutral hydrogen.

Conditions and nature of gas flow in spiral-elliptical binary galactic systems

The process of gas flow from a spiral to an elliptical galaxy has been modeled numerically in the framework of the circular restricted three-body problem. It has been found that the possibility of gas loss by the spiral galaxy depends strongly on the relationship between the direction of its axial rotation and the orbital motion of the galaxies of the binary system. When these motions are in opposite directions, gas crossflow does not occur. In addition, an important parameter of the problem is the size of the disk of the spiral galaxy. According to the calculations, the rate at which gas can pass to the elliptical galaxy for a statistically average S-E pair with signs of interaction in the catalog of I.D. Karachentsev is /approx/0.2M/circled dot/ per year. This value is quite sufficient to explain the enhanced abundance of neutral hydrogen (/approx/10/sup 8/ M/circled dot/) in a number of galaxies of early types. The calculations have also shown that during the time of one or two revolutions of the disk of the spiral galaxy there is a strong distortion of its shape by the tidal interaction. This fact must be taken into account when the individual masses of spiral galaxiesmore » belonging to binary systems are determined from the rotation curves.« less

Observations of F‐region Vertical Velocities at Millstone Hill 2. Evidence for Fluxes into and Out of the Protonosphere

The vertical flux of O+ in the F region has been obtained from incoherent scatter radar measurements at Millstone Hill. The paper reports an investigation of the altitude variation of the flux during two 24‐hour periods in March taken a year apart. We find that except for a brief period around sunrise the flux is always directed downwards at altitudes below 500 km and its magnitude seems governed simply by the competing processes of production, loss, and diffusion. On the quiet day above 535 km the flux is directed upward in the afternoon when the electron density is not changing with time. This upward flux reached a peak value of ∼108 ions cm−2 sec−1 near 700 km altitude and then decreased at higher altitudes. This decrease is attributed to charge exchange with hydrogen. During the night on this day there is evidence for a downward flux of 3 × 107 O+ ions cm−2 sec−1 from the protonosphere. Additional H+ ions may be diffusing through the 1000‐km level and contributing to the maintenance of the nighttime F layer, but they would not be detected in our observations. On the disturbed day there is no evidence of a flux of ionization into the protonosphere except at sunrise. We suggest that this change is brought about by a large reduction in the abundance of neutral hydrogen, resulting from increased escape either via the polar wind or from the heating of the atmosphere in the vicinity of the auroral zones. Thus, it is suggested that the abundance of neutral hydrogen at the base of the exosphere is reduced at high latitudes during magnetic storms.

Observations of F‐region Vertical Velocities at Millstone Hill 3. Determination of Altitude Distribution of H+

Measurements of the vertical flux of O+ in the F region have been made by the incoherent scatter technique at Millstone Hill. In part 2 of this paper, evidence was presented for the existence of fluxes of ≥ 108 cm−2 sec−1 into the protonosphere during the day. In this paper we employ the altitude variation of the O+ flux to infer the magnitude of the proton flux in the charge‐transition region. Based on continuity considerations and the theory of minor ion diffusion in the topside of the F layer, we have attempted to determine the abundance of H+ in the altitude interval where it is a minor ion. The results indicate that the abundance of neutral hydrogen near 600 km must exceed about 105 cm−3. However, the absence of detectable amounts of H+ in the signal spectra taken near 1000 km indicates that the transition altitude must be above 1000 km and in turn the neutral‐hydrogen abundance cannot exceed this estimate by more than a factor of 3.