Leo Blitz

High velocity clouds: building blocks of the local group

We suggest that the high-velocity clouds (HVCs) are large clouds, with typical diameters of 25 kpc, containing 3×107 M☉ of neutral gas and 3×108 M☉ of dark matter, falling onto the Local Group; altogether the HVCs contain 1010 M☉ of neutral gas. Our reexamination of the Local Group hypothesis for the HVCs connects their properties to the hierarchical structure formation scenario and to the gas seen in absorption toward quasars. We show that at least one HVC complex (besides the Magellanic Stream) must be extragalactic at a distance of more than 40 kpc from the Galactic center, with a diameter greater than 20 kpc and a mass of more than 108 M☉. We discuss a number of other clouds that are positionally associated with the Local Group galaxies, and we show that the entire ensemble of HVCs is inconsistent with a Galactic origin. The observed kinematics imply rather that the HVCs are falling toward the Local Group barycenter. We simulate the dynamical evolution of the Local Group and find that material falling onto the Local Group reproduces the location of two of the three most significant groupings of clouds and the kinematics of the entire cloud ensemble (excluding the Magellanic Stream). We interpret the third grouping (the A, C, and M complexes) as the nearest HVC. It is tidally unstable and is falling onto the Galactic disk. We interpret the more distant HVCs as gas contained within dark matter minihalos moving along filaments toward the Local Group. Most poor galaxy groups should contain similar H I clouds bound to the group at large distances from the individual galaxies. We suggest that the HVCs are local analogs of the Lyman limit absorbing clouds observed against distant quasars. Our picture implies that the chemical evolution of the Galactic disk is governed by episodic infall of metal-poor HVC gas that only slowly mixes with the rest of the interstellar medium. We argue that there is a Galactic fountain in the Milky Way, but that the fountain does not explain the origin of the HVCs. Our analysis of the H I data leads to the detection of a vertical infall of low-velocity gas toward the plane and implies that the H I disk is not in hydrostatic equilibrium. We suggest that the fountain is manifested mainly by relatively local neutral gas with characteristic velocities of 6 km s-1 rather than 100 km s-1. The Local Group infall hypothesis makes a number of testable predictions. The HVCs should have subsolar metallicities. Their Hα emission should be less than that seen from the Magellanic Stream. The clouds should not be seen in absorption against nearby stars. The clouds should be detectable in both emission and absorption around other galaxy groups. We show that current observations are consistent with these predictions and discuss future tests.

Atomic hydrogen in the outer milky way

The H I in the outer Galaxy is reanalyzed using the CO rotation curve and the full latitude extent of the gas from the Weaver and Williams survey. Gas with a surface density > or approx. =0.1 M/sub sun/ pc/sup -2/ is found to a distance of 30 kpc from the center. Three distinct, well-defined spiral features of roughly constant surface density are seen, two of which extend at least 20--25 kpc along their length and can be traced to 20 kpc from the center. If they are logarithmic spirals, the major arms have a pitch angle of approximately 22/sup 0/--25/sup 0/. A nearly circular radial corrugation is seen as a derivation from the large-scale warping at Rroughly-equal11 kpc. The outermost parts of the Galaxy show a remarkable scalloping with a large azimuthal wave number (mroughly-equal10). The scale height of the gas shows an almost linear increase from the solar vicinity to Rroughly-equal30 kpc. This increase implies that the large mass in the outer Galaxy implied by the rotation curve does not reside in the disk. The vertical distribution of the gas is shown to be not well descibed by either a Gaussian or an exponential.

Star Forming Giant Molecular Clouds

The properties of galactic giant molecular clouds (GMCs) in the solar vicinity and in the inner Galaxy are reviewed. Special attention is given to the role of the clouds in forming stars. The question of whether all GMCs form stars is raised and it is shown that there is little evidence that GMCs anywhere in the Galaxy are devoid of star formation, even O star formation. The angular momentum of local GMCs is then discussed. It is shown quantitatively that the specific angular momentum of GMCs is within the range expected if the clouds condense out of the diffuse interstellar medium. At least four GMCs in the solar neighborhood, however, have retrograde rotation in an inertial frame of reference, placing significant constraints on how they could have formed. Three GMCs in different evolutionary states are identified, and and some of the differences in their properties are identified. GMCs are shown to have atomic envelopes with masses comparable to their molecular masses. These envelopes are likely to pervade the interclump medium and be responsible for most of its mass.

Infrared cirrus and high-latitude molecular clouds

It is established that a close correlation exists between far-infrared cirrus emission observed with IRAS and the CO emission from high-latitude molecular clouds (HCLs). In all cases, the HLCs correspond to the central portions of 100-micron infrared cirrus features. This association firmly establishes at least some of the cirrus as features of the local interstellar medium with typical distances of 100 pc. The infrared energy distribution of the cirrus displays an excess of 12-micron and 25-micron emission over that expected from dust at equilibrium temperature, consistent with emission from very small (less than 10 A) transiently heated grains.

A molecular cloud in the local, hot interstellar medium

Echelle spectra recorded at the D lines of Na I are reported for nine A or F stars. Lying at approximate distances ranging from 25 to 230 pc, the stars are projected on or near the high-latitude molecular cloud MBM 12 at l = 159 deg, b = -34 deg. Among a subgroup of five of these stars separated by no more than 1.2 deg on the sky, four which are located at distances d more than 70 pc show strong interstellar D line absorption near the radial velocity of the CO emission observed in this general direction. The fifth star, at roughly 60 pc, shows no detectable absorption. MBM 12 therefore probably lies at roughly 65 pc, within the local region filled primarily by very hot, low-density gas, a conclusion supported by the large internal velocity dispersion of the molecular cloud complex. 15 references.

On the nearest molecular clouds. II - MBM 12 and 16

The paper presents echelle spectra recorded at the D lines of Na I for three stars projected on the high-latitude molecular cloud MBM 16 at l = 172 deg, b = -38 deg. The A stars HD 21142 at about 95 pc and HD 21134 at about 240 pc show strong D-line absorption at the same velocities as the CO emission observed at these positions. The distance to MBM 16 therefore is in the range of 60 to 95 pc. MBM 16 is only 11 deg away from MBM 12, previously placed by the same method at distance of about 65 pc. Consideration is given to the relationship between clouds 12 and 16 and the local hot low-density interstellar gas. 36 references.

The New Milky Way

Our understanding of the large-scale structure of the Milky Way has undergone considerable revision during the past few years. The Galaxy is larger and much more massive than was previously supposed; the newly discovered mass consists of nonluminous matter which is likely to be the dominant form of matter in the universe. New analyses of the atomic hydrogen gas show that the disk of the Galaxy is about twice as extended as was previously thought. Beyond the sun, the gas is concentrated in large-scale, coherent spiral arms indicative of a regular four-armed spiral pattern. The outer edge of the disk has a remarkable scalloping.

The Structure of Molecular Clouds

The structure and properties of small, local molecular clouds and giant molecular clouds as typified by the Rosette Molecular Cloud are reviewed. The small clouds are found at high galactic latitude with typical diameters of 2 pc, masses of 40 M0, and mean densities of ~ 200 cm -3. The internal motions of these clouds are shown to be inconsistent with their being bound by either gravity, pressure or magnetic fields. They are found to have considerable substructure, in some cases as small as 0.03 pc with substellar masses. Some clouds are found to have extraordinary CO line wings that are not powered by internal energy sources. The CO abundances are found to be typical of dark clouds even though the extincxad tion is similar to diffuse clouds. The clouds have high OH abundances, but H2CO appears to be normal. It is suggested that the internal kinematics and molecular abundances can be understood if the small molecular clouds are recently ( ;S 1Q6y ago) formed in interstellar shocks. The results of extensive 13CO mapping of the Rosette Molecular Cloud are presented. They show that most of the gas in the complex is gathered into clumps with a high density contrast: the clumps may be thought of as interstellar baseballs. The clumps are found to have a power law mass spectrum with density proportional to radius. The highest mass clumps are gravitationally bound, but the lowest mass clumps are far from being bound and appear to be either ephemeral structures, or they are pressure bound by interclump gas in hydrostatic equilibrium with the gravitational potential of the entire cloud complex. Broad CO and 13CO wings are found throughout the complex suggestxad ing that there is a pervasive interclump gas that is leaking out of the complex. Although this gas has a small column density, it is optically thick in CO.

CO in the Milky Way

If the CO distribution of the Milky Way is described as a truncated exponential rather than as a molecular ring with some gas at large radii, it becomes easier to understand the evolution of the disk of stars. The star formation rate per unit molecular gas mass is constant as a function of radius, and the H2 depletion time turns out to be only a few percent of the Hubble time. This very short timescale requires that the atomic gas act as a reservoir for the active star forming gas. Because the HI has such a different radial distribution, there must either be infall from outside the Galaxy, an efficient way for the atomic gas in the disk to lose angular momentum, or both, leading to measurable infall or inflow velocities. The truncation radius of CO is probably due to the recently identified stellar bar.

On the nearest molecular clouds. III - MBM 40, 53, 54, and 55

In an attempt to determine the distances to four high-latitude molecular clouds (HLCs), echelle spectra near the Na I D lines, accurate MK spectral types, and photoelectric photometry for 25 nearby stars have been obtained. Fairly firm distance limits may be placed on MBM 40 (d smaller than or equal to 140 pc) and MBM 53 (d greater than or equal to 110 pc and less than or equal to 155 pc), based on the presence or absence of strong interstellar Na I absorption towards stars projected on or near those HLCs. Weak interstellar absorption lines observed toward many of the stars located near MBM 54 and 55 make the distances to those clouds less certain (about 265 pc for both). Interstellar CH absorption at 4300 A was detected in the spectrum of HD 218662, located behind MBM 53 with a CH column density of 2.1 x 10 to the 13th per sq cm, thus implying a CH abundance comparable to that observed in other molecular clouds. Morphological and velocity agreement among CO emission, the Na I absorption, the 100 micron infrared cirrus emission, and the 21 cm H I emission near these HLCs suggest a close association of themorexa0» interstellar material responsible for those phenomena. 44 refs.«xa0less