Joshua Eli Goldston Peek

A parallactic distance of 389+24-21 parsecs to the orion nebula cluster from very long baseline array observations

We determine the parallax and proper motion of the flaring, non-thermal radio star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline Array observations. Based on the parallax, we measure a distance of 389 +24/-21 parsecs to the source. Our measurement places the Orion Nebula Cluster considerably closer than the canonical distance of 480 +/- 80 parsecs determined by Genzel et al. (1981). A change of this magnitude in distance lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We briefly discuss two effects of this change--an increase in the age spread of the pre-main sequence stars and better agreement between the zero-age main-sequence and the temperatures and luminosities of massive stars.

An Accurate Distance to High-Velocity Cloud Complex C

We report an accurate distance of -->d = 10 ? 2.5 kpc to the high-velocity cloud Complex C. Using high signal-to-noise ratio Keck HIRES spectra of two horizontal-branch stars, we have detected Ca II K absorption lines from the cloud. Significant nondetections toward a further three stars yield robust lower distance limits. The resulting H I mass of Complex C is -->MH I = 4.9+2.9?2.2 ? 106 M?; a total mass of -->Mtot = 8.2+ 4.6?2.6 ? 106 M? is implied, after corrections for helium and ionization. At 10 kpc, Complex C has physical dimensions -->3 ? 15 kpc, and if it is as thick as it is wide, then the average density is -->log n ?2.5. We estimate the contribution of Complex C to the mass influx may be as high as ~0.14 M? yr?1.

The Disruption and Fueling of M33

The disruption of the M33 galaxy is evident from its extended gaseous structure. We present new data from the Galactic Arecibo L-Band Feed Array H I (GALFA-H I) Survey that show the full extent and detailed spatial and kinematic structure of M33s neutral hydrogen. Over 18% of the H I mass of M33 ( M ☉) is found beyond the star-forming disk as traced in the far-ultraviolet (FUV). The most distinct features are extended warps, an arc from the northern warp to the disk, diffuse gas surrounding the galaxy, and a southern cloud with a filament back to the galaxy. The features extend out to 22xa0kpc from the galaxy center (18xa0kpc from the edge of the FUV disk), and the gas is directly connected to M33s disk. Only five discrete clouds (i.e., gas not directly connected to M33 in position-velocity space) are cataloged in the vicinity of M33, and these clouds show similar properties to Galactic and M31 halo clouds. M33s gaseous features most likely originate from the tidal disruption of M33 by M31 1-3xa0Gyr ago as shown by an orbit analysis which results in a tidal radius <15xa0kpc in the majority of M33s possible orbits. M33 is now beyond the disruptive gravitational influence of M31, and the gas appears to be returning to M33s disk and redistributing its star formation fuel. M33s high mean velocity dispersion in the disk (~18.5xa0kmxa0s–1) may also be consistent with the previous interaction and high rate of star formation. M33 will either exhaust its star formation fuel in the next few Gyrs or eventually become star formation fuel for M31. The latter represents the accretion of a large gaseous satellite by a spiral galaxy, similar to the Magellanic Clouds relationship to the Galaxy.

ONGOING GALACTIC ACCRETION : SIMULATIONS AND OBSERVATIONS OF CONDENSED GAS IN HOT HALOS

Ongoing accretion onto galactic disks has been recently theorized to progress via the unstable cooling of the baryonic halo into condensed clouds. These clouds have been identified as analogous to the High-Velocity Clouds (HVCs) observed in HI in our Galaxy. Here we compare the distribution of HVCs observed around our own Galaxy and extra-planar gas around the Andromeda galaxy to these possible HVC analogs in a simulation of galaxy formation that naturally generates these condensed clouds. We find a very good correspondence between these observations and the simulation, in terms of number, angular size, velocity distribution, overall flux and flux distribution of the clouds. We show that condensed cloud accretion only accounts for ~ 0.2 M_solar / year of the current overall Galactic accretion in the simulations. We also find that the simulated halo clouds accelerate and become more massive as they fall toward the disk. The parameter space of the simulated clouds is consistent with all of the observed HVC complexes that have distance constraints, except the Magellanic Stream which is known to have a different origin. We also find that nearly half of these simulated halo clouds would be indistinguishable from lower-velocity gas and that this effect is strongest further from the disk of the galaxy, thus indicating a possible missing population of HVCs. These results indicate that the majority of HVCs are consistent with being infalling, condensed clouds that are a remnant of Galaxy formation.

VELOCITY SPECTRUM FOR H I AT HIGH LATITUDES

In this paper, we present the results of the statistical analysis of high-latitude H I turbulence in the Milky Way. We have observed H I in the 21 cm line, obtained with the Arecibo3 L-Band Feed Array receiver at the Arecibo radio telescope. For recovering velocity statistics, we have used the velocity coordinate spectrum (VCS) technique. In our analysis, we have used direct fitting of the VCS model, as its asymptotic regimes are questionable for Arecibos resolution, given the restrictions from thermal smoothing of the turbulent line. We have obtained a velocity spectral index of 3.87 ± 0.11, an injection scale of 140 ± 80 pc, and an H I cold phase temperature of 52 ± 11 K. The spectral index is steeper than the Kolmogorov index and can be interpreted as being due to shock-dominated turbulence.

FIRST RESULTS FROM THE ARECIBO GALACTIC H i SURVEY: THE DISK/HALO INTERFACE REGION IN THE OUTER GALAXY

The consortium for Galactic studies with the Arecibo L-band Feed Array (ALFA) is conducting a neutral hydrogen (H I) survey of the whole Arecibo sky (declination range from -1° to 38°), with high angular (35) and velocity resolution (0.2 km s-1). The precursor observations with ALFA of a region in the Galactic anticenter reveal numerous isolated, small (a few parsecs in size), and cold (Tk < 400 K) H I clouds at low negative velocities, distinctly separated from the H I disk emission (low-velocity clouds [LVCs]). These clouds are most likely located in the transition region between the Galactic disk and halo (at scale heights of 60-900 pc), yet they have properties of typical cold neutral clouds. LVCs are colder and, most likely, smaller and less massive than Lockmans clouds in the disk/halo interface region of the inner Galaxy. Our observations demonstrate that the cloudy structure of the interface region is most likely a general phenomenon, not restricted to the inner Galaxy. LVCs have sizes and radial velocities in agreement with the expectations for clouds formed in low-temperature fountain flows, although we measure a factor of 10 higher H I column densities. Alternatively, LVCs could represent the final stages of the infalling intergalactic material in the ongoing construction of the Galaxy. In the same data set at higher negative velocities, we have discovered a companion H I cloud located 50 southwest of HVC 186+19-114. HVC 186+19-114 is a typical compact high-velocity cloud (HVC) with a well-defined core/envelope structure. The companion cloud has a diameter of only 7 × 9 and is one of the smallest HVCs known, most likely stripped from the main cloud through the interactions with the halo medium.

The Many Streams of the Magellanic Stream

We present results from neutral hydrogen (H I) observations of the tip of the Magellanic Stream (MS), obtained with the Arecibo telescope as a part of the ongoing survey by the consortium for Galactic studies with the Arecibo L-band Feed Array. We find four large-scale, coherent H I streams, extending continuously over a length of 20°, each stream possessing different morphology and velocity gradients. The newly discovered streams provide strong support for the tidal model of the MS formation by Connors et al. (2006), who suggested a spatial and kinematic bifurcation of the MS. The observed morphology and kinematics suggest that three of these streams could be interpreted as a three-way splitting of the main MS filament, while the fourth stream appears much younger and may have originated from the Magellanic Bridge. We find an extensive population of H I clouds at the tip of the MS. Two-thirds of clouds have an angular size in the range 3.5-10. We interpret this as being due to thermal instability, which would affect a warm tail of gas trailing through the Galactic halo over a characteristic timescale of a few Myr to a few hundred Myr. We show that thermal fragments can survive in the hot halo for a long time, especially if surrounded by a <106 K halo gas. If the observed clumpy structure is mainly due to thermal instability, then the tip of the MS is at a distance of ~70 kpc. A significant fraction of H I clouds at the tip of the MS show multiphase velocity profiles, indicating the coexistence of cooler and warmer gas.

RECONSTRUCTING DECONSTRUCTION: HIGH-VELOCITY CLOUD DISTANCE THROUGH DISRUPTION MORPHOLOGY

We present Arecibo L-band Feed Array 21 cm observations of a subcomplex of HVCs at the tip of the anticenter complex. These observations show morphological details that point to interaction with the ambient halo medium and differential drag within the cloud subcomplex. We develop a new technique for measuring cloud distances, which relies on these observed morphological and kinematic characteristics, and show that it is consistent with Hα distances. These results are consistent with distances to HVCs and halo densities derived from models in which HVCs are formed from cooling halo gas.

A Cold Nearby Cloud inside the Local Bubble

The high-latitude Galactic H I cloud toward the extragalactic radio source 3C 225 is characterized by very narrow 21 cm emission and absorption indicative of a very low H I spin temperature of about 20 K. Through high-resolution optical spectroscopy, we report the detection of strong, very narrow Na I absorption corresponding to this cloud toward a number of nearby stars. Assuming that the turbulent H I and Na I motions are similar, we derive a cloud temperature of 20 K (in complete agreement with the 21 cm results) and a line-of-sight turbulent velocity of 0.37 ± 0.08 km s-1 from a comparison of the H I and Na I absorption line widths. We also place a firm upper limit of 45 pc on the distance of the cloud, which situates it well inside the Local Bubble in this direction and makes it the nearest known cold diffuse cloud discovered to date.

LOW-VELOCITY HALO CLOUDS

Models that reproduce the observed high-velocity clouds (HVCs) also predict clouds at lower radial velocities that may easily be confused with Galactic disk (|z|< 1 kpc) gas. We describe the first search for these low-velocity halo clouds (LVHCs) using Infrared Astronomical Satellite (IRAS) data and the initial data from the Galactic Arecibo L-band Feed Array survey in H I. The technique is based upon the expectation that such clouds should, like HVCs, have very limited infrared (IR) thermal dust emission as compared to their H I column density. We describe our displacement-map technique for robustly determining the dust-to-gas ratio (DGR) of clouds and the associated errors that take into account the significant scatter in the IR flux from the Galactic disk gas. We find that there exist lower-velocity clouds that have extremely low DGRs, consistent with being in the Galactic halo—candidate LVHCs. We also confirm the lack of dust in many HVCs with the notable exception of complex M, which we consider to be the first detection of dust in HVCs. We do not confirm the previously reported detection of dust in complex C. In addition, we find that most intermediate- and low-velocity clouds that are part of the Galactic disk have a higher 60 μm/100 μm flux ratio than is typically seen in Galactic H I, which is consistent with a previously proposed picture in which fast-moving Galactic clouds have smaller, hotter dust grains.