Snezana Stanimirovic

THE SPITZER SURVEY OF THE SMALL MAGELLANIC CLOUD: FAR-INFRARED EMISSION AND COLD GAS IN THE SMALL MAGELLANIC CLOUD

We present new FIR maps of the SMC at 24, 70, and 160 μm obtained as part of the Spitzer Survey of the Small Magellanic Cloud (S^(3)MC). These maps cover most of the star formation in the SMC bar and wing. We combine our maps with literature data to derive the dust mass surface density across the SMC. We find a total dust mass of M_(dust) = 3 × 10^5 M_☉, implying a dust-to-hydrogen ratio over the region studied of log_(10)(D/H) = -2.86, or 1 : 700, which includes H_2. Assuming the dust to trace the total gas column, we derive H_2 surface densities across the SMC. We find a total H_2 mass M_(H_2) = 3.2 × 10^7 M_☉ in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H_2 around six molecular peaks; on average H_2 is more extended than CO by a factor of ~1.3. The implied CO-to-H_2 conversion factor over the whole SMC is X_(CO) = (13 ± 1) × 10^(21) cm^(-2) (K km s^(-1))^(-1). Over the volume occupied by CO the conversion factor is lower, X_(CO) = (6 ± 1) × 10^(21) cm^(-2) (K km s^(-1))^(-1), but still a few times larger than that found using virial mass methods. The molecular peaks have H_2 surface densities Σ_(H_2) ≈ 180 ± 30 M pc^(-2), similar to those in Milky Way GMCs, and correspondingly low extinctions, A_V ~ 1-2 mag. The theory of photoionization-regulated star formation predicts A_V ~ 6, which would require the GMCs to be ~3 times smaller than our 46 pc resolution element. For a given hydrostatic gas pressure, the SMC has a 2-3 times lower ratio of molecular to atomic gas than spiral galaxies. Combined with lower mean densities, this results in this galaxy having only 10% of its gas in the molecular phase.

Cold Atomic Gas in the Small Magellanic Cloud

In this project, we determine the mixture of thermal phases in the atomic medium of the Small Magellanic Cloud. The larger purpose is to study the difference in properties of the interstellar medium in a dwarf irregular galaxy compared with the Milky Way. The observations used the Australia Telescope Compact Array to measure absorption spectra of the 21 cm line through the SMC, for comparison with 21 cm emission spectra from a combination of data from the Parkes and Compact Array telescopes. Using the two together, we find the abundance of cool-phase gas to be less than 15% of the total H I. Thus, the cool-phase gas is very scarce, only about half as abundant relative to the total H I as in the solar neighborhood, a lower figure than that found in the LMC, M31, M33, or the solar neighborhood of the Milky Way. This is in qualitative agreement with the heating/cooling equilibrium calculations for conditions in a dwarf irregular galaxy. In addition, we find the temperature of the cool-phase clouds to be very low, typically 40 K or less. This is much less than the nominal diffuse cloud temperature in the Milky Way (50-100 K). This suggests that the H I may be more abundant in dense cold clouds in the SMC than in the Milky Way, where clouds of similar density and temperature are largely molecular.

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.

THE THINNEST COLD H I CLOUDS IN THE DIFFUSE INTERSTELLAR MEDIUM

We confirm and discuss recently discovered cold H I clouds with column densities among the lowest ever detected. The column densities of the cold neutral medium (CNM) toward 3C 286 and 3C 287 are ~1018 cm-2, below an observational lower limit and also below tiny-scale atomic clouds detected by VLBI and time-variable profiles against pulsars. These column densities are close to the minimum imposed by thermal evaporation. The ratios of the CNM to total H I toward 3C 286 and 3C 287 are ~4% and 2%, respectively. We discuss the CNM fraction and the CNM clouds in relation to several theoretical models.

Mueller Matrix Parameters for Radio Telescopes and Their Observational Determination

Modern digital cross-correlators permit the simultaneous measurement of all four Stokes parameters. However, the results must be calibrated to correct for the polarization transfer function of the receiving system. The transfer function for any device can be expressed by its Mueller matrix. We express the matrix elements in terms of fundamental system parameters that describe the voltage transfer functions (known as the Jones matrix) of the various system devices in physical terms and thus provide a means for comparing with engineering calculations and investigating the effects of design changes. We describe how to determine these parameters with astronomical observations. We illustrate the method by applying it to some of the receivers at the Arecibo Observatory.

The Turbulence Velocity Power Spectrum of Neutral Hydrogen in the Small Magellanic Cloud

We present the results of the Velocity Coordinate Spectrum (VCS) technique to calculate the velocity power spectrum of turbulence in the Small Magellanic Cloud (SMC) in 21cm emission. We have obtained a velocity spectral index of -3.85 and an injection scale of 2.3 kpc. The spectral index is steeper than the Kolmogorov index which is expected for shock-dominated turbulence which is in agreement with past works on the SMC gas dynamics. The injection scale of 2.3 kpc suggests that tidal interactions with the Large Magellanic Cloud are the dominate driver of turbulence in this dwarf galaxy. This implies turbulence maybe driven by multiple mechanisms in galaxies in addition to supernova injection and that galaxy-galaxy interactions may play an important role.

Cold and warm atomic gas around the Perseus molecular cloud. I. Basic properties

(Abridged) Using the Arecibo Observatory we have obtained neutral hydrogen (HI) absorption and emission spectral pairs in the direction of 26 background radio continuum sources in the vicinity of the Perseus molecular cloud. Strong absorption lines were detected in all cases allowing us to estimate spin temperature (T_s) and optical depth for 107 individual Gaussian components along these lines of sight. Basic properties of individual HI clouds (spin temperature, optical depth, and the column density of the cold and warm neutral medium, CNM and WNM) in and around Perseus are very similar to those found for random interstellar lines of sight sampled by the Millennium HI survey. This suggests that the neutral gas found in and around molecular clouds is not atypical. However, lines of sight in the vicinity of Perseus have on average a higher total HI column density and the CNM fraction, suggesting an enhanced amount of cold HI relative to an average interstellar field. Our estimated optical depth and spin temperature are in stark contrast with the recent attempt at using Planck data to estimate properties of the optically thick HI. Only ~15% of lines of sight in our study have a column density weighted average spin temperature lower than 50 K, in comparison with >85% of Plancks sky coverage. The observed CNM fraction is inversely proportional to the optical-depth weighted average spin temperature, in excellent agreement with the recent numerical simulations by Kim et al. While the CNM fraction is on average higher around Perseus relative to a random interstellar field, it is generally low, 10-50%. This suggests that extended WNM envelopes around molecular clouds, and/or significant mixing of CNM and WNM throughout molecular clouds, are present and should be considered in the models of molecule and star formation.

Discovery of Pulsed OH Maser Emission Stimulated by a Pulsar

Stimulated emission of radiation has not been directly observed in astrophysical situations up to this time. Here we demonstrate that photons from pulsar B1641–45 stimulate pulses of excess 1720-megahertz line emission in an interstellar hydroxyl (OH) cloud. As this stimulated emission is driven by the pulsar, it varies on a few-millisecond time scale, which is orders of magnitude shorter than the quickest OH maser variations previously detected. Our 1612-megahertz spectra are inverted copies of the 1720-megahertz spectra. This “conjugate line” phenomenon enables us to constrain the properties of the interstellar OH line–producing gas. We also show that pulsar signals undergo significantly deeper OH absorption than do other background sources, which confirms earlier tentative findings that OH clouds are clumpier on small scales than are neutral hydrogen clouds.

All‐Stokes Parameterization of the Main Beam and First Sidelobe for the Arecibo Radio Telescope

Radio astronomical measurements of extended emission require knowledge of the beam shape and response because the measurements need correction for quantities such as beam efficiency and beamwidth. We describe a scheme that characterizes the main beam and sidelobe in all Stokes parameters employing parameters that allow reconstruction of the complete beam patterns and, also, afford an easy way to see how the beam changes with azimuth, zenith angle, and time. For the main beam in Stokes I, the parameters include the beamwidth, ellipticity and its orientation, coma and its orientation, the point-source gain, and the integrated gain (or, equivalently, the main-beam efficiency); for the other Stokes parameters, the beam parameters include beam squint and beam squash. For the first sidelobe ring in Stokes I, the parameters include an eight-term Fourier series describing the height, radius, and radial width; for the other Stokes parameters they include only the sidelobes fractional polarization.We illustrate the technique by applying it to the Arecibo telescope. The main-beam width is smaller and the sidelobe levels higher than for a uniformly illuminated aperture of the same effective area. These effects are modeled modestly well by a blocked aperture, with the blocked area equal to about 10% of the effective area (this corresponds to 5% physical blockage). In polarized emission, the effects of beam squint (difference in pointing direction between orthogonal polarizations) and squash (difference in beamwidth between orthogonal polarizations) do not correspond to theoretical expectation and are higher than expected; these effects are almost certainly caused by the blockage. The first sidelobe is highly polarized because of blockage. These polarization effects lead to severe contamination of maps of polarized emission by spatial derivatives in brightness temperature.

Spitzer Space Telescope Detection of the Young Supernova Remnant 1E 0102.2–7219

We present infrared observations of the young, oxygen-rich supernova remnant 1E 0102.2-7219 (E0102) in the Small Magellanic Cloud, obtained with the Spitzer Space Telescope. The remnant is detected at 24 μm but not at 8 or 70 μm and has a filled morphology with two prominent filaments. We find evidence for the existence of up to 8 × 10-4 M☉ of hot dust (Td ~ 120 K) associated with the remnant. Most of the hot dust is located in the central region of E0102, which appears significantly enhanced in infrared and radio continuum emission relative to the X-ray emission. Even if all of the hot dust was formed in the explosion of E0102, the estimated mass of dust is at least 100 times lower than what is predicted by some recent theoretical models.