Snežana Stanimirović

A NEW LOOK AT THE KINEMATICS OF NEUTRAL HYDROGEN IN THE SMALL MAGELLANIC CLOUD

We have used the latest H I observations of the Small Magellanic Cloud (SMC), obtained with the Australia Telescope Compact Array and the Parkes telescope, to reexamine the kinematics of this dwarf irregular galaxy. A large velocity gradient is found in the H I velocity field, with a significant symmetry in isovelocity contours, suggestive of a differential rotation. A comparison of H I data with the predictions from tidal models for the SMC evolution suggests that the central region of the SMC corresponds to the central disklike or barlike component left from the rotationally supported SMC disk prior to its last two encounters with the Large Magellanic Cloud. In this scenario, the velocity gradient is expected as a leftover from the original, preencounter angular momentum. We have derived the H I rotation curve and the mass model for the SMC. This rotation curve rapidly rises to about 60 km s-1 up to the turnover radius of ~3 kpc. A stellar mass-to-light ratio of about unity is required to match the observed rotation curve, suggesting that a dark matter halo is not needed to explain the dynamics of the SMC. A set of derived kinematic parameters agrees well with the assumptions used in tidal theoretical models that led to a good reproduction of observational properties of the Magellanic System. The dynamical mass of the SMC, derived from the rotation curve, is 2.4 × 109 M☉.

The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wave Bands

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S^3MC), which imaged the star-forming body of the SMC in all seven MIPS and IRAC wave bands. We find that the F_8/F_(24) ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 μm brightness and F_(24)/F_(70) ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: (1) bright 5.8 μm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] - [8.0] > 1.2), which we identify as YSOs; (2) bright mid-infrared sources with mildly red colors (0.16 ≾ [5.8] - [8.0] < 0.6), identified as carbon stars; (3) bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars; and (4) unreddened early B stars (B3-O9) with a large 24 μm excess. This excess is reminiscent of debris disks and is detected in only a small fraction of these stars (≾5%). The majority of the brightest infrared point sources in the SMC fall into groups 1-3. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%).

Velocity and Density Spectra of the Small Magellanic Cloud

This Letter reports results on the statistical analysis of H I turbulence in the Small Magellanic Cloud (SMC). We use 21 cm channel maps, obtained with the Australia Telescope Compact Array and the Parkes telescope, and analyze the spectrum of observed intensity fluctuations as a function of the velocity slice thickness. We confirm recent predictions by Lazarian & Pogosyan on the change of the power-law index and establish the spectra of three-dimensional density and velocity. The obtained spectral indices, -3.3 and -3.4, are slightly more shallow than the predictions for the Kolmogorov spectrum. This contrasts with the predictions for the shock-type spectra that are steeper than the Kolmogorov one. The nature of the energy injection in the SMC is unclear, as no distinct energy injection scales are observed up to the entire scale of the SMC.

The Structure of a Low-metallicity Giant Molecular Cloud Complex

To understand the impact of low metallicities on giant molecular cloud (GMC) structure, we compare far-infrared dust emission, CO emission, and dynamics in the star-forming complex N83 in the Wing of the Small Magellanic Cloud (SMC). Dust emission (measured by Spitzer as part of the Spitzer Survey of the SMC and Surveying the Agents of a Galaxys Evolution in the SMC surveys) probes the total gas column independent of molecular line emission and traces shielding from photodissociating radiation. We calibrate a method to estimate the dust column using only the high-resolution Spitzer data and verify that dust traces the interstellar medium in the H I-dominated region around N83. This allows us to resolve the relative structures of H2, dust, and CO within a GMC complex, one of the first times such a measurement has been made in a low-metallicity galaxy. Our results support the hypothesis that CO is photodissociated while H2 self-shields in the outer parts of low-metallicity GMCs, so that dust/self-shielding is the primary factor determining the distribution of CO emission. Four pieces of evidence support this view. First, the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11 × 1021 cm–2 (K km s–1)–1, or 20-55 times the Galactic value. Second, the CO-to-H2 conversion factor varies across the complex, with its lowest (most nearly Galactic) values near the CO peaks. Third, bright CO emission is largely confined to regions of relatively high line-of-sight extinction, AV 2 mag, in agreement with photodissociation region models and Galactic observations. Fourth, a simple model in which CO emerges from a smaller sphere nested inside a larger cloud can roughly relate the H2 masses measured from CO kinematics and dust.

The Spitzer Survey of the Small Magellanic Cloud (S3MC): Insights into the Life Cycle of Polycyclic Aromatic Hydrocarbons

We present the results of modeling dust spectral energy distributions (SEDs) across the Small Magellanic Cloud (SMC) with the aim of mapping the distribution of polycyclic aromatic hydrocarbons (PAHs) in a low-metallicity environment. Using Spitzer Survey of the SMC photometry from 3.6 to 160 μm over the main star-forming regions of the Wing and Bar of the SMC along with spectral mapping observations from 5 to 38 μm from the Spitzer Spectroscopic Survey of the SMC in selected regions, we model the dust SED and emission spectrum to determine the fraction of dust in PAHs across the SMC. We use the regions of overlapping photometry and spectroscopy to test the reliability of the PAH fraction as determined from SED fits alone. The PAH fraction in the SMC is low compared to the Milky Way and variable—with relatively high fractions (q PAH ~1%-2%) in molecular clouds and low fractions in the diffuse interstellar medium (ISM; average q PAH = 0.6%). We use the map of PAH fraction across the SMC to test a number of ideas regarding the production, destruction, and processing of PAHs in the ISM. We find weak or no correlation between the PAH fraction and the distribution of carbon asymptotic giant branch stars, the location of supergiant H I shells and young supernova remnants, and the turbulent Mach number. We find that the PAH fraction is correlated with CO intensity, peaks in the dust surface density and the molecular gas surface density as determined from 160 μm emission. The PAH fraction is high in regions of active star formation, as predicted by its correlation with molecular gas, but is suppressed in H II regions. Because the PAH fraction in the diffuse ISM is generally very low—in accordance with previous work on modeling the integrated SED of the SMC—and the PAH fraction is relatively high in molecular regions, we suggest that PAHs are destroyed in the diffuse ISM of the SMC and/or PAHs are forming in molecular clouds. We discuss the implications of these observations for our understanding of the PAH life cycle, particularly in low-metallicity and/or primordial galaxies.

The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the H II Region NGC 346

We use Spitzer Space Telescope observations from the Spitzer Survey of the Small Magellanic Cloud (S^(3)MC) to study the young stellar content of N66, the largest and brightest H II region in the SMC. In addition to large numbers of normal stars, we detect a significant population of bright, red infrared sources that we identify as likely to be young stellar objects (YSOs). We use spectral energy distribution (SED) fits to classify objects as ordinary (main-sequence or red giant) stars, asymptotic giant branch stars, background galaxies, and YSOs. This represents the first large-scale attempt at blind source classification based on Spitzer SEDs in another galaxy. We firmly identify at least 61 YSOs, with another 50 probable YSOs; only one embedded protostar in the SMC was reported in the literature prior to the S^(3)MC. We present color selection criteria that can be used to identify a relatively clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the infrared-bright YSOs in N66 have stellar masses ranging from 2 to 17 M_☉, and that approximately half of the objects are stage II protostars, with the remaining YSOs roughly evenly divided between stage I and stage III sources. We find evidence for primordial mass segregation in the H II region, with the most massive YSOs being preferentially closer to the center than lower mass objects. Despite the low metallicity and dust content of the SMC, the observable properties of the YSOs appear consistent with those in the Milky Way. Although the YSOs are heavily concentrated within the optically bright central region of N66, there is ongoing star formation throughout the complex, and we place a lower limit on the star formation rate of 3.2 × 10^(-3) M_☉ yr^(-1) over the last ~1 Myr.

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.

The Small-Scale Structure of the Magellanic Stream

We have mapped two regions at the northern tip of the Magellanic Stream in neutral hydrogen 21 cm emission, using the Arecibo telescope. The new data are used to study the morphology and properties of the Stream far away from the Magellanic Clouds, as well as to provide indirect constraints on the properties of the Galactic halo. We investigate confinement mechanisms for the Stream clouds and conclude that these clouds cannot be gravitationally confined or in free expansion. The most likely mechanism for cloud confinement is pressure support from the hot Galactic halo gas. This allows us to place an upper limit on the halo density: nh(15 kpc) = 10-3 cm-3 and/or nh (45 kpc) = 3 × 10-4 cm-3, depending on the distance. These values are significantly higher than predicted for an isothermal stratified halo.

A High-resolution Study of the H I-H2 Transition across the Perseus Molecular Cloud

To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the H I and H2 surface density (?H I and ?H2) images of the Perseus molecular cloud on sub-pc scales (~0.4?pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the ?H2 distribution across Perseus. We find a relatively uniform ?H I ~ 6-8 M ??pc?2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ?H2/?H I and ?H I + ?H2. The transition between the H I- and H2-dominated regions occurs at N(H I) + 2N(H2) ~ (8-14)?? 1020?cm?2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of CO-dark gas, f DG ~ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V-band extinction.

High resolution H(I) observations of the western Magellanic Bridge

The 21-cm line emission from a 7 × 6d eg 2 region east of and adjoining the Small Magellanic Cloud (SMC) has been observed with the Australia Telescope Compact Array and the Parkes telescopes. This region represents the westernmost part of the Magellanic Bridge, a gas-rich tail extending ∼14 ◦ to the Large Magellanic Cloud. A rich and complex neutral hydrogen (H I) structure containing shells, bubbles and filaments is revealed. On the larger scale, the H I of the Bridge is organized into two velocity components. This bimodality, which appears to originate in the SMC, converges to a single velocity component within the observed region. A census of shell-like structures suggests a shell population with characteristics similar to that of the SMC. The mean kinematic age of the shells is ∼6 Myr, in agreement with the SMC shell population, but not with ages of OB clusters populating the Magellanic Bridge, which are approximately a factor of 3 older. In general, the projected spatial correlation of Bridge H I shells with OB associations is poor and as such, there does not appear to be a convincing relationship between the positions of OB associations and that of expanding spherical H I structures. This survey has found only one H I shell that has an identifiable association with a known Hα shell. The origin of the expanding structures is therefore generally still uncertain, although current theories regarding their formation include gravitational and pressure instabilities, high-velocity cloud collisions and ram pressure effects.