Northern Arm Research Articles

Stars and gas in the very large interacting galaxy NGC 6872

The dynamical evolution of the large (> 100 kpc), barred spiral galaxy NGC 6872 and its small companion IC 4970 in the southern group Pavo is investigated. We present N-body simulations with stars and gas and 21 cm HI observations carried out with the Australia Telescope Compact Array of the large-scale distribution and kinematics of atomic gas. HI is detected toward the companion, corresponding to a gas mass of ca 1.3 10^9 Msun. NGC 6872 contains ca 1.4 10^{10} Msun of HI gas, distributed in an extended rotating disk. Massive concentrations of gas (10^9 Msun) are found at the tip of both tidal tails and towards the break seen in the optical northern arm near the companion. We detect no HI counterpart to the X-ray trail between NGC 6872 and NGC 6876, the dominant elliptical galaxy in the Pavo group located 8 arcmin to the southeast. At the sensitivity and the resolution of the observations, there is no sign in the overall HI distribution that NGC 6876 has affected the evolution of NGC 6872. There is no evidence of ram pressure stripping either. The X-ray trail could be due to gravitational focusing of the hot gas in the Pavo group behind NGC 6872 as the galaxy moves supersonically through the hot medium. The simulations of a gravitational interaction with a small nearby companion on a low-inclination prograde passage are able to reproduce most of the observed features of NGC 6872, including the general morphology of the galaxy, the inner bar, the extent of the tidal tails and the thinness of the southern tail.

Desulfohalobium utahense sp. nov., a moderately halophilic, sulfate-reducing bacterium isolated from Great Salt Lake

A novel moderately halophilic, sulfate-reducing bacterium, strain EtOH3(T), was isolated from anoxic hypersaline (270 g NaCl l(-1)) sediment of the northern arm of the Great Salt Lake, Utah, USA. Cells of strain EtOH3(T) were oval to rod-shaped, non-motile, non-sporulating and stained Gram-negative. The strain required sodium and magnesium ions for growth and grew at salinities of up to 240 g NaCl l(-1) and 121 g MgCl(2).6H(2)O l(-1). The optimum NaCl concentration was 80-100 g l(-1). Strain EtOH3(T) grew at temperatures ranging from 15 to 44 degrees Celsius (optimum 37 degrees Celsius). The pH range for growth was 6.5-8.3 (optimum around pH 6.8). Only sulfate and thiosulfate served as electron acceptors for a broad range of electron donors including various short-chain fatty acids and primary (C(1-5)) alcohols, amino acids, H(2)/acetate and H(2)/yeast extract. The G+C content of the genomic DNA was 51.4 mol%. Phylogenetic analysis of dsrAB [genes encoding the major subunits of dissimilatory (bi)sulfite reductase] and 16S rRNA gene sequence data placed strain EtOH3(T) within the deltaproteobacterial family Desulfohalobiaceae. Strain EtOH3(T) shared 76 and 91 % dsrAB and 16S rRNA gene sequence similarity, respectively, with the type strain of the phylogenetically most closely related species with a validly published name, Desulfohalobium retbaense DSM 5692(T). High 16S rRNA gene sequence similarity ( approximately 97 %) was shared with the recently described strain 'Desulfovermiculus halophilus' VKM B-2364. Strain EtOH3(T), however, clearly differed from this strain in both genomic G+C content and in several of its phenotypic properties. On the basis of phenotypic and genotypic characteristics, the novel species Desulfohalobium utahense sp. nov. is proposed, with strain EtOH3(T) (=VKM B-2384(T)=DSM 17720(T)) as the type strain.

Nitrogen sources and cycling in the San Francisco Bay Estuary: A nitrate dual isotopic composition approach

We used the dual isotopic composition of nitrate (δ15N and δ18O) within the estuarine system of San Francisco (SF) Bay, California, to explore the utility of this approach for tracing sources and cycling of nitrate (NO2−). Surface water samples from 49 sites within the estuary were sampled during July–August 2004. Spatial variability in the isotopic composition suggests that there are multiple sources of nitrate to the bay ecosystem including seawater, several rivers and creeks, and sewage effluent. The spatial distribution of nitrate from these sources is heavily modulated by the hydrodynamics of the estuary. Mixing along the estuarine salinity gradient is the main control on the spatial variations in isotopic composition of nitrate within the northern arm of SF Bay. However, the nitrate isotopic composition in the southern arm of SF Bay exhibited a combination of source mixing and phytoplankton drawdown due mostly to the long residence time during the summer study period. Very low δ18ONO3 values (as low as −5.0%) at the Sacramento–San Joaquin River delta region give rise to a wide range of δ18ONO3 values in the SF Bay system. The range in δ18ONO3 values is more than twice that of δ15NNO3, suggesting that δ18ONO3 is an even more sensitive tool for tracing nitrate sources and cycling than δ15NNO3.

Dusty Sources at the Galactic Center theN‐ andQ‐Band Views with VISIR

We present mid-infrared N- and Q-band photometry of the Galactic center from images obtained with the mid-infrared camera VISIR at the ESO VLT in 2004 May. The high resolution and sensitivity possible with VISIR enable us to investigate a total of over 60 pointlike sources, an unprecedented number for the Galactic center at these wavelengths. Combining these data with previous results at shorter wavelengths (Viehmann and coworkers) enables us to construct SEDs covering the H- to Q-band regions of the spectrum, i.e., 1.6-19.5 μm. We find that the SEDs of certain types of Galactic center sources show characteristic features. We can clearly distinguish between luminous Northern Arm bow shock sources, lower luminosity bow shock sources, hot stars, and cool stars. This characterization may help clarify the status of currently unclassified sources.

Zooplankton and ichthyoplankton communities in a temperate estuary: spatial and temporal patterns

Zooplankton and ichthyoplankton assemblages were studied from January 2003 to June 2004 in a temperate shallow estuary (Mondego estuary, Portugal). Monthly sampling was performed at five stations at high and low tides, with subsurface tows with 335 and 500 µm mesh Bongo nets. Analysis of variance (ANOVA) showed a significant effect (P < 0.05) of the mesh size of the net on the abundance of main zooplankton groups. On average, the abundance of the 500 µm taxocenosis was 67 and 102 times lower than the 335 µm taxocenosis at high and low tides respectively, especially in the south arm. More than 80 species were identified in the zooplankton samples. The upper reaches of northern arm are dominated by freshwater crustacean mesozooplankton like Acanthocyclops robustus and Diaptomus spp. and the cladocerans Daphnia, Ceriodaphnia and Bosmina, often being codominant. In the southern arm, the resident estuarine copepod Acartia tonsa was dominant, eventually being the most abundant taxon. Marine reaches of estuary are usually dominated by the marine calanoid copepods Acartia clausi and Temora longicornis and the siphonophores Muggiaea atlantica. Concerning the ichthyoplankton, this was dominated by the larvae of estuarine resident species, mainly Pomatoschistus sp., and eggs of Engraulis encrasicolus. The abundance of Pomatoschistus sp. larvae was positively correlated with water temperature. Statistical analysis (canonical correspondence analysis) used to determine the spatiotemporal structure of the zooplankton assemblages and its correlation with environmental variables showed that salinity and temperature were the main factors influencing the distribution of zooplankton. The results obtained also showed that abundance was strongly influenced by the hydrological circulation pattern and direct or indirect human impacts that occur in each arm of the estuary.

Tectonic geomorphology of the Red Rock fault, insights into segmentation and landscape evolution of a developing range front normal fault

The Red Rock fault, an active normal fault in southwest Montana, is reevaluated for fault segmentation using a multiproxy approach. Field characterization of soils coupled with radiocarbon ages in offset hanging wall fans and terraces constrain the rupture history at a number of locations along strike. These data are coupled with morphometric analysis of dated alluvial fan deposits and adjacent 2nd and 3rd order footwall drainage basins to further constrain fault kinematics and segmentation. Morphometric analysis of the Big Sheep Creek catchment, which drains a large area of the footwall range, provides a control on the spatial extent of fault influence on footwall topography. Three fault segments are recognized by this study, compared with the two segments recognized previously. Data from offset alluvial surfaces were synthesized with hanging wall fan and footwall drainage basin data to demonstrate a southward increase in displacement along the Red Rock fault since at least the latest Pleistocene, at a maximum rate of 1.5 mm/yr. Red Rock ruptures were mainly confined to segments, suggesting a fault behaving as several discrete slip surfaces; however, simultaneous rupture of adjacent segments during one event indicates some fault zone coalescence. A youthful Red Rock fault is supported by the absence of tectonic influences on footwall topography and is consistent with an eastward progression of normal fault development along the northern arm of the Yellowstone hot spot.

Stellar Bow Shocks in the Northern Arm of the Galactic Center: More Members and Kinematics of the Massive Star Population

We present new 2.2 μm diffraction-limited images from the W. M. Keck 10 m and Gemini 8 m telescopes of the cool Galactic center sources, IRS 1W, 5, 8, 10W, and 21, along with new proper motions for IRS 1W, 10W, and 21. These observations were carried out to test the bow shock hypothesis presented by Tanner et al. as an alternative to a very recent (104 yr) epoch of star formation within the tidal stream of gas and dust known as the northern arm. Resolved asymmetric structure is detected in all the sources, with bow shock morphologies associated with IRS 1W, 5, 8, and 10W. For IRS 1W and 10W, there is an agreement between the position angle of the asymmetry and that of the inferred relative velocity vector of the near-infrared source with respect to the northern arm gas, strengthening the bow shock hypothesis. We therefore conclude that the observed morphology is indeed a bow shock generated by sources plowing through the northern arm. Furthermore, the large extent of the resolved structures (310-1340 AU), along with their luminosities (~104-105 L☉), suggests that their central sources are Wolf-Rayet stars, comparable to the broad He emission-line stars, which have strong winds on the order of 1000 km s-1. The bow shock geometry, along with the proper motion measurements, provide three-dimensional orbital solutions for this enigmatic class of objects. The orientations of the orbital planes of IRS 1W and 10W are consistent with that of the putative clockwise plane which has been proposed as a solution for the He I emission-line stars. While these observations eliminate the need to invoke star formation within the northern arm, they increase by 10% the total known population of massive, young stars with strong winds, whose origin remains unexplained in the context of the nearby supermassive black hole.

Warm Molecular Gas Traced with COJ= 7→6 in the Galaxy’s Central 2 Parsecs: Dynamical Heating of the Circumnuclear Disk

We present an 11 resolution map of the central 2 pc of the Galaxy in the CO J = 7 --> 6 rotational transition. The CO emission shows rotation about Sgr A* but also evidence for noncircular turbulent motion and a clumpy morphology. We combine our data set with available CO measurements to model the physical conditions in the disk. We find that the molecular gas in the region is both warm and dense, with T approx. 200-300 K and n(sub H2) approx. (5-7) x 10(exp 4) cm(exp -3). The mass of warm molecular gas we measure in the central 2 pc is at least 2000 M(solar), about 20 times the UV-excited atomic gas mass, ruling out a UV heating scenario for the molecular material. We compare the available spectral tracers with theoretical models and conclude that molecular gas is heated with magnetohydrodynamic shocks with v approx. 10-20 km s(exp -1) and B approx. 0.3- 0.5 mG. Using the conditions derived with the CO analysis, we include the other important coolants, neutral oxygen and molecular hydrogen, to estimate the total cooling budget of the molecular material. We derive a mass-to-luminosity ratio of approx. 2-3 M(solar)(L(solar)exp -1), which is consistent with the total power dissipated via turbulent decay in 0.1 pc cells with v(sub rms) approx. 15 kilometers per second. These size and velocity scales are comparable to the observed clumping scale and the velocity dispersion. At this rate, the material near Sgr A* is dissipating its orbital energy on an orbital timescale and cannot last for more than a few orbits. Our conclusions support a scenario in which the features near Sgr A* such as the circumnuclear disk and northern arm are generated by infalling clouds with low specific angular momentum.

HCN and HCO+Observations of the Galactic Circumnuclear Disk

We present high spatial resolution (51 × 27) Owens Valley Radio Observatory (OVRO) millimeter array observations of HCN (J = 1-0) and HCO+ (J = 1-0) emission in the inner 3 pc of the Galaxy (0.04 pc ~ 1''). The HCN and HCO+ emission of the circumnuclear disk (CND) is distributed in a well-defined ring with a peak at a radius of 1.6 pc. The observed radial velocities are generally consistent with rotation at ~110 km s-1 (except along the western edge of the CND). The HCO+/HCN emission ratio is typically ~0.4 but with significant variations. The variations in the HCO+/HCN emission and absorption ratios can be attributed to greater abundances of HCO+ in lower density regions both within the CND and along the line of sight. The HCN emission is well correlated with the H2 emission at 2.12 μm both in the main emission lobes of the CND and also in four H2 and HCN filaments. Multiple areas of interaction between the ionized gas and the CND are also seen; the western arm of the minispiral is spatially and kinematically consistent with being the ionized inner edge of the CND, and the northern arm may be connected to the CND northeastern extension. With the enhanced spatial resolution of the HCN maps, we resolve numerous dense molecular gas cores within the CND with characteristic diameter ~7'' (0.25 pc). For 26 of the more isolated cores, we have measured sizes, velocity widths, and integrated fluxes. From these properties we estimated three masses for each core: a virial mass assuming the cores are gravitationally bound, an optically thick mass from observed column densities of HCN, and a lower limit mass assuming the HCN emission is optically thin and shock excitation is negligible. The virial and optically thick masses are in good agreement with a typical mass of (2-3) × 104 M☉ and a total CND gas mass of 106 M☉. The internal densities implied by these core masses (assuming a uniform density distribution for each core) are on average (3-4) × 107 cm-3. The core densities are high enough to be stable against tidal disruption from Sgr A* and the central stellar concentration. This tidal stability suggests a longer lifetime for the CND. The high densities and masses within the cores might support star formation either in the CND itself or within a core infalling toward the inner parsec, thus providing a mechanism for the formation of the young stellar population observed in the inner arcseconds of the Galaxy.

GALEX Ultraviolet Observations of the Interacting Galaxy NGC 4438 in the Virgo Cluster

We present GALEX near-ultraviolet (2310 Å) and far-ultraviolet (1530 Å) images of the interacting galaxy NGC 4438 (Arp 120) in the center of the Virgo Cluster. These images show an extended (20 kpc) tidal tail at the northwest edge of the galaxy that was previously undetected at other wavelengths; this tail is 15-25 kpc from NGC 4438's nucleus. Except for in the nucleus, the UV morphology of NGC 4438 is totally different from the Hα + [N II] morphology, which is more similar to the X-ray emission, confirming its gas cooling origin. We study the star formation history of NGC 4438 by combining spectrophotometric data in the UV-visible-near-IR wavelength range with population synthesis and galaxy evolution models. The data are consistent with a recent (~10 Myr), instantaneous burst of star formation in the newly discovered UV northwestern tail that is significantly younger than the age of the tidal interaction with NGC 4435, dated by dynamical models at ~100 Myr ago. Recent star formation events are also present at the edge of the northern arm and in the southern tail, while totally lacking in the other regions, which are dominated by the old stellar population that was perturbed during the dynamical interaction with NGC 4435. The contribution of this recent starburst to the total galaxy stellar mass is lower than 0.1%, an extremely low value for such a violent interaction. High-velocity, off-center tidal encounters such as that observed in Arp 120 are thus not sufficient to significantly increase the star formation activity of cluster galaxies.

Kinematic and structural analysis of the Minispiral in the Galactic Center from BEAR spectro-imagery

Integral field spectroscopy of the inner region of the Galactic Center, over a field of roughly 40 �� ×40 �� was obtained at 2.06 µ m( He) and 2.16 µ m( Brγ) using BEAR, an imaging Fourier Transform Spectrometer, at spectral resolutions respectively of 52.9 km s −1 and 21.3 km s −1 , and a spatial resolution of � 0.5 �� . The analysis of the data was focused on the kinematics of the gas flows, traditionally called the Minispiral, concentrated in the neighborhood of the central black hole, Sgr A � .F rom the decomposition into several velocity components (up to four) of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, and five additional, coherently-moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright, narrow North-South lane seen on intensity images, but it consists instead of a weak, continuous, triangular-shaped surface, drawn out into a narrow stream in the vicinity of Sgr Awhere it shows a strong velocity gradient, and a bright western rim. The Eastern Arm is split into three components (a Ribbon and a Tip, separated by a cavity, and an elongated feature parallel to the Ribbon: the Eastern Bridge). We also report extinction of some interstellar structures by other components, providing information on their relative position along the line of sight. A system of Keplerian orbits can be fitted to most of the Northern Arm, and the bright rim of this feature can be interpreted in terms of line-of-sight orbit crowding caused by the warping of the flowing surface at the western edge facing Sgr A � . These results lead to a new picture of the gas structures in Sgr A West, in which large-scale gas flows and isolated gas patches coexist in the gravitational field of the central Black Hole. The question of the origin of the ionized gas is addressed and a discussion of the lifetime of these features is presented.

Mechanisms of Malaysian Rainfall Anomalies

The behavior of the Malaysian anomalous rainfall is examined in relation to the El Niño–Southern Oscillation (ENSO) and local air–sea influences. The behavior is consistent with the northward migration of the ENSO-related coherence in the Maritime Continent rainfall. During the June–July–August (JJA) period, the ENSO-related coherence is low in the Malaysian rainfall but higher in the Indonesian rainfall. During the September– October–November (SON) period, the behavior of Malaysian anomalous rainfall is similar to that of the Indonesian rainfall since the ENSO-related coherence progresses northward to include the Malaysian region. It is proposed that a mechanism that operates during this period is similar to that in Indonesia, where local anomalous sea surface temperatures (SSTs) act to strengthen the remote influence of anomalous SST associated with ENSO in the eastern central Pacific. However, during the December–January–February (DJF) period, the behavior is markedly different from that of the Indonesian rainfall. During this period, the ENSO-related coherence has shifted northward across the equator. This shift results in the strengthening of ENSO–Malaysia rainfall relationship and the weakening of ENSO–Indonesia rainfall relationship. It is proposed that a different mechanism in which the anomalous cyclonic circulation over the South China Sea, northern Borneo, and the Philippine Sea is directly responsible for modulating the anomalous Malaysian rainfall during the DJF period. Finally, it is postulated that the anomalous cyclonic circulation is a response to the SST dipole associated with the strengthening of the northern arm of the boomerang-shaped SST in the northwestern Pacific Ocean.

Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data

In this paper, we present new results from an extensive set of Hubble Space Telescope (HST) and ground-based observations of the Seyfert galaxy NGC 2110. The HST data sets include Wide Field Planetary Camera 2 (WFPC2) observations as well as the first high-spatial resolution spectroscopy ([O i], [N ii], Hα and [S ii] lines) of this galaxy obtained using the Space Telescope Imaging Spectrograph (STIS). The ground-based data are three-dimensional (x, y, λ) spectrographic observations obtained using the integral field spectrograph OASIS on the Canada–France–Hawaii Telescope, complemented by near-infrared long-slit spectroscopy obtained using NIRSPEC on the Keck–II telescope. The OASIS observations cover regions containing both stellar absorption lines and major optical emission lines. The NIRSPEC observations cover the H and K bands. Combined with archival HST observations, the WFPC2 data provide us with a high-spatial resolution extinction map. The OASIS data allowed bidimensional mapping of the stellar and gaseous kinematics, as well as of the spectral properties of the ionized gas. These results are compared to those obtained in the near-infrared with NIRSPEC/Keck. Last, we used the STIS data to probe the ionized gas kinematics and properties in the inner 4 arcsec along PA = 156° at unprecedented spatial resolution. Our two-dimensional (2D) map of the stellar velocity field and the near-infrared stellar velocity profile are symmetric about the nucleus, confirming the results of previous long-slit observations. The asymmetry of the velocity field of the ionized gas is present at the same level for visible and near-infrared lines, indicating this is not a reddening effect. Multiple Gaussian fitting of the emission-line profile allowed the contributions of the broad and a narrow components to be disentangled. The intensity peak of the [O iii] narrow component is located north of the nucleus, indicating that the bulk of the narrow [O iii] emission comes from the jet-like structure (Mulchaey et al.) and not from the nucleus itself. We suggest that the northern arm is the anomalous one, contrary to what has been claimed earlier. Last, we also show that the elongated region of high gas velocity dispersion located close to the nucleus and discovered by Gonzalez Delgado et al. is intrinsic to the narrow component.

Magnetic fields in barred galaxies

Observations of the southern peculiar galaxy NGC 2442 with the Australia Telescope Compact Array in total and linearly polarized radio continuum at λ6 cm are presented and compared with previously obtained Hα data. The distribution of polarized emission, a signature of regular magnetic fields, reveals some physical phenomena which are unusual among spiral galaxies. We find evidence for tidal interaction and/or ram pressure from the intergalactic medium compressing the magnetic field at the northern and western edges of the galaxy. The radial component of the regular magnetic field in the northern arm is directed away from the centre of the galaxy, a finding which is in contrast to the majority of galaxies studied to date. The oval distortion caused by the interaction generates a sudden jump of the magnetic field pattern upstream of the inner northern spiral arm, similar to galaxies with long bars. An unusual “island” of strong regular magnetic field east of the galaxy is probably the brightest part of a magnetic arm similar to those seen in some normal spiral galaxies, which appear to be phase-shifted images of the preceding optical arm. The strong magnetic field of the “island” may indicate a past phase of active star formation when the preceding optical arm was exposed to ram pressure.

The infrared L'-band view of the Galactic Center with NAOS-CONICA at VLT

We report on Galactic Center L'-band observations made during NAOS/CONICA Science Verification. Colors of the inner 2´´ stars reveal an infrared excess of S2, the closest star to the black hole, that could sign the first thermal infrared detection of Sgr A*. A multi-wavelength maximum likelihood analysis has allowed us to eliminate all but two of the candidates for gravitational lensing proposed by [CITE]. Our observations of the thin and intersecting structures of the Northern Arm could trace several shocks heating the neighbooring dust rather than a stream of matter in orbit around the central mass as previously thought.

The infrared L'-band view of the Galactic Center with NAOS-CONICA at VLT

We report on Galactic Center L'-band observations made during NAOS/CONICA Science Verification. Colors of the inner 2 stars reveal an infrared excess of S2, the closest star to the black hole, that could sign the first thermal infrared detection of Sgr A*. A multi-wavelength maximum likelihood analysis has allowed us to eliminate all but two of the candidates for gravitational lensing proposed by Alexander (2001). Our observations of the thin and intersecting structures ofthe Northern Arm could trace several shocks heating the neighbooring dust rather than a stream of matter in orbit around the central mass as previously thought.

A Bow Shock of Heated Dust Surrounding Galactic Center Source IRS 8

High-resolution images in the H and K bands obtained by the Gemini North Telescope of the peculiar Galactic center source, IRS 8, reveal a central pointlike object enveloped in a remarkable bow shock, whose apex is located 02 to the northeast. The H-K' color of the bow shock is considerably redder than that of the central star. A United Kingdom Infrared Telescope (UKIRT) K-band spectrum reveals that the combined spectrum of the point source and bow shock is nearly featureless and that no shocked line emission (e.g., from H2) is physically associated with the bow. We interpret the bow as resulting from the interaction of the envelope or wind of the central star of IRS 8 with the extension of the northern arm of Sgr A West and/or the circumnuclear disk, and its emission as coming from radiatively heated, and possibly shock heated, dust.

Structural analysis of the Minispiral from high-resolution Br? data

Integral field spectroscopy of a roughly 40″ × 40″ region about the Galactic Center was obtained at 2.16 μm (Br ) using BEAR, an imaging Fourier Transform Spectrometer, at a spectral resolution of 21.3 km s–1and a spatial resolution of ≃0.5″. The analysis of the data was focused on the kinematics of the gas flows concentrated in the neighborhood of SgrA* , traditionally called the “Minispiral”. From the decomposition into several velocity components (up to four) of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, as well as five additional moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright north-south lane seen on intensity images, but consists instead of a continuous, weakly-emitting, triangular-shaped surface having a bright western rim, and narrowed at its forward apex in the vicinity of SgrA* where a strong velocity gradient is observed. The gravitational field of the central Black Hole can account for both the strong acceleration in this region and the tidal compression of the forward tip of the Northern Arm. Keplerian orbits can be fitted to the velocity field of the bright lane, which can be interpreted as formed by the bending of the western edge of the flowing surface. These results raise questions regarding the formation of the Sgr A West gas structures.

Resolving The Northern Arm Sources at the Galactic Center

Diffraction limited images obtained with the W. M. Keck telescopes have spatially resolved the cool luminous Galactic Center sources IRS 21, 1W, 2, 5, and 10W at wavelengths ranging from 2 to 25 μm. Their gaussian convolved sizes (∼ 2000 AU or 0″. 25 in diameter), along with their mid-infrared color temperatures, favor the hypothesis that they are centrally heated stellar sources rather than externally heated dust clumps. The near-infrared Keck speckle and AO images as well as 2.2 μm Gemini AO images of IRS 8 reveal asymmetric structures indicative of bow shock structures around all the Northern Arm sources. The presence of such large bow shocks around these objects suggests that their central heating sources have large winds requiring them to be young, massive stars like those observed in the nearby IRS 16 cluster. This number of windy stars is expected considering the a 10% volume coverage of the Northern Arm and the number of windy stars detected over a smaller region of the central parsec by Paumard et al (2001). This increases the total population of such stars to fifteen, which is comparable to the number of similar stars found in the nearby Quintuplet cluster.

The First Measurement of Radial Acceleration of Ionized Gas Near Sagittarius A*

Motivated by the presence of a massive black hole coincident with Sgr A* at the center of our Galaxy, a number of observers have successfully detected proper motion of ionized gas and stars at the Galactic center. Here we compare the radial velocities of ionized gas in the inner parsec of the Galaxy in three epochs (1993, 1999 & 2002). VLA observations were carried out in the H92α radio recombination line from Sgr A West; each observation had identical velocity coverage as well as velocity and spatial resolutions. Accurate velocity fields were derived from Gaussian fitting the data from the three epochs; radial acceleration was determined by comparing the fitted velocity fields. In the Northern Arm of Sgr A West, we report the the detection of radial acceleration (+0.21 kms−1 yr−1) consistent with motion dominated by gravity. In the minicavity, the measured acceleration is too large to be the result of gravity alone and suggest that non-gravitational effects are significant. Additionally, we detect a single region that shows peculiar motion of +4.1 kms−1 yr−1 southeast of Sgr A*. The first evidence of the acceleration of ionized gas in the Galactic center is presented.