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Register a new userThe Interstellar Medium toward the Galactic Center Source 2MASS J17470898-2829561
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We describe and discuss remarkable infrared spectra, covering key portions of the $2-5$ $mu$m wavelength interval, of the probable OH/IR supergiant 2MASS J17470898$-$2829561 (2M1747), located in direction of the Sgr B molecular cloud complex within the Central Molecular Zone (CMZ) of the Galaxy. This star was originally singled out for examination based on its suitability for spectroscopy of lines of H$_3^+$ in the CMZ. Analysis of the spectra shows that 2M1747 is deeply embedded within Sgr B1, with A$_V$ $gtrsim$ 100 mag, making it the only star within Sgr B for which infrared spectra have been obtained at present, and thereby a unique infrared probe of the dense interstellar medium within the CMZ. Despite the high extinction, spectra of 2M1747 reveal a veiled photosphere in the $K$ band and circumstellar gas in the $M$ band, giving clues as to its nature. Its $ 3.5-4.0$ $mu$m spectrum contains the strongest absorption lines of H$_3^+$ observed toward any object to date. The $4.5-4.8$ $mu$m spectrum has impressively deep and wide absorption lines of interstellar CO, most of which arise in dense gas within Sgr B1. The $3-5$ $mu$m spectrum also contains several solid state absorption features, which are characteristic of both dense and diffuse clouds, and which raise questions about the identifications of some of these features. We discuss the nature of the star, the extinction to it, the extinction law for dust in the CMZ, and the identifications of the various solid-state features and where they are produced along this complex line of sight.
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Near-infrared polarimetric imaging observations toward the Galactic center have been carried out to examine the efficiency and wavelength dependence of interstellar polarization. A total area of about 5.7 deg$^2$ is covered in the $J$, $H$, and $K_S$ bands. We examined the polarization efficiency, defined as the ratio of degree of polarization to color excess. The interstellar medium between the Galactic center and us shows the polarization efficiency lower than that in the Galactic disk by a factor of three. Moreover we investigated the spatial variation of the polarization efficiency by comparing it with those of color excess, degree of polarization, and position angle. The spatial variations of color excess and degree of polarization depend on the Galactic latitude, while the polarization efficiency varies independently of the Galactic structure. Position angles are nearly parallel to the Galactic plane, indicating the longitudinal magnetic field configuration between the Galactic center and us. The polarization efficiency anticorrelates with dispersions of position angles. The low polarization efficiency and its spatial variation can be explained by the differences of the magnetic field directions along the line-of-sight. From the lower polarization efficiency, we suggest a higher strength of a random component relative to a uniform component of the magnetic field between the Galactic center and us. We also derived the ratios of degree of polarization $p_H/p_J$ = 0.581 $pm$ 0.004 and $p_{K_S}/p_H$ = 0.620 $pm$ 0.002. The power law indices of the wavelength dependence of polarization are $beta_{JH}$ = 2.08 $pm$ 0.02 and $beta_{HK_S}$ = 1.76 $pm$ 0.01. Therefore the wavelength dependence of interstellar polarization exhibits flattening toward longer wavelengths in the range of 1.25$-$2.14 $micron$. The flattening would be caused by aligned large-size dust grains.
This paper deals with the heating and the ionization of the interstellar medium (ISM) in the 500 central pc of the Milky Way (hereafter Galactic center, GC). We review the results of the Infrared Space Observatory (ISO) observations of a sample of GC molecular clouds located far from thermal radiocontinuum or far-infrared sources. For the first time, we have been able to study in detail the dust continuum spectra from 40 to 190 microns founding a warm (30-40 K) dust component in addition to the well known 15-20 K component. Fine-structure lines observations have revealed the presence of diffuse ionized gas associated with the molecular clouds. The effective temperature of the ionizing radiation is higher than 33000 K. ISO has also allow us to measure the fraction of warm (~ 150 K) H2 in the GC clouds, which is on average of 30 %. The observations of the warm (a few 100 K) neutral gas are compatible with a Photon Dominated Region (PDR) scenario.
We have used the Wide Field Spectrograph on the Australian National University 2.3-m telescope to perform the integral field spectroscopy for a sample of the Galactic planetary nebulae. The spatially resolved velocity distributions of the H$alpha$ emission line were used to determine the kinematic features and nebular orientations. Our findings show that some bulge planetary nebulae toward the Galactic center have a particular orientation.
The Galactic Center lobe is a degree-tall shell seen in radio continuum images of the Galactic center (GC) region. If it is actually located in the GC region, formation models would require massive energy input (e.g., starburst or jet) to create it. At present, observations have not strongly constrained the location or physical conditions of the GC lobe. This paper describes the analysis of new and archival single-dish observations of radio recombination lines toward this enigmatic object. The observations find that the ionized gas has a morphology similar to the radio continuum emission, suggesting that they are associated. We study averages of several transitions from H106alpha to H191epsilon and find that the line ratios are most consistent with gas in local thermodynamic equilibrium. The radio recombination line widths are remarkably narrow, constraining the typical electron temperature to be less than about 4000 K. These observations also find evidence of pressure broadening in the higher electronic states, implying a gas density of n_e=910^{+310}_{-450} cm^{-3}. The electron temperature, gas pressure, and morphology are all consistent with the idea that the GC lobe is located in the GC region. If so, the ionized gas appears to form a shell surrounding the central 100 parsecs of the galaxy with a mass of roughly 10^5 Msun, similar to ionized outflows seen in dwarf starbursts.
We present near-infrared (IR) spectra of two planetary nebula (PN) candidates in close lines of sight toward the Galactic center (GC) using the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini North. High-resolution images from radio continuum and narrow-band IR observations reveal ringlike or barrel-shaped morphologies of these objects, and their mid-IR spectra from the Spitzer Space Telescope exhibit rich emission lines from highly-excited species such as [S IV], [Ne III], [Ne V], and [O IV]. We also derive elemental abundances using the Cloudy synthetic models, and find an excess amount of the $s$-process element Krypton in both targets, which supports their nature as PN. We estimate foreground extinction toward each object using near-IR hydrogen recombination lines, and find significant visual extinctions ($A_V > 20$). The distances inferred from the size versus surface brightness relation of other PNe are $9.0pm1.6$ kpc and $7.6pm1.6$ kpc for SSTGC 580183 and SSTGC 588220, respectively. These observed properties along with abundance patterns and their close proximity to Sgr A$^*$ (projected distances $<20$ pc) make it highly probable that these objects are the first confirmed PN objects in the nuclear stellar disk. The apparent scarcity of such objects resembles the extremely low rate of PN formation in old stellar systems, but is in line with the current rate of the sustained star formation activity in the Central Molecular Zone.
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