towards and within the Galactic centre

2006 ◽  
Vol 364 (1848) ◽  
pp. 3035-3042 ◽  
Author(s):  
T.R Geballe
Keyword(s):  
Rotational Level ◽  
Ionization Rate ◽  
High Resolution Spectroscopy ◽  
Line Of Sight ◽  
Galactic Centre ◽  
Long Line ◽  
Wide Range ◽  
The Galaxy ◽  
Infrared Sources ◽  
Diffuse Clouds

High-resolution spectroscopy of bright infrared sources in the centre of the Galaxy has resulted in the detection of in a remarkable array of dense and diffuse clouds along the 8000 parsec long line of sight, at a wide range of distances from the centre. Most prominent among these is a previously undetected, but very large amount of warm ( T ∼250 K) and diffuse ( n ∼100 cm 2 ) gas within a few hundred parsecs of the centre. The key to understanding the environment of the in this region is an absorption line at 3.53 μm from the metastable (3,3) rotational level, which has not been detected in dense or diffuse clouds outside of the Galactic centre (GC). We have used spectroscopy of this line along with other lines of and CO to characterize all of the clouds along the line of sight to the GC. The high abundance of in the central few hundred parsecs implies an ionization rate there that is several times larger than estimated for diffuse clouds outside the GC, and nearly two orders of magnitude greater than originally predicted for diffuse clouds.

scholarly journals H 3 + , the ideal probe for in situ measurement of the Galactic cosmic rays

2019 ◽  
Vol 377 (2154) ◽  
pp. 20180402 ◽  
Author(s):  
Takeshi Oka
Keyword(s):  
Cosmic Ray ◽  
Ionization Rate ◽  
Galactic Cosmic Rays ◽  
Molecular Ions ◽  
Galactic Centre ◽  
Galactic Disc ◽  
The Galaxy ◽  
Diffuse Clouds ◽  
The Ideal

The reason why H 3 + is an ideal probe for in situ measurement of cosmic ray energy density in the Galaxy is discussed. The variation of measured values of cosmic ray ionization rate ζ of H 2 since the theoretical prediction by Spitzer & Tomasko (Spitzer & Tomasko 1968 Astrophys. J. 152 , 971–986) is reviewed. How the measurements by H 3 + established ζ as of the order of 10 −17  s −1 for dense clouds, 10 −16  s −1 for diffuse clouds in the Galactic disc, and 10 −14  s −1 for warm and diffuse gas in the Galactic centre is discussed. This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H 3 + , H 5 + and beyond’.

scholarly journals The distribution of cosmic-ray ionization rates in diffuse molecular clouds as probed by H 3 +

2012 ◽  
Vol 370 (1978) ◽  
pp. 5142-5150 ◽  
Author(s):  
Nick Indriolo
Keyword(s):  
Molecular Clouds ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Line Of Sight ◽  
Interstellar Clouds ◽  
Upper Limits ◽  
The Galaxy ◽  
Cosmic Ray Flux ◽  
Made In ◽  
Ionization Rates

Owing to its simple chemistry, H is widely regarded as the most reliable tracer of the cosmic-ray ionization rate in diffuse interstellar clouds. At present, H observations have been made in over 50 sight lines that probe the diffuse interstellar medium (ISM) throughout the Galaxy. This small survey presents the opportunity to investigate the distribution of cosmic-ray ionization rates in the ISM, as well as any correlations between the ionization rate and line-of-sight properties. Some of the highest inferred ionization rates are about 25 times larger than the lowest upper limits, suggesting variations in the underlying low-energy cosmic-ray flux across the Galaxy. Most likely, such variations are caused predominantly by the distance between an observed cloud and the nearest site of particle acceleration.

scholarly journals Origin and evolution of the Galactic inventories of interstellar dust and its composition

2020 ◽  
Vol 494 (3) ◽  
pp. 4149-4167
Author(s):  
Anuj Gupta ◽  
Sandeep Sahijpal
Keyword(s):  
Interstellar Dust ◽  
Galactic Centre ◽  
Dust Grains ◽  
Thermodynamical Equilibrium ◽  
Origin And Evolution ◽  
Isotopic Signatures ◽  
Interstellar Dust Grains ◽  
Wide Range ◽  
Dust Mass ◽  
The Galaxy

ABSTRACT Interstellar dust is a significant component of matter in the galaxies. The dust owns its origin and reprocessing in a wide range of astrophysical environments. In order to understand the origin and evolution of the distinct types of interstellar dust grains, we have attempted a comprehensive correlated study of the thermodynamics condensation of dust grains in distinct stellar environments with the Galactic chemical evolution of the Milky Way Galaxy. The Galaxy is evolved in terms of elemental evolution resulting from stellar nucleosynthetic contributions of several generations of stars. Based on the elemental composition of the evolving Galaxy, the relative abundances of the major constituents of interstellar dust are assessed. The major aim is to redistribute the various condensable elements at any epoch during the evolution of the Galaxy into various grain constituents and understand their abundance evolution based on a mass-balance formalism. We also performed thermodynamical equilibrium condensation calculations to understand the stellar origin of various grain constituents that could carry the isotopic signatures of the various stellar nucleosynthetic sources. This is perhaps a novel attempt to estimate the bulk dust mass budget in the evolving Galaxy. The normalized mass of the Galactic dust is predicted to decrease with the increase in distance from the Galactic centre. It increases over time. The supernovae SNe Ia are predicted as the most prominent sources of Fe-dust mass, the supernova SN II+Ib/c produces oxides- and silicate-dust mass, and the AGB stars contribute to carbonaceous dust mass.

scholarly journals Interstellar Scattering Towards Cyg X-3

1988 ◽  
Vol 129 ◽  
pp. 305-306
Author(s):  
P. N. Wilkinson ◽  
R. E. Spencer ◽  
R. F. Nelson
Keyword(s):  
Flux Density ◽  
Basic Result ◽  
Model Fitting ◽  
Line Of Sight ◽  
Galactic Centre ◽  
Circular Symmetry ◽  
Interstellar Scattering ◽  
The Galaxy ◽  
Short Baselines ◽  
Ngc 6334

MERLIN observations at 0.408 GHz of the December 1985 outburst of Cyg X-3 have enabled us to characterise the angular broadening, caused by interstellar scattering, very accurately. The scattering along the line of sight towards Cyg X-3 is exceeded by only two other lines of sight through the Galaxy: towards the Galactic centre (see e.g. Lo et al. 1985. Nature, 315, 124) and towards NGC 6334 (Rodriguez et al. 1982. Astrophys. J., 225, 103); as a result quite short baselines are needed to study the scattering disk at 73 cm wavelength (0.408 GHz). The projected MERLIN baselines for our observations cover the range from a few km (on which the source was unresolved and had a correlated flux density ∼3.6 Jy) to ∼130 km (on which the source was totally resolved i.e. a correlated flux density <0.030 Jy). The basic result from these observations is that over this range of baselines the scattering appears to be purely diffractive in character. The scattering disk is, to quite a good approximation, a circular gaussian and shows no evidence of fine scale substructure. Model fitting to the visibility amplitudes, assuming circular symmetry, yields θ0.408GHz = 2.85±0.05 arcsec (FWHM).

scholarly journals Exploring the central molecular zone of the Galaxy using spectroscopy of H 3 + and CO

2012 ◽  
Vol 370 (1978) ◽  
pp. 5151-5161 ◽  
Author(s):  
T. R. Geballe
Keyword(s):  
Large Fraction ◽  
Level Structure ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Radiative Properties ◽  
Molecular Gas ◽  
Interstellar Gas ◽  
Energy Level Structure ◽  
Wide Range ◽  
The Galaxy

The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of and CO is a powerful way to determine the basic properties of molecular interstellar gas, because the abundance ratio of to CO in ‘dense’ clouds is quite different from that in ‘diffuse’ clouds. Moreover, the energy-level structure and the radiative properties of combined with the unusually warm temperatures of molecular gas in the CMZ make a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200 km s −1 and that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy.

scholarly journals GALACTICNUCLEUS: A high angular-resolution JHKs imaging survey of the Galactic centre

2020 ◽  
Vol 641 ◽  
pp. A141
Author(s):  
F. Nogueras-Lara ◽  
R. Schödel ◽  
N. Neumayer ◽  
E. Gallego-Cano ◽  
B. Shahzamanian ◽  
...  
Keyword(s):  
Stellar Population ◽  
Near Infrared ◽  
Angular Resolution ◽  
Interstellar Extinction ◽  
Extinction Curve ◽  
Line Of Sight ◽  
Galactic Centre ◽  
High Angular Resolution ◽  
The Galaxy ◽  
Red Clump

Context. The characterisation of the extinction curve in the near-infrared (NIR) is fundamental to analysing the structure and stellar population of the Galactic centre (GC), whose analysis is hampered by the extreme interstellar extinction (AV ~ 30 mag) that varies on arc-second scales. Recent studies indicate that the behaviour of the extinction curve might be more complex than previously assumed, pointing towards a variation of the extinction curve as a function of wavelength. Aims. We aim to analyse the variations of the extinction index, α, with wavelength, line-of-sight, and absolute extinction, extending previous analyses to a larger area of the innermost regions of the Galaxy. Methods. We analysed the whole GALACTICNUCLEUS survey, a high-angular resolution (~0.2″) JHKs NIR survey specially designed to observe the GC in unprecedented detail. It covers a region of ~6000 pc2, comprising fields in the nuclear stellar disc, the inner bulge, and the transition region between them. We applied two independent methods based on red clump (RC) stars to constrain the extinction curve and analysed its variation superseding previous studies. Results. We used more than 165 000 RC stars and increased the size of the regions analysed significantly to confirm that the extinction curve varies with the wavelength. We estimated a difference Δα = 0.21 ± 0.07 between the obtained extinction indices, αJH = 2.44 ± 0.05 and αHKs = 2.23 ± 0.05. We also concluded that there is no significant variation of the extinction curve with wavelength, with the line-of-sight or the absolute extinction. Finally, we computed the ratios between extinctions, AJ∕AH = 1.87 ± 0.03 and AH/AKs = 1.84 ± 0.03, consistent with all the regions of the GALACTICNUCLEUS catalogue.

Pulsar scintillation as a physical tool

1992 ◽  
Vol 341 (1660) ◽  
pp. 167-176 ◽  
Keyword(s):  
Spatial Coherence ◽  
Line Of Sight ◽  
Interstellar Gas ◽  
Pulsar Emission ◽  
Pulsar Radiation ◽  
Wide Range ◽  
Random Modulation ◽  
Radio Frequency Spectrum ◽  
The Galaxy ◽  
Diffraction Patterns

The interstellar gas contains irregularities of electron density having a wide range of physical scales. Pulsar radiation propagating through this inhomogeneous medium suffers a random modulation of phase which causes the received intensity to scintillate on a variety of timescales. Observations of the radio frequency spectrum and temporal variation of scintillation give information on the form of the irregularity spectrum and the distribution of density structure across the Galaxy. The high spatial coherence of pulsar radiation leads to the formation of extremely fine-scale diffraction patterns which also provide information on the motion of sources across the line of sight and the size of pulsar emission regions. Some uses of scintillation as a means of probing the interstellar gas and elucidating the physical properties of pulsars will be discussed.

scholarly journals Extending the view of ArH+ chemistry in diffuse clouds

2020 ◽  
Vol 643 ◽  
pp. A91
Author(s):  
Arshia M. Jacob ◽  
Karl M. Menten ◽  
Friedrich Wyrowski ◽  
Benjamin Winkel ◽  
David A. Neufeld
Keyword(s):  
Interstellar Medium ◽  
Crab Nebula ◽  
Cosmic Ray ◽  
Neutral Medium ◽  
Galactic Centre ◽  
Chemical Models ◽  
The Galaxy ◽  
Molecular Fraction ◽  
Atomic Gas ◽  
Diffuse Clouds

Context. One of the surprises of the Herschel mission was the detection of ArH+ towards the Crab Nebula in emission and in absorption towards strong Galactic background sources. Although these detections were limited to the first quadrant of the Galaxy, the existing data suggest that ArH+ ubiquitously and exclusively probes the diffuse atomic regions of the interstellar medium. Aims. In this study, we extend the coverage of ArH+ to other parts of the Galaxy with new observations of its J = 1−0 transition along seven Galactic sight lines towards bright sub-millimetre continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack of correlation as suggested by chemical models) with other well-known atomic gas tracers such as OH+ and H2O+ and the molecular gas tracer CH. Methods. The observations of the J = 1−0 line of ArH+ near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. Furthermore, the two sidebands of this receiver allowed us to observe the NKaKc = 11,0−10,1 transitions of para-H2O+ at 607.227 GHz simultaneously with the ArH+ line. Results. We modelled the optically thin absorption spectra of the different species and subsequently derived their column densities. By analysing the steady state chemistry of OH+ and o-H2O+, we derive on average a cosmic-ray ionisation rate, ζp(H), of (2.3 ± 0.3) × 10−16 s−1 towards the sight lines studied in this work. Using the derived values of ζp(H) and the observed ArH+ abundances we constrain the molecular fraction of the gas traced by ArH+ to lie below 2 × 10−2 with a median value of 8.8 × 10−4. Combined, our observations of ArH+, OH+, H2O+, and CH probe different regimes of the interstellar medium, from diffuse atomic to diffuse and translucent molecular clouds. Over Galactic scales, we see that the distribution of N(ArH+) is associated with that of N(H), particularly in the inner Galaxy (within 7 kpc of the Galactic centre) with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average ortho-to-para ratio for H2O+ of 2.1 ± 1.0, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.

scholarly journals Variability of the near-infrared extinction curve towards the Galactic centre

2019 ◽  
Vol 630 ◽  
pp. L3 ◽  
Author(s):  
F. Nogueras-Lara ◽  
R. Schödel ◽  
F. Najarro ◽  
A. T. Gallego-Calvente ◽  
E. Gallego-Cano ◽  
...  
Keyword(s):  
Near Infrared ◽  
Stellar Structure ◽  
Extinction Curve ◽  
Line Of Sight ◽  
Galactic Centre ◽  
High Angular Resolution ◽  
Mean Values ◽  
The Galaxy ◽  
Red Clump ◽  
Significant Line

Context. Due to the extreme extinction towards the Galactic centre (AV ∼ 30 mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. Aims. We studied the dependence of the extinction index (αλ) in the NIR on the line of sight, wavelength, and extinction. Methods. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue (0.2″) for the inner part of the Galaxy in JHKs, and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. Results. We did not detect any significant line-of-sight or extinction variation in α within the studied region in the nuclear stellar disc. The extinction index between JH and HKs differs by 0.19 ± 0.05. We obtained mean values for the extinction indices αJH = 2.43 ± 0.03 and αHKs = 2.23 ± 0.03. The dependence of the extinction index on the wavelength could explain the differences obtained for αλ in the literature since it was assumed constant for the NIR regime.

scholarly journals Hot, metastable hydronium ion in the Galactic centre: formation pumping in X-ray-irradiated gas?

2012 ◽  
Vol 370 (1978) ◽  
pp. 5162-5173 ◽  
Author(s):  
Dariusz C. Lis ◽  
Peter Schilke ◽  
Edwin A. Bergin ◽  
Martin Emprechtinger ◽  
Keyword(s):  
Direct Detection ◽  
Rotational Temperature ◽  
Rotational Level ◽  
Building Blocks ◽  
High Spectral Resolution ◽  
High Resolution Spectroscopy ◽  
Galactic Centre ◽  
X Rays ◽  
Hydronium Ion ◽  
Rotational Transitions

With a 3.5 m diameter telescope passively cooled to approximately 80 K, and a science payload comprising two direct detection cameras/medium resolution imaging spectrometers (PACS and SPIRE) and a very high spectral resolution heterodyne spectrometer (HIFI), the Herschel Space Observatory is providing extraordinary observational opportunities in the 55–670 μm spectral range. HIFI has opened for the first time to high-resolution spectroscopy the submillimetre band that includes the fundamental rotational transitions of interstellar hydrides, the basic building blocks of astrochemistry. We discuss a recent HIFI discovery of metastable rotational transitions of the hydronium ion (protonated water, H 3 O + ), with rotational level energies up to 1200 K above the ground state, in absorption towards Sagittarius B2(N) in the Galactic centre. Hydronium is an important molecular ion in the oxygen chemical network. Earlier HIFI observations have indicated a general deficiency of H 3 O + in the diffuse gas in the Galactic disc. The presence of hot H 3 O + towards Sagittarius B2(N) thus appears to be related to the unique physical conditions in the central molecular zone, manifested, for example, by the widespread presence of abundant H . One intriguing theory for the high rotational temperature characterizing the population of the H 3 O + metastable levels may be formation pumping in molecular gas irradiated by X-rays emitted by the Galactic centre black hole. Alternatively, the pervasive presence of enhanced turbulence in the central molecular zone may give rise to shocks in the lower-density medium that is exposed to energetic radiation.

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