Academic literature on the topic 'Galactic Centre Lobe (GCL)'

AbstractRadio continuum (cont) and radio recombination line (RRL) observations with the Yamaguchi 32-m radio telescope toward the lower part of the Galactic center lobe (GCL) in the Galactic center region are presented. While two ridges of the GCL were seen in both continuum and RRL images, the spatial coverage of the ridges of the continuum and RRL is not coincident. We distinguish the continuum emission of the GCL into thermal and non-thermal emission by assuming an electron temperature of the ionized gas of 4370 K, estimated based on the line width (14.1 km s−1). The thermal emission was found to be located inside and surrounded by the non-thermal emission.

AbstractWe analyze a high resolution (114″ × 60″) 74 MHz image of the Galactic center taken with the Very Large Array (VLA). We have identified several absorption and emission features in this region, and we discuss preliminary results of two Galactic center sources: the Sgr D complex (G1.1–0.1) and the Galactic center lobe (GCL).The 74 MHz image displays the thermal and nonthermal components of Sgr D and we argue the Sgr D supernova remnant (SNR) is consistent with an interaction with a nearby molecular cloud and the location of the Sgr D Hii region on the near side of the Galactic center. The image also suggests that the emission from the eastern side of the GCL contains a mixture of both thermal and nonthermal sources, whereas the western side is primarily thermal.

AbstractWe discuss the nature of the Galactic center lobe (GCL), a degree-tall, loop-like structure apparently erupting from the central few hundred parsecs of our Galaxy. Although its coincidence with the Galactic center has inspired diverse models for its origin, the observational evidence connecting this structure to the GC region has been thin. We describe a multiwavelength observing campaign with the VLA, GBT, Spitzer, and other telescopes that finds compelling evidence that the structure is likely formed by a mass outflow from the central tens of parsecs of our Galaxy. The size and mass of the putative outflow is consistent with that expected from the observed supernova rate and gas pressure in the GC region. If the GCL is a mass outflow, its relative proximity offers a unique opportunity for studying these structures in unprecedented detail.

ABSTRACT Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2° × 4° field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic Centre (GC). This mapping shows that the northern plume or fountain is offset from a well-known radio lobe (or the GCL), which however may represent a foreground H ii region, and that the southern plume is well wrapped by a corresponding radio lobe recently discovered by MeerKAT. In particular, we find that a distinct X-ray thread, G0.17−0.41, is embedded well within a non-thermal radio filament, which is locally inflated. This thread with a width of ∼1.6 arcsec (FWHM) is ∼2.6 arcmin or 6 pc long at the distance of the GC and has a spectrum that can be characterized by a power law or an optically-thin thermal plasma with temperature ≳ 3 keV. The X-ray-emitting material is likely confined within a strand of magnetic field with its strength ≳ 1 mG, not unusual in such radio filaments. These morphological and spectral properties of the radio/X-ray association suggest that magnetic field re-connection is the energy source. Such re-connection events are probably common when flux tubes of antiparallel magnetic fields collide and/or become twisted in and around the diffuse X-ray plumes, representing blowout superbubbles driven by young massive stellar clusters in the GC. The understanding of the process, theoretically predicted in analog to solar flares, can have strong implications for the study of interstellar hot plasma heating, cosmic ray acceleration and turbulence.

I critically discuss three possible MHD mechanisms for the formation of Galactic center lobes (GCL) found by Sofue and Handa (1984) from the theoretical point of view. The three mechanisms I shall discuss are: (1) sweeping-magnetic-twist mechanism, (2) explosion in a disk with a vertical magnetic field, and (3) nonlinear Parker instability. I review the characteristics of these mechanisms, which are mainly obtained from nonlinear 2D MHD numerical simulations, and discuss their merits and demerits as possible mechanisms for the formation of GCL and related magnetic structures.

AbstractHigh resolution imaging at radio frequencies has revealed several long, filamentary, non-thermal sources in the Galactic Centre region. One of these, known as the Snake, is unique in that it lies outside the Galactic Centre Lobe, and has two kinks along its length, one of which appears to be associated with a small, resolved source. For this work the Snake is assumed to be embedded in a region where both the magnetic field and the particle energy spectrum are uniform. The Snake is then modelled as an enhancement over the background of the particle energy spectrum. Some preliminary results from this model are presented here.

AbstractThe 3.5 meter diameter Herschel Space Observatory conducted a ∼720 square-degree survey of the Galactic plane, the Herschel Galactic plane survey (Hi-GAL). These data provide the most sensitive and highest resolution observations of the far-IR to sub-mm continuum from the central molecular zone (CMZ) at λ = 70, 160, 250, 350, and 500 μm obtained to date. Hi-GAL can be used to map the distributions of temperature and column density of dust in CMZ clouds, warm dust in Hii regions, and identify highly embedded massive protostars and clusters and the dusty shells ejected by supergiant stars. These data enable classification of sources and re-evaluation of the current and recent star-formation rate in the CMZ. The outer CMZ beyond |l| = 0.9 degrees (Rgal > 130 pc) contains most of the dense (n > 104 cm−3 gas in the Galaxy but supports very little star formation. The Hi-GAL and Spitzer data show that almost all star formation occurs in clouds moving on x2 orbits at Rgal < 100 pc. While the 106 M⊙ Sgr B2 complex, the 50 km s−1 cloud near Sgr A, and the Sgr C region are forming clusters of massive stars, other clouds are relatively inactive star formers, despite their high densities, large masses, and compact sizes. The asymmetric distribution of dense gas about Sgr A* on degree scales (most dense CMZ gas and dust is at positive Galactic longitudes and positive VLSR) and compact 24 μm sources (most are at negative longitudes) may indicate that eposidic mini-starbursts occasionally ‘blow-out’ a portion of the gas on these x2 orbits. The resulting massive-star feedback may fuel the compact 30 pc scale Galactic center bubble associated with the Arches and Quintuplet clusters, the several hundred pc scale Sofue-Handa lobe, and the kpc-scale Fermi/LAT bubble, making it the largest ‘superbubble’ in the Galaxy. A consequence of this model is that in our Galaxy, instead of the supermassive black hole (SMBH) limiting star formation, star formation may limit the growth of the SMBH.

Giant radio galaxies are thought to be massive ellipticals powered by accretion of interstellar matter onto a supermassive black hole. Interactions with gas rich galaxies may provide the interstellar matter to feed the active galactic nucleus (AGN). To power radio lobes that extend up to distances of hundreds of kiloparsecs, gas has to be funneled from kiloparsec size scales down to the AGN at rates of ˜1 M⊙ yr−1 during ≥108 years. Therefore, large and massive quasi-stable structures of gas and dust should exist in the deep interior of the giant elliptical hosts of double lobe radio galaxies. Recent mid-infrared observations with ISO revealed for the first time a bisymmetric spiral structure with the dimensions of a small galaxy at the centre of Centaurus A (Mirabel et al. 1999). The spiral was formed out of the tidal debris of accreted gas-rich object(s) and has a dust morphology that is remarkably similar to that found in barred spiral galaxies. The observations of the closest AGN to Earth suggest that the dusty hosts of giant radio galaxies like CenA, are “symbiotic” galaxies composed of a barred spiral inside an elliptical, where the bar serves to funnel gas toward the AGN.

Abstract An observational result of a radio continuum and H92α radio recombination line of the Galactic center lobe (GCL), using the Yamaguchi 32 m radio telescope, is reported. The obtained spatial intensity distribution of the radio recombination line shows two distinctive ridge-like structures extending from the Galactic plane vertically to the north at the eastern and western sides of the Galactic center, which are connected to each other at a latitude of ${1{^{\circ}_{.}}2}$ to form a loop-like structure as a whole. This suggests that most of the radio continuum emission of the GCL is free–free emission, and that the GCL is filled with thermal plasma. The east ridge of the GCL observed with the radio recombination line separates 30 pc from the radio arc, which has been considered a part of the GCL, but coincides with a ridge of the radio continuum at a Galactic longitude of 0°. The radial velocity of the radio recombination line is found to be between −4 and +10 km s−1 across the GCL. This velocity is much smaller than expected from the Galactic rotation, and hence indicates that the GCL exists apart from the Galactic center. These characteristics of the GCL suggest that the long-standing hypothesis that the GCL was created by explosive activity in the Galactic center is unlikely, but favor that the GCL is a giant H ii region.

Abstract The Galactic Center Lobe (GCL) is a peculiar object widely protruding from the Galactic plane toward the positive Galactic latitude, which had been found toward the Galactic Center (GC) in the early days of the radio observation. The peculiar shape has suggested a relation with historical events, star burst, large explosion, and so on in the GC. However, the issue of whether the GCL is a single large structure located in the GC region is not yet settled conclusively. In the previous observations, the silhouette against the low-frequency emission was found in the western part of the GCL (WPGCL); this suggests that the part is located in front of the GC region. On the other hand, the Local Standard of Rest (LSR) velocity of the radio recombination line toward it was found to be as low as 0 km s−1. However, these observations cannot determine the exact position on the line-of-sight. There is still another possibility that it is in the near-side area of the GC region. In this analysis, we compare these results with the visual extinction map toward the GC. We found that the distribution of the visual extinction larger than 4 mag clearly corresponds to the silhouette of the WPGCL. The WPGCL must be located at most within a few kpc from us and not in the GC region. This would be a giant H ii region in the Galactic disk.

Galactic outflows play an important role in the formation and evolution of galaxies by regulating the star formation rate (SFR) within them and by throwing out metals into the intergalactic medium (IGM). They are key to understand the relation between the stellar and the dark matter halo mass, mass-metallicity relation of galaxies, intergalactic metal enrichment, formation of high velocity clouds and much more. Galactic outflows have been observed to be present in galaxies at all redshifts either in emission or in absorption of the stellar continuum. Outflows have been also detected in the immediate vicinity of galaxies by probing absorption lines in the spectrum of background Active Galactic Nuclei. In this thesis we explore the interactions between supernovae (SNe) driven outflows and the circumvallate medium (CGM), an extended hot gas atmosphere believed to be present in the haloes of massive (stellar mass, M? & 1010 M_) galaxies. Given the complexity of geometry and multiphase nature of outflows, we use numerical simulations to study gas interactions. Our results shine light on many interesting aspects of the galactic outflows, such as, i) the effect of the circumgalactic medium on the mass outflow rate and velocity of the outflowing gas on large scales, ii) origin of high velocity cold (_ 104 K) gas in outflows iii) origin of X-ray emission in different scenarios. We connect our numerical and analytical work with the X-ray data. We also use our numerical set up to understand the origin and nature of two giant -ray bubbles, called the Fermi Bubbles, at the centre of our Galaxy. We compare our synthetic emission models to the observed -rays, X-rays, radio and UV absorption data and constrain the energetics and age of these bubbles. Below we outline the investigations undertaken in this thesis and point out our main results. Interaction of circumgalactic medium and outflows In a standard SNe driven outflow scenario, SNe ejected gas is a continuous outflow that expands freely with or without the gravity of the galaxy (Chevalier & Clegg 1985; Sharma & Nath 2013). The multiphase nature of the outflowing gas and the resistance provided by the CGM is often neglected while estimating the total mass outflow rate from galaxies (Arribas et al. 2014; Heckman et al. 2015). In the presence of a CGM, this scenario can change completely as the wind does not remain in a steady state anymore and involves far more complexities than typically considered, such as mixing with the hot CGM. The dynamics of the cold gas is expected to be different in such a non-steady state compared to the calculations in which the cold clumps move under the effect of a steady state wind. To study these effects, we perform hydrodynamical simulations of SNe driven outflows in a Milky-Way type galaxy that includes a CGM. We assess the effects of the CGM on the outflow by varying the star formation rate. We find that the total mass outflow rate is divided almost equally in two phases that peak at _ 105 K (warm) and at _ 3_106 K (hot). This means that observations in optical/UV or X-ray only probe a fraction of the outflowing mass. We also find that the mass loading factor (_), defined as the ratio between mass outflow rate to the star formation rate, at outer radii (_ 100 kpc) of a galaxy can be much higher than the rate observed in warm gas (_ _ 0:3-0:5). We present simple scaling relations between the mass loading factor in warm gas and the total mass loading factor at the virial radius (_v) that can be used to estimate the total mass outflow rate from such galaxies. We also find that warm gas can be entrained by _ 1000 km s􀀀1 free wind to reach velocities as large as _ 700 km s􀀀1. Cold clouds also form at the interaction zone between the outflow and the CGM. Some of these clouds keep moving outwards while some of them fall back to the stellar disc due to gravity. This galactic fountain gas which falls back can lead to further star formation in the disc. X-rays from galaxies Diffuse X-ray emission in case of a standard SNe driven outflow is dominated by the central part of the wind where temperature is _ 107 K and density is & 0:1 MP cm􀀀3 . Since density at the centre of a standard SNe driven outflow is simply proportional to the star formation rate (SFR), the X-ray luminosity (LX) is expected to be proportional to the SFR2. Observations, however, indicate a linear, or even a sub linear relation between LX and SFR (Mineo et al. 2012b; Wang et al. 2016). We used analytical results and numerical simulations to understand the origin of the X-ray emission from the star forming galaxies. We find that for highly star forming galaxies with no CGM, the diffuse X-ray mainly comes from the centre of the SNe wind as expected. However, for massive galaxies with low star formation rate (. 1 M_ yr􀀀1), the emission is dominated by the contribution from the CGM. This contamination results in a flatter LX-SFR relation than typically expected from a pure SNe driven outflow. Even after we increased the contribution from the outflowing wind by enhancing the mass loading factor to its maximum value, the CGM contamination could not be ignored. We further argue that these high LX values of low star forming, massive galaxies could be inverted to study the properties of the CGM itself Multi-wavelength properties of outflow and Fermi Bubbles in our Galaxy Observations reveal two giant (_ 50_) gamma-ray bubbles, called the Fermi Bubbles (FBs) toward the centre of our Galaxy (Su et al., 2010; Ackermann et al., 2014) the origin of which is still a mystery. Observations in other wavebands such as X-ray, radio and UV (absorption lines) also revealed many other interesting features associated with the FBs. There have been a number of attempts to explain the gamma-ray brightness and spectrum by considering feedback from the Galactic centre black hole (GCBH) and cosmic ray diffusion (Guo et al., 2012; Yang et al., 2012; Zubovas & Nayakshin, 2012). The required mechanical luminosity in these models exceeds the value that is achievable with the current accretion rate by a few orders of magnitude. Star formation driven wind models have been, however, under-investigated so far with much less attention to explain the multi-wavelength features related to the FBs. To understand the origin and nature of these bubbles, we simulate SNe driven wind scenario appropriate for the Milky-Way. By using the information about morphology and X-ray emission, we find that the required star formation rate at the centre of our Galaxy is _ 0:5 M_ yr􀀀1. After comparing the synthetic microwave surface brightness from our simulation with the observed data, we constrain the magnetic field inside the bubbles to be _ 4_G. We also find that the gamma-ray morphology and spectral signatures in our simulated bubbles closely resemble the observed ones. The cold gas (< 105 K) kinematics in our simulations also have a similar behaviour, to some extent, as observed in UV absorption lines through the northern bubbles. O viii and O vii line ratio through Fermi Bubbles Most of the models of the Fermi Bubbles focus on getting a reasonable gamma-ray morphology and spectrum by varying the mechanical luminosity of the central source. Other ways to determine the origin of the FBs include probing the bubbles in X-rays to obtain information about the strength of the explosion at the Galactic centre. X-ray spectral analysis by Kataoka et al. (2013) suggests that the shock velocity is _ 300 km s􀀀1 with an age of _ 20 Myr for the bubble, whereas, by analysing the O viii and O vii line ratio Miller & Bregman (2016) obtained a shock speed of _ 500 km s􀀀1 , indicating an age of _ 4 Myr. We simulate both star formation driven and GCBH driven wind scenarios in our Galaxy with varying strength of star formation and accretion rate. We consider a self consistent gas distribution for the Milky-Way CGM that is close to the observations. We compare the synthetic O viii and O vii lines from our simulations with the observations of Miller & Bregman (2016) and find that the data indicates a shock velocity of _ 300 km s􀀀1 and a corresponding age of the bubbles to be 15-25 Myr. After considering possible electron-proton non-equilibrium in the shocked gas that can affect the observability of the X-ray lines, we rule out mechanical luminosities & 1041 erg s􀀀1 as the possible driver of the Fermi Bubbles.