Juergen Ott

Wild at Heart: The particle astrophysics of the Galactic Centre

We consider the high-energy astrophysics of the inner ∼200 pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to (i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the regions non-thermal radio and TeV γ-ray emission; (ii) drive a powerful wind that advects non-thermal particles out of the inner Galactic Centre; (iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm envelope H 2 detected throughout the Central Molecular Zone; (iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ∼150 pc scales above and below the plane (the Galactic Centre lobe); and (v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼ 10 kpc), generate the recently identified Wilkinson Microwave Anisotropy Probe (WMAP) haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/μG < 400 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the regions dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this starburst-like environment - act to substantially modify the conditions of star formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511 keV line emission. We present extensive appendices reviewing the environmental conditions in the Galactic Centre, deriving the star formation and supernova rates there, and setting out the extensive prior evidence that exists, supporting the notion of a fast outflow from the region.

A Chandra X-ray survey of nearby dwarf starburst galaxies — II. Starburst properties and outflows

We present a comprehensive comparison of the X-ray properties of a sample of eight dwarf starburst galaxies observed with Chandra (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253, He2–10). In PaperI we presented in detail the data reduction and analysis of the individual galaxies. For the unresolved X-ray sources we find the following: point sources are in general located close to bright H ii regions, rims of superbubbles, or young stellar clusters. The number of X-ray point sources appears to be a function of the current star formation rate and the blue luminosity of the hosts. Ultraluminous X-ray sources are only found in those dwarf galaxies which are currently interacting. The power law index of the combined cumulative X-ray point source luminosity function is α = 0.24 ± 0.06, shallower than that of more massive starburst galaxies (α = 0.4 − 0.8) and of non-starburst galaxies (α ∼ 1.2). For those galaxies showing extended X-ray emission (6 out of the 8 galaxies), we derive the following: Superwinds develop along the steepest gradient of the H i distribution with volume densities of 0.02 −0.06cm 3 , pressures of 1 −3 ×10 5 Kcm 3 , thermal energies of 2 − 30 × 10 54 erg, and hot gas masses of 2 − 20 × 10 6 M⊙ (∼ 1 per cent of the H i masses. On global scales, the distribution of the X-ray emission looks remarkably similar to that seen in Hα (comparing azimuthal averages); locally however their distribution is clearly distinct in many cases – this can be explained by the different emission mechanisms (forward vs. reverse shocks). Mass-loading of order 1 to 5 is required to explain the differences between the amount of hot gas and and the modelled mass-loss from massive stars. The metallicity of the dwarf galaxies correlates with the diffuse X-ray luminosity and anti-correlates with the cooling time of the hot gas. The diffuse X-ray luminosity is also a function of the current star formation rate. The mechanical luminosities of the developing superwinds are energetic enough to overcome the gravitational potentials of their host galaxies. This scenario is supported by the overpressures of the hot gas compared to the ambient ISM. Extended H i envelopes such as tidal tails, however, may delay outflows on timescales exceeding those of the cooling time of the hot gas.

Multiwavelength observations of southern hot molecular cores traced by methanol masers - I. Ammonia and 24-GHz continuum data

We present observations of the (1,1), (2,2), (4,4) and (5,5) inversion transitions of para-ammonia (NH 3 ) and 24-GHz continuum, taken with the Australia Telescope Compact Array towards 21 southern Galactic hot molecular cores traced by 6.7-GHz methanol maser emission. We detect NH 3 (1,1) emission towards all 21 regions and 24-GHz continuum emission towards 12 of the regions, including six with no reported 8-GHz continuum counterparts. In total, we find the 21 regions contain 41 NH 3 (1,1) cores but around half of the regions only contain a single core. We extract characteristic spectra for every core at each of the NH 3 transitions and present both integrated intensity maps and channel maps for each region. NH 3 (2,2) emission was detected towards all NH 3 (1,1) cores. NH 3 (4,4) emission was detected in 13 of the NH 3 (1,1) cores with NH 3 (5,5) emission coincident with 11 of these. The NH 3 (4,4) and (5,5) emission is always unresolved and found at the methanol maser position. An analysis of the NH 3 (1,1) and (2,2) line ratios suggests that the cores with NH 3 (4,4) and (5,5) emission are warmer than the remaining cores rather than simply containing more ammonia. The coincidence of the maser emission with the higher spatial resolution NH 3 (4,4) and (5,5) emission indicates that the methanol masers are found at the warmest part of the core. In all cores detected at NH 3 (4,4) (with the exception of G12.68-0.18 core 4), the measured linewidth increases with transition energy. The NH 3 (1,1) spectra of several cores show an emission and absorption component slightly offset in velocity but it is unclear whether or not this is due to systematic motion of the gas. We observe large asymmetries in the NH 3 (1,1) hyperfine line profiles and conclude that this is due to non-local thermodynamic equilibrium conditions arising from a number of dense, small clumps within the beam, rather than systematic motions of gas in the cores. Assuming that the 24-GHz continuum emission is optically-thin bremsstrahlung, we derive properties of the ionized gas. The rate of Lyman continuum photons required to ionize the gas of 10 45 -10 48 s -1 suggests that the continuum emission is powered by stars of mass >8 M ⊙ . We investigate the nature of the 24-GHz continuum sources which were not detected at 8 GHz and find that these are always coincident with both ammonia and methanol maser emission. This is in contrast to those detected at both 8 and 24 GHz which are generally offset from the methanol maser emission. We investigate the possibility that these may be hypercompact H II regions. Finally, we separate the cores into five groups, based on their association with NH 3 , methanol maser and continuum emission. From the different physical properties of the cores in the groups, we discuss the possibility that these groups may represent cores at different evolutionary stages of the massive star formation process.

The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253

We present interferometric maps of ammonia (NH3) of the nearby starburst galaxy NGC 253. The observations have been taken with the Australia Telescope Compact Array and include the para-NH3 (1, 1) and (2, 2) and the ortho-NH3 (3, 3) and (6, 6) inversion lines. Six major complexes of dense ammonia are identified, three of them on either side of the starburst center, out to projected galactocentric radii of ~250 pc. Rotational temperatures are derived toward selected individual positions, as well as for the entire southeastern and northwestern molecular complexes. The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 K toward the southwest and northeast, respectively. The temperatures under which ammonia was formed in the past are with 30 K also warmer toward the southwest than toward the northeast (~15-20 K). This is indicated by the ortho-ammonia-to-para-ammonia ratio, which is ~1 and 1.5-2.5 toward the southwest and northeast, respectively. Ammonia column densities in the brightest complexes are in the range of (6-11) × 1014 cm-2, which adds up to a total ammonia mass of ~20 M☉, about evenly distributed toward both sides of the nucleus. Ammonia abundances relative to H2 are ~3 × 10-8. In the southwestern complex, the ammonia abundances increase from the starburst center to larger galactocentric radii. Toward the center of NGC 253, NH3 (1, 1), (2, 2), and (6, 6) are detected in absorption against an unresolved continuum source. At the same location, however, ammonia (3, 3) is found in emission, which indicates maser activity. This would be the first detected extragalactic NH3 maser. Evidence for an expanding shell in the southwestern complex is provided. The shell, with a dynamical age of ~1.3 Myr, is centered on an X-ray point source that must be located within the dense gas of NGC 253. The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ~105 M☉, which also heats the dense gas. A current star formation rate of ~2.8 M☉ yr-1 is derived for the nuclear starburst in NGC 253 based on its 1.2 cm continuum emission.

A Chandra X‐ray survey of nearby dwarf starburst galaxies – I. Data reduction and results

We present an analysis of Chandra X-ray observations of a sample of eight dwarf starburst galaxies (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253, and He2–10). Extended, diffuse X-ray emission is detected in all but two of the objects. Unresolved sources were found within all dwarf galaxies (total: 55 sources). These point sources are well fit by power law, thermal plasma, or black body models. Ten of the point sources exceed an X-ray luminosity of 10 39 erg s 1 (ultraluminous X-ray sources). In those galaxies where diffuse X-ray emission is detected, this emission (with X-ray luminosities ranging from 4 × 10 38 erg s 1 to 2 × 10 40 erg s 1 ) contains most (60-80 per cent) of the X-ray photons. This diffuse emission can be well fit by MeKaL one-temperature thermal plasma models once the contribution from the unresolved point sources is subtracted properly. The diffuse X-ray component is significantly extended, reaching as far as 0.5 5kpc into the outskirts of their hosts. Azimuthally averaged X-ray surface brightness profiles are well approximated by exponential functions. Temperatures of various regions within the galaxies range from 1.6 7.6 × 10 6 K. With few exceptions, temperatures of the hot gas are remarkably uniform, hovering around 2 3×10 6 K. Temperatures of the coronal gas in the outer regions are in general � 2 3 times lower than those found in the central regions. Fits to the diffuse emission do not allow strong constraints to be put on the metallicities of the emitting plasmas. However, the derived metallicities are compatible with those determined from their H ii regions. An α/F e ratio of � 2 is indicated for the hot gas within at least three objects (NGC1569, NGC4449, and He2–10). Shadowing of the diffuse X-ray emission by the cooler disc gas is used to constrain the orientation of the galaxies.

Evidence for BlowOut in the Low-Mass Dwarf Galaxy Holmberg I

We present radio and optical observations of Holmberg I (Ho I), a member of the M81 group of galaxies (distance D3.6 Mpc). Ho I is a low-mass, low surface brightness dwarf galaxy. High-resolution, multiarray, Very Large Array observations in the line of neutral hydrogen (H I) reveal a supergiant shell (diameter 1.7 kpc) that covers about half the optical extent of Ho I and that comprises 75% of the total H I content (total H I mass: 1.1 ] 108 We estimate the scale height of the H I layer to be 250 M _ ). pc. We set a tentative upper limit to the dark matter content of The H I pc [ h [ 550

The Formation of Kiloparsec-scale H I Holes in Dwarf Galaxies

The origin of kpc-scale holes in the atomic hydrogen (H i) distributions of some nearby dwarf irregular galaxies presents an intriguing problem. Star formation histories (SFHs) derived from resolved stars give us the unique opportunity to study past star-forming events that may have helped shape the currently visible Hi distribution. Our sample of five nearby dwarf irregular galaxies spans over an order of magnitude in both total Hi mass and absolute B-band magnitude and is at the low-mass end of previously studied systems. We use Very Large Array Hi line data to estimate the energy required to create the centrally dominant hole in each galaxy. We compare this energy estimate to the past energy released by the underlying stellar populations computed from SFHs derived from data taken with the Hubble Space Telescope. The inferred integrated stellar energy released within the characteristic ages exceeds our energy estimates for creating the holes in all cases, assuming expected efficiencies. Therefore, it appears that stellar feedback provides sufficient energy to produce the observed holes. However, we find no obvious signature of single star-forming events responsible for the observed structures when comparing the global SFHs of each galaxy in our sample to each other or to those of dwarf irregular galaxies reported in the literature. We also fail to find evidence of a central star cluster in FUV or Hα imaging. We conclude that large Hi holes are likely formed from multiple generations of star formation and only under suitable interstellar medium conditions.

γ-rays and the far-infrared–radio continuum correlation reveal a powerful Galactic Centre wind

We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost star-burst-like region is ultimately driven dominantly by on-going massive star formation. We show that this region’s radio continuum (RC) emission is in relative decit with respect to the expectation aorded by the Farinfrared{Radio Continuum Correlation (FRC). Likewise we show that the region’s -ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km/s) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic eld falls in the range 100-300 G and that { in the time they remain in the region { GC cosmic rays do not penetrate into the region’s densest molecular material.

The influence of far-ultraviolet radiation on the properties of molecular clouds in the 30 Dor region of the Large Magellanic Cloud

We present a complete ^(12)CO J = 1 → 0 map of the prominent molecular ridge in the Large Magellanic Cloud (LMC) obtained with the 22 m ATNF Mopra Telescope. The region stretches southward by ~2° (or 1.7 kpc) from 30 Doradus, the most vigorous star-forming region in the Local Group. The location of this molecular ridge is unique insofar as it allows us to study the properties of molecular gas as a function of the ambient radiation field in a low-metallicity environment. We find that the physical properties of CO-emitting clumps within the molecular ridge do not vary with the strength of the far-ultraviolet radiation field. Since the peak CO brightness of the clumps shows no correlation with the radiation field strength, the observed constant value for CO-to-H_2 conversion factor along the ridge seems to require an increase in the kinetic temperature of the molecular gas that is offset by a decrease in the angular filling factor of the CO emission. We find that the difference between the CO-to-H_2 conversion factor in the molecular ridge and the outer Milky Way is smaller than has been reported by previous studies of the CO emission: applying the same cloud identification and analysis methods to our CO observations of the LMC molecular ridge and CO data from the outer Galaxy survey by Dame et al., we find that the average CO-to-H_2 conversion factor in the molecular ridge is X_(CO) ≃ (3.9 ± 2.5) × 10^(20) cm^(–2) (K km s^(–1))^(–1), approximately twice the value that we determine for the outer Galaxy clouds. The mass spectrum and the scaling relations between the properties of the CO clumps in the molecular ridge are similar, but not identical, to those that have been established for Galactic molecular clouds.

Interpretation of radio continuum and molecular line observations of Sgr B2: free-free and synchrotron emission, and implications for cosmic rays

Recent ammonia (1,1) inversion line data on the Galactic star-forming region Sgr B2 show that the column density is consistent with a radial Gaussian density profile with a standard deviation of 2.75 pc. Deriving a formula for the virial mass of spherical Gaussian clouds, we obtain M_(vir) = 1.9 x 10^6 M⊙ for Sgr B2. For this matter distribution, a reasonable magnetic field and an impinging flux of cosmic rays of solar neighbourhood intensity, we predict the expected synchrotron emission from the Sgr B2 giant molecular cloud due to secondary electrons and positrons resulting from cosmic-ray interactions, including effects of losses due to pion production collisions during diffusive propagation into the cloud complex. We assemble radio-continuum data at frequencies between 330 MHz and 230 GHz. From the spectral-energy distribution, the emission appears to be thermal at all frequencies. Before using these data to constrain the predicted synchrotron flux, we first model the spectrum as free-free emission from the known ultra compact H II regions plus emission from an envelope or wind with a radial-density gradient, and obtain an excellent fit. We thus find the spectrum at all frequencies to be dominated by thermal emission, and this severely constrains the possible synchrotron emission by secondary electrons to quite low-flux levels. The absence of a significant contribution by secondary electrons is almost certainly due to multi-GeV energy cosmic rays being unable to penetrate far into giant molecular clouds. This would also explain why 100 MeV-GeV gamma-rays (from neutral pion decay or bremsstrahlung by secondary electrons) were not observed from Sgr B2 by the EGRET instrument on the Compton Gamma Ray Observatory, while TeV energy gamma-rays were observed by the High Energy Stereoscopic System (HESS), being produced by higher energy cosmic rays which more readily penetrate giant molecular clouds.