Modelling the bar in the centre of the starburst galaxy M82

We present VLA A-array 21-cm atomic hydrogen (H  i ) absorption observed against the central region of the starburst galaxy M82 with an angular resolution of ∼1.3 arcsec (≃20 pc). These observations, together with MERLIN H  i absorption measurements, are compared with the molecular (CO) and ionized ([Ne  ii ]) gas distributions and are used to constrain the dynamics and structure of the ionized, neutral and molecular gas in this starburst.
A position–velocity diagram of the H  i distribution reveals an unusual 'hole' feature which, when previously observed in CO, has been interpreted as an expanding superbubble contained within a ring of gas in solid body rotation. However, we interpret this feature as a signature of a nearly edge-on barred galaxy. In addition, we note that the CO, H  i and [Ne  ii ] position–velocity diagrams reveal two main velocity gradients, and we interpret these as gas moving on x₁- and x₂-orbits within a bar potential. We find the best fit to the data to be produced using a bar potential with a flat rotation curve velocity v _b=140 km s⁻¹ and a total length of 1 kpc, a non-axisymmetry parameter q =0.9, an angular velocity of the bar Ω_b=217 km s⁻¹ arcsec⁻¹, a core radius R _c=25 pc, an inclination angle i =80° and a projected angle between the bar and the major axis of the galaxy φ '=4°. We also discuss the orientation of the disc and bar in M82.     

Neutral hydrogen absorption against supernova remnants in M82

We present MERLIN neutral hydrogen absorption measurements against supernova remnants in the central starburst region of M82 with an angular resolution of ∼ 0.4 arcsec. We detect H  I absorption or set significant upper limits against 33 supernova remnants from which we have been able to deduce column densities. Hence, using these measurements, we are able to probe the neutral hydrogen distribution and dynamics of the interstellar medium in M82 along 33 lines of sight on linear scales of order 1 pc.   Our results show column densities ranging from <1.6 to >30 × 10²¹ atom cm⁻² with the highest values seen towards the edge of the 250-pc 'ring'. The absorption velocities show a gradient of 7.3 ± 4 km s⁻¹ arcsec⁻¹, consistent with rotation parameters of this 'ring' inferred from other measurements. The absorption velocities against individual remnants show deviations of typically 30 km s⁻¹ from simple solid body rotation, and a number show multiple velocity absorption features. Although some of these deviations may be the result of the remnants being embedded at different depths within the neutral gas, the velocities cannot be explained by a simple rotating ring.     

A high-resolution radio study of neutral gas in the starburst galaxy NGC 520

We present subarcsec angular resolution observations of the neutral gas in the nearby starburst galaxy NGC 520. The central kpc region of NGC 520 contains an area of significantly enhanced star formation. The radio continuum structure of this region resolves into ∼10 continuum components. By comparing the flux densities of the brightest of these components at 1.4 GHz with published 15-GHz data we infer that these components detected at 1.4 and 1.6 GHz are related to the starburst and are most likely to be collections of several supernova remnants within the beam. None of these components is consistent with emission from an active galactic nuclei. Both neutral hydrogen (H  i ) and hydroxyl (OH) absorption lines are observed against the continuum emission, along with a weak OH maser feature probably related to the star formation activity in this galaxy. Strong H  i absorption  ( N _H∼ 10²² atoms cm⁻²)  traces a velocity gradient of 0.5 km s⁻¹ pc⁻¹ across the central kpc of NGC 520. The H  i absorption velocity structure is consistent with the velocity gradients observed in both the OH absorption and in CO emission observations. The neutral gas velocity structure observed within the central kpc of NGC 520 is attributed to a kpc-scale ring or disc. It is also noted that the velocity gradients observed for these neutral gas components appear to differ with the velocity gradients observed from optical ionized emission lines. This apparent disagreement is discussed and attributed to the extinction of the optical emission from the actual centre of this source hence implying that optical ionized emission lines are only detected from regions with significantly different radii to those sampled by the observations presented here.     

A hard X-ray constraint on the presence of an AGN in the ultraluminous infrared galaxy Arp 220

We present X-ray results on the ultraluminous infrared galaxy Arp 220 obtained with BeppoSAX . X-ray emission up to 10 keV is detected. No significant signal is detected with the PDS detector in the higher energy band. The 2–10 keV emission has a flat spectrum (Γ∼1.7) , similar to M82, and a luminosity of ∼ 1×10⁴¹ erg s⁻¹ . A population of X-ray binaries may be a major source of this X-ray emission. The upper limit of an iron K line equivalent width at 6.4 keV is ≃600 eV. This observation imposes the tightest constraint so far on an active nucleus if present in Arp 220. We find that a column density of X-ray absorption must exceed 10²⁵ cm⁻² for an obscured active nucleus to be significant in the energetics, and the covering factor of the absorption should be almost unity. The underluminous soft X-ray starburst emission may need a good explanation, if the bolometric luminosity is primarily powered by a starburst.     

H i and dark matter in the windy starburst dwarf galaxy NGC 1705

We present 21-cm H  i line observations of the blue compact dwarf galaxy NGC 1705. Previous optical observations show a strong outflow powered by an ongoing starburst dominating the H  ii morphology and kinematics. In contrast, most of the H  i lies in a rotating disc. An extraplanar H  i spur accounts for ∼8 per cent of the total H  i mass, and is possibly associated with the H  ii outflow. The inferred mass loss rate out of the core of the galaxy is significant, ∼0.2 − 2 M_⊙ yr⁻¹, but does not dominate the H  i dynamics. Mass model fits to the rotation curve show that the dark matter (DM) halo is dominant at nearly all radii and has a central density ρ₀ ≈ 0.1 M_⊙ pc⁻³: ten times higher than typically found in dwarf irregular galaxies, but similar to the only other mass-modelled blue compact dwarf, NGC 2915. This large difference strongly indicates that there is little evolution between dwarf irregular and blue compact dwarf types. Instead, dominant DM haloes may regulate the morphology of dwarf galaxies by setting the critical surface density for disc star formation. Neither our data nor catalogue searches reveal any likely external trigger to the starburst in NGC 1705.     

Neutral hydrogen absorption observations of the central region of NGC 3628

Using MERLIN with 0.2-arcsec resolution we have observed neutral hydrogen absorption against the central region of the starburst galaxy NGC 3628. The central region resolves into ∼16 continuum components at 1.4 GHz. From comparison with published 15-GHz data, we infer that these components are supernova remnants, although three components may be consistent with a weak active galactic nucleus. Neutral hydrogen absorption is seen against the continuum emission with column densities ∼10²² cm⁻². The absorption appears to be from two separate absorbing structures. Assuming a simple morphology, the main velocity structure can be attributed to a ring of neutral gas with a radius 130 pc rotating around a central starburst with a velocity gradient of 1270 km s⁻¹ kpc⁻¹. From simple assumptions, the mass interior to this ring is 0.9 × 10⁹ M_⊙. The second absorption structure may represent outflow from the starburst region or a large-scale galactic structure. Alternatively the absorption structure may be non-axisymmetric, such as a bar.     

A radio study of the superwind galaxy NGC 1482

We present multifrequency radio continuum as well as H  i observations of the superwind galaxy NGC 1482, with both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). This galaxy has a remarkable hourglass-shaped optical emission-line outflow as well as bipolar soft X-ray bubbles on opposite sides of the galactic disc. The low-frequency, lower-resolution radio observations show a smooth structure. From the non-thermal emission, we estimate the available energy in supernovae, and examine whether this would be adequate to drive the observed superwind outflow. The high-frequency, high-resolution radio image of the central starburst region located at the base of the superwind bi-cone shows one prominent peak and more extended emission with substructure. This image has been compared with the infrared, optical red continuum, Hα, and soft and hard X-ray images from Chandra to understand the nature and relationship of the various features seen at different wavelengths. The peak of the infrared emission is the only feature that is coincident with the prominent radio peak, and possibly defines the centre of the galaxy.
The H  i observations with the GMRT show two blobs of emission on opposite sides of the central region. These are rotating about the centre of the galaxy and are located at ∼2.4 kpc from it. In addition, these observations also reveal a multicomponent H  i absorption profile against the central region of the radio source, with a total width of ∼250 km s⁻¹. The extreme blue- and redshifted absorption components are at 1688 and 1942 km s⁻¹, respectively, while the peak absorption is at 1836 km s⁻¹. This is consistent with the heliocentric systemic velocity of  1850 ± 20 km s⁻¹  , estimated from a variety of observations. We discuss possible implications of these results.     

ROSAT ASCA observations of X-ray luminous starburst galaxies: NGC 3310 and 3690

We present ROSAT [High Resolution Imager (HRI) and Position Sensitive Proportional Counter (PSPC)] and ASCA observations of the two luminous ( L _x ∼ 10⁴¹⁻⁴² erg s⁻¹) star-forming galaxies NGC 3310 and 3690. The HRI shows clearly that the sources are extended with the X-ray emission in NGC 3690 coming from at least three regions. The combined 0.1–10 keV spectrum of NGC 3310 can be described by two components, a Raymond–Smith plasma with temperature kT  = 0.81^+0.09_−0.12 keV and a hard power law, Γ = 1.44^−0.20_−0.11 (or alternatively a harder Raymond–Smith plasma with kT  ∼ 15 keV), while there is no substantial excess absorption above the Galactic column value. The soft component emission is probably a super wind while the nature of the hard emission is more uncertain with the likely origins being X-ray binaries, inverse Compton scattering of infrared photons, an active galactic nucleus or a very hot gas component (∼10⁸ K). The spectrum of NGC 3690 is similar, with kT  = 0.83^+0.02_−0.04 keV and Γ = 1.56^+0.11_−0.11. We also employ more complicated models such as a multi-temperature thermal plasma, a non-equilibrium ionization code or the addition of a third softer component, which improve the fit but not at a statistically significant level (2σ). These results are similar to recent results on the archetypal star-forming galaxies M82 and NGC 253.     

Australia Telescope Compact Array H i observations of the NGC 6845 galaxy group

We present the results of Australia Telescope Compact Array (ATCA) H  i line and 20-cm radio continuum observations of the galaxy quartet NGC 6845. The H  i emission extends over all four galaxies but can only be associated clearly with the two spiral galaxies, NGC 6845A and B, which show signs of strong tidal interaction. We derive a total H  i mass of at least  1.8 × 10¹⁰ M_⊙  , most of which is associated with NGC 6845A, the largest galaxy of the group. We investigate the tidal interaction between NGC 6845A and B by studying the kinematics of distinct H  i components and their relation to the known H  ii regions. No H  i emission is detected from the two lenticular galaxies, NGC 6845C and D. A previously uncatalogued dwarf galaxy, ATCA  J2001−4659  , was detected 4.4 arcmin NE from NGC 6845B and has an H  i mass of  ∼5 × 10⁸ M_⊙  . No H  i bridge is visible between the group and its newly detected companion. Extended 20-cm radio continuum emission is detected in NGC 6845A and B as well as in the tidal bridge between the two galaxies. We derive star formation rates of  15–40 M_⊙ yr⁻¹  .     

X-ray and radio observations of the poor cluster A3581 which hosts the radio galaxy PKS 1404 − 267

We present new X-ray and H  I 21-cm data on the poor cluster of galaxies Abell 3581. The ASCA spectrum requires a low temperature, has a strong requirement for excess absorption and shows evidence for multi-temperature components. The ROSAT HRI image shows the strongly peaked emission indicative of a cooling flow. Despite the low temperature (∼ 1.5–2.0 keV) and low luminosity (∼ 2 × 10⁴² erg s⁻¹ in the 2–10 keV band), Abell 3581 has a mass deposition rate ∼ 80 M⊙ yr⁻¹ which is larger than found for other nearby low-luminosity objects. VLA observations in the 21-cm band set velocity width and spin temperature dependent limits on the column density of atomic hydrogen.     

Gas in early-type galaxies: cross-fuelling in late-type–early-type pairs?

We present  ¹²CO ( J = 1–0)  and  ¹²CO ( J = 2–1)  observations of eight early-type galaxies, forming part of a sample of interacting galaxies, each consisting of one late- and one early-type system. All of the early-type galaxies observed are undetected in CO to low levels, allowing us to place tight constraints on their molecular gas content. Additionally, we present H  i absorption data for one system. The implications for possible gas transfer from the late- to the early-type galaxy during the interaction are discussed.     

H i and optical spectroscopy towards the M15 intermediate-velocity cloud

We present single-dish Arecibo 21-cm H  i observations, covering a 0675×0625 RA–Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V _LSR=70 km s⁻¹ are investigated; it is found to be clumpy and has a peak surface density N _{H  i} ∼8×10¹⁹ cm⁻². Additionally, we have performed a long H  i integration towards HD 203664, a Galactic halo star some 31 from M15, in which optical IVC absorption has previously been detected. No H  i with a velocity exceeding 60 km s⁻¹ was found to a brightness temperature limit of 0.05 K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo H  i pointings and low-resolution spectra in the Ca  ii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity H  i is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤ d ≤1350 pc. No Ca  ii absorption is observed, either because the IVC is further away than ∼1350 pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations.     

The shape of the dark matter halo in the early-type galaxy NGC 2974

We present H  i observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected H  i disc in this galaxy is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape.
We construct mass models of NGC 2974 by combining the H  i rotation curve with the central kinematics of the ionized gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionized gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the H  i gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and modified Newtonian dynamics, which fit the data marginally worse.
The mass-to-light ratio M / L _I increases in NGC 2974 from 4.3 M_⊙/L_⊙, _I at one effective radius to 8.5 M_⊙/L_{⊙, I} at 5  R _e. This increase of M / L already suggests the presence of dark matter: we find that within 5  R _e at least 55 per cent of the total mass is dark.     

Chandra High-Resolution Camera observations of the luminous X-ray source in the starburst galaxy M82

We analyse Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous X-ray source. We find a position for the source of         (J2000) with a 1 σ radial error of 0.7 arcsec. The accurate X-ray position shows that the luminous source is neither at the dynamical centre of M82 nor coincident with any suggested radio AGN candidate. The source is highly variable between observations, which suggests that it is a compact object and not a supernova or remnant. There is no significant short-term variability within the observations. Dynamical friction and the off-centre position place an upper bound of 10⁵–10⁶ M_⊙ on the mass of the object, depending on its age. The X-ray luminosity suggests a compact object mass of at least 500 M_⊙. Thus the luminous source in M82 may represent a new class of compact object with a mass intermediate between those of stellar-mass black hole candidates and supermassive black holes.     

The binary star population of the young cluster NGC 1818 in the Large Magellanic Cloud

We determine the binary star fraction as a function of radius in NGC 1818, a young rich cluster in the Large Magellanic Cloud, using Hubble Space Telescope images in bands F336W (∼ U ) and F555W (∼ V ). Our sample includes binaries with M _primary ∼ 2–5.5 M_⊙ and M _secondary ≳ 0.7 M_primary. The binary fraction increases towards the cluster centre, from ∼ 20 ± 5 per cent in the outer parts, to ∼ 35 ± 5 per cent inside the core. This increase is consistent with dynamical mass segregation and need not be primordial. We compare our results with expectations from N -body models, and discuss the implications for the formation and early evolution of such clusters.     

Iron line profiles including emission from within the innermost stable orbit of a black hole accretion disc

We propose a model for the source of the X-ray background (XRB) in which low-luminosity active nuclei ( L  ∼ 10⁴³ erg s⁻¹) are obscured ( N  ∼ 10²³ cm⁻²) by nuclear starbursts within the inner ∼ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ∼ few × 100 km s⁻¹ and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star-forming cluster itself and the central black hole. A hot ( T  ∼ 10⁶ − 10⁷ K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ∼ 10⁷ M⊙ through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.     

The high-velocity molecular stream in W51B: a ring structure with compact H ii regions

We present ¹³ CO J  = 1 − 0 line observations of the H  ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ∼ + 65 km s⁻¹ of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ∼ 48 × 17 pc² and ∼ 2.4 × 10⁵ M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v _LSR=+ 60 to +73 km s⁻¹. The velocity gradient of this ring is ∼ 0.5 km s⁻¹ pc⁻¹, and the mass is ∼ 6.2 × 10⁴ M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ∼ 7 km s⁻¹, ∼ 13 pc and ∼ 3.0 × 10 ⁴⁹ erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H  ii regions, G49.0−0.3 and G48.9−0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure.     

Observations and modelling of the interacting galaxies NGC 3395 and 3396

We present both observations and modelling of the atomic hydrogen in the closely interacting galaxies NGC 3395 and 3396. The observations were made with the VLA in both C- and D-arrays. We detect a large 'tail' of H  i extending to a projected distance of 63 kpc (10 arcmin) south-west of the pair, as well as two smaller galaxies, IC 2604, 14 arcmin to the south-west, and IC 2608, 14 arcmin to the south-east. However, these galaxies appear to have had at most a minor influence on the dynamics of NGC 3395/6. By means of N -body simulation we show that the tail is gas that has been stripped from NGC 3395 during a prograde encounter with NGC 3396, and that the pair has had one previous close approach. It is shown that the galaxies are within 5 × 10⁷ yr of their second perigalactic passage. Comparison of the time-scales for starburst activity with those from the simulations shows that the current starbursts are a result of the current close approach and not the first one. The interaction between NGC 3395 and 3396 has flattened the rotation curve of NGC 3396 owing to the parameters of the interaction. This naturally explains the more nucleated radio continuum structure observed in this galaxy, as significant infall and a subsequent central starburst would be expected in this scenario. The velocity structure and line profiles of the H  i are best explained if both cloud–cloud collisions between the two gas discs and tidal forces have been important.     

The optical velocity of the Antlia dwarf galaxy

We present the results of a Very Large Telescope observing programme carried out in service mode using fors 1 on ANTU (UT1) in long slit mode to determine the optical velocities of nearby low surface brightness galaxies. Outlying Local Group galaxies are of paramount importance in placing constraints on the dynamics and thus on both the age and the total mass of the Local Group. Optical velocities are also necessary to determine if the observations of H  i gas in and around these systems are the result of gas associated with these galaxies or a chance superposition with high-velocity H  i clouds or the Magellanic Stream. The data were of a sufficient signal-to-noise ration to enable us to obtain a reliable result in one of the galaxies we observed – Antlia – for which we have found an optical heliocentric radial velocity of 351±15 km s⁻¹.     

Towards a precision cosmology from starburst galaxies at z > 2

This work investigates the use of a well-known empirical correlation between the velocity dispersion, metallicity and luminosity in Hβ of nearby H  ii galaxies to measure the distances to H  ii -like starburst galaxies at high redshifts. This correlation is applied to a sample of 15 starburst galaxies with redshifts between   z = 2.17  and   z = 3.39  to constrain  Ω_m  , using data available from the literature. A best-fitting value of  Ω_m= 0.21^+0.30_−0.12  in a Λ-dominated universe and of  Ω_m= 0.11^+0.37_−0.19  in an open universe is obtained. A detailed analysis of systematic errors, their causes and their effects on the values derived for the distance moduli and  Ω_m  is carried out. A discussion of how future work will improve constraints on  Ω_m  by reducing the errors is also presented.