Starburst Superwinds Research Articles

Understanding the origin of radio outflows in Seyfert galaxies using radio polarimetry

AbstractThe role of starburst winds versus active galactic nuclei (AGN) jets/winds in the formation of the kiloparsec scale radio emission seen in Seyferts is not yet well understood. In order to be able to disentangle the role of various components, we have observed a sample of Seyfert galaxies exhibiting kpc-scale radio emission suggesting outflows, along with a comparison sample of starburst galaxies, with the EVLA B-array in polarimetric mode at 1.4 GHz and 5 GHz. The Seyfert galaxy NGC 2639, shows highly polarized secondary radio lobes, not observed before, which are aligned perpendicular to the known pair of radio lobes. The additional pair of lobes represent an older epoch of emission. A multi-epoch multi-frequency study of the starburst-Seyfert composite galaxy NGC 3079, reveals that the jet together with the starburst superwind and the galactic magnetic fields might be responsible for the well-known 8-shaped radio lobes observed in this galaxy. We find that many of the Seyfert galaxies in our sample show bubble-shaped lobes, which are absent in the starburst galaxies that do not host an AGN.

Dusty Superwind from a Galaxy with a Compact Obscured Nucleus: Optical Spectroscopic Study of NGC 4418

Abstract We report our optical spectroscopic study of the nucleus and its surrounding region of a nearby luminous infrared galaxy NGC 4418. This galaxy has been known to host a compact obscured nucleus, showing distinct characteristics such as a very compact (∼20 pc) submillimeter and mid-infrared core and dusty circumnuclear region with massive molecular gas concentration. We detected dusty superwind outflow at ≳1 kpc scale along the disk semiminor axis in both shock-heated emission lines and enhanced interstellar Na D absorption. This superwind shows basic characteristics similar to those of the prototypical superwind in the starburst galaxy M82, such as a kiloparsec-scale extended structure of gas and dust along the disk minor axis, outflowing components (multiphase gas and dust), physical conditions of the ionized gas, and monotonically blueshifting radial velocity field with increasing distance from the nucleus on the front side of the superwind. We also detected a moderately extinct starburst population in the Sloan Digital Sky Survey nuclear spectrum with a burst age of ≃10 Myr and stellar mass of ≃1 × 107 M ☉. It is powerful enough to drive the superwind within the dynamical age of the superwind (≃10 Myr). On the basis of comparison between this starburst–superwind scenario and the observations in terms of the burst age, stellar mass, infrared luminosity, and obscuration in the optical bands, we argue that this superwind-driving starburst is separate from the submillimeter core even if the core is a very young star cluster. Therefore, this galaxy hosts both the enshrouded compact core and the superwind-driving circumnuclear starburst.

Acceleration of ultrahigh-energy cosmic rays in starburst superwinds

The sources of ultrahigh-energy cosmic rays (UHECRs) have been stubbornly elusive. However, the latest report of the Pierre Auger Observatory provides a compelling indication for a possible correlation between the arrival directions of UHECRs and nearby starburst galaxies. We argue that if starbursts are sources of UHECRs, then particle acceleration in the large-scale terminal shock of the superwind that flows from the starburst engine represents the best known concept model in the market. We investigate new constraints on the model and readjust free parameters accordingly. We show that UHECR acceleration above about $1{0}^{11}\text{ }\text{ }\mathrm{GeV}$ remains consistent with observation. We also show that the model could accommodate hard source spectra as required by Auger data. We demonstrate how neutrino emission can be used as a discriminator among acceleration models.

Numerical simulations of shocks encountering clumpy regions

We present numerical simulations of the adiabatic interaction of a shock with a clumpy region containing many individual clouds. Our work incorporates a sub-grid turbulence model which for the first time makes this investigation feasible. We vary the Mach number of the shock, the density contrast of the clouds, and the ratio of total cloud mass to inter-cloud mass within the clumpy region. Cloud material becomes incorporated into the flow. This "mass-loading" reduces the Mach number of the shock, and leads to the formation of a dense shell. In cases in which the mass-loading is sufficient the flow slows enough that the shock degenerates into a wave. The interaction evolves through up to four stages: initially the shock decelerates; then its speed is nearly constant; next the shock accelerates as it leaves the clumpy region; finally it moves at a constant speed close to its initial speed. Turbulence is generated in the post-shock flow as the shock sweeps through the clumpy region. Clouds exposed to turbulence can be destroyed more rapidly than a similar cloud in an "isolated" environment. The lifetime of a downstream cloud decreases with increasing cloud-to-intercloud mass ratio. We briefly discuss the significance of these results for starburst superwinds and galaxy evolution.

CO + in M82: A Consequence of Irradiation by X-Rays

Based on its strong CO+ emission, it is argued that the M82 starburst galaxy is exposed to a combination of FUV and X-ray radiation. The latter is likely to be the result of the starburst superwind, which leads to diffuse thermal emission, and a compact hard source (but not an AGN). Although a photon-dominated region (FUV) component is clearly present in the nucleus of M82, and capable of forming CO+, only X-ray irradiated gas of density 103-105 cm-3 can reproduce the large, ~(1-4) × 1013 cm-2, columns of CO+ that are observed toward the prototypical starburst M82. The total X-ray luminosity produced by M82 is weak, ~1041 ergs s-1, but this is sufficient to drive the formation of CO+.

High‐Resolution Imaging of Warm and Dense Molecular Gas in the Nuclear Region of the Luminous Infrared Galaxy NGC 6240

We present ~2'' resolution CO(3-2), HCO+(4-3) and 880micron continuum images of the luminous infrared galaxy NGC6240 obtained at the Submillimeter Array. We find that the spatially resolved CO(3-2), HCO+(4-3) and the 880micron emission peaks between the two nuclear components that are both known to harbor AGNs. Our Large Velocity Gradient (LVG) analysis performed on each velocity channel suggests that the peak of the molecular gas emission traced in our observations is warm (T = 20 - 100K), dense (nH2 = 10^(5.0 - 5.4) cm^-3) and moderately optically thin (tau = 0.2 - 2) in the central 1 kpc. We also find large column densities of ~10^(23) cm^(-2). Such extreme conditions are observed over ~300 km s^(-1) centered around the CO derived systemic velocity. The derived molecular gas mass from the CO(3-2) emission and a CO-to-H2 conversion factor commonly used for ULIRGs is (6.9 +/- 1.7) x 10^9 Msun, and this is consistent with the mass derived from previous CO(2-1) observations. The gas is highly turbulent in the central kpc (Delta v_(FWZI) ~ 1175 km s^(-1)). Furthermore, possible inflow or outflow activity is suggested from the CO(3-2) velocity distribution. We tentatively state that 3.5 x 10^8 Msun of isolated CO(3-2) emission seen west of the northern disk may be associated with outflows from starburst superwinds, but the gas outflow scenario from one of the central AGN is not completely ruled out. Piecing all of the information together, the central region of NGC 6240 harbors 2 AGNs, ~10^(10) Msun of molecular gas mass, 5 x 10^7 Msun of dust mass, and has possible evidence of inflow and outflow activity.

The absorbing haloes of high‐redshift radio galaxies and Lyα blobs

AbstractResults are summarized from a program of VLT‐UVES echelle spectroscopy on the HI Lyα absorption in z > 2 radio galaxies. The nature and incidence of this absorption are reviewed, along with scenarios for its origin. Comparison is made with integral field spectroscopy of the two radio‐quiet 100 kpc‐scale Lyα haloes in the z = 3.09 SSA22 protocluster–the Steidel Lyα blobs. The emphasis is on using such absorption to study the injection of energy and metals from radio jets and starburst superwinds, processes considered vital to the formation of the most massive galaxies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamical and pressure structures in winds with multiple embedded evaporating clumps - I. Two-dimensional numerical simulations

Because of its key role in feedback in star formation and galaxy formation, we examine the nature of the interaction of a flow with discrete sources of mass injection. We show the results of two-dimensional numerical simulations in which we explore a range of configurations for the mass sources and study the effects of their proximity on the downstream flow. The mass sources act effectively as a single source of mass injection if they are so close together that the ratio of their combined mass injection rate is comparable to or exceeds the mass flux of the incident flow into the volume that they occupy. The simulations are relevant to many diffuse sources, such as planetary nebulae and starburst superwinds, in which a global flow interacts with material evaporating or being ablated from the surface of globules of cool, dense gas.

ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240

We present Infrared Space Observatory mid- and far-infrared spectroscopy of the merging galaxy NGC 6240, an object presenting many aspects of importance for the role of star formation and AGN activity in [ultra]luminous infrared galaxies. The mid-infrared spectrum shows starburst indicators in the form of low excitation fine-structure line emission and aromatic `PAH' features. A strong high excitation [OIV] line is observed which most likely originates in the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely powerful emission in the pure rotational lines of molecular hydrogen. We argue that this emission is mainly due to shocks in its turbulent central gas component and its starburst superwind. The total shock cooling in infrared emission lines accounts for ~0.6% of the bolometric luminosity, mainly through rotational H_2 emission and the [OI] 63micron line. We analyse several ways of estimating the luminosities of the starburst and the AGN in NGC 6240 and suggest that the contributions to its bolometric luminosity are most likely in the range 50-75% starburst and 25-50% AGN.

Cooling in Coronal Gas in the M82 Starburst Superwind

We have used the Far Ultraviolet Spectroscopic Explorer to search for O VI λ1031.926 emission at four locations in the starburst superwind of M82. No O VI emission was detected at any of the four pointings, with upper limits less than or equal to the 0.3-2 keV X-ray flux. These observations limit the energy lost through radiative cooling of coronal phase (T ~ 105.5 K) gas to roughly the same magnitude as that lost in the hot phase through X-ray emission, which has been shown to be small. The wind material retains most of its energy and should be able to escape from the gravitational potential of M82, enriching the intergalactic medium with energy and metals. The lack of coronal gas in the wind and observations of spatially correlated X-ray and Hα emission are consistent with a scenario in which the hot wind material overruns cold clouds in the halo or with one in which the Hα and X-ray emission arise at the interface between the hot wind and a cool shell of swept-up interstellar medium, as long as the shock velocity is 150 km s-1. The observed limits on the O VI/Hα and C III/Hα flux ratios rule out shock heating as the source of the T = 104 K gas unless the shock velocity is 90 km s-1.

Giant Lyα Nebulae Associated with High‐Redshift Radio Galaxies

We report deep, Keck narrowband Lyα images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc scale, emission-line nebulae, centered on these galaxies, that exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, cone-shaped structures and (indirect) evidence for extended Lyα absorption. We discuss these features within a general scenario in which the nebular gas cools gravitationally in large cold dark matter halos, forming stars and multiple stellar systems. Merging of these building blocks triggers large-scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds supermassive black holes, which produce the powerful radio jets and lobes. The radio sources, starburst superwinds, and radiation pressure from active galactic nucleus then disrupt the accretion process, limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lyα nebulae.

The absorbing haloes around high-redshift radio galaxies: the UVES view

We summarise the properties of the absorbing haloes around two high-redshift radio galaxies studied with the VLT-UVES echelle spectrograph. The absorbers in 0943−242 ( z=2.92) exhibit little further structure compared with previous data—the main absorber still has N( HI)≃10 19 cm −2—reinforcing the picture in which the absorbing gas occupies a smooth shell beyond the emission line halo. In 0200+015 ( z=2.23) the main absorber now splits into two spatially-resolved systems with N( HI)≃4×10 14 cm −2, one of which has strong CIV absorption, implying a metallicity of Z≃10 Z ⊙. We explain the differences between the two sources with an evolutionary scenario, invoking interaction with the expanding radio source and metal enrichment via a starburst superwind.

A Mapping Survey of the 13 CO and 12 CO Emission in Galaxies

We present spectra of the extended 12CO and 13CO J = 1 → 0 emission along the major axes of 17 nearby galaxies. Spatial variations in the ratio of CO and 13CO integrated intensities, , are found in nearly every galaxy observed. There is an overall variation in of 20%-40% from the inner 2 kpc to the disk. Roughly one-third of the survey galaxies have such gradients in detected above the 2 σ confidence level. Though some galaxies show a lower central value of , on average inside 2 kpc is 10%-30% higher than outside of 2 kpc. The average CO/13CO intensity ratio within the central 2 kpc of the survey sources is 11.6 ± 0.4 (based on the noise) ± 1.5 (based on systematic uncertainties estimated from daily variations in CO and 13CO intensities). The 1 σ dispersion in between galactic nuclei of 4.2 is also quite large. The average value of outside 2 kpc is 9.8 ± 0.6 ± 1.2 with a standard deviation of 4.5. An increase in the CO/13CO intensity ratio from disk to nucleus may imply that the conversion factor between CO intensity and H2 column density, X, is lower in galactic nuclei. In addition, variations in physical conditions, most notably the gas kinetic temperature, affect both and X. Abundance variations probably do not cause the gradient in , though we do not rule out a decrease in effective cloud column densities in galactic nuclei possibly caused by destructive starburst superwinds. A modest rise in temperature (less than a factor of 2 or 3) from outside a 2 kpc radius toward the nucleus can easily account for the observed gradient. These results support previous work implying that X is lower in the center of the Milky Way and probably most galactic nuclei. Therefore, calculating H2 masses using the standard Galactic X-factor, especially within the central few kiloparsecs of galaxies, overestimates the true mass by factors of a few. The standard X-factor still appears to be appropriate for galactic disks.

Diffuse Ionized Gas in Edge‐on Spiral Galaxies: Extraplanar and Outer Disk Hα Emission

We present Hα images of five edge-on galaxies: NGC 891, NGC 4631, NGC 4244, NGC 3003, and UGC 9242. We also analyze [S II] λλ6717, 6731 and [O III] λ5007 images of NGC 4631. For several of these galaxies, these images are the most sensitive to date. We analyze the ionized gas content, with particular attention to the diffuse ionized gas (DIG). The DIG layer in NGC 891 is traced out to at least 5 kpc from the midplane, confirming an earlier spectroscopic detection. The DIG in four of these galaxies contributes 40%-50% of the total Hα luminosity, similar to the contribution in face-on galaxies, but in NGC 891 the DIG contributes 80%-90%. This is likely due to the higher dust content in the disk of NGC 891, which obscures the H II regions but may also reflect the extraordinary prominence of the DIG layer in that galaxy. Our very deep image of UGC 9242 shows very low surface brightness emission, as low as 0.3 pc cm-6, reaching 4-5 kpc above the midplane. This galaxy also exhibits filaments near the bright Hα nucleus, an indication of a starburst superwind. In NGC 4631 we see a very large shell of emission extending 3.5 kpc into the halo. The [S II]/Hα and [O III]/Hα ratios in NGC 4631 are consistent with the ratios seen in other galaxies, and with photoionization models. There is a region on the southeast side of disk where the [O III]/(Hα + [N II]) ratio reaches over 1.0 in the DIG, which coincides with an H I supershell. We use our very deep images of NGC 3003 and UGC 9242 to search for ionized gas in the outer disks as a test of the strength of the metagalactic ionizing radiation field. We find no outer disk emission down to our 1 σ limit of 0.13 pc cm-6 on scales of 1.5 kpc in NGC 3003. Based on this limit we rule out a metagalactic ionizing radiation field stronger than 11 × 10-23 ergs cm-2 s-1 Hz-1 sr-1. There is an indication of extended disk emission in UGC 9242, which would imply a stronger radiation field, but various concerns, most importantly flat-fielding uncertainties due to foreground stars in the image, lead us to question whether this feature is real.

Mass Injection Rates Due to Supernovae and Cloud Evaporation in Starburst Superwinds

We show that, for reasonable assumptions, conductively driven and ablation-driven evaporation of clouds in the core of the starburst galaxy M82 lead to a rate of mass injection into the superwind that is several times greater than that due to supernovae. This is in harmony with the results of efforts to construct a global superwind model consistent with the X-ray emission of M82. We examine the possibility that the increase of pressure in a starburst causes the burst to develop by inducing gravitational instability of the clouds.

Dynamics and X-ray emission of a galactic superwind interacting with disk and halo gas

There is a general agreement that the conspicuous extranuclear X-ray, optical-line, and radio-contiuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However, despite the significant advances in observational studies of superwinds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the starburst superwind regions. To address this issue, we have conducted two-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to represent the ISM of the starburst galaxy, we analyze the interaction of the wind with a two-component disk-halo ambient interstellar medium and argue that this two-component representation is crucial for adequate modeling of starbursts. The emphasis of this study is on the geometry and structure of the wind region and the X-ray emission arising in the wind material and the shocked gas in the disk and the halo of the galaxy. The simulation results have shown that a clear-cut bipolar wind can easily develop under a range of very different conditions. On the other hand, a complex 'filamentary' structure associated with the entrained dense disk material is found to arise within the hot bubble blown out by the wind. The flow pattern within the bubble is dominated equally by the central biconic outflow and a system of whirling motions r elated to the origin and development of the 'filaments'. The filament parameters make them a good candidate for optical-emission-line filamentary gas observed in starburst halos. We find that the history of mass and energy deposition in the starburst region of the galaxy is crucial for wind dynamics. A 'mild' early wind, which arises as a result of the cumulative effect of stellar winds from massive stars, produces a bipolar vertical cavity in the disk and halo gas without strongly affecting the gaseous disk, thus creating conditions for virtually free vertical escape of the hot gas at the later, much more violent supernova-dominated phases of the starburst. We calculate the luminosity, mass, and effective temperature of the X-ray emitting gas in the 'soft' (0.1 to 0.7 keV, 0.7 to 2.2 keV, and 0.1 to 2.2 keV) and 'hard' (1.6 to 8.3 keV) energy bands and estimate the contribution of different gaseous components to the X-ray flux in these bands. Analysis of these parameters enables us to make conclusions regarding the nature of the X-ray-emitting material. We have inferred that the bulk of the soft thermal X-ray emission from starbursts arises in the wind-shocked material of the disk and halo gas rather than in the wind material itself. This enables us to predict that the integrated soft X-ray spectra of starbursts need not show an overabundance of heavy elements which are believed to be produced copiously in the centers of starbursts. Unlike soft X-ray emission, the hard component of thermal X-ray emission is found to originate in the wind material ejected from the starburst region. However, the derived ratio of hard-to-soft X-ray luminosities is too small compared to that observed in starbursts. We conclude therefore that the observed hard X-ray emission of starbursts is probably not associated with the thermal emission of hot wind or ambient shocked gas. Typical temperatures of the bulk of the soft X-ray-emitting material in our very different models have been found to agree well with the ones estimated on the basis of the ROSAT data for the soft component of X-ray emission of nearby starbursts. We predict that temperatures of the extranuclear soft X-ray-emitting gas in starburst galaxies with heavy element abundances near solar should be close to T(sub Xs = 2 to 5 x 10(exp 6)K.