Curtis Struck

Multi stage three-dimensional sweeping and annealing of disc galaxies in clusters

We present new three-dimensional, hydrodynamic simulations of the ram pressure stripping of disc galaxies via interaction with a hot intracluster medium (ICM). The simulations were carried with the smoothed-particle hydrodynamics, adaptive mesh ‘HYDRA’ code (SPH-AP 3 M), with model galaxies consisting of gas and stellar disc components and dark haloes. The simulations also include radiative cooling, which is important for keeping the warm, diffuse gas of moderate density from being unrealistically heated by the ICM. We examine the role that wind velocity, density and galaxy tilt play in gas stripping. We include cases with lower ram pressures than other recent studies. In accord with previous studies, we find that low column density gas is promptly removed from the outer disc. However, we also find that not all of the gas stripped from the disc escapes immediately from the halo, some of material can linger for times of order 10 8 yr. We use a simple analytic model to demonstrate that gas elements in the ICM wind feel an effective potential with a minimum displaced downstream from the halo centre. The onset of the ICM wind has a profound effect on the disc gas that is not immediately stripped. This remnant disc is displaced relative to the halo centre and compressed. This can trigger gravitational instability and the formation of numerous flocculent spirals. These waves transport angular momentum outward, resulting in further compression of the inner disc and the formation of a prominent gas ring. This ‘annealing’ process makes the inner disc, which contains much of the total gas mass, resistant to further stripping, but presumably susceptible to global starbursts. Spirals in the outer disc stretch, shear and are eventually stripped on timescales of a few times 10 8 yr, after which time, mass and angular momentum loss effectly cease. For inclined galaxies, these effects are considerably modified over the same time-scale. The amount of mass loss is reduced. In addition, we find that a higher galaxy tilt couples the wind and the rotating disc, and produces a higher degree of angular momentum removal. Temperature and line-of-sight velocity maps from several of the simulations are presented for comparison with observation. When the mass loss and annealing processes go to completion, we find that the total amount of mass lost from a fixed target galaxy is well-fitted by a simple power-law function of a dimensionless parameter that combines the ram pressure and internal properties of the galaxy. Ramifications for the cluster galaxy evolution are discussed.

The Spitzer Spirals, Bridges, and Tails Interacting Galaxy Survey: Interaction-Induced Star Formation in the Mid-Infrared

We present Spitzer mid-infrared images from a survey of three dozen pre-merger strongly interacting galaxy pairs selected from the Arp Atlas. The global mid-infrared colors of these galaxies and their tidal tails and bridges are similar to those of normal spiral galaxies, thus this optically selected sample of interacting galaxies does not have strongly enhanced normalized star formation rates in their disks or tidal features. Despite distortion and disturbance these systems continue to form stars at a normal rate on average. The morphology of these galaxies is generally smoother in the shorter wavelength IRAC bands than at 8

Simulations of Collisions between Two Gas-rich Galaxy Disks with Heating and Cooling

\mu

Dust Spirals and Acoustic Noise in the Nucleus of the Galaxy NGC 2207

m, where dozens of clumps of star formation are detected.

SHOCK-ENHANCED C+ EMISSION AND THE DETECTION OF H2O FROM THE STEPHAN'S QUINTET GROUP-WIDE SHOCK USING HERSCHEL

Particle hydrodynamics (SPH) simulations are presented of direct collisions between two model galaxies, most consisting of a rigid halo and a gas disk. Local self-gravity is also computed in the gas. The companion galaxy in these simulations is about one-third of the mass of the primary, and its disk is half the size. An adiabatic equation of state is combined with simple approximations for the effects of radiative cooling and local heating due to young star activity, which allows a continuous range of thermal phases to develop. These terms and multiple phases have not generally been included in galaxy collision simulations to date. Their effects are assessed in part by repeating runs with an isothermal equation of state and comparing the results. One model with a star plus gas disk is also included for comparison. These models are most relevant to interactions involving low surface brightness, or other late-type galaxies with extensive gas disks, including the precursors to well-known ring galaxies like the Cartwheel and VII Zw 466. In the simulations, the companion impact is slightly off center in the target disk, as is probably the case in these systems. In all cases, clear ring waves develop in the primary despite the disruption of parts of the disk by impact shocks. The gas density in the disk of the primary is initialized to values slightly below the gravitational instability threshold throughout, and the ring waves induce star formation in all the heating and cooling models. The structure of the waves and other interaction morphologies are found to be quite similar on large scales in both isothermal and heating/cooling cases, despite the fact that at certain stages large quantities of gas are heated above the initial temperature in the latter. On a finer scale, there are clear differences, including the fact that star formation heating in ring waves increases the vertical scale height of the primary gas disk and delays spoke development. The companion disk is largely disrupted in most of these simulations, and a substantial mass of gas is splashed out into a bridge connecting the two potential centers. The companion disk reforms by accreting gas out of the bridge, though generally in a different plane than its initial one. There is also a good deal of infall back onto the primary disk. Although heated by impact, the gas in the bridge cools rapidly. However, kinematic expansion prevents it from reaching threshold density, and there is no star formation heating there. A comparison run with a diskless companion produced no significant bridge, so in this type of collision the bridge is primarily a hydrodynamic phenomenon. The amount of material pushed out into the splash bridge and how much of it comes from each galaxy depends on the relative orientation of the disks at impact. This orientation also affects how much bridge material accretes onto each galaxy. The onset of accretion is initially delayed but then accelerates to a peak and declines thereafter in both galaxies. The infall is spatially asymmetric and is primarily located in well-defined streams. Most of the accreted gas ends up in the central regions of the model galaxies, but only after spiraling around the center and passing through one or more shocks. Accretion heating is substantial, and is shown to inhibit or delay global star formation enhancements. The thermal effects of the impact between galaxies are short-lived, but the models predict that accretion and young star heating effect the global thermal phase balance for a much longer period. The magnitude and duration of these effects also depend on the relative orientation of the disks at impact. Thus, the postcollision Hubble type of the companion is a sensitive function of initial orientation.

Spitzer Space Telescope IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission

Observations with the Hubble Space Telescope reveal an irregular network of dust spiral arms in the nuclear region of the interacting disk galaxy NGC 2207. The spirals extend from ~50 to ~300 pc in galactocentric radius, with a projected width of ~20 pc. Radiative transfer calculations determine the gas properties of the spirals and the inner disk and imply a factor of ~4 local gas compression in the spirals. The gas is not strongly self-gravitating, nor is there a nuclear bar, so the spirals could not have formed by the usual mechanisms applied to main galaxy disks. Instead, they may result from acoustic instabilities that amplify at small galactic radii. Such instabilities may promote gas accretion into the nucleus.

Models of the Morphology, Kinematics, and Star Formation History of the Prototypical Collisional Starburst System NGC 7714/7715 = ARP 284

We present the first Herschel spectroscopic detections of the [OI]63µm and [CII]158µm fine-structure transitions, and a single para-H_2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephans Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s^(-1)) luminous [CII] line profiles, as well as fainter [OI]63µm emission. SPIRE FTS observations reveal water emission from the p-H_2O (1_(11)-0_(00)) transition at several positions in the filament, but no other molecular lines. The H_2O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH_(tot) and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII region excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H_2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H_2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.

CANDIDATE TIDAL DWARF GALAXIES IN Arp 305: LESSONS ON DWARF DETACHMENT AND GLOBULAR CLUSTER FORMATION

IC 2163 and NGC 2207 are interacting galaxies that have been well studied at optical and radio wavelengths and simulated in numerical models to reproduce the observed kinematics and morphological features. Spitzer IRAC and MIPS observations reported here show over 200 bright clumps from young star complexes. The brightest IR clump is a morphologically peculiar region of star formation in the western arm of NGC 2207. This clump, which dominates the Hα and radio continuum emission from both galaxies, accounts for ~12% of the total 24 μm flux. Nearly half of the clumps are regularly spaced along some filamentary structure, whether in the starburst oval of IC 2163 or in the thin spiral arms of NGC 2207. This regularity appears to influence the clump luminosity function, making it peaked at a value nearly a factor of 10 above the completeness limit, particularly in the starburst oval. This is unlike the optical clusters inside the clumps, which have a luminosity function consistent with the usual power-law form. The giant IR clumps presumably formed by gravitational instabilities in the compressed gas of the oval and the spiral arms, whereas the individual clusters formed by more chaotic processes, such as turbulence compression, inside these larger scale structures.IC 2163 and NGC 2207 are interacting galaxies that have been well studied at optical and radio wavelengths and simulated in numerical models to reproduce the observed kinematics and morphological features. Spitzer IRAC and MIPS observations reported here show over 200 bright clumps from young star complexes. The brightest IR clump is a morphologically peculiar region of star formation in the western arm of NGC 2207. This clump, which dominates the Halpha and radio continuum emission from both galaxies, accounts for ~12% of the total 24mu m flux. Nearly half of the clumps are regularly spaced along some filamentary structure, whether in the starburst oval of IC 2163 or in the thin spiral arms of NGC 2207. This regularity appears to influence the clump luminosity function, making it peaked at a value nearly a factor of 10 above the completeness limit, particularly in the starburst oval. This is unlike the optical clusters inside the clumps, which have a luminosity function consistent with the usual power law form. The giant IR clumps presumably formed by gravitational instabilities in the compressed gas of the oval and the spiral arms, whereas the individual clusters formed by more chaotic processes, such as turbulence compression, inside these larger-scale structures.

Slowly breaking waves: the longevity of tidally induced spiral structure

We present new N-body, hydrodynamical simulations of the interaction between the starburst galaxy NGC 7714 and its poststarburst companion NGC 7715, focusing on the formation of the collisional features, including (1) the gas-rich star-forming bridge, (2) the large gaseous loop (and stellar tails) to the west of the system, (3) the very extended H I tail to the west and north of NGC 7714, and (4) the partial stellar ring in NGC 7714. Our simulations confirm the results of earlier work that an off-center inclined collision between two disk galaxies is almost certainly responsible for the peculiar morphologies of this system. However, we have explored a wider set of initial galaxy and collisional encounter parameters than previously and have found a relatively narrow range of parameters that reproduce all the major morphologies of this system. The simulations suggest specific mechanisms for the development of several unusual structures. We find that the complex gas bridge has up to four distinct components, with gas contributed from two sides of NGC 7715, as well as from NGC 7714. The observed gas-star offset in this bridge is accounted for in the simulations by the dissipative evolution of the gas. The models suggest that the most recently formed gas bridge component from NGC 7715 is interacting with gas from an older component. This interaction may have stimulated the band of star formation on the north side of the bridge. The models also indicate that the low surface brightness H I tail to the far west of NGC 7714 is the end of the NGC 7715 countertail, curved behind the two galaxies. The sensitivity of the tidal structures to collision parameters is demonstrated by comparisons between models with slightly different parameter values. Comparison of model and observational (H I) kinematics provides an important check that the morphological matches are not merely fortuitous. Line-of-sight velocity and dispersion fields from the model are found to match those of the observations reasonably well at current resolutions. Spectral evolutionary models of the NGC 7714 core by Lancon et al. suggest the possibility of multiple starbursts in the last 300 Myr. Our hydrodynamic models suggest that bursts could be triggered by induced ringlike waves and a postcollision buildup of gas in the core of the galaxy.

Hubble Space Telescope Observations of Dust and Star-forming Regions in the Ocular Galaxy IC 2163 and Its Spiral Companion NGC 2207

To search for Tidal Dwarf Galaxies (TDGs) and to study star formation (SF) in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the Galaxy Evolution Explorer (GALEX) telescope. As part of that study, we present a GALEX UV and Sloan Digital Sky Survey and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and SF outside the disks. This includes a luminous star-forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star-forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages ~6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.