Shock-induced Star Formation Research Articles

Passive spirals and shock influenced star formation in the merging cluster A3376

ABSTRACT We present a detailed analysis of star formation properties of galaxies in a nearby (z ∼ 0.046) young (∼0.6 Gyr) post-merger cluster system A3376, with a moderate merger shock front (vs ∼1630 km s−1; $\mathcal {M}$ ∼ 2) observed as symmetric radio relics. Exploiting the spectroscopic data from the wide-field OmegaWINGS survey and the associated photometric information, our investigations reveal the plausible effects of the dynamic post-merger environment differing from the high-density cluster environment experienced prior to the merging activity. The remnants of the pre-merger relaxed cluster environment are realized through the existence of passive spiral galaxies located in the central regions of the cluster between the two brightest cluster galaxies. We discover A3376 to contain a population of massive (log (M*/M⊙) > 10) blue regular star-forming spirals in regions of maximum merger shock influence but exhibiting star formation rates similar to those in relaxed clusters at similar epoch. We further discover low-mass (log (M*/M⊙) ≤ 10) late-type blue post-starburst galaxies which could either be formed as a result of rapid quenching of low-mass spirals following the shock-induced star formation or due to the intense surge in the intracluster medium pressures at the beginning of the merger. With the possibility of the merger shock affecting high- and low-mass spirals differently, our results bridge the seemingly contradictory results observed in known merging cluster systems so far and establish that different environmental effects are at play right from pre- to post-merger stages.

The Double Galaxy Cluster A2465. III. X-Ray and Weak-lensing Observations∗

Abstract We report Chandra X-ray observations and optical weak-lensing measurements from Subaru/Suprime-Cam images of the double galaxy cluster A2465 (z = 0.245). The X-ray brightness data are fit to a β model to obtain the radial gas density profiles of the northeast (NE) and southwest (SW) subcomponents, which are seen to differ in structure. We determine core radii, central temperatures, the gas masses within r 500c, and the total masses for the broader NE and sharper SW components assuming hydrostatic equilibrium. There is no large X-ray excess between the two components. The central entropy of the NE subcluster is about two times higher than the SW. Along with its structural properties and an apparent radio halo that is a sign of a merger, this suggests that the NE component has undergone merging on its own. The weak-lensing analysis gives virial masses for each substructure, which compare well with earlier dynamical results. The derived outer mass contours of the SW sub-component from weak lensing are more irregular and extended than those of the NE. Although there is a weak enhancement and small offsets between X-ray gas and mass centers from weak lensing, the lack of large amounts of gas between the two subclusters indicates that A2465 is in a pre-merger state. We discuss star formation enhancement in this system resulting from its dynamics and shock-induced star formation scenarios. A dynamical model that is consistent with the observed cluster data, based on the FLASH program and the radial infall model, is constructed, where the subclusters currently separated by ∼1.2 Mpc are approaching each other at ∼2000 km s−1 and will meet in ∼0.4 Gyr.

THE SHOCK-INDUCED STAR FORMATION SEQUENCE RESULTING FROM A CONSTANT SPIRAL PATTERN SPEED

We utilize a suite of multiwavelength data, of 9 nearby spirals, to analyze the shock-induced star formation sequence, that may result from a constant spiral pattern speed. The sequence involves tracers as the HI, CO, 24um, and FUV, where the spiral arms were analyzed with Fourier techniques in order to obtain their azimuthal phases as a function of radius. It was found that only two of the objects, NGC 628 and NGC 5194, present coherent phases resembling the theoretical expectations, as indicated by the phase shifts of CO-24um. The evidence is more clear for NGC 5194, and moderate for NGC 628. It was also found that the phase shifts are different for the two spiral arms. With the exception on NGC 3627, a two-dimensional Fourier analysis showed that the rest of the objects do not exhibit bi-symmetric spiral structures of stellar mass, i.e., grand design spirals. A phase order inversion indicates a corotation radius of ~ 89" for NGC 628, and ~ 202" for NGC 5194. For these two objects, the CO-Halpha phase shifts corroborate the CO-24um azimuthal offsets. Also for NGC 5194, the CO-70um, CO-140um, and CO-250um phase shifts indicate a corotation region.

The role of cluster mergers and travelling shocks in shaping the Hα luminosity function at z ∼ 0.2: ‘sausage’ and ‘toothbrush’ clusters

The most extreme cluster mergers can lead to massive cluster-wide travelling shock waves. The CIZA J2242.8+5301 ('sausage') and 1RXS J0603.3+4213 (`toothbrush') clusters ($z\sim0.2$) host enormous radio-emitting shocks with simple geometry. We investigate the role of mergers and shocks in shaping the H$\alpha$ luminosity function, using custom-made narrow-band filters matching the cluster redshifts mounted on the INT. We surveyed $\sim0.28$ deg$^2$ for each cluster and found $181$ line emitters in the `sausage' (volume of $3.371\times10^3$ Mpc$^3$ for H$\alpha$ at $z=0.1945$) and $141$ in the `toothbrush' ($4.546\times10^3$ Mpc$^3$ for H$\alpha$ at $z=0.225$), out of which $49$ (`sausage') and $30$ (`toothbrush') are expected to be H$\alpha$. We build luminosity functions for the field-of-view down to an average limiting star formation rate of $0.14$ M$_{\odot}$ yr$^{-1}$, find good agreement with field luminosity functions at $z=0.2$, but significant differences between the shapes of the luminosity functions for the two clusters. We discover extended, tens-of-kpc-wide H$\alpha$ haloes in galaxies neighbouring relics, which were possibly disrupted by the passage of the shock wave. By comparing the `sausage' cluster with blank fields and other clusters, we also uncover an order of magnitude boost (at $9\sigma$ level) in the normalisation $\phi^*$ of the luminosity function in the relic areas. Our results suggest that cluster mergers may play an important role in the evolution of cluster galaxies through shock-induced star formation.

Dynamically driven star formation in models of NGC 7252

We present new dynamical models of the merger remnant NGC 7252 which include star formation simulated according to various phenomenological rules. By using interactive software to match our model with the observed morphology and gas velocity field, we obtain a consistent dynamical model for NGC 7252. In our models, this proto-elliptical galaxy formed by the merger of two similar gas-rich disk galaxies which fell together with an initial pericentric separation of ~2 disk scale lengths approximately 620 Myr ago. Results from two different star formation rules--- density-dependent and shock-induced--- show significant differences in star formation during and after the first passage. Shock-induced star formation yields a prompt and wide-spread starburst at the time of first passage, while density-dependent star formation predicts a more slowly rising and centrally concentrated starburst. A comparison of the distributions and ages of observed clusters with results of our simulations favors shock-induced mechanism of star formation in NGC 7252. We also present simulated color images of our model of NGC 7252, constructed by incorporating population synthesis with radiative transfer and dust attenuation. Overall the predicted magnitudes and colors of the models are consistent with observations, although the simulated tails are fainter and redder than observed. We suggest that a lack of star formation in the tails, reflected by the redder colors, is due to an incomplete description of star formation in our models rather than insufficient gas in the tails.

GALAXY MERGERS WITH ADAPTIVE MESH REFINEMENT: STAR FORMATION AND HOT GAS OUTFLOW

In hierarchical structure formation, merging of galaxies is frequent and known to dramatically affect their properties. To comprehend these interactions high-resolution simulations are indispensable because of the nonlinear coupling between pc and Mpc scales. To this end, we present the first adaptive mesh refinement (AMR) simulation of two merging, low mass, initially gas-rich galaxies (1.8e10 Ms each), including star formation and feedback. With galaxies resolved by ~2e7 total computational elements, we achieve unprecedented resolution of the multiphase interstellar medium, finding a widespread starburst in the merging galaxies via shock-induced star formation. The high dynamic range of AMR also allows us to follow the interplay between the galaxies and their embedding medium depicting how galactic outflows and a hot metal-rich halo form. These results demonstrate that AMR provides a powerful tool in understanding interacting galaxies.

The Cosmic Evolution Survey (COSMOS): A Large‐Scale Structure at z = 0.73 and the Relation of Galaxy Morphologies to Local Environment

We have identified a large-scale structure at z ≃ 0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak-lensing convergence map, and the distribution of extended X-ray sources in a mosaic of XMM-Newton observations. The main peak seen in these maps corresponds to a rich cluster with TX = 3.51 keV and LX = (1.56 ± 0.04) × 1044 ergs s-1 (0.1-2.4 keV band). We estimate an X-ray mass within r500 corresponding to M500 ≃ 1.6 × 1014 M☉ and a total lensing mass (extrapolated by fitting a NFW profile) MNFW = (6 ± 3) × 1015 M☉. We use an automated morphological classification of all galaxies brighter than IAB = 24 over the structure area to measure the fraction of early-type objects as a function of local projected density Σ10, based on photometric redshifts derived from ground-based deep multiband photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions" of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to (the colder side of) a possible shock region detected in the X-ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two subclusters. Our analysis reveals the value of combining various measures of the projected mass density to locate distant structures and their potential for elucidating the physical processes at work in the transformation of galaxy morphologies.

Multiobject Spectroscopy of Young Star Clusters in NGC 4676

Galaxy interactions are known to trigger starbursts. The young star clusters formed in mergers may be young globular clusters. The ages of these young star clusters yield the timing of interaction-triggered star formation and provide an important way to reconstruct the history of merging galaxies. Here we present the first results from our investigation into age and metallicity of 12 young clusters in the merging galaxy pair NGC 4676, using spectra from the multiobject Low Resolution Imaging Spectrometer (LRIS) on Keck. For 10 clusters, comparison of the Balmer emission lines with model equivalent widths (EWs) yields ages less than 10 Myr. Two spectra display Balmer absorption lines typical of star clusters dominated by A-type stars, with estimated ages of about 170 Myr. These ages are comparable to the dynamical age of the tidal tails and are consistent with star formation triggered during the first passage of the pair. The locations of these two clusters in the tidal tails are generally consistent with predictions of shock-induced star formation models. One of these older objects appears unresolved on the image and is luminous enough to qualify as a young globular cluster. Using EWs of the diagnostic lines [O II] and [O III], we obtain oxygen abundances in the range 7.3 < 12 + log(O/H) < 9.0. These values show a nearly flat distribution along the northern tail, suggesting efficient gas mixing in the tail.

Detecting shock waves in cosmological smoothed particle hydrodynamics simulations

We develop a formalism for the identification and accurate estimation of the strength of structure formation shocks during cosmological smoothed particle hydrodynamics simulations. Shocks play a decisive role not only for the thermalization of gas in virializing structures but also for the acceleration of relativistic cosmic rays (CRs) through diffusive shock acceleration. Our formalism is applicable both to ordinary non-relativistic thermal gas, and to plasmas composed of CRs and thermal gas. To this end, we derive an analytic solution to the one-dimensional Riemann shock tube problem for a composite plasma of CRs and thermal gas. We apply our methods to study the properties of structure formation shocks in high-resolution hydrodynamic simulations of the Lambda cold dark matter (� CDM) model. We find that most of the energy is dissipated in weak internal shocks with Mach numbers M ∼ 2 which are predominantly central flow shocks or merger shock waves traversing halo centres. Collapsed cosmological structures are surrounded by external shocks with much higher Mach numbers up to M ∼ 1000, but they play only a minor role in the energy balance of thermalization. This is because of the higher pre-shock gas densities within non-linear structures, and the significant increase of the mean shock speed as the characteristic halo mass grows with cosmic time. We show that after the epoch of cosmic reionization the Mach number distribution is significantly modified by an efficient suppression of strong external shock waves due to the associated increase of the sound speed of the diffuse gas. Invoking a model for CR acceleration in shock waves, we find that the average strength of shock waves responsible for CR energy injection is higher than that for shocks that dominate the thermalization of the gas. This implies that the dynamical importance of shock-injected CRs is comparatively large in the low-density, peripheral halo infalling regions, but is less important for the weaker flow shocks occurring in central highdensity regions of haloes. When combined with radiative dissipation and star formation, our formalism can also be used to study CR injection by supernova shocks, or to construct models for shock-induced star formation in the interstellar medium.

Shock-induced star formation in a model of the Mice

Star formation plays an important role in the fate of interacting galaxies. To date, most galactic simulations including star formation have used a density-dependent star formation rule designed to approximate a Schmidt law. Here, I present a new star formation rule which is governed by the local rate of energy dissipation in shocks. The new and old rules are compared using self-consistent simulations of NGC 4676; shock-induced star formation provides a better match to the observations of this system.

A search for possible interactions between ejections from GRS 1915+105 and the surrounding interstellar medium

We have observed an extended region surrounding the first discovered galactic superluminal source GRS 1915+105, seeking evidence of interaction between the relativistic ejecta of that object and the interstellar medium. We find two radio sources axisymmetrically aligned along the sub-arcsecond relativistic ejecta of GRS 1915+105 and roughly 17' distant from it, which coincide with the luminous IRAS sources 19124+1106 and 19132+1035. We have observed these sources at centimeter (VLA), millimeter (IRAM 30m), and infrared (ISO, UKIRT, ESO/MPI 2.2m) wavelengths in both line and continuum emission. At centimeter wavelengths a non-thermal jet-like feature aligned along the outflow axis is located adjacent to the inner edge of the southern source. Strong density enhancements are found in the millimeter tracers CO and H13CO+ at the positions of both sources and some of the morphology is reminiscent of shock-like interactions; however, linewidths are narrow. At infrared wavelengths strong hydrogen recombination lines and weak lines of molecular hydrogen are observed at the southern source. We discuss these results as possible evidence of the sought-after interaction, both in terms of the regions undergoing ongoing shock-heating and in terms of them being locations of shock-induced star formation. The evidence for each of these is inconclusive. Millimeter line mapping of a portion of W 50 where the relativistic jets of the X-ray binary SS 433 interact with the interstellar medium shows roughly similar morphology as GRS 1915+105, suggesting that the phenomena observed at the IRAS sources may not be unusual for such a long distance interaction.

Age, Evolution, and Dispersion of the Loose Groups of Blue Stars in the Northeast Radio Lobe of Centaurus A

The association of shocked gas and loose groups of young stars in the northeast radio lobe of Centaurus A (NGC 5128) provides direct evidence for shock-induced star formation. In this paper, we present UBV photometry of the blue stars based on observations made with the 2.5 m du Pont Telescope at Las Campanas Observatory. In a field covering 88 × 88, three groups are identified. Color-magnitude and color-color diagrams are plotted for each group. Two-color photometry of the many foreground field stars belonging to our own Galaxy indicates a foreground reddening EB-V = 0.14 mag ± 0.02 mag. The new photometry is compared with that carried out in a small section of the field with the Hubble Space Telescope. The HST frames, in conjunction with the ground-based material allow compact star clusters to be identified, some of which are exciting small H II regions. Positions, finding charts, and photometry are given for 50 stars with V < 23.0 mag and (B-V) < 0.4 mag. The brightest blue star, at V = 19.94 mag, has an estimated absolute visual magnitude MV = -8.2 mag, which is comparable to that of the brightest A-type supergiants in the Large Magellanic Cloud. Most of the blue stars lie just inside the 1.4 GHz contours of the large-scale jetlike feature recently detected by Morganti et al. It is suggested that rather than being a component of the jet that powers the radio lobes, this feature, which resembles a spiral arm of a normal galaxy, results from compression of ambient gas by the jet. Bolometric magnitudes and effective temperatures are derived for the bright blue stars and plotted. Isochrones based on the Geneva evolutionary tracks are fitted and indicate a significant age spread for the stars extending up to 15 million years. The older stars are distributed more widely than the younger stars indicating that star formation extends over time and that the blue stars eventually disperse into the extended stellar envelope of the galaxy.

Spiral Density Wave Shock-induced Star Formation at High Galactic Latitudes.

We have modeled the gas response to a spiral density wave (SDW) in a thick, magnetized galactic disk. The inclusion in the model of the vertically extended galactic warm ionized gas layer alters the conventional view of the SDW scenario for star formation: whereas marked density enhancements still occur in the midplane, the shock and a prominent high column density structure extend to high z (the height above the galactic midplane) above the arm. We argue that if the SDW mechanism indeed triggers molecular cloud and star formation, it should do so not only at the midplane but also at distances well above the star-forming thin disk of the conventional picture. The resulting structure resembles a hydraulic jump, or bore, in which gas entering the spiral arm rises suddenly on the upstream side of the arm, then accelerates and angles downward, finally landing on a large downfall region downstream of the arm.

Bow Shock Induced Star Formation in the LMC

The supergiant shell LMC 4 is known for its ring of HII regions and the deep minimum in HI in the interior (Rohlfs et al. 1984). LMC 4 shows X-ray emission detected by ROSAT and highly ionised gas (C IV) detected with HST (Bomans et al. 1994, 1996). The velocities of its neutral gas indicate it cannot have been formed by the collision with a high velocity cloud (Domgörgen et al. 1995).

Shocked Gas and Star Formation in the Centaurus A Radio Galaxy

Observational evidence for shock-induced star formation is found in the northeast radio lobe of the nearby radio galaxy Centaurus A (NGC 5128). A gas cloud, recently detected in H I, is impacted by the adjacent radio jet to the extent that cloud collapse is triggered and loose chains of blue supergiant stars are formed. Diffuse clouds and filaments of ionized gas have been observed near the interface of the H I cloud and the radio jet. These show velocities that cover a range of more than 550 km s-1. Line intensities in their spectra are characteristic of a shock-related origin with strong [N II] and [S II] relative to Hα. The [O III]/Hα line ratio indicates a large range in excitation that is not correlated with velocity. Distinct from this component is a group of four apparently normal H II regions that are excited by embedded young stars and whose velocities are very close to that of the H I cloud. Star formation will continue for as long as the gas cloud remains close to the radio jet. The loose chains of blue stars in the area are resolved only because NGC 5128 is so close. The reported faint blue extensions and plumes in more distant analogs probably have similar origins.

A High‐Resolution VLA Study of M17‐UC1

Using the Very Large Array, we have observed the H66α radio recombination line (RRL) emission at 1.3 cm and the H52α RRL emission at 7 mm toward the ultracompact H II region M17-UC1 and the nearby arc-shaped structure to the east. The angular resolution of the data is 1''-2''. The compact H II region (deconvolved size ~06 or 0.006 pc) is one of a number of H II regions with broad (≥35 km s-1) RRLs. The line parameters of the nearby arc of ionized emission (hereafter the Arc), about 1'' or 0.01 pc to the east of UC1, are remarkably constant over an area of 165 × 3'' (0.17 pc × 0.03 pc). The derived LTE electron temperature from the H66α line (T) is 6600 K in M17-UC1 and 8200 K in the Arc region. H2O maser emission at 1.3 cm from the region of M17-UC1 was observed in the D array with a beam of 4''. Observations of the H2O maser emission toward M17-UC1 reveal four masers within 30'' of M17-UC1; two of these sources were previously known. We discuss the region near M17-UC1 as an example of shock-induced star formation consisting of a hot, young, massive star surrounded by ionized material, perhaps resulting from a stellar wind outflow. We compare M17-UC1 with the six other known examples of broad-line RRL emission from ultracompact H II regions.

Whence the Local Bubble, Gum, Orion? GSH 238+00+09, A Nearby Major Superbubble toward Galactic Longitude 238o

We examine H I, IR, radio continuum, and 1/4 keV X-ray data and find evidence for a nearby (~0.8 kpc) superbubble in all four data sets. GSH 238+00+09 follows the official IAU naming convention and specifies the central Galactic coordinates and the LSR velocity. Its mass and energy required for production rank it among the major supershells in the Galaxy. It is at least partially responsible for the apparent extreme elongation of the Local Bubble toward l~230°, and it might intersect the Local Bubble or the Lindblad Ring. It cleared out a large cavity into which the Eridanus Loop and the Gum Nebula currently expand, and it may have been responsible for shock-induced star formation in the Gum, Orion, and Mon R2 regions. There are several candidates for the originating star cluster of GSH 238+00+09, of which the most interesting is Collinder 121.

A Morphological Method to Determine Corotation Radii in Spiral Galaxies

Shock-induced star formation in a stellar density wave scenario produces an azimuthal gradient of ages across the spiral arms that has opposite signs on either side of the corotation resonance (CR). We present a method based on the Fourier analysis of azimuthal profiles to locate the CR and determine the arm character (trailing or leading) in spiral galaxies. Basically, we compare the behavior of the phase angle of the two-armed spiral in blue and infrared colors that pick out, respectively, young and older disk stellar population. We illustrate the method using theoretical leading and trailing spirals. We have also applied the method to the spiral galaxies NGC 7479, for which we confirm the reported leading arms, and NGC 1832. In these galaxies, we find two and three CRs, respectively.

On the Origin of Blue Lobes in Central Cooling Flow Galaxies

view Abstract Citations (15) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Origin of Blue Lobes in Central Cooling Flow Galaxies De Young, David S. Abstract Recent observations of the central galaxy in two rich clusters with probable cooling flows have shown double-lobed distributions of blue light. In each case the blue light is coincident with the emission from a central double radio source. Moreover, in the one case so far observed, the blue continuum appears to be unpolarized. The possibility that this blue continuum is due to young stars produced by jet-induced star formation is explored here. Numerical simulation of the jet propagation through the ambient ISM, followed by shock-induced star formation and subsequent tracking of the stellar orbits in the galaxy and cluster potential allows a study of the development of the blue continuum as a function of space and time. It is found that agreement with observations can be obtained either if the radio jets are seen at an early stage (∼107 yr), with total energy fluxes of order 1044 ergs s-1, or if the jets are robust but short lived, with lifetimes less than 107 yr. In the first case these events must be intrinsically rare; if the second case is true, then the events can be more common and may account for the anomalous color gradients seen in central galaxies residing in rich clusters. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175810 Bibcode: 1995ApJ...446..521D Keywords: GALAXIES: CLUSTERS: GENERAL; GALAXIES: COOLING FLOWS; GALAXIES: JETS; GALAXIES: PHOTOMETRY; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (3)

Shock propagation and the generation of magnetohydrodynamic wave fields in inhomogeneous molecular clouds

We develop a simple one-dimensional model for the interaction of a steady, thin, planar shock wave with a nonrigid cloud which may be in motion relative to the surrounding medium, and we apply the model to shocks impinging on, and propagating through, molecular clouds. Both 'adiabatic' (gamma = 5/3) and radiative (gamma = 1) shocks are considered and we allow for the presence of a uniform magnetic field directed either parallel or perpendicular to the shock normal. The former field orientation is equivalent to the hydrodynamic case, and the latter involves only fast MHD shocks. We focus on the manner in which such shocks can generate internal kinetic motions in the cloud on a range of size and density scales through the direct acceleration of cores and clumps by shocks transmitted into them and through the generation of an MHD wavefield via the reflection of the incident shock at clump boundaries. We find that stronger incident Mach numbers and smaller density contrasts lead to more efficient cloud acceleration, as do isothermal intercloud shocks and small intercloud magnetic field strengths. The acceleration efficiency is insensitive to the adiabatic index and the magnetic field strength in the cloud itself. For typical parameter choices, the direct acceleration of clouds and clumps by strong shocks is found to be substantial and could at least in part account for their observed velocity dispersions. If the shocks are moderately weak, the final velocity of the cloud is linearly related to its initial velocity, with higher acceleration giving shallower slopes (i.e., final velocity distributions which are less sensitive to the initial distribution). Compared to the kinetic energy of the postshock cloud, the energy given to the wavefield at each encounter is small, and the heating of the interclump medium by the dissipation of this wavefield is found to be insufficient to balance the cooling rate in the cloud as a whole (although it may be important in particular regions), even if this medium is warm, unless it is also extremely tenuous (n approximately less than 0.1/cu cm). Nevertheless, the correction for the velocity imparted to the cloud leads to a substantial increase in the critical incident Mach number for wave emission over that reported by Spitzer for the rigid case. The implications of our model for shock-induced star formation are discussed briefly.