Violent Merger Research Articles

LUMINOUS INFRARED GALAXIES WITH THE SUBMILLIMETER ARRAY. II. COMPARING THE CO (3-2) SIZES AND LUMINOSITIES OF LOCAL AND HIGH-REDSHIFT LUMINOUS INFRARED GALAXIES

We present a detailed comparison of the CO(3-2) emitting molecular gas between a local sample of luminous infrared galaxies (U/LIRGs) and a high redshift sample that comprises submm selected galaxies (SMGs), quasars, and Lyman Break Galaxies (LBGs). The U/LIRG sample consists of our recent CO(3-2) survey using the Submillimeter Array while the CO(3-2) data for the high redshift population are obtained from the literature. We find that the L(CO(3-2)) and L(FIR) relation is correlated over five orders of magnitude, which suggests that the molecular gas traced in CO(3-2) emission is a robust tracer of dusty star formation activity. The near unity slope of 0.93 +/- 0.03 obtained from a fit to this relation suggests that the star formation efficiency is constant to within a factor of two across different types of galaxies residing in vastly different epochs. The CO(3-2) size measurements suggest that the molecular gas disks in local U/LIRGs (0.3 - 3.1 kpc) are much more compact than the SMGs (3 - 16 kpc), and that the size scales of SMGs are comparable to the nuclear separation (5 - 40 kpc) of the widely separated nuclei of U/LIRGs in our sample. We argue from these results that the SMGs studied here are predominantly intermediate stage mergers, and that the wider line-widths arise from the violent merger of two massive gas-rich galaxies taking place deep in a massive halo potential.

Galaxies in from the cold

Computer simulations of the cosmos suggest that cold streams of gas could underlie the unexpectedly high star-formation activity of many massive galaxies found to exist a few billion years after the Big Bang. Recent observations suggest that the massive galaxies that were at the height of their star-forming activity in the young Universe ten billion years ago formed their stars at surprisingly high rates. While such rates are commonly attributed to violent galaxy mergers, many of these galaxies are rotating discs, as extended as today's Milky Way, a structure that is incompatible with such a history. A new cosmological simulation suggests that these galaxies were 'stream fed', acquiring the material that was needed to fuel star formation as a steady flow of cold gas from the extended dark-matter haloes surrounding the galaxies. It is the rarer submillimetre galaxies, which form stars even more intensely, that are largely merger-induced starbursts.

Cold streams in early massive hot haloes as the main mode of galaxy formation

Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids. Cosmological simulations and clustering theory are used to explore how these galaxies acquired their gas. Here we report that they are 'stream-fed galaxies', formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes. A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid. This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.

Suzaku broad-band spectroscopy of RX J1347.5–1145: constraints on the extremely hot gas and non-thermal emission

We present the results from the analysis of long Suzaku observations of the most X-ray luminous galaxy cluster RX J1347.5-1145 at z=0.451. Aims: We study physical properties of the hot (~20 keV) gas clump in the south-east (SE) region discovered by the Sunyaev-Zel'dovich (SZ) effect observations, to understand the gas physics of a violent cluster merger. We also explore a signature of non-thermal emission using the hard X-ray data. Results: We find that the single-temperature model fails to reproduce the continuum emission and Fe-K lines measured by XIS simultaneously. The two-temperature model with a very hot component improves the fit, although the XIS data can only give a lower bound on its temperature. We detect the hard X-ray emission in the 12-40 keV band at the 7 sigma level; however, the significance becomes marginal when the systematic error in the background estimation is included. With the Suzaku + Chandra joint analysis, we determine the temperature of the SE excess component to be 25.3^{+6.1}_{-4.5} ^{+6.9}_{-9.5} keV (90% statistical and systematic errors), which is in an excellent agreement with the previous SZ + X-ray analysis. This is the first time that the X-ray spectroscopy alone gives a good measurement of the temperature of the hot component in the SE region, which is made possible by Suzaku's unprecedented sensitivity to the wide X-ray band. These results strongly indicate that the cluster has undergone a recent, violent merger. The spectral analysis shows that the SE component is consistent with being thermal. We find the 3 sigma upper limit on the non-thermal flux, F 0.007 micro G.

The Environment of M85 Optical Transient 2006‐1: Constraints on the Progenitor Age and Mass

M85 Optical Transient 2006-1 (M85 OT 2006-1) is the most luminous member of the small family of V838 Mon-like objects, whose nature is still a mystery. This event took place in the Virgo Cluster of galaxies and peaked at an absolute magnitude of MI ≈ -13. Here we present Hubble Space Telescope images of M85 OT 2006-1 and its environment, taken before and after the eruption, along with a spectrum of the host galaxy at the transient location. We find that the progenitor of M85 OT 2006-1 was not associated with any star-forming region. The g- and z-band absolute magnitudes of the progenitor were fainter than about –4 and –6 mag, respectively. Therefore, we can set a lower limit of ~50 Myr on the age of the youngest stars at the location of the progenitor that corresponds to a mass of <7 M⊙. Previously published line indices suggest that M85 has a mean stellar age of 1.6 ± 0.3 Gyr. If this mean age is representative of the progenitor of M85 OT 2006-1, then we can further constrain its mass to be less than 2 M⊙. We compare the energetics and mass limit derived for the M85 OT 2006-1 progenitor with those expected from a simple model of violent stellar mergers. Combined with further modeling, these new clues may ultimately reveal the true nature of these puzzling events.

Violent stellar merger model for transient events

We derive the constraints on the mass ratio for a binary system to merge in a violent process. We find that the secondary-to-primary stellar mass ratio should be 0.003 ≤ (M 2 /M 1 ) ≤ 0.15. A more massive secondary star will keep the primary stellar envelope in synchronized rotation with the orbital motion until merger occurs. This implies a very small relative velocity between the secondary star and the primary stellar envelope at the moment of merger, and therefore very weak shock waves, and low-flash luminosity. A too low-mass secondary will release small amount of energy, and will expel small amount of mass, which is unable to form an inflated envelope. It can, however, produce a quite luminous but short flash when colliding with a low-mass main-sequence star. Violent and luminous mergers, which we term mergebursts, can be observed as V838 Monocerotis-type events, where a star undergoes a fast brightening lasting days to months, with a peak luminosity of up to ∼10 6 L ⊙ followed by a slow decline at very low effective temperatures.

Spectroscopic Metallicities for Fornax Ultracompact Dwarf Galaxies, Globular Clusters, and Nucleated Dwarf Elliptical Galaxies

Various formation channels for the puzzling ultra-compact dwarf galaxies (UCDs) have been proposed in the last few years. To better judge on some of the competing scenarios, we present spectroscopic [Fe/H] estimates for a sample of 26 compact objects in the central region of the Fornax cluster, covering the magnitude range of UCDs and bright globular clusters. We find a break in the metallicity distribution of compact objects at M_V ~ -11 mag (~3*10^6 M_sun): for M_V<-11 mag the mean metallicity is [Fe/H]=-0.62 +/- 0.05 dex, 0.56 +/- 0.15 dex higher than for M_V>-11 mag. This metallicity break is accompanied by a change in the size-luminosity relation for compact objects, as deduced from HST-imaging: for M_V<-11 mag, r_h scales with luminosity, while for M_V>-11 mag, r_h is almost luminosity-independent. We therefore assume a limiting absolute magnitude of M_V=-11 mag between UCDs and globular clusters. The mean metallicity of five Fornax dE,N nuclei included in our study is about 0.8 dex lower than that of the UCDs, at 4.5 sigma significance. Because of this large metallicity discrepancy we disfavor the hypothesis that most of the Fornax UCDs are the remnant nuclei of tidally stripped dE,Ns. Our metallicity estimates for UCDs are closer to but slightly below those derived for young massive clusters (YMCs) of comparable masses. We therefore favor a scenario where most UCDs in Fornax are successors of merged YMCs produced in the course of violent galaxy-galaxy mergers. It is noted that in contrast to that, the properties of Virgo UCDs are more consistent with the stripping scenario, suggesting that different UCD formation channels may dominate in either cluster.

Dynamical Properties of Ultraluminous Infrared Galaxies. I. Mass Ratio Conditions for ULIRG Activity in Interacting Pairs

We present first results from our Very Large Telescope large program to study the dynamical evolution of ultraluminous infrared galaxies (ULIRGs), which are the products of mergers of gas-rich galaxies. The full data set consists of high-resolution long-slit H- and K-band spectra of 38 ULIRGs and 12 QSOs (in the range 0.042 < z < 0.268). In this paper, we present the sources that have not fully coalesced and therefore have two distinct nuclei. This subsample consists of 21 ULIRGs, the nuclear separation of which varies between 1.6 and 23.3 kpc. From the CO band heads that appear in our spectra, we extract the stellar velocity dispersion, σ, and the rotational velocity, Vrot. The stellar dispersion equals 142 km s-1 on average, while Vrot is often of the same order. We combine our spectroscopic results with high-resolution infrared (IR) imaging data to study the conditions for ULIRG activity in interacting pairs. We find that the majority of ULIRGs are triggered by almost equal-mass major mergers of 1.5 : 1 average ratio. Less frequently, 3 : 1 encounters are also observed in our sample. However, less violent mergers of mass ratio >3 : 1 typically do not force enough gas into the center to generate ULIRG luminosities.

Contraction of Dark Matter Halos in Response to Condensation of Baryons

The cooling of baryons in the centers of dark matter halos leads to a more concentrated dark matter distribution. This effect has traditionally been calculated using the model of adiabatic contraction, which assumes spherical symmetry, while in hierarchical formation scenarios halos grow via multiple violent mergers. We test the adiabatic contraction model in high-resolution cosmological simulations and find that the dissipation of gas indeed increases the density of dark matter and steepens its radial profile compared to the case without cooling. Although the standard model systematically overpredicts the increase of dark matter density, a simple modification of the assumed invariant from M(r)r to M( )r, where is the orbit-averaged particle position, reproduces the simulated profiles within 10%.

The Physical Connections among Infrared QSOs, Palomar‐Green QSOs, and Narrow‐Line Seyfert 1 Galaxies

We study the properties of infrared-selected QSOs (IR QSOs), optically selected Palomar-Green QSOs (PG QSOs), and narrow-line Seyfert 1 galaxies (NLS1s). We compare their properties from the IR to the optical and examine various correlations among the black hole mass, accretion rate, star formation rate, and optical and IR luminosities. We find that the IR excess in IR QSOs is mostly in the far-IR, and their IR spectral indices suggest that the excess emission is from low-temperature dust heated by starbursts rather than active galactic nuclei (AGNs). The IR excess is therefore a useful criterion to separate the relative contributions of starbursts and AGNs. We further find a tight correlation between the star formation rate and the accretion rate of central AGNs for IR QSOs. The ratio of the star formation rate and the accretion rate is about several hundred for IR QSOs but decreases with the central black hole mass. This shows that the tight correlation between the stellar mass and the central black hole mass is preserved in massive starbursts during violent mergers. We suggest that the higher Eddington ratios of NLS1s and IR QSOs imply that they are in the early stage of evolution toward classical Seyfert 1 galaxies and QSOs, respectively.

Response of Dark Matter Halos to Condensation of Baryons: Cosmological Simulations and Improved Adiabatic Contraction Model

The cooling of gas in the centers of dark matter halos is expected to lead to a more concentrated dark matter distribution. The response of dark matter to the condensation of baryons is usually calculated using the model of adiabatic contraction, which assumes spherical symmetry and circular orbits. In contrast, halos in the hierarchical structure formation scenarios grow via multiple violent mergers and accretion along filaments, and particle orbits in the halos are highly eccentric. We study the effects of the cooling of gas in the inner regions of halos using high-resolution cosmological simulations which include gas dynamics, radiative cooling, and star formation. We find that the dissipation of gas indeed increases the density of dark matter and steepens its radial profile in the inner regions of halos compared to the case without cooling. For the first time, we test the adiabatic contraction model in cosmological simulations and find that the standard model systematically overpredicts the increase of dark matter density in the inner 5% of the virial radius. We show that the model can be improved by a simple modification of the assumed invariant from M(r)r to M(r_av)r, where r and r_av are the current and orbit-averaged particle positions. This modification approximately accounts for orbital eccentricities of particles and reproduces simulation profiles to within 10-20%. We present analytical fitting functions that accurately describe the transformation of the dark matter profile in the modified model and can be used for interpretation of observations.

Exploring Cluster Physics with High-Resolution Sunyaev-Zel’dovich Effect Images and X-Ray Data: The Case of the Most X-Ray-Luminous Galaxy Cluster RX J1347—1145

Abstract Foreseeing the era of high spatial resolution measurements of the Sunyaev–Zel’dovich effect (SZE) in clusters of galaxies, we present a prototype analysis of this sort combined with Chandra X-ray data. It is applied specifically to RX J1347$-$1145 at $z=0.451$, the most X-ray-luminous galaxy cluster known, for which the highest resolution SZE and X-ray images are currently available. We demonstrate that the combined analysis yields a unique probe of complex structures in the intracluster medium, offering determinations of their temperature, density, and line-of-sight extent. For a subclump in RX J1347$-$1145, previously discovered in our SZE map, the temperature inferred after removing the foreground and background components is well in excess of 20 keV, indicating that the cluster has recently undergone a violent merger. Excluding the region around this subclump, the SZE signals in submillimeter to centimeter bands (350, 150, and 21 GHz) are all consistent with those expected from Chandra X-ray observations. We further present a temperature deprojection technique based on the SZE and X-ray images, without any knowledge of spatially resolved X-ray spectroscopy. The methodology presented here will be applicable to a statistical sample of clusters available in the future SZE surveys.

Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars

Based on spectra from F and G dwarf stars, we present elementalabundance trends in the Galactic thin and thick disks in the metallicityregime -0.8< [Fe/H < +0.4. Our findings can besummarized as follows. 1) Both the thin and the thick disks show smoothand distinct abundance trends that, at sub-solar metallicities, areclearly separated. 2) For the alpha -elements the thick disk showssignatures of chemical enrichment from SNe type Ia. 3) The age of thethick disk sample is in the mean older than the thin disk sample. 4)Kinematically, there exist thick disk stars with super-solarmetallicities. Based on these findings, together with other constraintsfrom the literature, we discuss different formation scenarios for thethick disk. We suggest that the currently most likely formation scenariois a violent merger event or a close encounter with a companion galaxy.Based on kinematics the stellar sample was selected to contain starswith high probabilities of belonging either to the thin or to the thickGalactic disk. The total number of stars are 66 of which 21 belong tothe thick disk and 45 to the thin disk. The analysis is based onhigh-resolution spectra with high signal-to-noise (R 48 000 and S/Ngtrsim 150, respectively) recorded with the FEROS spectrograph on LaSilla, Chile. Abundances have been determined for four alpha -elements(Mg, Si, Ca, and Ti), for four even-nuclei iron peak elements (Cr, Fe,Ni, and Zn), and for the light elements Na and Al, from equivalent widthmeasurements of 30 000 spectral lines. An extensive investigation ofthe atomic parameters, log gf-values in particular, have been performedin order to achieve abundances that are trustworthy. Noteworthy is thatwe find for Ti good agreement between the abundances from Ti I and TiIi. Our solar Ti abundances are in concordance with the standardmeteoritic Ti abundanceBased on observations collected at the European Southern Observatory, LaSilla, Chile, Proposals #65.L-0019(B) and 67.B-0108(B).Full Tables eftab:linelist and eftab:abundances are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/527 (Less)

Signatures of SN Ia in the galactic thick disk

We present the first results of a larger study into the stellar abundances and chemical trends in long-lived dwarf stars in the solar neighbourhood that belong to (based on their kinematics) the thin and thick galactic disk, respectively. We confirm that the trends of α-elements in the thin and thick disk are distinct (this has previously been shown for Mg by Fuhrmann [CITE], but e.g. Chen et al. [CITE] claimed the trends to follow smoothly upon each other). We find that the thick disk show the typical signature of contribution from SN Ia (i.e. the “knee”) to the enrichment of the interstellar gas out of which the later generations of thick disk stars formed. The trend starts out as at and continue on this level with increasing [Fe/H] until -0.4 dex where a decline in [Mg/Fe] starts and steadily continues down to 0 dex at solar metallicity. The same is true for the other α-elements (e.g. Si). Using ages from the literature we find that the thick disk in the mean is older than the thin disk. Combining our results with other observational facts we suggest that the most likely formation scenario for the thick disk is, still, a violent merger event. We also suggest that there might be tentative evidence for diffusion of orbits in todays thin disk (based on kinematics in combination with elemental abundances).

On the Origin of Radio Halos in Galaxy Clusters

Previously, it has been recognized that radio halos in galaxy clusters are preferentially associated with merging systems, as indicated by substructure in the X-ray images and temperature maps. Since, however, many clusters without radio halos also possess substructure, the role of mergers in the formation of radio halos has remained unclear. By using power ratios to relate gravitational potential fluctuations to substructure in X-ray images, we provide the first quantitative comparison of the dynamical states of clusters possessing radio halos. A correlation between the 1.4 GHz power (P1.4) of the radio halo (or relic) and the magnitude of the dipole power ratio (P1/P0) is discovered such that approximately P1.4 ∝ P1/P0; i.e., the strongest radio halos appear only in those clusters currently experiencing the largest departures from a virialized state. From the additional consideration of a small number of highly disturbed clusters without radio halos detected at 1.4 GHz and recalling that radio halos are more common in clusters with high X-ray luminosity (Giovannini, Tordi, & Feretti), we argue that radio halos form preferentially in massive (LX 0.5 × 1045 ergs s-1) clusters experiencing violent mergers (P1/P0 0.5 × 10-4) that have seriously disrupted the cluster core. The association of radio halos with massive, large-P1/P0, core-disrupted clusters can account for both the vital role of mergers in accelerating the relativistic particles responsible for the radio emission as well as the rare occurrence of radio halos in cluster samples.

Cluster gas temperature maps with ASCA and beyond

During the past several years, ASCA has nearly completed mapping of gas temperatures in bright nearby clusters of galaxies. In several clusters, it discovered strong and irregular spatial temperature variations indicative of ongoing violent subcluster mergers. In other systems, the temperature distribution appears symmetric, indicating a relaxed state which is a necessary condition for reliable hydrostatic total mass measurements. I present a (very incomplete) review of these results, discuss their implications for understanding of physical processes in the intracluster gas and for the cluster mass measurements. I also mention some interesting studies that will become possible with the forthcoming Chandra X-ray observatory.

Violent Tidal Disruptions of Atomic Hydrogen Gas in Quasar Host Galaxies

Violent galactic encounters or mergers are the leading contenders for triggering luminous quasar activity at low redshifts: such interactions can lead to the concentration of gas in the host galactic nucleus, thus fueling the suspected central supermassive black hole. Although optical images show a number of violently interacting systems, in many cases, the evidence for such interactions is only circumstantial (e.g., asymmetric optical morphologies, projected nearby companion galaxies) or not at all apparent. Here we image quasar host galaxies for the first time in the redshifted 21 cm line emission of neutral atomic hydrogen (H I) gas, which, in nearby galaxies, has proved to be a particularly sensitive as well as enduring tracer of tidal interactions. The three quasars studied have different optical environments that are normally seen around low-redshift quasars, ranging from a perhaps mildly interacting system to a relatively undisturbed host with a projected neighboring galaxy to an isolated and apparently serene host galaxy. By contrast with their optical appearances, all three quasar host galaxies exhibit ongoing or remnant tidal H I disruptions tracing galactic encounters or mergers. These observations demonstrate the utility of H I at revealing tidal interactions in quasar host galaxies and, combined with optical studies, provide a fuller understanding of the likely stage of the interaction.

A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas

A universal mechanism for cluster formation in all epochs and environments is found to be consistent with the properties and locations of young and old globular clusters, open clusters and unbound associations, and interstellar clouds. The primary structural differences between various cluster types result from differences in pressure at the time of formation, combined with different ages for subsequent evolution. All clusters begin with a mass distribution similar to that for interstellar clouds, which is approximately n(M)dM M-2 dM. Old halo globulars have a current mass distribution that falls off at low mass because of a Hubble time of cluster destruction. Young globulars have not yet had time for a similar loss, and some old open clusters have survived because of their low densities. The peak in the halo cluster luminosity function depends only on age, and is independent of the host galaxy luminosity, as observed. The peak globular cluster mass is not a characteristic or Jeans mass in the primordial galaxy, as previously suggested. The initial mass distribution functions for young and old globular clusters, open clusters and associations, and interstellar clouds are all power laws with a slope of ~ -2. This distribution could be the result of fractal structure in turbulent gas. New data on clusters in the LMC also follow this power law. The slope is so steep that it implies a significant fraction of star formation occurs in small clusters. Numerous halo field stars should come from the evaporation of small halo clusters, and a high fraction of disk field stars should arise in small unbound disk clusters. This differs significantly from previous suggestions that most disk stars form in large OB associations. Globular clusters of all ages preferentially form in high-pressure regions. This is directly evident today in the form of large kinematic pressures from the densities and relative velocities of member stars. High pressures at the time of globular cluster formation are either the result of a high background virial density in that part of the galaxy (as in dwarf galaxies or galactic nuclei and nuclear rings), turbulence compression (in halo globulars), or large-scale shocks (in interacting galaxies). Massive clusters that form in such high-pressure environments are more likely to be bound than low-mass clusters or clusters of equal mass in low-pressure regions. This is because virialized clouds are more tightly bound at high pressure. A simple model illustrates this effect. One implication of this result is that starburst regions preferentially make globular clusters, in which case some elliptical galaxies could have formed by the violent merger of spiral galaxies.

Distribution functions for clusters of galaxies from N-body simulations

We present the results of an attempt to adapt the distribution function formalism to characterize large-scale structures like clusters of galaxies that form in a cosmological N-body simulation. While galaxy clusters are systems that are not strictly in equilibrium, we show that their evolution can nevertheless be studied using a physically motivated extension of the language of equilibrium stellar dynamics. Restricting our analysis to the virialized region, a prescription to limit the accessible phase-space is presented, which permits the construction of both the isotropic and the anisotropic distribution functions $f(E)$ and $f(E,L)$. The method is applied to models extracted from a catalogue of simulated clusters. Clusters evolved in open and flat background cosmologies are followed during the course of their evolution, and are found to transit through a sequence of what we define as `quasi-equilibrium' states. An interesting feature is that the computed $f(E)$ is well fit by an exponential form. We conclude that the dynamical evolution of a cluster, undergoing relaxation punctuated by interactions and violent mergers with consequent energy-exchange, can be studied both in a qualitative and quantitative fashion by following the time evolution of $f(E)$.

The Galaxy Mass Distribution from Mergers in a Collapsing Spherical Cluster

We examine the role of mergers in the formation and statistics of galaxies and clusters of galaxies. First steps in this direction were carried out by Shaviv &amp; Shaviv (1993, ApJ.Lett., 412, L25) where the OD case was treated. The work is extended to examine the effect of self gravity. Thus, we essentially explore the combined effect of violent relaxation and mergers on the resulting structure and mass distribution of galaxies in clusters of galaxies. We developed a numerical method to treat the spherical 2D problem (radial position and velocity) of the evolution of a collapsing cluster of gas clouds (Shaviv &amp; Shaviv, 1995, ApJ., 448, 514).