Asymmetric Galaxy Research Articles

Redshifting galaxies from DESI to JWST CEERS: Correction of biases and uncertainties in quantifying morphology

Observations of high-redshift galaxies with unprecedented detail have now been rendered possible with the James Webb Space Telescope (JWST). However, accurately quantifying their morphology remains uncertain due to potential biases and uncertainties. To address this issue, we used a sample of 1816 nearby DESI galaxies, with a stellar mass range of 109.75 − 11.25 M⊙, to compute artificial images of galaxies of the same mass located at 0.75 ≤ z ≤ 3 and observed at rest-frame optical wavelength in the Cosmic Evolution Early Release Science (CEERS) survey. We analyzed the effects of cosmological redshift on the measurements of Petrosian radius (Rp), half-light radius (R50), asymmetry (A), concentration (C), axis ratio (q), and Sérsic index (n). Our results show that Rp and R50, calculated using non-parametric methods, are slightly overestimated due to PSF smoothing, while R50, q, and n obtained through fitting a Sérsic model does not exhibit significant biases. By incorporating a more accurate noise effect removal procedure, we improve the computation of A over existing methods, which often overestimate, underestimate, or lead to significant scatter of noise contributions. Due to PSF asymmetry, there is a minor overestimation of A for intrinsically symmetric galaxies. However, for intrinsically asymmetric galaxies, PSF smoothing dominates and results in an underestimation of A, an effect that becomes more significant with higher intrinsic A or at lower resolutions. Moreover, PSF smoothing also leads to an underestimation of C, which is notably more pronounced in galaxies with higher intrinsic C or at lower resolutions. We developed functions based on resolution level, defined as Rp/FWHM, for correcting these biases and the associated statistical uncertainties. Applying these corrections, we measured the bias-corrected morphology for the simulated CEERS images and we find that the derived quantities are in good agreement with their intrinsic values – except for A, which is robust only for angularly large galaxies where Rp/FWHM ≥ 5. Our correction functions can be applied to other surveys, offering valuable tools for future studies.

Morphological signatures of mergers in the TNG50 simulation and the Kilo-Degree Survey: the merger fraction from dwarfs to Milky Way-like galaxies

ABSTRACT Using the TNG50 cosmological simulation and observations from the Kilo-Degree Survey (KiDS), we investigate the connection between galaxy mergers and optical morphology in the local Universe over a wide range of galaxy stellar masses (8.5 ≤ log (M*/M⊙) ≤ 11). To this end, we have generated over 16 000 synthetic images of TNG50 galaxies designed to match KiDS observations, including the effects of dust attenuation and scattering, and used the statmorph code to measure various image-based morphological diagnostics in the r-band for both data sets. Such measurements include the Gini–M20 and concentration–asymmetry–smoothness statistics. Overall, we find good agreement between the optical morphologies of TNG50 and KiDS galaxies, although the former are slightly more concentrated and asymmetric than their observational counterparts. Afterwards, we trained a random forest classifier to identify merging galaxies in the simulation (including major and minor mergers) using the morphological diagnostics as the model features, along with merger statistics from the merger trees as the ground truth. We find that the asymmetry statistic exhibits the highest feature importance of all the morphological parameters considered. Thus, the performance of our algorithm is comparable to that of the more traditional method of selecting highly asymmetric galaxies. Finally, using our trained model, we estimate the galaxy merger fraction in both our synthetic and observational galaxy samples, finding in both cases that the galaxy merger fraction increases steadily as a function of stellar mass.

A Simulation-driven Deep Learning Approach for Separating Mergers and Star-forming Galaxies: The Formation Histories of Clumpy Galaxies in All of the CANDELS Fields

Being able to distinguish between galaxies that have recently undergone major-merger events, or are experiencing intense star formation, is crucial for making progress in our understanding of the formation and evolution of galaxies. As such, we have developed a machine-learning framework based on a convolutional neural network to separate star-forming galaxies from post-mergers using a data set of 160,000 simulated images from IllustrisTNG100 that resemble observed deep imaging of galaxies with Hubble. We improve upon previous methods of machine learning with imaging by developing a new approach to deal with the complexities of contamination from neighboring sources in crowded fields and define a quality control limit based on overlapping sources and background flux. Our pipeline successfully separates post-mergers from star-forming galaxies in IllustrisTNG 80% of the time, which is an improvement by at least 25% in comparison to a classification using the asymmetry (A) of the galaxy. Compared with measured Sérsic profiles, we show that star-forming galaxies in the CANDELS fields are predominantly disk-dominated systems while post-mergers show distributions of transitioning disks to bulge-dominated galaxies. With these new measurements, we trace the rate of post-mergers among asymmetric galaxies in the universe, finding an increase from 20% at z = 0.5 to 50% at z = 2. Additionally, we do not find strong evidence that the scattering above the star-forming main sequence can be attributed to major post-mergers. Finally, we use our new approach to update our previous measurements of galaxy merger rates .

On the relationship between gas content, star formation, and global H i asymmetry of galaxies on the star-forming main-sequence

ABSTRACT Observations have revealed that disturbances in the cold neutral atomic hydrogen (H i) in galaxies are ubiquitous, but the reasons for these disturbances remain unclear. While some studies suggest that asymmetries in integrated H i spectra (global H i asymmetry) are higher in H i-rich systems, others claim that they are preferentially found in H i-poor galaxies. In this work, we utilize the Arecibo Legacy Fast ALFA (ALFALFA) and extended GALEX Arecibo SDSS Survey (xGASS) surveys, plus a sample of post-merger galaxies, to clarify the link between global H i asymmetry and the gas properties of galaxies. Focusing on star-forming galaxies in ALFALFA, we find that elevated global H i asymmetry is not associated with a change in the H i content of a galaxy, and that only the galaxies with the highest global H i asymmetry show a small increase in specific star formation rate (sSFR). However, we show that the lack of a trend with H i content is because ALFALFA misses the ‘gas-poor’ tail of the star-forming main-sequence. Using xGASS to obtain a sample of star-forming galaxies that is representative in both sSFR and H i content, we find that global H i asymmetric galaxies are typically more gas-poor than symmetric ones at fixed stellar mass, with no change in sSFR. Our results highlight the complexity of the connection between galaxy properties and global H i asymmetry. This is further confirmed by the fact that even post-merger galaxies show both symmetric and asymmetric H i spectra, demonstrating that merger activity does not always lead to an asymmetric global H i spectrum.

A massive stellar bulge in a regularly rotating galaxy 1.2 billion years after the Big Bang.

Cosmological models predict that galaxies forming in the early Universe experience a chaotic phase of gas accretion and star formation, followed by gas ejection due to feedback processes. Galaxy bulges may assemble later via mergers or internal evolution. Here we present submillimeter observations (with spatial resolution of 700 parsecs) of ALESS 073.1, a starburst galaxy at redshift [Formula: see text] when the Universe was 1.2 billion years old. This galaxy's cold gas forms a regularly rotating disk with negligible noncircular motions. The galaxy rotation curve requires the presence of a central bulge in addition to a star-forming disk. We conclude that massive bulges and regularly rotating disks can form more rapidly in the early Universe than predicted by models of galaxy formation.

Early assembly of a galaxy disk and bulge

Galaxies Galaxy formation in the early Universe is thought to have been a chaotic process, producing disturbed and asymmetric galaxy morphologies. Over billions of years, galaxies dynamically relaxed to form stable morphological features. Lelli et al. observed a distant galaxy at a redshift when the Universe was 1.2 billion years old (see the Perspective by Wardlow). They used gas and dust emission to measure its kinematics, and then modeled the mass distribution within the galaxy. The authors found that the galaxy contains a massive stellar bulge and a uniformly rotating disk, features that models predict take billions of years to form. These results indicate that galaxy evolution is a more rapid process than previously thought. Science , this issue p. [713][1]; see also p. [674][2] [1]: /lookup/doi/10.1126/science.abc1893 [2]: /lookup/doi/10.1126/science.abg2907

Modelling the asymmetry of the halo cross-correlation function with relativistic effects at quasi-linear scales

ABSTRACT The observed galaxy distribution via galaxy redshift surveys appears distorted due to redshift-space distortions (RSD). While one dominant contribution to RSD comes from the Doppler effect induced by the peculiar velocity of galaxies, the relativistic effects, including the gravitational redshift effect, are recently recognized to give small but important contributions. Such contributions lead to an asymmetric galaxy clustering along the line of sight, and produce non-vanishing odd multipoles when cross-correlating between different biased objects. However, non-zero odd multipoles are also generated by the Doppler effect beyond the distant-observer approximation, known as the wide-angle effect, and at quasi-linear scales, the interplay between wide-angle and relativistic effects becomes significant. In this paper, based on the formalism developed by Taruya et al., we present a quasi-linear model of the cross-correlation function taking a proper account of both the wide-angle and gravitational redshift effects, as one of the major relativistic effects. Our quasi-linear predictions of the dipole agree well with simulations even at the scales below $20\, h^{-1}\,$Mpc, where non-perturbative contributions from the halo potential play an important role, flipping the sign of the dipole amplitude. When increasing the bias difference and redshift, the scale where the sign flip happens is shifted to a larger scale. We derive a simple approximate formula to quantitatively account for the behaviours of the sign flip.

Star Formation in Massive Galaxies at Redshift z ∼ 0.5

Abstract It is believed that massive galaxies have quenched their star formation because of active galactic nucleus feedback. However, recent studies have shown that some massive galaxies are still forming stars. We analyze the morphology of star formation regions for galaxies of stellar masses larger than 1011.3 M ⊙ at around redshift z r = 0.5 using u − z color images. We find that about 20% of the massive galaxies are star-forming (SF) galaxies, and most of them (∼85%) have asymmetric structures induced by recent mergers. Moreover, for these asymmetric galaxies, we find that the asymmetry of the SF regions becomes larger for bluer galaxies. Using the Illustris simulation, we can qualitatively reproduce the observed relation between asymmetry parameter and color. Furthermore, using the merger trees in the simulation, we find a correlation between the color of the main branch galaxies at z r = 0.5 and the sum of the star formation rates of the recently accreted galaxies, which implies that star formation of the accreted galaxies has contributed to the observed star formation of the massive (host) galaxies (ex situ star formation). Furthermore, we find two blue and symmetric galaxies, candidates for massive blue disks, in our observed sample, which indicates that about ∼10% of massive SF galaxies are forming stars in the normal mode of disk star formation (in situ star formation). With the simulation, we find that the disk galaxies at z r ≈ 0.5 should have experienced few major mergers during the last 4.3 Gyr.

XGASS: Robust quantification of asymmetries in global H i spectra and their relationship to environmental processes

ABSTRACT We present an analysis of asymmetries in global H i spectra from the extended GALEX Arecibo SDSS Survey (xGASS), a stellar mass-selected and gas fraction-limited survey which is representative of the H i properties of galaxies in the local Universe. We demonstrate that the asymmetry in a H i spectrum is strongly linked to its signal-to-noise meaning that, contrary to what was done in previous works, asymmetry distributions for different samples cannot be compared at face value. We develop a method to account for noise-induced asymmetry and find that the typical galaxy detected by xGASS exhibits higher asymmetry than what can be attributed to noise alone, with 37 per cent of the sample showing asymmetry greater than 10 per cent at an 80 per cent confidence level. We find that asymmetric galaxies contain, on average, 29 per cent less H i mass compared to their symmetric counterparts matched in both stellar mass and signal-to-noise. We also present clear evidence that satellite galaxies, as a population, exhibit more asymmetric H i spectra than centrals and that group central galaxies show a slightly higher rate of H i asymmetries compared to isolated centrals. All these results support a scenario in which environmental processes, in particular those responsible for gas removal, are the dominant driver of asymmetry in xGASS.

SDSS-IV MaNGA: spatially resolved star formation histories and the connection to galaxy physical properties

A key task of observational extragalactic astronomy is to determine where -- within galaxies of diverse masses and morphologies -- stellar mass growth occurs, how it depends on galaxy properties and what processes regulate star formation. Using spectroscopic indices derived from the stellar continuum at $\sim 4000$\AA, we determine the spatially resolved star-formation histories of 980000 spaxels in 2404 galaxies in the SDSS-IV MaNGA IFU survey. We examine the spatial distribution of star-forming, quiescent, green valley, starburst and post-starburst spaxels as a function of stellar mass and morphology to see where and in what types of galaxy star formation is occurring. The spatial distribution of star-formation is dependent primarily on stellar mass, with a noticeable change in the distribution at \mstar$>10^{10}$\msun. Galaxies above this mass have an increasing fraction of regions that are forming stars with increasing radius, whereas lower mass galaxies have a constant fraction of star forming regions with radius. Our findings support a picture of inside-out growth and quenching at high masses. We find that morphology (measured via concentration) correlates with the fraction of star-forming spaxels, but not with their radial distribution. We find (post-)starburst regions are more common outside of the galaxy centre, are preferentially found in asymmetric galaxies, and have lower gas-phase metallicity than other regions, consistent with interactions triggering starbursts and driving low metallicity gas into regions at $<1.5R_e$.

The SAMI Galaxy Survey: asymmetry in gas kinematics and its links to stellar mass and star formation

We study the properties of kinematically disturbed galaxies in the SAMI Galaxy Survey using a quantitative criterion, based on kinemetry (Krajnovic et al.). The approach, similar to the application of kinemetry by Shapiro et al. uses ionised gas kinematics, probed by H{\alpha} emission. By this method 23+/-7% of our 360-galaxy sub-sample of the SAMI Galaxy Survey are kinematically asymmetric. Visual classifications agree with our kinemetric results for 90% of asymmetric and 95% of normal galaxies. We find stellar mass and kinematic asymmetry are inversely correlated and that kinematic asymmetry is both more frequent and stronger in low-mass galaxies. This builds on previous studies that found high fractions of kinematic asymmetry in low mass galaxies using a variety of different methods. Concentration of star forma- tion and kinematic disturbance are found to be correlated, confirming results found in previous work. This effect is stronger for high mass galaxies (log(M*) > 10) and indicates that kinematic disturbance is linked to centrally concentrated star formation. Comparison of the inner (within 0.5Re) and outer H{\alpha} equivalent widths of asymmetric and normal galaxies shows a small but significant increase in inner equivalent width for asymmetric galaxies.

Beyond spheroids and discs: classifications of CANDELS galaxy structure at 1.4 &lt;z&lt; 2 via principal component analysis

Important but rare and subtle processes driving galaxy morphology and star-formation may be missed by traditional spiral, elliptical, irregular or S\'ersic bulge/disk classifications. To overcome this limitation, we use a principal component analysis of non-parametric morphological indicators (concentration, asymmetry, Gini coefficient, $M_{20}$, multi-mode, intensity and deviation) measured at rest-frame $B$-band (corresponding to HST/WFC3 F125W at 1.4 $< z <$ 2) to trace the natural distribution of massive ($>10^{10} M_{\odot}$) galaxy morphologies. Principal component analysis (PCA) quantifies the correlations between these morphological indicators and determines the relative importance of each. The first three principal components (PCs) capture $\sim$75 per cent of the variance inherent to our sample. We interpret the first principal component (PC) as bulge strength, the second PC as dominated by concentration and the third PC as dominated by asymmetry. Both PC1 and PC2 correlate with the visual appearance of a central bulge and predict galaxy quiescence. PC1 is a better predictor of quenching than stellar mass, as as good as other structural indicators (S\'ersic-n or compactness). We divide the PCA results into groups using an agglomerative hierarchical clustering method. Unlike S\'ersic, this classification scheme separates compact galaxies from larger, smooth proto-elliptical systems, and star-forming disk-dominated clumpy galaxies from star-forming bulge-dominated asymmetric galaxies. Distinguishing between these galaxy structural types in a quantitative manner is an important step towards understanding the connections between morphology, galaxy assembly and star-formation.

Photometric studies of Abell 1664: the subtle effect a minor merger has on cluster galaxies

A combination of $BRI$ photometry and archival Chandra X-ray data have been used to analyse the effects a minor merger has on the galaxy population of A1664. We utilise adaptive smoothing techniques in the 2D spatial distribution of cluster galaxies to reveal substructure $\sim$ 800 kpc South of the cluster core. We identify this substructure as most likely the remnant core of a merging group which has passed pericentre and responsible for triggering a cold front in the cluster core. We define two samples to represent two different environments within A1664 in accordance with the location of the substructure. We apply a morphological analysis using CAS, M$_{20}$ and Gini to these samples to deduce if there has been any significant effect on the cluster galaxies due to this interaction. We find there are more asymmetric galaxies found in the inner sample (at the 3.7$\sigma$ level) which is likely due to galaxy-galaxy interactions as the merging group passed through core passage. No other differences were found between the inner and outer cluster in our morphological analysis, which we attribute to the limited resolution of our imagery. The colour profiles of the galaxies are found to be consistent with the morphology-density relation suggesting there is no unique environmental effect in A1664 that has enhanced galaxy transformations. This study favours the star formation of cluster galaxies being quenched well before it is able to interact with the merging group and demonstrates that a minor cluster merger has little effect on the observable parameters of cluster galaxies such as morphology and colour.

H I asymmetry in the isolated galaxy CIG 85 (UGC 1547)

We present the results from the Giant Metrewave Radio Telescope (GMRT) interferometric HI and 20 cm radio continuum observations of CIG 85, an isolated asymmetric galaxy from the AMIGA (Analysis of the Interstellar Medium of Isolated GAlaxies) sample. Despite being an isolated galaxy, CIG 85 showed an appreciable optical and HI spectral asymmetry and therefore was an excellent candidate for resolved HI studies to understand the reasons giving rise to asymmetries in isolated galaxies. The galaxy was imaged in HI and 20 cm radio continuum using the GMRT. For a detailed discussion of the results we also made use of multi-wavelength data from archival SDSS, GALEX and Halpha imaging. We find the HI in CIG 85 to have a clumpy, asymmetric distribution which in the NW part is correlated with optical tail like features, but the HI velocity field displays a relatively regular rotation pattern. Evaluating all the observational evidence, we come to a conclusion that CIG 85 is most likely a case of a disturbed spiral galaxy which now appears to have the morphology of an irregular galaxy. Although it is currently isolated from major companions, the observational evidence is consistent with HI asymmetries, a highly disturbed optical disk and recent increase in star formation having been caused by a minor merger, remnants of which are now projected in front of the optical disk. If this is correct, the companion will be fully accreted by CIG 85 in the near future.

Galaxy pairs in the Sloan Digital Sky Survey - II. The effect of environment on interactions

We use a sample of close galaxy pairs selected from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4) to investigate in what environments galaxy mergers occur and how the results of these mergers depend on differences in local galaxy density. The galaxies are quantified morphologically using two-dimensional bulge-plus-disk decompositions and compared to a control sample matched in stellar mass, redshift and local projected density. Lower density environments have fractionally more galaxy pairs with small projected separations (r_p) and relative velocities (Delta v), but even high density environments contain significant populations of pairs with parameters that should be conducive to interactions. Metrics of asymmetry and colour are used to identify merger activity and triggered star formation. The location of star formation is inferred by distinguishing bulge and disk colours and calculating bulge fractions from the SDSS images. Galaxies in the lowest density environments show the largest changes in star formation rate, asymmetry and bulge-total fractions at small separations, accompanied by bluer bulge colours. At the highest local densities, the only galaxy property to show an enhancement in the closest pairs is asymmetry. We interpret these results as evidence that whilst interactions (leading to tidal distortions) occur at all densities, triggered star formation is seen only in low-to-intermediate density environments. We suggest that this is likely due to the typically higher gas fractions of galaxies in low density environments. Finally, by cross-correlating our sample of galaxy pairs with a cluster catalogue, we investigate the dependence of interactions on clustercentric distance. It is found that for close pairs the fraction of asymmetric galaxies is highest in the cluster centres.

COMPARATIVE STUDY OF ASYMMETRY ORIGIN OF GALAXIES IN DIFFERENT ENVIRONMENTS. II. NEAR-INFRARED OBSERVATIONS

In this second paper of two analyses, we present near-infrared (NIR) morphological and asymmetry studies performed in sample of 92 galaxies found in different density environments: galaxies in Compact Groups (HCGs), Isolated Pairs of Galaxies (KPGs), and Isolated Galaxies (KIGs). Both studies have proved useful for identifying the effect of interactions on galaxies. In the NIR, the properties of the galaxies in HCGs, KPGs, and KIGs are more similar than they are in the optical. This is because the NIR band traces the older stellar populations, which formed earlier and are more relaxed than the younger populations. However, we found asymmetries related to interactions in both KPG and HCG samples. In HCGs, the fraction of asymmetric galaxies is even higher than what we found in the optical. In the KPGs the interactions look like very recent events, while in the HCGs galaxies are more morphologically evolved and show properties suggesting they suffered more frequent interactions. The key difference seems to be the absence of star formation in the HCGs; while interactions produce intense star formation in the KPGs, we do not see this effect in the HCGs. This is consistent with the dry merger hypothesis (Coziol & Plauchu-Frayn 2007); the interaction between galaxies in compact groups, (CGs), is happening without the presence of gas. If the gas was spent in stellar formation (to build the bulge of the numerous early-type galaxies), then the HCGs possibly started interacting sometime before the KPGs. On the other hand, the dry interaction condition in CGs suggests that the galaxies are on merging orbits, and consequently such system cannot be that much older either. [abridge]

Shape measurement biases from underfitting and ellipticity gradients

Precision weak gravitational lensing experiments require measurements of galaxy shapes accurate to <1 part in 1000. We investigate measurement biases, noted by Voigt & Bridle and Melchior et al., that are common to shape measurement methodologies that rely upon fitting elliptical-isophote galaxy models to observed data. The first bias arises when the true galaxy shapes do not match the models being fit. We show that this ‘underfitting bias’ is due, at root, to these methods' attempts to use information at high spatial frequencies that have been destroyed by the convolution with the point spread function (PSF) and/or by sampling. We propose a new shape-measurement technique that is explicitly confined to observable regions of k-space. A second bias arises for galaxies whose ellipticity varies with radius. For most shape-measurement methods, such galaxies are subject to ‘ellipticity gradient bias’. We show how to reduce such biases by factors of 20–100 within the new shape-measurement method. The resulting shear estimator has multiplicative errors <1 part in 103 for high signal-to-noise ratio images, even for highly asymmetric galaxies. Without any training or recalibration, the new method obtains Q= 3000 in the GREAT08 Challenge of blind shear reconstruction on low-noise galaxies, several times better than any previous method.

The effect of gas fraction on the morphology and time-scales of disc galaxy mergers

Gas-rich galaxy mergers are more easily identified by their disturbed morphologies than mergers with less gas. Because the typical gas fraction of galaxy mergers is expected to increase with redshift, the under-counting of low gas-fraction mergers may bias morphological estimates of the evolution of galaxy merger rate. To understand the magnitude of this bias, we explore the effect of gas fraction on the morphologies of a series of simulated disc galaxy mergers. With the resulting g-band images, we determine how the time-scale for identifying major and minor galaxy mergers via close projected pairs and quantitative morphology (the Gini coefficient G, the second-order moment of the brightest 20 per cent of the light M20 and asymmetry A) depends on baryonic gas fraction fgas. Strong asymmetries last significantly longer in high gas-fraction mergers of all mass ratios, with time-scales ranging from ≤300 Myr for fgas∼ 20 per cent to ≥1 Gyr for fgas∼ 50 per cent. Therefore, the strong evolution with redshift observed in the fraction of asymmetric galaxies may reflect evolution in the gas properties of galaxies rather than the global galaxy merger rate. On the other hand, the time-scale for identifying a galaxy merger via G–M20 is weakly dependent on gas fraction (∼200–400 Myr), consistent with the weak evolution observed for G–M20 mergers.

The Millennium Galaxy Catalogue: The Connection between Close Pairs and Asymmetry; Implications for the Galaxy Merger Rate

We compare the use of galaxy asymmetry and pair proximity for measuring galaxy merger fractions and rates for a volume limited sample of 3184 galaxies with � 21 < MB � 5 log h < � 18 mag. and 0:010 < z < 0:123 drawn from the Millenium Galaxy Catalogue. Our findings are the following: (1) Galaxies in close pairs are generally more asymmetric than isolated galaxies and the degree of asymmetry increases for closer pairs. At least 35% of close pairs (with projected separation of less than 20 h � 1 kpc and velocity difference of less than 500 km s � 1 ) show significant asymmetry and are therefore likely to bephysically bound.(2) Among asymmetric galaxies, wefindthat at least 80% are either interacting systems or merger remnants. However, a significant fraction of galaxies initially identified as asymmetric are contaminated by nearby stars or are fragmented by the source extraction algorithm. Merger rates calculated via asymmetry indices need careful attention in order to remove the above sources of contamination, but are very reliable once this is carried out. (3) Close pairs and asymmetries represent two complementary methods of measuringthemergerrate.Galaxiesinclosepairsidentifyfuturemergers,occurringwithinthedynamicalfrictiontimescale, while asymmetries are sensitive to the immediate premerger phase and identify remnants. (4) The merger fraction derivedviatheclosepairfractionandasymmetriesisabout2%foramergerrateof (5:2 � 1:0) ; 10 � 4 h 3 Mpc � 3 Gyr � 1 . These results are marginally consistent with theoretical simulations (depending on the merger timescale), but imply a flat evolution of the merger rate with redshift up to z � 1. Subject headingg galaxies: interactions — galaxies: structure Online material: color figures

AHubble Space TelescopeSnapshot Survey of Dynamically Close Galaxy Pairs in the CNOC2 Redshift Survey

We compare the structural properties of two classes of galaxies at intermediate redshift: those in dynamically close galaxy pairs, and those that are isolated. Both samples are selected from the Canadian Network for Observational Cosmology field galaxy redshift survey (CNOC2) and have redshifts in the range 0.1 < z < 0.6. Hubble Space Telescope WFPC2 images were acquired as part of a snapshot survey and were used to measure bulge fraction and asymmetry for these galaxies. We find that paired and isolated galaxies have identical distributions of bulge fractions. Conversely, we find that paired galaxies are much more likely to be asymmetric (RT + RA ≥ 0.13) than isolated galaxies. Assuming that half of these pairs are unlikely to be close enough to merge, we estimate that 40% ± 11% of merging galaxies are asymmetric, compared with 9% ± 3% of isolated galaxies. The difference is even more striking for strongly asymmetric (RT + RA ≥ 0.16) galaxies: 25% ± 8% for merging galaxies versus 1% ± 1% for isolated galaxies. We find that strongly asymmetric paired galaxies are very blue, with rest-frame B-R colors close to 0.80, compared with a mean (B-R)0 of 1.24 for all paired galaxies. In addition, asymmetric galaxies in pairs have strong [O II] λ3727 emission lines. We conclude that close to half of the galaxy pairs in our sample are in the process of merging and that most of these mergers are accompanied by triggered star formation.