Galaxy Pairs Research Articles

Defeating stochasticity: coalescence time-scales of massive black holes in galaxy mergers

ABSTRACT The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three orders of magnitude. To understand this variation, we simulate the merger of equal-mass, eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We model the formation and hardening of a central BHB using the fast multiple method as a force solver, which features a O(N) scaling with the number N of particles and obtains results equivalent to direct-summation simulations. At N ∼ 5 × 105, typical for contemporary studies, the eccentricity of the BHBs can vary significantly for different random realizations of the same initial condition, resulting in a substantial variation of the merger time-scale. This scatter owes to the stochasticity of stellar encounters with the BHB and decreases with increasing N. We estimate that N ∼ 107 within the stellar half-light radius suffices to reduce the scatter in the merger time-scale to ∼10 per cent. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.

ALMA [N ii] 205 μm Imaging Spectroscopy of the Lensed Submillimeter Galaxy ID 141 at Redshift 4.24

Abstract We present an Atacama Large Millimeter/submillimeter Array observation of the Submillimeter galaxy ID 141 at z = 4.24 in the [N ii] 205 μm line (hereafter [N ii] ) and the underlying continuum at (rest-frame) 197.6 μm. Benefiting from lensing magnification by a galaxy pair at z = 0.595, ID 141 is one of the brightest SMGs. At the angular resolutions of –1.″5 (1″ ∼6.9 kpc), our observation clearly separates and moderately resolves the two lensed images in both continuum and line emission at a signal-to-noise ratio >5. Our continuum-based lensing model implies an averaged amplification factor of ∼5.8 and reveals that the delensed continuum image has a Sérsic index and Sérsic radius of ∼0.″18 (∼1.24 kpc). Furthermore, the reconstructed [N ii] velocity field in the source plane is dominated by a rotation component with a maximum velocity of ∼300 km s−1 at large radii, indicating a dark matter halo mass of ∼10 . This, together with the reconstructed velocity dispersion field being smooth and modest in value (<100 km s−1) over much of the outer parts of the galaxy, favors the interpretation of ID 141 being a disk galaxy dynamically supported by rotation. The observed [N ii] /CO (7−6) and [N ii] /[C ii] 158 μm line-luminosity ratios, which are consistent with the corresponding line ratio versus far-infrared color correlation from local luminous infrared galaxies, imply a delensed star formation rate of (1.8 yr−1 and provide an independent estimate of the size of the star-forming region kpc in radius.

Larger λR in the disc of isolated active spiral galaxies than in their non-active twins

We present a comparison of the spin parameter λR, measured in a region dominated by the galaxy disc, between 20 pairs of nearby (0.005 < z < 0.03) seemingly isolated twin galaxies differing in nuclear activity. We find that 80−82% of the active galaxies show higher values of λR than their corresponding non-active twin(s), indicating larger rotational support in the active galactic nuclei (AGN) discs. This result is driven by the 11 pairs of unbarred galaxies, for which 100% of the AGN show larger λR than their twins. These results can be explained by a more efficient angular momentum transfer from the inflowing gas to the disc baryonic matter in the case of the active galaxies. This gas inflow could have been induced by disc or bar instabilities, although we cannot rule out minor mergers if these are prevalent in our active galaxies. This result represents the first evidence of galaxy-scale differences between the dynamics of active and non-active isolated spiral galaxies of intermediate stellar masses (1010 < M* < 1011 M⊙) in the Universe.

Lopsided Satellite Distributions around Isolated Host Galaxies

Abstract We investigate the spatial distribution of the satellites of bright, isolated host galaxies. In agreement with previous studies, we find that, on average, the satellites of red hosts are found preferentially close to their hosts’ major axes, while the satellites of blue hosts are distributed isotropically. We compute the pairwise clustering of the satellites and find a strong tendency for pairs of satellites to be located on the same side of their host, resulting in lopsided spatial distributions. The signal is most pronounced for the satellites of blue hosts, where the number of pairs on the same side of their host exceeds the number of pairs on opposite sides of their host by a factor of 1.8 ± 0.1. For the satellites of red hosts, the number of pairs on the same side of their host exceeds the number of pairs on opposite sides of their host by a factor of 1.08 ± 0.03. Satellites that are far from their hosts (r p ≳ 300 kpc) show a strong preference for being located on the same side of their hosts; satellites that are near to their hosts (r p ≲ 100 kpc) show a weak preference for being located on opposite sides of their hosts. While lopsided distributions have been found previously for the satellites of bright pairs of galaxies, ours is the first study to find lopsided distributions for the satellites of isolated bright galaxies.

Is this an early stage merger? A case study on molecular gas and star formation properties of Arp 240

ABSTRACT We present new high-resolution 12CO J = 1–0, J = 2–1, and 13CO J = 1–0 maps of the early stage merger Arp 240 (NGC 5257/8) obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Simulations in the literature suggest that the merger has just completed its first passage; however, we find that this system has a lower global gas fraction but a higher star formation efficiency (SFE) compared to typical close galaxy pairs, which suggests that this system may already be in an advanced merger stage. We combine the ALMA data with 12CO J = 3–2 observations from the Submillimeter Array and carry out RADEX modelling on several different regions. Both, the RADEX modelling and a local thermal equilibrium (LTE) analysis show that the regions are most likely to have a CO-to-H2 conversion factor αCO close to or perhaps even smaller than the typical value for (ultra)luminous infrared galaxies. Using 33-GHz data from the Very Large Array to measure the star formation rate, we find that most star-forming regions have molecular gas depletion times of less than 100 Myr. We calculated the SFE per free-fall time for different regions and find some regions appear to have values greater than 100 per cent. We find these regions generally show evidence for young massive clusters (YMCs). After exploring various factors, we argue that this is mainly due to the fact that radio continuum emission in those regions is dominated by that from YMCs, which results in an overestimate of the SFE per free-fall time.

A systematic search for galaxy proto-cluster cores at z ∼ 2

ABSTRACT A proto-cluster core is the most massive dark matter halo (DMH) in a given proto-cluster. To reveal the galaxy formation in core regions, we search for proto-cluster cores at z ∼ 2 in ${\sim}1.5\, \mathrm{deg}^{2}$ of the COSMOS field. Using pairs of massive galaxies [log (M*/M⊙) ≥ 11] as tracers of cores, we find 75 candidate cores, among which 54 per cent are estimated to be real. A clustering analysis finds that these cores have an average DMH mass of $2.6_{-0.8}^{+0.9}\times 10^{13}\, \mathrm{M}_{\odot }$, or $4.0_{-1.5}^{+1.8}\, \times 10^{13} \, \mathrm{M}_{\odot }$ after contamination correction. The extended Press–Schechter model shows that their descendant mass at z = 0 is consistent with Fornax-like or Virgo-like clusters. Moreover, using the IllustrisTNG simulation, we confirm that pairs of massive galaxies are good tracers of DMHs massive enough to be regarded as proto-cluster cores. We then derive the stellar mass function (SMF) and the quiescent fraction for member galaxies of the 75 candidate cores. We find that the core galaxies have a more top-heavy SMF than field galaxies at the same redshift, showing an excess at log (M*/M⊙) ≳ 10.5. The quiescent fraction, $0.17_{-0.04}^{+0.04}$ in the mass range 9.0 ≤ log (M*/M⊙) ≤ 11.0, is about three times higher than that of field counterparts, giving an environmental quenching efficiency of $0.13_{-0.04}^{+0.04}$. These results suggest that stellar mass assembly and quenching are accelerated as early as z ∼ 2 in proto-cluster cores.

The Cosmic Ultraviolet Baryon Survey (CUBS) – I. Overview and the diverse environments of Lyman limit systems at z < 1

ABSTRACT We present initial results from the Cosmic Ultraviolet Baryon Survey (CUBS). CUBS is designed to map diffuse baryonic structures at redshift z ≲ 1 using absorption-line spectroscopy of 15 UV-bright QSOs with matching deep galaxy survey data. CUBS QSOs are selected based on their NUV brightness to avoid biases against the presence of intervening Lyman limit systems (LLSs) at zabs < 1. We report five new LLSs of $\log \, N({\mathrm{ H} \,{\small I}})/{{\rm cm^{-2}}}\gtrsim 17.2$ over a total redshift survey path-length of $\Delta \, z_{\mathrm{ LL}}=9.3$, and a number density of $n(z)=0.43_{-0.18}^{+0.26}$. Considering all absorbers with $\log \, N({{\mathrm{ H} \,{\small I}}})/{{\rm cm^{-2}}}\gt 16.5$ leads to $n(z)=1.08_{-0.25}^{+0.31}$ at zabs < 1. All LLSs exhibit a multicomponent structure and associated metal transitions from multiple ionization states such as C ii, C iii, Mg ii, Si ii, Si iii, and O vi absorption. Differential chemical enrichment levels as well as ionization states are directly observed across individual components in three LLSs. We present deep galaxy survey data obtained using the VLT-MUSE integral field spectrograph and the Magellan Telescopes, reaching sensitivities necessary for detecting galaxies fainter than $0.1\, L_*$ at d ≲ 300 physical kpc (pkpc) in all five fields. A diverse range of galaxy properties is seen around these LLSs, from a low-mass dwarf galaxy pair, a co-rotating gaseous halo/disc, a star-forming galaxy, a massive quiescent galaxy, to a galaxy group. The closest galaxies have projected distances ranging from d = 15 to 72 pkpc and intrinsic luminosities from ${\approx} 0.01\, L_*$ to ${\approx} 3\, L_*$. Our study shows that LLSs originate in a variety of galaxy environments and trace gaseous structures with a broad range of metallicities.

An [O iii] search for extended emission around AGN with H i mapping: a distant cloud ionized by Mkn 1

ABSTRACT Motivated by the discovery of large ionized clouds around AGN, and particularly the large fraction of those that are consistent with photoionized gaseous tidal debris, we searched for [O iii] emission around Seyfert galaxies previously mapped in H i, many with extended gas features. Of 26 Seyfert galaxies, we find one spatially extended emission feature, a discrete cloud projected ≈12 kpc SW from the centre of Mkn 1 and spanning a transverse extent of 8 kpc. Optical spectroscopy (Kast/Lick and SCORPIO/BTA) of this cloud confirms its association with the Mkn 1–NGC 451 galaxy pair, closely matching the kinematics of nearby H i structures, and reveals emission-line ratios requiring photoionization by the AGN at roughly the direct observed luminosity of the nucleus. For the entire sample, the full opening angle of the ionization cones (bicones) must be <20° if the AGNs are continuously bright for scales longer than the light-traveltimes to the H i structures. Since typical AGN ionization cones are observed to be much broader than this, our low detection fraction may add to evidence for the ubiquity of strong variations in AGN luminosity on scales 104–105 yr.

Supernova Siblings: Assessing the Consistency of Properties of Type Ia Supernovae that Share the Same Parent Galaxies

Abstract While many studies have shown a correlation between properties of the light curves of SNe Ia and properties of their host galaxies, it remains unclear what is driving these correlations. We introduce a new direct method to study these correlations by analyzing “parent” galaxies that host multiple SNe Ia “siblings.” Here, we search the Dark Energy Survey SN sample, one of the largest samples of discovered SNe, and find eight galaxies that hosted two likely SNe Ia. Comparing the light-curve properties of these SNe and recovered distances from the light curves, we find no better agreement between properties of SNe in the same galaxy as any random pair of galaxies, with the exception of the SN light-curve stretch. We show at 2.8σ significance that at least one-half of the intrinsic scatter of SNe Ia distance modulus residuals is not from common host properties. We also discuss the robustness with which we could make this evaluation with LSST, which will find 100× more pairs of galaxies, and pave a new line of study on the consistency of SNe Ia in the same parent galaxies. Finally, we argue that it is unlikely that some of these SNe are actually single, lensed SN with multiple images.

A Pair of Tidally Disrupted Galaxies from The Hubble Legacy Archive

Abstract We describe archival, serendipitously obtained Hubble Space Telescope (HST) observations of a tidally disturbed galaxy pair (AM 0436–472 NED01/LEDA 503707) located at z = 0.058 (240 Mpc). The system likely belongs to peculiar ring galaxies of Solitaire type. We also analyze an archival optical spectrum of AM 0436–472 NED01 and report an upper limit in X-rays (0.2–10 keV).

Properties of the environment around active galactic nucleus / luminous galaxy pairs through the HSC wide survey

Abstract We investigated the properties of active galactic nucleus (AGN) environments, particularly environments where the association of a luminous galaxy (LG) is found within 4 Mpc from an AGN with redshift 0.8–1.1. For comparison, three additional AGN environments, (namely, AGNs of all types, type 1 AGNs with X-ray and/or radio detection, and type 2 AGNs) and an environment of blue M*, the characteristic luminosity of the Schechter function, galaxies were investigated. The cross-correlation function with the surrounding galaxies was measured and compared between the AGN and blue galaxy samples. We also compared the distributions of color, absolute magnitude, and stellar mass of the galaxies around such target objects. The properties of clusters detected using surrounding galaxies selected based on a photometric redshift were examined and compared for different samples. The target AGNs were drawn from the Million Quasars (MILLIQUAS) catalog, and the blue galaxies were drawn from six redshift survey catalogs (SDSS, WiggleZ, DEEP2, VVDS, VIPERS, and PRIMUS). The galaxies used as a measure of the environment around the targets were drawn from the S18a internal data released by the Hyper Suprime-Cam Subaru Strategic Program. We found that, among the five AGN and blue galaxy samples considered, the environment of AGN–LG pairs is the most enriched with luminous galaxies. We also found an enhancement in the number of mass-selected clusters in the AGN–LG pair sample against those in the other samples. The results obtained in this study indicate that existence of multiple clusters is the major driver in the association of AGNs and LGs, rather than a single large-mass dark matter halo hosting the AGN.

Weak lensing measurement of filamentary structure with the SDSS BOSS and Subaru Hyper Suprime-Cam data

ABSTRACT We report the weak lensing measurement of filaments between Sloan Digital Sky Survey (SDSS) III/Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxy pairs at z ∼ 0.55, using the Subaru Hyper Suprime-Cam (HSC) first-year galaxy shape catalogue. Despite of the small overlap of 140 deg2 between these surveys, we detect the filament lensing signal at 3.9σ significance, which is the highest signal-to-noise lensing measurement of filaments between galaxy-scale haloes at this redshift range. We derive a theoretical prediction and covariance using mock catalogues based on full-sky ray-tracing simulations. We find that the intrinsic scatter of filament properties and the fluctuations in a large-scale structure along the line of sight are the primary component of the covariance and the intrinsic shape noise from source galaxies no longer limits our lensing measurement. This fact demonstrates the statistical power of the HSC survey due to its deep observations and high number density of source galaxies. Our result is consistent with the theoretical prediction and supports the ‘thick’ filament model. As the HSC survey area increases, we will be able to study detailed filament properties such as the dark matter distributions and redshift evolution of filaments.

Numerical simulations of colliding jets in an external wind: application to 3C 75

ABSTRACT The radio galaxy 3C 75 is remarkable because it contains a pair of radio-loud active galaxies, each of which produces a two-sided jet, with the jet beams appearing to collide and merge to the west of the galaxies. Motivated by 3C 75, we have conducted three-dimensional hydrodynamic simulations of jet collisions. We have extended previous studies by modelling the physical properties of the cluster atmosphere, including an external wind, and using realistic jet powers obtained from observational data. We are able to produce a morphology similar to that of 3C 75. The simulations imply that direct contact between the bulk jet flows on the west of the source is required to produce a morphology consistent with 3C 75. We quantify how the merging jets decelerate, how the wind deflects the jets and cocoons, the entrainment of intra-cluster material into the cocoons, the cocoon energetics, and how the jet interactions generate enstrophy. By comparing simulations of pairs of two-sided jets with those of single two-sided sources, we determine how the interaction between two bipolar jets changes their evolution. The unprecedented sensitivity and angular resolution of upcoming observatories will lead to the detection of many more complex sources at high redshift, where interacting jets are expected to be more numerous. The morphology of these complex sources can provide significant insight into the conditions in their environments.

Galaxy pairs in the Sloan Digital Sky Survey – XIV. Galaxy mergers do not lie on the fundamental metallicity relation

ABSTRACT In recent observational studies, star-forming galaxies have been shown to follow a relation often dubbed the fundamental metallicity relation (FMR). This relation links the stellar mass of a galaxy with its star formation rate (SFR) and its gas-phase metallicity. Specifically, the FMR predicts that galaxies, at a given stellar mass, exhibit lower metallicities for higher SFRs. This trend is qualitatively consistent with observations of galaxy pairs, which have been robustly shown to experience increasing gas-phase metallicity dilution and enhanced star formation activity with decreasing projected separation. In this work, we show that, despite the qualitative consistency with FMR expectations, the observed O/H dilution in galaxy pairs of the Sloan Digital Sky Survey is stronger than what is predicted by the FMR. We conclude that the evolutionary phase of galaxies interacting with companions is not encoded in the FMR, and thus, mergers constitute a clearly defined population of outliers. We find that galaxies in pairs are consistent with the FMR only when their separation is larger than 110 kpc. Finally, we also quantify the local environment of the pairs using the number of galaxy neighbours within 2 Mpc, N2, and the projected separation to the second closest galaxy, r2. We find that pairs are more sensitive to a second companion than to the local galaxy density, displaying less elevated SFRs with smaller values of r2.

Observing the Effects of Galaxy Interactions on the Circumgalactic Medium

Abstract We continue our empirical study of the emission line flux originating in the cool (T ∼ 104 K) gas that populates the halos of galaxies and their environments. Specifically, we present results obtained for a sample of galaxy pairs with a range of projected separations, , and mass ratios <1:5, intersected by 5443 Sloan Digital Sky Survey lines of sight at projected radii of 10–50 kpc from either or both of the two galaxies. We find significant enhancement in Hα emission and a moderate enhancement in [N ii]6583 emission for low-mass pairs (mean stellar mass per galaxy, ) relative to the results from a control sample. This enhanced Hα emission comes almost entirely from sight lines located between the galaxies, consistent with a short-term, interaction-driven origin for the enhancement. We find no enhancement in Hα emission, but significant enhancement in [N ii]6583 emission for high-mass ( ) pairs. Furthermore, we find a dependence of the emission line properties on the galaxy pair mass ratio such that those with a mass ratio below 1:2.5 have enhanced [N ii]6583 and those with a mass ratio between 1:2.5 and 1:5 do not. In all cases, departures from the control sample are only detected for close pairs (S p < 100 kpc). Attributing an elevated [N ii]6583/Hα ratio to shocks, we infer that shocks play a role in determining the circumgalactic medium properties for close pairs that are among the more massive and have mass ratios closer to 1:1.

Hα KINEMATICS OF THE ISOLATED INTERACTING GALAXY PAIR KPG 486 (NGC 6090)

In optical images, the barely studied isolated interacting galaxy pair KPG 486 (NGC 6090) displays similar features to the Antennae (NGC 4038/39) galaxy pair. To compare the distribution of ionized hydrogen gas, morphology and kinematic and dynamic behaviour of both galaxy pairs, we present observations in the Hα emission line of NGC 6090 acquired with the scanning Fabry-Perot interferometer, PUMA. For each galaxy in NGC 6090 we obtained several kinematic parameters, its velocity field and its rotation curve. We also analysed some of the perturbations induced by their encounter. We verified the consistency of our results by comparing them with kinematic results from the literature. The comparison of our results on NGC 6090 with those obtained in a previous similar kinematic analysis of the Antennae highlighted great differences between these galaxy pairs.

“Semi-Dark” Dwarf Galaxy Coma P on the Periphery of the Virgo Galaxy Cluster

Abstract—Based on archived images from the Hubble Space Telescope, stellar photometry of the dwarf galaxy Coma P, located near the Virgo galaxy cluster, was performed. The selection of weakly luminous regions of the galaxy showed that Coma P consists of two interacting dwarf galaxies, one of which has a weak star formation, and the second galaxy has a very low surface brightness, there is no star formation in it, and it can be called a “dark” galaxy. The distance to this pair of galaxies, measured by the TRGB method (D = 12.7 ± 0.9 Mpc), showed that Coma P is located on the periphery of the Virgo galaxy cluster.

SDSS-IV MaNGA: Kinematic Asymmetry as an Indicator of Galaxy Interaction in Paired Galaxies

Abstract The interaction between galaxies is believed to be the main origin of the peculiarities of galaxies, which disturbs not only the morphology but also their kinematics. These disturbed and asymmetric features are the indicators of galaxy interaction. We study the velocity field of the ionized gas of the paired galaxies in the SDSS-IV MaNGA IFU survey. Using the kinemetry package, we fit the velocity field of the ionized gas to quantify the degree of kinematic asymmetry. We find that the star formation rate (SFR) of the paired galaxies with high kinematic asymmetry is significantly enhanced even when the projected separation between the pair members is quite large ( ). On the contrary, no significant SFR enhancement is found for the paired galaxies with low kinematic asymmetry even when their projected separation is small ( ). Moreover, we also find that the fraction of galaxies with high kinematic asymmetry is much higher in close pairs ( ) than those with larger , which explains well the early statistical finding of the significant SFR enhancement in close pairs. Our new findings illustrate that the kinematic asymmetry is an excellent indicator of galaxy–galaxy interaction strength, which helps us better understand the merging stage of the observed galaxy pairs.

Outflow from Outer-arm Starburst in a Grazing Collision between Galaxies

Abstract Gemini NIFS K-band spectra and Atacama Large Millimeter/submillimeter Array 12CO , HCO+, and 100 GHz continuum observations are used to study a bright starburst clump on an outer arm of the interacting galaxy NGC 2207. This clump emits 23% of the total 24 μm flux of the galaxy pair and has an optically opaque dust cone extending out of its 170 pc core. The measured CO accounts for the dark cone extinction if almost all the gas and dust there are in front of the star clusters. An associated approaching CO outflow has v z ∼ 16 km s−1, an estimated molecular mass 8 × 106 M ⊙, and rises to heights ∼0.9 kpc. A receding CO outflow on the far side with v z ∼ 28 km s−1 is less extensive. The observed star formation in the core over 10 Myr can supply the dark cone kinetic energy of roughly 2 × 1052 erg via supernovae and stellar winds. Other signs of intense activity are a variable radio continuum, suggesting an embedded supernova or other outburst; X-ray emission possibly from an X-ray binary or intermediate-mass black hole, depending on the extinction; and Brγ and He i lines with 82 km s−1 line widths and fluxes consistent with excitation by embedded O-type stars. According to previous models, the retrograde encounter suffered by NGC 2207 caused the loss of angular momentum. This compressed its outer disk. We suggest that the resulting inward crashing stream hit a massive H i clump on the preexisting spiral arm and triggered the observed starburst.

A New Method to Constrain the Origins of Dark-matter-free Galaxies and Their Unusual Globular Clusters

Abstract We present a novel method to constrain the past collisional evolution of observed globular cluster (GC) systems, in particular their mass functions. We apply our method to a pair of galaxies hypothesized to have recently undergone an episode of violent relaxation due to a strong galaxy–galaxy interaction, namely, NGC 1052-DF2 and NGC 1052-DF4. We begin by exploring the observational evidence for a collisional origin for these two recently discovered ultradiffuse galaxies observed in the NGC 1052 group, posited in the literature to be dark matter free. We compute the timescales for infall to the central nucleus due to dynamical friction for the GCs in these galaxies, using the shortest of these times to constrain how long ago a galaxy–galaxy interaction could have occurred. We go on to quantify the initial GC numbers and densities needed for significant collisional evolution to occur within the allotted times and show that, if the hypothesis of a previous galaxy–galaxy interaction is correct, a paucity of low-mass GCs should be revealed by deeper observational surveys. If any are found, they should be more spatially extended than the currently observed GC population. Finally, we apply our method to these galaxies, in order to illustrate its efficacy in constraining their dynamical evolution. Our results motivate more complete observations of the GC luminosity functions in these galaxies, in addition to future studies aimed at combining the method presented here with a suite of numerical simulations in order to further constrain the origins of the curious GC populations in these (and other) galaxies.