Companion Galaxy Research Articles

Broad-band photometry and long-slit spectroscopy of the peculiar ring galaxy FM 287-14

Aims. We report detailed morphological and kinematical insights into the disturbed galaxy FM 287-14, which is reported in the literature as a member of strongly interacting galaxies with a clear case of mergers. The main purpose of this paper is to provide a first observational study of this double-nuclei object based on photometry and spectroscopy. Methods. The study is based on BVR photometry with the 1.6-m telescope at OPD, and long-slit spectroscopy with Gemini South’s GMOS spectrograph, with two-gratings centered at 5 011 A and 6 765 A. Image enhancement techniques were applied to the direct images, and the main lines in the spectra were used to determine the radial velocity. Line ratios and color-color diagnostic diagrams helped determine the nature of this object. Results. The external NE section is populated with well-defined H ii regions and is bluer than the SW region with less evidence of H ii regions. A double nucleus has been identified. The (B − R) × (B − V) diagram shows that the main nucleus is redder than the second one; the internal “ring-like” structure is redder than the external one. The main object redshift is z = 0.0311, corresponding to a line-of-sight velocity of 9334 km s −1 (a distance of 133.3 Mpc), and the second nucleus has a redshift of z = 0.0310, corresponding to a line-of-sight velocity of 9300 km s −1 (a distance of 132.9 Mpc). Both nuclei show characteristics of early-type galaxies, but the main one also shows modest emission evidence of the [N ii]λ6583 line. Conclusions. The peculiarities of FM 287-14 can be adequately interpreted as an ongoing merging-stage of interaction of the main object with a companion galaxy.

The formation of Local Group planes of galaxies

The confinement of most satellite galaxies in the Local Group to thin planes presents a challenge to the theory of hierarchical galaxy clustering. The PAndAS collaboration has identified a particularly thin configuration with kinematic coherence among companions of M31 and there have been long standing claims that the dwarf companions to the Milky Way lie in a plane roughly orthogonal to the disk of our galaxy. This discussion investigates the possible origins of four Local Group planes: the plane similar, but not identical to that identified by PAndAS, an adjacent slightly tilted plane, and two planes near the Milky Way: one with nearer galaxies and the other with more distant ones. Plausible orbits are found by using a combination of Numerical Action methods and a backward in time integration procedure. For M31, M33, IC10, and LeoI, solutions are found that are consistent with measurements of their proper motions. For galaxies in planes, there must be commonalities in their proper motions, and this constraint greatly limits the number of physically plausible solutions. Key to the formation of the planar structures has been the evacuation of the Local Void and consequent build-up of the Local Sheet, a wall of this void. Most of the M31 companion galaxies were born in early-forming filamentary or sheet-like substrata that chased M31 out of the void. M31 is a moving target because of its attraction toward the Milky Way, and the result has been alignments stretched toward our galaxy. In the case of the configuration around the Milky Way, it appears that our galaxy was in a three-way competition for companions with M31 and Centaurus A. Only those within a modest band fell our way. The Milky Ways' attraction toward the Virgo Cluster resulted in alignments along the Milky Way-Virgo Cluster line.

DUAL SUPERMASSIVE BLACK HOLE CANDIDATES IN THE AGN AND GALAXY EVOLUTION SURVEY

Dual supermassive black holes (SMBHs) with kiloparsec scale separations in merger-remnant galaxies are informative tracers of galaxy evolution, but the avenue for identifying them in large numbers for such studies is not yet clear. One promising approach is to target spectroscopic signatures of systems where both SMBHs are fueled as dual active galactic nuclei (AGNs), or where one SMBH is fueled as an offset AGN. Dual AGNs may produce double-peaked narrow AGN emission lines, while offset AGNs may produce single-peaked narrow AGN emission lines with line-of-sight velocity offsets relative to the host galaxy. We search for such dual and offset systems among 173 Type 2 AGNs at z<0.37 in the AGN and Galaxy Evolution Survey (AGES), and we find two double-peaked AGNs and five offset AGN candidates. When we compare these results to a similar search of the DEEP2 Galaxy Redshift Survey and match the two samples in color, absolute magnitude, and minimum velocity offset, we find that the fraction of AGNs that are dual SMBH candidates increases from z=0.25 to z=0.7 by a factor of ~6 (from 2/70 to 16/91, or 2.9% to 18%). This may be associated with the rise in the galaxy merger fraction over the same cosmic time. As further evidence for a link with galaxy mergers, the AGES offset and dual AGN candidates are tentatively ~3 times more likely than the overall AGN population to reside in a host galaxy that has a companion galaxy (from 16/173 to 2/7, or 9% to 29%). Follow-up observations of the seven offset and dual AGN candidates in AGES will definitively distinguish velocity offsets produced by dual SMBHs from those produced by narrow-line region kinematics, and will help sharpen our observational approach to detecting dual SMBHs.

A SEARCH FOR RR LYRAE STARS IN SEGUE 2 AND SEGUE 3

We present an extensive search for RR Lyrae stars in and around the ultra-faint Milky Way companions Segue 2 and Segue 3. The former (M_V = -2.5, Belokurov et al. 2009) appears to be an extremely faint dwarf galaxy companion of the Milky Way. The latter (M_V = 0.0, Fadely et al. 2011) is among the faintest star clusters known. We use B and V band time-series imaging obtained at the WIYN 0.9 meter telescope at Kitt Peak National Observatory to search for RR Lyrae in these objects. In our Segue 2 observations, we present a previously unknown fundamental mode (RRab) RR Lyrae star with a period of P_ab = 0.748 days. With this measurement, we revisit the inverse correlation between <P_ab> and <[Fe/H]> established in the literature for Milky Way dwarf galaxies and their RR Lyrae. In this context, the long period of Segue 2's RRab star as well as the known significant spread in metallicity in this dwarf galaxy are consistent with the observed trend in <P_ab> and <[Fe/H]>. We derive the first robust distance to Segue 2, using both its RRab star and spectroscopically confirmed blue horizontal branch stars. Using [Fe/H] = -2.16 and -2.44 dex, we find d_RRL = 36.6 +2.5/-2.4 and 37.7 +/- 2.7 kpc; assuming [Fe/H] = -2.257 dex, we find d_BHB = 34.4 +/- 2.6 kpc. Although no RR Lyrae were present in the Segue 3 field, we found a candidate eclipsing binary star system.

H i in HO: Hoag's Object revisited

We present new HI observations of Hoag's Object obtained with the Westerbork Synthesis Radio Telescope (WSRT). The data show that the luminous optical ring around the elliptical body has a bright HI counterpart that shares the kinematical properties of the optical ring. The entire HI structure is twice as large as the optical ring and shows a mild warp in its outer regions relative to the inner ring. We detect two additional HI sources close in redshift to that of Hoag's Object, and report on a newly identified SDSS optical companion galaxy. The HI sources are ~0.3 and ~1 Mpc away in projected distance, and the companion galaxy is also ~1 Mpc away. Our main conclusion is that the HI detected in Hoag's Object shows no indication that this galaxy has experienced a recent (less than ~1 Gyr ago) accretion event. At least one of the two additional HI detected objects does not have an optical counterpart. One possibility is that this object is an HI filament left over from an interaction shaping Hoag's Object, in which case this interaction must also have occurred at least 1-2 Gyr ago.

Growth of brightest cluster galaxies via mergers since z=1

Hierarchical assembly within clusters of galaxies is tied directly to the evolution of the Brightest Cluster Galaxies (BCGs), which dominate the stellar light in the centres of rich clusters. In this paper we investigate the number of mergers onto BCGs in 14 X-ray selected clusters over the redshift range 0.8 < z < 1.4 using HST imaging data. We find significant differences in the numbers of companion galaxies to BCGs between the clusters in our sample indicating that BCGs in similar mass clusters can have very different merging histories. Within a 50 kpc radius around the BCGs we find an average of 6.45 \pm 1.15 companion galaxies with mass ratios (companion:BCG) between 1:1 and 1:20. The infalling companions show a 50/50 split between major (1:1 - 1:2) and minor (1:3 - 1:20) mergers. When compared to similar work using lower redshift clusters, these results demonstrate that both major and minor merging was more common in the past. Since the dynamical timescales for merging onto the BCG are relatively short compared with the look-back time to z ~ 1 our results suggest that the BCG stellar mass may increase by as much as 1.8 times since z = 1. However the growth rate of BCGs will be substantially less if stripped material from nearby companions ends up in the diffuse intracluster light.

VGS31b: a highly inclined ring along a filament in a void. Implication for the cold accretion

VGS31b is a highly-inclined ring galaxy found along a filament in a void (Kreckel et al. 2012). Detailed photometry, by using u, g, r, i, z SDSS images, has shown that the overall morphology of VGS31b is very tricky, due to (i) the presence of a highly inclined (72 deg) ring-like structure, which reaches the galaxy center tracing a "spiral-like" pattern, (ii) a one sided tail towards North-East and (iii) a bar in the central regions (Beygu et al. 2013). Such structure is reasonably the result of a "second event" in the evolution history of this galaxy, which could be a gravitational interaction with a companion galaxy or with the environment. The main aim of the present work is to address the most reliable formation scenario for this object, by comparing the observed properties, i.e. structure, baryonic mass, kinematics and chemical abundances, with the theoretical predictions. In particular, we have used archival spectroscopic data, to derive the metallicity in the ring: we found a very low, sub-solar average value of Z = 0.3 Zsun, comparable with other polar ring/disk galaxies, but lower than those measured for ordinary spirals of similar luminosity. The study of the chemical abundances in polar ring/disk galaxies and related objects has received an increasing attention in recent years, since it has revealed to be a key-parameter to disentangle among the formation scenarios suggested for this class of objects: major merging, tidal accretion or cold accretion. In the present work we check the cold accretion of gas through a "cosmic filament" as a possible scenario for the formation of the ring-like structure in VGS31b.

Study of Possible Local Quasars and Search for a Quasar-Stellar Connection. III. A Sample of 341 QSOs

A new sample of 341 possible local QSOs are presented, which are in the vicinity of low redshift galaxies. Physical characteristics of the sample quasars are determined and previously reported relations are confirmed: density - redshift, abso- lute mag - radius, absolute mag - mass, mass - radius, mass - luminosity, and mass - density. These relations seem to sup- port the basic assumptions that quasars are single, compact objects, with dimensions close to their respective gravitational ra- dius. Redshifts of quasars are dominated by a gravitational redshift component, and the gravitational redshifts seem to be quantized according to the Karlsson - sequence. Evidence is found in favour of the Arp's evolutionary scenario: QSOs are ejected from their respective parent galaxy and evolve as they recede, building new small mass companion galaxies. Evidence is found that in the course of evolution the quasar density and redshift decrease, while dimensions and luminosity increase. Relation luminosity - density is found in the sense that more dense quasars are less luminous. Simple linear density equation is found, which seems to apply to quasars, but also to stars and may be even to planets, providing a possible link between these seemingly very different structures in the Universe. Evidence is found of possible increase of quasar mass and luminosity with increasing distance to about z=0.03. The physics behind all these processes and relations remain unclear, but a revival of the old Ambartsumian's hypotheses seems possible, suggesting disintegra- tion of an unknown primordial dense matter. The properties of quasars may invoke the need for deeper changes in our cur- rent theories. Most fascinating seems the possibility that a link may exist between quasars and stars.

HUBBLE SPACE TELESCOPEOBSERVATIONS OF DUSTY FILAMENTS IN HERCULES A: EVIDENCE FOR ENTRAINMENT

We present U, V, and I-band images of the host galaxy of Hercules A (3C 348) obtained with HST/WFC3/UVIS. We find a network of dusty filaments which are more complex and extended than seen in earlier HST observations. The filaments are associated with a faint blue continuum light (possibly from young stars) and faint H-alpha emission. It seems likely that the cold gas and dust has been stripped from a companion galaxy now seen as a secondary nucleus. There are dusty filaments aligned with the base of the jets on both eastern and western sides of the galaxy. The morphology of the filaments is different on the two sides - the western filaments are fairly straight, while the eastern filaments are mainly in two loop-like structures. We suggest that despite the difference in morphologies, both sets of filaments have been entrained in a slow moving boundary layer outside the relativistic flow. As suggested by Fabian et al. (2008), magnetic fields in the filaments may stabilize them against disruption. We consider a speculative scenario to explain the relation between the radio source and the shock and cavities in the hot ICM seen in the Chandra data (Nulsen et al. 2005). We suggest the radio source originally (~60 Myr ago) propagated along a position angle of ~35 degrees where it created the shock and cavities. The radio source axis changed to its current orientation (~100 degrees) possibly due to a supermassive black hole merger and began its current epoch of activity about 20 Myr ago.

The environment of HII galaxies revisited

We present a study of the close (< 200 kpc) environment of 110 relatively local (z< 0.16) HII galaxies, selected from the Sloan Digital Sky Survey (SDSS; DR7). We use available spectroscopic and photometric redshifts in order to investigate the presence of a close and possibly interacting companion galaxy. Our aim is to compare the physical properties of isolated and interacting HII galaxies and investigate possible systematic effects in their use as cosmological probes. We find that interacting HII galaxies tend to be more compact, less luminous and have a lower velocity dispersion than isolated ones, in agreement with previous studies on smaller samples. However, as we verified, these environmental differences do not affect the cosmologically important L_{H{\beta}}-{\sigma} correlation of the HII galaxies.

Star formation in galaxies: the role of spiral arms

AbstractStudying star formation in spiral arms tells us not only about the evolution of star formation, and molecular clouds, but can also tell us about the nature of spiral structure in galaxies. I will address both these topics using the results of recent simulations and observations. Galactic scale simulations are beginning to examine in detail the evolution of Giant Molecular Clouds (GMC) as they form in spiral arms, and then disperse by stellar feedback or shear. The overall timescale for this process appears comparable to the crossing time of the GMCs, a few Myrs for 105 M⊙ clouds, 20 Myr or so for more massive GMCs. Both simulations and observations show that the massive clouds are found in the spiral arms, likely as a result of cloud-cloud collisions. Simulations including stars should also tell us about the stellar age distribution in GMCs, and across spiral arms. More generally, recent work on spiral galaxies suggests that the dynamics of gas flows in spiral arms are different in longlived and transient spiral arms, resulting in different age patterns in the stars. Such results could be used to help establish the main driver of spiral structure in the Milky Way (Toomre instabilities, the bar, or nearby companion galaxies) in conjunction with future surveys.

Galactic interaction as the trigger for the young radio galaxy MRC B1221−423★

Mergers between a massive galaxy and a small gas-rich companion (minor mergers) have been proposed as a viable mechanism for triggering radio emission in an active galaxy. Until now the problem has been catching this sequence of events as they occur. With MRC B1221$-$423 we have an active radio galaxy that has only recently been triggered, and a companion galaxy that provides the "smoking gun". Using spectroscopic data taken with the VIMOS Integral Field Unit detector on the European Southern Observatory's Very Large Telescope, we have examined the distribution, ionization state and kinematics of ionized gas in this interacting system. We have also modelled the stellar continuum with synthesised spectra of stellar populations of different ages. From our study of the ionized gas, we have derived preliminary models for the geometry of the interaction, analysed the kinematic behaviour of the ionized gas, and examined the ionization mechanisms at work throughout the system. Our modelling of the stellar continuum allowed us to identify and date distinct stellar populations within the galaxy pair. We find evidence of multiple episodes of widespread starburst activity, and by dating these populations, we provide tentative insight into the history of the interaction.

ENHANCED OFF-CENTER STELLAR TIDAL DISRUPTIONS BY SUPERMASSIVE BLACK HOLES IN MERGING GALAXIES

Off-center stellar tidal disruption flares have been suggested to be a powerful probe of recoiling supermassive black holes (SMBHs) out of galactic centers due to anisotropic gravitational wave radiations. However, off-center tidal flares can also be produced by SMBHs in merging galaxies. In this paper, we computed the tidal flare rates by dual SMBHs in two merging galaxies before the SMBHs become self-gravitationally bounded. We employ an analytical model to calculate the tidal loss-cone feeding rates for both SMBHs, taking into account two-body relaxation of stars, tidal perturbations by the companion galaxy, and chaotic stellar orbits in triaxial gravitational potential. We show that for typical SMBHs with mass 10^7 M_\sun, the loss-cone feeding rates are enhanced by mergers up to \Gamma ~ 10^{-2} yr^{-1}, about two order of magnitude higher than those by single SMBHs in isolated galaxies and about four orders of magnitude higher than those by recoiling SMBHs. The enhancements are mainly due to tidal perturbations by the companion galaxy. We suggest that off-center tidal flares are overwhelmed by those from merging galaxies, making the identification of recoiling SMBHs challenging. Based on the calculated rates, we estimate the relative contributions of tidal flare events by single, binary, and dual SMBH systems during cosmic time. Our calculations show that the off-center tidal disruption flares by un-bound SMBHs in merging galaxies contribute a fraction comparable to that by single SMBHs in isolated galaxies. We conclude that off-center tidal disruptions are powerful tracers of the merging history of galaxies and SMBHs.

Galaxy And Mass Assembly (GAMA): the life and times of L★ galaxies

In this work we investigate in detail the effects local environment (groups and pairs) has on galaxies with stellar mass similar to the Milky-Way (L* galaxies). A volume limited sample of 6,150 galaxies is classified to determine emission features, morphological type and presence of a disk. This sample allows for characteristics of galaxies to be isolated (e.g. stellar mass and group halo mass), and their codependencies determined. We observe that galaxy-galaxy interactions play the most important role in shaping the evolution within a group halo, the main role of halo mass is in gathering the galaxies together to encourage such interactions. Dominant pair galaxies find their overall star formation enhanced when the pair's mass ratio is close to 1, otherwise we observe the same galaxies as we would in an unpaired system. The minor galaxy in a pair is greatly affected by its companion galaxy, and whilst the star forming fraction is always suppressed relative to equivalent stellar mass unpaired galaxies, it becomes lower still when the mass ratio of a pair system increases. We find that, in general, the close galaxy-galaxy interaction rate drops as a function of halo mass for a given amount of stellar mass. We find evidence of a local peak of interactions for Milky-Way stellar mass galaxies in Milky-Way halo mass groups. Low mass halos, and in particular Local Group mass halos, are an important environment for understanding the typical evolutionary path of a unit of stellar mass. We find compelling evidence for galaxy conformity in both groups and pairs, where morphological type conformity is dominant in groups, and emission class conformity is dominant in pairs. This suggests that group scale conformity is the result of many galaxy encounters over an extended period of time, whilst pair conformity is a fairly instantaneous response to a transitory interaction.

Deep multiband surface photometry on a sample of 24 blue compact galaxies – I

[Abridged] We present deep optical and near-infrared UBVRIHKs imaging data for 24 blue compact galaxies (BCGs). The sample contains luminous dwarf and intermediate-mass BCGs which are predominantly metal-poor, although a few have near-solar metallicities. We have analyzed isophotal and elliptical integration surface brightness and color profiles, extremely deep (mu_B<29 mag arcsec^{-2}) contour maps and RGB images for each galaxy in the sample. The colors are compared to different spectral evolutionary models. We detect extremely extended low surface brightness (LSB) components dominant beyond the Holmberg radius as well as optical bridges between companion galaxies at the mu_V~28th mag arcsec^{-2} isophotal level. The central surface brightness mu_0 and scale length h_r are derived from two radial ranges typically assumed to be dominated by the underlying host galaxy. We find that mu_0 and h_r of the BCGs host deviate from those of dwarf ellipticals (dE) and dwarf irregulars (dI) solely due to a strong burst contribution to the surface brightness profile almost down to the Holmberg radius. Structural parameters obtained from a fainter region, mu_B=26-28 mag arcsec^{-2}, are consistent with those of true LSB galaxies for the starbursting BCGs in our sample, and with dEs and dIs for the BCGs with less vigorous star formation.

THE AFTERGLOW AND ULIRG HOST GALAXY OF THE DARK SHORT GRB 120804A

We present the optical discovery and sub-arcsecond optical and X-ray localization of the afterglow of the short GRB 120804A, as well as optical, near-IR, and radio detections of its host galaxy. X-ray observations with Swift/XRT, Chandra, and XMM-Newton to ~19 d reveal a single power law decline. The optical afterglow is faint, and comparison to the X-ray flux indicates that GRB 120804A is "dark", with a rest-frame extinction of A_V~2.5 mag (at z~1.3). The intrinsic neutral hydrogen column density inferred from the X-ray spectrum, N_H~2x10^22 cm^-2, is commensurate with the large extinction. The host galaxy exhibits red optical/near-IR colors. Equally important, JVLA observations at 0.9-11 d reveal a constant 5.8 GHz flux density and an optically-thin spectrum, unprecedented for GRB afterglows, but suggestive instead of emission from the host galaxy. The optical/near-IR and radio fluxes are well fit with the scaled spectral energy distribution of the local ultra-luminous infrared galaxy (ULIRG) Arp 220 at z~1.3, with a resulting star formation rate of ~300 Msun/yr. The inferred extinction and small projected offset (2.2+/-1.2 kpc) are also consistent with the ULIRG scenario, as is the presence of a companion galaxy at a separation of about 11 kpc. The limits on radio afterglow emission, in conjunction with the observed X-ray and optical emission, require a circumburst density of ~10^-3 cm^-3 an isotropic-equivalent energy scale of E_gamma,iso ~ E_K,iso ~ 7x10^51 erg, and a jet opening angle of >8 deg. The expected fraction of luminous infrared galaxies in the short GRB host sample is ~0.01-0.3 (for pure stellar mass and star formation weighting, respectively). Thus, the observed fraction of 2 events in about 25 hosts (GRBs 120804A and 100206A), provides additional support to our previous conclusion that short GRBs track both stellar mass and star formation activity.

The puzzling merging cluster Abell 1914: new insights from the kinematics of member galaxies

We analyze the dynamical state of Abell 1914, a merging cluster hosting a radio halo, quite unusual for its structure. Our study considers spectroscopic data for 119 galaxies obtained with the Italian Telescopio Nazionale Galileo. We select 89 cluster members from spatial and velocity distributions. We also use photometry Canada-France-Hawaii Telescope archives. We compute the mean cluster redshift, <z>=0.168, and the velocity dispersion which shows a high value, sigma_v=1210_{-110}^{+125} km/s. From the 2D analysis we find that Abell 1914 has a NE-SW elongated structure with two galaxy clumps, that mostly merge in the plane of the sky. Our best, but very uncertain estimate of the velocity dispersion of the main system is sigma_v~1000 km/s. We estimate a virial mass M_sys=1.4--2.6 10^{15} h_{70}^{-1} Msun for the whole system. We study the merger through a simple two-body model and find that data are consistent with a bound, outgoing substructure observed just after the core crossing. By studying the 2D distribution of the red galaxies, photometrically selected, we show that Abell 1914 is contained in a rich large scale structure, with two close companion galaxy systems, known to be at z~0.17. The system at SW supports the idea that the cluster is accreting groups from a filament aligned in the NE-SW direction, while that at NW suggests a second direction of the accretion NW-SE. We conclude that Abell 1914 well fits among typical clusters with radio halos. We argue that the unusual radio emission is connected to the complex cluster accretion and suggest that Abell 1914 resembles the well-known nearby merging cluster Abell 754 for its particular observed phenomenology.

Variations in the fundamental constants in the QSO host J1148+5251 at z = 6.4 and the BR1202−0725 system at z = 4.7

We use sensitive observations of three high redshift sources; [CII] fine structure and CO(2-1) rotational transitions for the z=6.4 Quasar host galaxy (QSO) J1148+5251, and [CII] and CO(5-4) transitions from the QSO BR1202-0725 and its sub-millimeter companion (SMG) galaxy at z=4.7. We use these observations to place constraints on the quantity Dz = z(CO) - z(CII) for each source where z(CO) and z(CII) are the observed redshifts of the CO rotational transition and [CII] fine structure transition respectively, using a combination of approaches; 1) By modelling the emission line profiles using `shapelets' to compare both the emission redshifts and the line profiles themselves, in order to make inferences about the intrinsic velocity differences between the molecular and atomic gas, and 2) By performing a marginalisation over all model parameters in order to calculate a non-parametric estimate of Dz. We derive 99% confidence intervals for the marginalised posterior of Dz of (-1.9 pm 1.3) x10^-3, (-3 pm 8) x10^-4 and (-2 pm 4) x10^-3 for J1148+5251, and the BR1202-0725 QSO and SMG respectively. We show the [CII] and CO(2-1) line profiles for J1148+5251 are consistent with each other within the limits of the data, whilst the [CII] and CO(5-4) line profiles from the BR1202-0725 QSO and SMG respectively have 65 and >99.9% probabilities of being inconsistent, with the CO(5-4) lines ~ 30% wider than the [CII] lines. Therefore whilst the observed values of Dz can correspond to variations in the quantity Delta F/F with cosmic time, where F=alpha^2/mu, with alpha the fine structure constant, and mu the proton-to-electron mass ratio, of both (-3.3 pm 2.3) x10^-4 for a look back time of 12.9 Gyr and of (-5 pm 15) x10^-5 for a look back time of 12.4 Gyr we propose that they are the result of the two species of gas being spatially separated as indicated by the inconsistencies in their line profiles.

Isolated ellipticals and their globular cluster systems

The globular cluster (GC) systems of isolated elliptical galaxies have only recently begun to be studied in detail, and may exhibit morphological connections to the evolutionary histories of their hosts. Here we present the first in a series of wide-field analyses of the GC systems of the isolated ellipticals - Washington C and R photometry of NGC 3585 and NGC 5812 down to ~24 mag. The GC systems are characterised, with each system displaying both the "Universal" blue peak at (C-R)~1.3, and a red peak, but each with differing strengths. The total number of GCs in each system, and their specific frequencies, are estimated. The GC colours and specific frequencies are highly indicative that the host galaxy environment plays a role in shaping its GC system. We produce, and subtract, accurate models of each galaxy, revealing interesting underlying features, including the first definitive evidence that NGC 5812 is interacting with a dwarf companion galaxy. From the galaxy models we also determine surface brightness and colour profiles. Both colour profiles appear quite flat and with (C-R)~1.7 and we discuss the apparent youth of NGC 3585 in the context of this work.

Revealing the Character of Orbits in a Binary System Consisting of a Primary Galaxy and a Satellite Companion

AbstractIn this article, we present a galactic gravitational model of three degrees of freedom (3D), in order to study and reveal the character of the orbits of the stars, in a binary stellar system composed of a primary quiet or active galaxy and a small satellite companion galaxy. Our main dynamical analysis will be focused on the behaviour of the primary galaxy. We investigate in detail the regular or chaotic nature of motion, in two different cases: (i) the time-independent model in both 2D and 3D dynamical systems and (ii) the time-evolving 3D model. For the description of the structure of the 2D system, we use the classical method of the Poincaré (x, px), y = 0, py &lt; 0 phase plane. In order to study the structure of the phase space of the 3D system, we take sections in the plane y = 0 of the 3D orbits, whose initial conditions differ from the plane parent periodic orbits, only by the z component. The set of the four-dimensional points in the (x, px, z, pz) phase space is projected on the (z, pz) plane. The maximum Lyapunov characteristic exponent is used in order to make an estimation of the chaoticity of our galactic system, in both 2D and 3D dynamical models. Our numerical calculations indicate that the percentage of the chaotic orbits increases when the primary galaxy has a dense and massive nucleus. The presence of the dense galactic core also increases the stellar velocities near the center of the galaxy. Moreover, for small values of the distance R between the two bodies, low-energy stars display chaotic motion, near the central region of the galaxy, while for larger values of the distance R, the motion in active galaxies is entirely regular for low-energy stars. Our simulations suggest that in galaxies with a satellite companion, the chaotic nature of motion is not only a result of the galactic interaction between the primary galaxy and its companion, but also a result caused by the presence of the dense nucleus in the core of the primary galaxy. Theoretical arguments are presented in order to support and interpret the numerically derived outcomes. Furthermore, we follow the 3D evolution of the primary galaxy, when mass is transported adiabatically from the disk to the nucleus. Our numerical results are in satisfactory agreement with observational data obtained from the M51-type binary stellar systems. A comparison between the present research and similar and earlier work is also made.