A multiwavelength view of the ISM in the merger remnant galaxy Fornax A

Aims. Shells in Ellipticals are peculiar faint sharp edged features that are thought to be formed by galaxy mergers. We determine the shell and dust distributions, and colours of a well-resolved shell and the underlying galaxy in NGC 5982, and compare the spatial distributions of the dust and gas phases, in order to investigate the origin of shells and dust in NGC 5982. Methods. We use Spitzer data in the wavelength range from 3.6 to 160 mu m and HST/ACS optical data. Surface photometry, galaxy models and residual images are determined using IRAF task ELLIPSE and GALPHOT task ELLIPFIT. After subtracting the galaxy models, residual images are used to identify the shells. Excess emission is identified by subtracting a scaled stellar photosphere emission template. Colours [3.6]-[4.5], [3.6]-[8.0] and [5.8]-[8.0] of the underlying galaxy and [5.8]-[8.0] of the excess emission are obtained using techniques similar to those of Pahre et al. The [3.6]-[4.5] and V -I colours of a well-resolved shell are determined using techniques of Sikkema et al. Results. Shells are visible in the 3.6 mu m image, fainter at 4.5 mu m and undetected at 5.8 mu m. We find two new shells, the outermost identified so far in NGC 5982. All shells apart from shell 24 are on the photometric major axis. Shell 24 is the only one for which reliable colours are obtained. It has colours [3.6]-[4.5] and V - I bluer than the underlying galaxy. Excess emission at 4.5, 5.8 and 8 mu m is widely extended. The ratio of excess to total emission decreases towards the centre, which could be related to a lower mass loss rate from AGB stars. The [3.6]-[4.5] colour indicates a major contribution from late-type (K2-M0III) stars towards the centre. Dust traced by 24 mu m emission follows the stellar spatial distribution, supporting a circumnuclear origin. We find extended 160 mu m emission from cold dust, possibly forming a disk inclined to the principal axes, with a mass of a few 10(5) M-circle dot. The warmer big grains and/or smaller grains traced by the 70 mu m emission are confined to a small region elongated along the minor axis and shifted a few kpc from the centre. Conclusions. We detect for the first time shells from mid-infrared data. The very different distributions of dust, warm gas and HI gas together with the presence of shells and a kinematically decoupled core suggest a minor merger in NGC 5982.

We report the detection of of molecular gas in the central region of the S0/E7 galaxy NGC 4550, inferred from observations of CO(1-0) emission. Dust is detected in HST WFPC2 images and found to be asymmetrically distributed around the nucleus, only extending to a galactocentric distance of 7'' (600 pc). The shape of the CO emission profile is consistent with a molecular gas distribution following the dust. The distribution of the dust and gas in the center could be the result of an instability, which is the fastest growing unstable mode in counterrotating stellar disks. On a global scale the molecular gas in NGC 4550 is stable against gravitational collapse but nevertheless star formation appears to be ongoing with normal star formation efficiency and gas consumption time scales. The stellar velocity dispersion in NGC 4550 resembles that of elliptical galaxies. It is therefore likely that a hot X-ray emitting plasma limits the lifetime of the molecular gas, that must arise from a recent (≪1 Gyr) accretion event.

The molecular gas in (some) early type galaxies holds important clues to the history and the future of these galaxies. In pursuit of these clues we have used the BIMA millimeter array to map CO emission in the giant elliptical galaxies NGC 83 and NGC 2320 and to search for CO emission from the S0 galaxy NGC 5838. We also present V and R images of NGC 83 and NGC 2320 which trace their dust distributions and enable a search for disky stellar structures. The molecular gas in NGC 83 is well relaxed, but both CO and dust in NGC 2320 show asymmetric structures which may be linked to a recent acquisition of the gas. However, the specific angular momentum distribution of molecular gas in NGC 2320 is consistent with that of the stars. Internal origin of the gas (stellar mass loss) cannot, therefore, be ruled out on angular momentum grounds alone. We also consider the evidence for star formation activity and disk growth in these two elliptical galaxies. Radio continuum and FIR fluxes of NGCv83 suggest star formation activity. NGC 2320 has bright [O III] emission, but its large radio/FIR flux ratio and the mismatch between the kinematics of CO and [O III] suggest that the ionized gas should not be attributed to star formation. The origin and future of these two CO-rich early type galaxies are thus complex, multi-faceted stories.

One of the central challenges to establishing the role of mergers in galaxy evolution is the selection of pure and complete merger samples in observations. In particular, while large and reasonably pure interacting galaxy pair samples can be obtained with relative ease via spectroscopic criteria, automated selection of post-coalescence merger remnants is restricted to the physical characteristics of remnants alone. Furthermore, such selection has predominantly focused on imaging data – whereas kinematic data may offer a complimentary basis for identifying merger remnants. Therefore, we examine the theoretical utility of both the morphological and kinematic features of merger remnants in distinguishing galaxy merger remnants from other galaxies. Deep classification models are calibrated and evaluated using idealized synthetic images and line-of-sight stellar velocity maps of a heterogeneous population of galaxies and merger remnants from the TNG100 cosmological hydrodynamical simulation. We show that even idealized stellar kinematic data have limited utility compared to imaging and underperforms by $2.1 \pm 0.5 {{\ \rm per\ cent}}$ in completeness and $4.7 \pm 0.4 {{\ \rm per\ cent}}$ in purity for our fiducial model architecture. Combining imaging and stellar kinematics offers a small boost in completeness (by $1.8 \pm 0.4 {{\ \rm per\ cent}}$, compared to $92.7 \pm 0.2 {{\ \rm per\ cent}}$ from imaging alone) but no change in purity ($0.1\pm 0.3 {{\ \rm per\ cent}}$ improvement compared to $92.7 \pm 0.2 {{\ \rm per\ cent}}$, evaluated with equal numbers of merger remnant and non-remnant control galaxies). Classification accuracy of all models is particularly sensitive to physical companions at separations ≲ 40 kpc and to time-since-coalescence. Taken together, our results show that the stellar kinematic data have little to offer in compliment to imaging for merger remnant identification in a heterogeneous galaxy population.

We construct a physically motivated model for predicting the properties of the remnants of gaseous galaxy mergers, given the properties of the progenitors and the orbit. The model is calibrated using a large suite of SPH merger simulations. It implements generalized energy conservation while accounting for dissipative energy losses and star formation. The dissipative effects are evaluated from the initial gas fractions and from the orbital parameters via an impulse parameter, which characterizes the strength of the encounter. Given the progenitor properties, the model predicts the remnant stellar mass, half-mass radius, and velocity dispersion to an accuracy of 25%. The model is valid for both major and minor mergers. We provide an explicit recipe for semi-analytic models of galaxy formation.

We use numerical models to investigate the nature of induced population gradients in merger remnants. As noted by White, we find that the population mixing in stellar-dynamical mergers is rather moderate, leading to metallicity gradients in the remnant that are closely tied to gradients in the progenitor galaxies. Furthermore, the resultant metallicity gradients are poorly fitted by power laws, showing significant nonlinearities within an effective radius. If constant metallicity bulges are added to the progenitor disks, the strong relation between final and initial metallicity gradients is diluted, but the final gradients are still not well fitted by power laws. Detailed studies of the shape of the metallicity gradients in elliptical galaxies should help determine the degree to which stellar-dynamical mergers have contributed to the population of present-day ellipticals. We also consider the effects of centrally concentrated starbursts on metallicity gradients. Using simple models for metallicity enhancement, we find that addition of a metal-rich starburst population can reproduce the magnitude and shape of metallicity gradients observed in elliptical galaxies. However, even in such cases the metallicity gradients in the merger remnant should steepen significantly beyond an effective radius. Modelling such merger-induced starbursts with population synthesis techniques, we find that if the starburst is characterized by solar metallicity, it should be detectable as a bluing of the nuclear regions for several Gyr. However, if the starburst is metal-rich, the reddening effects of higher metallicity makes the broad-band signature of the starburst much more difficult to detect.

We present a new technique for observationally identifying galaxy mergers spectroscopically rather than through host galaxy imaging. Our technique exploits the dynamics of supermassive black holes (SMBHs) powering active galactic nuclei (AGNs) in merger-remnant galaxies. Because structure in the universe is built up through galaxy mergers and nearly all galaxies host a central SMBH, some galaxies should possess two SMBHs near their centers as the result of a recent merger. These SMBHs spiral to the center of the resultant merger-remnant galaxy, and one or both of the SMBHs may power AGNs. Using the DEEP2 Galaxy Redshift Survey, we have examined 1881 red galaxies, of which 91 exhibit [O III] and Hbeta emission lines indicative of Seyfert 2 activity. Of these, 32 AGNs have [O III] emission-line redshifts significantly different from the redshifts of the host galaxies' stars, corresponding to velocity offsets of ~50 km/s to ~300 km/s. Two of these AGNs exhibit double-peaked [O III] emission lines, while the remaining 30 AGNs each exhibit a single set of velocity-offset [O III] emission lines. After exploring a variety of physical models for these velocity offsets, we argue that the most likely explanation is inspiralling SMBHs in merger-remnant galaxies. Based on this interpretation, we find that roughly half of the red galaxies hosting AGNs are also merger remnants, which implies that mergers may trigger AGN activity in red galaxies. The AGN velocity offsets we find imply a merger fraction of ~30% and a merger rate of ~3 mergers/Gyr for red galaxies at redshifts 0.34 < z < 0.82.

We present the results of a 6.7 GHz methanol maser survey from the Effelsberg 100 m radio telescope. A sample of 404 sources from the Bolocam Galactic Plane Survey (BGPS) 1.1 mm dust clump survey that met specific Galactic Legacy Infrared Mid-Plane Survey Extraodinaire (GLIMPSE) point-source color criteria was selected and 318 of these were observed. The new observations resulted in the detection of 29 methanol masers, including 12 new ones. Together with the additional 74 detections from the literature, this means that a total of 103 methanol masers are coincident with 1.1 mm dust clumps, yielding an overall detection rate of 26%. A comparison of the properties of a 1.1 mm dust clump and a 6.7 GHz methanol maser indicates that methanol masers with a higher flux density and/or luminosity are generally associated with more massive but less dense 1.1 mm dust clumps. The overall detection rate of 26% appears to vary as a function of the derived H-2 column density of the associated 1.1 mm dust clump. The methanol masers were primarily detected toward the brighter and more massive 1.1 mm dust clumps. A subsample of 194 sources that overlapped sources with observations of the 95 GHz methanol line was investigated in more detail for the properties of 1.1 mm dust clumps. The statistical analysis reveals that 1.1 mm dust clumps with both class I and II counterparts have much higher mean and median values of mass, column density, and flux density than those with only class I or II counterparts. Based on our much larger sample, we slightly revise the boundary defined previously for selecting BGPS sources associated with a class II methanol maser, wherein similar to 80% of expected class II methanol masers will be detected with a detection rate in the range of 40-50%.

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 hours at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity HI (up to about 100 km/s) and find 121 HI holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with HI holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the HI holes. There are also indications, especially in the outer regions, -an extended HI plume, velocity anomalies, sharp edges, and large-scale asymmmetries- pointing to tidal encounters and recent gas accretion.

The physical state of ionized gas in NGC 7793 was studied by spectroscopical means: the electronic temperature isTe∼104K while the electronic density ranges fromNe∼1400 cm−3 in the nucleus toNe∼1000 cm−3 in the emission regions. There are also indications of an excess of nitrogen in the nucleus. TheM/L ratio suggests for NGC 7793, a high proportion of young objects quite uniformly distributed over its body, with a slight concentration towards the nuclear region.

Using reductive perturbation method with appropriate boundary conditions, coupled evolution equations for first and second order potentials are derived for ion-acoustic waves in a collisionless, un-magnetized plasma consisting of hot isothermal electrons, cold ions, and massive mobile charged dust grains. The boundary conditions give rise to renormalization term, which enable us to eliminate secular contribution in higher order terms. Determining the non secular solution of these coupled equations, expressions for wave phase velocity and averaged non-linear ion flux associated with ion-acoustic cnoidal wave are obtained. Variation of the wave phase velocity and averaged non-linear ion flux as a function of modulus (k2) dependent wave amplitude are numerically examined for different values of dust concentration, charge on dust grains, and mass ratio of dust grains with plasma ions. It is found that for a given amplitude, the presence of positively (negatively) charged dust grains in plasma decreases (increases) the wave phase velocity. This behavior is more pronounced with increase in dust concentrations or increase in charge on dust grains or decrease in mass ratio of dust grains. The averaged non-linear ion flux associated with wave is positive (negative) for negatively (positively) charged dust grains in the plasma and increases (decreases) with modulus (k2) dependent wave amplitude. For given amplitude, it increases (decreases) as dust concentration or charge of negatively (positively) charged dust grains increases in the plasma.

We have conducted a sensitive water maser search with the ATCA towards 267 1.2-mm dust clumps presented in the literature. We combine our new observations with previous water maser observations to extend our sample to 294 1.2-mm dust clumps, towards which we detect 165 distinct water maser sites towards 128 1.2-mm dust clumps. Within the fields of our observations, we additionally find four water masers with no apparent associated 1.2-mm dust continuum emission. Our overall detection rate of 44 per cent appears to vary as a function of Galactic longitude. We find that there is an excellent correspondence between the locations of the detected water masers with the peak of the target 1.2-mm dust clump sources. As expected from previous similar studies, the water masers are chiefly detected towards the bigger, brighter and more massive 1.2-mm dust clumps. We find further evidence that the water masers tend to increase in flux density (and therefore luminosity), as well as velocity range, as the sources evolve. We also show that the current sample of water maser sources suffer less from evolutionary biases than previous targeted searches. A higher fraction of dust clump sources in our sample are only associated with water masers (41) than only associated with methanol masers (13). This suggests that water masers can be present at an even earlier evolutionary stage than 6.7-GHz methanol masers. Comparison of the water maser detection rates associated with different combinations of methanol maser and radio continuum, as well as those with neither tracer, shows that the highest detection rate is towards those sources which also exhibit methanol maser emission. We have tested a previously hypothesised model for water maser presence towards 1.2-mm dust clumps. We suggest refinements and future work which will further constrain the nature of the driving sources associated with water masers.

ABSTRACTWe follow trajectories of recoiling supermassive black holes (SMBHs) in analytical and numerical models of galaxy merger remnants with masses of 1011 M⊙ and 1012 M⊙. We construct various merger remnant galaxies in order to investigate how the central SMBH mass and the mass ratio of progenitor galaxies influence escape velocities of recoiling SMBHs. Our results show that static analytical models of major merger remnant galaxies overestimate the SMBHs escape velocities. During major mergers violent relaxation leads to the decrease of galaxy mass and lower potential at large remnant radii. This process is not depicted in static analytical potential but clearly seen in our numerical models. Thus, the evolving numerical model is a more realistic description of dynamical processes in galaxies with merging SMBHs. We find that SMBH escape velocities in numerical major merger remnant galaxies can be up to 25 per cent lower compared to those in analytical models. Consequently, SMBHs in numerical models generally reach greater galactocentric distances and spend more time on bound orbits outside of the galactic nuclei. Thus, numerical models predict a greater number of spatially offset active galactic nuclei (AGNs).

Large debate still exists on the origin of the elliptical galaxies in low density environments and on the evolutionary fate of merger remnant galaxies. The aim of this paper is to propose a new model grid to interpret the spectrophotometric data of merger remnants. The model grid focuses on the near-IR (NIR hereafter) properties of intermediate age populations — signatures of recent star formation events — dominated in the NIR by the AGB stars. In our calculations the stars evolve up to the end of the early AGB following the tracks calculated by the Padova group (Fagotto et al., 1994). Particular care was taken to calculate the evolutionary properties of the AGB stars. A new synthetic TP-AGB star evolution model was developed. This model includes the relevent physical processes that are controlling the TP-AGBs evolution, covering a large range of masses (0.8M ⊙ ≤ M initial ≤5 — 6M ⊙) and metallicities (1/50 ≤ Z/Z ⊙ ≤ 2.5). The models use a new stellar library that includes, for the first time, the spectral features that charactrize only the AGB population (Mouhcine and Lancon, 1999). The observational strategy based on those features is the most efficient one to look for the intermediate age population (Lancon et al., 1999)

We propose that the rotational kinematics of the globular cluster system (GCS) in M31 can result from a past major merger event that could have formed its bulge component. We numerically investigate kinematical properties of globular clusters (GCs) in remnants of galaxy mergers between two discs with GCs in both their disc and halo components. We find that the GCS formed during major merging can show strongly rotational kinematics with the maximum rotational velocities of ∼140–170 km s−1 for a certain range of orbital parameters of merging. We also find that a rotating stellar bar, which can be morphologically identified as a boxy bulge if seen edge-on, can be formed in models for which the GCSs show strongly rotational kinematics. We thus suggest that the observed rotational kinematics of GCs with different metallicities in M31 can be closely associated with the ancient major merger event. We discuss whether the formation of the rotating bulge/bar in M31 can be due to the ancient merger.