Gas-rich Minor Mergers Research Articles

Molecular Gas Reservoirs in Massive Quiescent Galaxies at z ∼ 0.7 Linked to Late-time Star Formation

We explore how the presence of detectable molecular gas depends on the inferred star formation histories (SFHs) in eight massive, quiescent galaxies at z ∼ 0.7. Half of the sample have clear detections of molecular gas, traced by CO(2–1). We find that the molecular gas content is unrelated to the rate of star formation decline prior to the most recent 1 Gyr, suggesting that the gas reservoirs are not left over from their primary star formation epoch. However, the recent SFHs of CO-detected galaxies demonstrate evidence for secondary bursts of star formation in their last Gyr. The fraction of stellar mass formed in these secondary bursts ranges from f burst ≈ 0.3%–6% and ended between t end-burst ≈ 0–330 Myr ago. The CO-detected galaxies form a higher fraction of mass in the last Gyr () compared to the CO-undetected galaxies (). The galaxies with gas reservoirs have enhanced late-time star formation, highlighting this as a contributing factor to the observed heterogeneity in the gas reservoirs in high-redshift quiescent galaxies. We find that the amount of gas and star formation driven by these secondary bursts are inconsistent with that expected from dry minor mergers, and instead are likely driven by recently accreted gas, i.e., gas-rich minor mergers. This conclusion would not have been made based on SFRUV+IR measurements alone, highlighting the power of detailed SFH modeling in the interpretation of gas reservoirs. Larger samples are needed to understand the frequency of low-level rejuvenation among quiescent galaxies at intermediate redshifts, and to what extent this drives the diversity of molecular gas reservoirs.

Extremely massive disc galaxies in the nearby Universe form through gas-rich minor mergers

ABSTRACT In our hierarchical structure-formation paradigm, the observed morphological evolution of massive galaxies – from rotationally supported discs to dispersion-dominated spheroids – is largely explained via galaxy merging. However, since mergers are likely to destroy discs, and the most massive galaxies have the richest merger histories, it is surprising that any discs exist at all at the highest stellar masses. Recent theoretical work by our group has used a cosmological, hydrodynamical simulation to suggest that extremely massive (M* > 1011.4 M⊙) discs form primarily via minor mergers between spheroids and gas-rich satellites, which create new rotational stellar components and leave discs as remnants. Here, we use UV-optical and H i data of massive galaxies, from the Sloan Digital Sky Survey, Galaxy Evolution Explorer, Dark Energy Camera Legacy Survey (DECaLS), and Arecibo Legacy Fast ALFA surveys, to test these theoretical predictions. Observed massive discs account for ∼13 per cent of massive galaxies, in good agreement with theory (∼11 per cent). ∼64 per cent of the observed massive discs exhibit tidal features, which are likely to indicate recent minor mergers, in the deep DECaLS images (compared to ∼60 per cent in their simulated counterparts). The incidence of these features is at least four times higher than in low-mass discs, suggesting that, as predicted, minor mergers play a significant (and outsized) role in the formation of these systems. The empirical star formation rates agree well with theoretical predictions and, for a small galaxy sample with H i detections, the H i masses and fractions are consistent with the range predicted by the simulation. The good agreement between theory and observations indicates that extremely massive discs are indeed remnants of recent minor mergers between spheroids and gas-rich satellites.

PGC 38025: A Star-forming Lenticular Galaxy with an Off-nuclear Star-forming Core

Abstract Lenticular galaxies (S0s) were considered mainly as passive evolved spirals due to environmental effects for a long time; however, most S0s in the field cannot fit into this common scenario. In this work, we study one special case, Sloan Digital Sky Survey J120237.07+642235.3 (PGC 38025), a star-forming field S0 galaxy with an off-nuclear blue core. We present optical integral field spectroscopic observation with the 3.5 meter telescope at Calar Alto Observatory, and high-resolution millimeter observation with the NOrthern Extended Millimeter Array. We estimated the star formation rate (SFR = 0.446 M ⊙ yr−1) and gaseous metallicity (12 + log(O/H) = 8.42) for PGC 38025, which follows the star formation main sequence and stellar mass–metallicity relation. We found that the ionized gas and cold molecular gas in PGC 38025 show the same spatial distribution and kinematics, while rotating misaligned with stellar component. The off-nuclear blue core is locating at the same redshift as PGC 38025 and its optical spectrum suggest it is H ii region. We suggest that the star formation in PGC 38025 is triggered by a gas-rich minor merger, and the off-nuclear blue core might be a local star formation happened during the accretion/merger process.

Searching for Local Counterparts of High-redshift Poststarburst Galaxies in Integral Field Unit Spectroscopic Surveys of Nearby Galaxies

Abstract Searching in the MaNGA IFU survey, I identify nine galaxies that have strong Balmer absorption lines and weak nebular emission lines measured from the spectra integrated over the entire IFU. The spectral features measured from the bulk of the stellar light make these galaxies local analogs of high-redshift spectroscopically selected poststarburst galaxies and thus proxies to understand the mechanisms producing poststarburst galaxies at high redshifts. I present the distributions of absorption line indices and emission line strengths, as well as the stellar kinematics of these local poststarburst galaxies. Almost all local poststarburst galaxies have central compact emission line regions at the central <1 kpc, mostly powered by weak star formation activities. The age-sensitive absorption line indices EW(Hδ) and D n 4000 indicate that the stellar populations at the outskirts are older. Toy stellar population synthesis models suggest that the galaxies as a whole are experiencing a rapid decline of star formation with residual star formation activities at the centers. These features indicate that most poststarburst galaxies are the aftermath of highly dissipative processes that drive gas into centers, invoke centrally concentrated star formation, and then quench the galaxies. Meanwhile, when measurable, poststarburst galaxies have the directions of maximum stellar velocity gradients aligned with photometric major axes, which suggest against major mergers being the principal driving mechanism, while gas-rich minor mergers are plausible. While directly obtaining the same quality of spatially resolved spectra of high-redshift poststarburst galaxies is very difficult, finding proper local counterparts provides an alternative to understand quenching processes in the distant universe.

Circumnuclear Molecular Gas in Low-redshift Quasars and Matched Star-forming Galaxies

Abstract A series of gravitational instabilities in a circumnuclear gas disk (CND) are required to trigger gas transport to a central supermassive black hole and ignite active galactic nuclei (AGNs). A test of this scenario is to investigate whether an enhanced molecular gas mass surface density (Σmol) is found in the CND-scale of quasars relative to a comparison sample of inactive galaxies. Here we performed subkiloparsec resolution CO(2–1) observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of four low-redshift (z ∼ 0.06), luminous (∼1045 erg s−1) quasars with each matched to a different star-forming galaxy, having similar redshift, stellar mass, and star formation rate. We detected CO(2–1) emission from all quasars, which show diverse morphologies. Contrary to expectations, Σmol of the quasar sample, computed from the CO(2–1) luminosity, tends to be smaller than the comparison sample at r < 500 pc; there is no systematic enhancement of Σmol in our quasars. We discuss four possible scenarios that would explain the lower molecular gas content (or CO(2–1) luminosity as an actual observable) at the CND-scale of quasars, i.e., AGN-driven outflows, gas-rich minor mergers, time-delay between the onsets of a starburst-phase and a quasar-phase, and X-ray-dominated region (XDR) effects on the gas chemical abundance and excitation. While not extensively discussed in the literature, XDR effects can have an impact on molecular mass measurements particularly in the vicinity of luminous quasar nuclei; therefore higher resolution molecular gas observations, which are now viable using ALMA, need to be considered.

The assembly history of the nearest S0 galaxy NGC 3115 from its kinematics out to six half-light radii

ABSTRACT Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to ∼6.5 half-light radii (Re) from its stars (integrated starlight), globular clusters (GCs), and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at ∼0.2 Re from a dispersion-dominated bulge (Vrot/σ < 1) to a fast-rotating disc (Vrot/σ > 1), and then an additional transition from the disc to a slowly rotating spheroid at $\sim 2\!-\!2.5\, R_\mathrm{e}$, as traced by the red GCs and PNe (and possibly by the blue GCs beyond $\sim 5\, R_\mathrm{e}$). From comparison with simulations, we propose an assembly history in which the original progenitor spiral galaxy undergoes a gas-rich minor merger that results in the embedded kinematically cold disc that we see today in NGC 3115. At a later stage, dwarf galaxies, in mini mergers (mass ratio < 1:10), were accreted building up the outer slowly rotating spheroid, with the central disc kinematics largely unaltered. Additionally, we report new spectroscopic observations of a sample of ultracompact dwarfs (UCDs) around NGC 3115 with the Keck/KCWI instrument. We find that five UCDs are inconsistent with the general rotation field of the GCs, suggesting an ex situ origin for these objects, i.e. perhaps the remnants of tidally stripped dwarfs. A further seven UCDs follow the GC rotation pattern, suggesting an in situ origin and, possibly a GC-like nature.

The Physical Properties of S0 Galaxy PGC 26218: The Origin of Starburst and Star Formation

We present 2D-spectroscopic observations from Centro Astron\'omico Hispano Alem\'an (CAHA) 3.5 m telescope and the millimetre observation from NOrthern Extended Millimeter Array (NOEMA) of the nearby S0 galaxy PGC 26218, which shows central star-formation activity and post-starburst outside in the disk. We estimate the star formation rate (SFR = $0.28\pm0.01$ $M_{\odot} \rm yr^{-1}$) and molecular gas mass ($\rm log\ $$M_{\rm H_{2}}=7.60\pm0.15\ M_{\odot}$) of PGC 26218 based on the extinction-corrected H$\alpha$ emission line and the CO-$\rm H_{2}$ conversion factor ($\alpha_{\rm CO}$) of the Milky Way, respectively. We find that PGC 26218 follows the star forming main sequence (SFMS) and the Kennicutt-Schmidt law. Comparing the kinematics of CO($J$=1-0), stars and H$\alpha$, we find that the rotational axis of CO($J$=1-0) is 45$^{\circ}$ different from that of H$\alpha$. In addition, the profile of the CO($J$=1-0) emission line shows asymmetry and has an inflow component of $\sim$ 46 $\rm km\ s^{-1}$. With the decomposition of the optical image, we confirm that PGC 26218 shows multiple nuclear structures. The projected offset between the most luminous optical center and the center of CO($J$=1-0) is $5.2\arcsec$ ($\sim$ 0.6 kpc) and the latter overlaps with one of the optical cores. These results support that PGC 26218 may have experienced a gas-rich minor merger, extending its star formation and locating it in the SFMS.

Color Dispersion as an Indicator of Stellar Population Complexity: Insights from the Pixel Color–Magnitude Diagrams of 32 Bright Galaxies in Abell 1139 and Abell 2589

Abstract We investigate the properties of bright galaxies of various morphological types in Abell 1139 and Abell 2589, using pixel color–magnitude diagram (pCMD) analysis. The sample contains 32 galaxies brighter than M r = −21.3 mag with spectroscopic redshifts, which are deeply imaged in the g and r bands using the MegaCam mounted on the Canada–France–Hawaii Telescope. After masking contaminants with two-step procedures, we examine how the detailed properties in the pCMDs depend on galaxy morphology and infrared color. The mean g − r color as a function of surface brightness (μ r ) in the pCMD of a galaxy shows good performance in distinguishing between early- and late-type galaxies, but it is not perfect because of the similarity between elliptical galaxies and bulge-dominated spiral galaxies. On the other hand, the g − r color dispersion as a function of μ r works better. We find that the best set of parameters for galaxy classification is a combination of the minimum color dispersion at μ r ≤ 21.2 mag arcsec−2 and the maximum color dispersion at 20.0 ≤ μ r ≤ 21.0 mag arcsec−2; the latter reflects the complexity of stellar populations at the disk component in a typical spiral galaxy. Finally, the color dispersion measurements of an elliptical galaxy appear to be correlated with the Wide-field Infrared Survey Explorer infrared color ([4.6]–[12]). This indicates that the complexity of stellar populations in an elliptical galaxy is related to its recent star formation activities. From this observational evidence, we infer that gas-rich minor mergers or gas interactions may have usually occurred during the recent growth of massive elliptical galaxies.

Deep spectroscopy of nearby galaxy clusters – IV. The quench of the star formation in galaxies in the infall region of Abell 85

Our aim is to understand the role of the environment in the quenching of star formation of galaxies located in the infall cluster region of Abell 85 (A85). This is achieved by studying the post-starburst galaxy population as tracer of recent quenching. By measuring the equivalent width (EW) of the [OII] and Hdelta spectral lines, we classify the galaxies in three groups: passive (PAS), emission line (EL), and post-starburst (PSB) galaxies. The PSB galaxy population represents about 4.5% of the full sample. Dwarf galaxies (Mr > -18.0) account for about 70 - 80% of PSBs, which indicates that most of the galaxies undergoing recent quenching are low-mass objects. Independently of the environment, PSB galaxies are disk-like objects with g - r colour between the blue ELs and the red PAS ones. The PSB and EL galaxies in low-density environments show similar luminosities and local galaxy densities. The dynamics and local galaxy density of the PSB population in high density environments are shared with PAS galaxies. However, PSB galaxies inside A85 are at shorter clustercentric radius than PAS and EL ones. The value of the EW(Hdelta) is larger for those PSBs closer to the cluster centre. We propose two different physical mechanisms producing PSB galaxies depending on the environment. In low density environments, gas-rich minor mergers or accretions could produce the PSB galaxies. For high density environments like A85, PSBs would be produced by the removal of the gas reservoirs of EL galaxies by ram-pressure stripping when they pass near to the cluster centre.

The H ix galaxy survey – II. H i kinematics of H i eXtreme galaxies

By analysing a sample of galaxies selected from the HI Parkes All Sky Survey (HIPASS) to contain more than 2.5 times their expected HI content based on their optical properties, we investigate what drives these HI eXtreme (HIX) galaxies to be so HI-rich. We model the HI kinematics with the Tilted Ring Fitting Code TiRiFiC and compare the observed HIX galaxies to a control sample of galaxies from HIPASS as well as simulated galaxies built with the semi-analytic model Dark Sage. We find that (1) HI discs in HIX galaxies are more likely to be warped and more likely to host HI arms and tails than in the control galaxies, (2) the average HI and average stellar column density of HIX galaxies is comparable to the control sample, (3) HIX galaxies have higher HI and baryonic specific angular momenta than control galaxies, (4) most HIX galaxies live in higher-spin haloes than most control galaxies. These results suggest that HIX galaxies are HI-rich because they can support more HI against gravitational instability due to their high specific angular momentum. The majority of the HIX galaxies inherits their high specific angular momentum from their halo. The HI content of HIX galaxies might be further increased by gas-rich minor mergers. This paper is based on data obtained with the Australia Telescope Compact Array (ATCA) through the large program C 2705.

An ALMA view of star formation efficiency suppression in early-type galaxies after gas-rich minor mergers

Gas-rich minor mergers contribute significantly to the gas reservoir of early-type galaxies (ETGs) at low redshift, yet the star formation efficiency (SFE; the star formation rate divided by the molecular gas mass) appears to be strongly suppressed following some of these events, in contrast to the more well-known merger-driven starbursts. We present observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of six ETGs, which have each recently undergone a gas-rich minor merger, as evidenced by their disturbed stellar morphologies. These galaxies were selected because they exhibit extremely low SFEs. We use the resolving power of ALMA to study the morphology and kinematics of the molecular gas. The majority of our galaxies exhibit spatial and kinematical irregularities, such as detached gas clouds, warps, and other asymmetries. These asymmetries support the interpretation that the suppression of the SFE is caused by dynamical effects stabilizing the gas against gravitational collapse. Through kinematic modelling we derive high velocity dispersions and Toomre Q stability parameters for the gas, but caution that such measurements in edge-on galaxies suffer from degeneracies. We estimate merger ages to be about 100~Myr based on the observed disturbances in the gas distribution. Furthermore, we determine that these galaxies lie, on average, two orders of magnitude below the Kennicutt-Schmidt relation for star-forming galaxies as well as below the relation for relaxed ETGs. We discuss potential dynamical processes responsible for this strong suppression of star formation surface density at fixed molecular gas surface density.

Structural analysis of star-forming blue early-type galaxies

Star-forming blue early-type galaxies at low redshift can give insight to the stellar mass growth of L$*$ elliptical galaxies in the local Universe. We wish to understand the reason for star formation in these otherwise passively evolving red and dead stellar systems. The fuel for star formation can be acquired through recent accretion events such as mergers or flyby. The signatures of such events should be evident from a structural analysis of the galaxy image. We carried out structural analysis on SDSS $r$-band imaging data of 55 star-forming blue elliptical galaxies, derived the structural parameters, analysed the residuals from best-fit to surface brightness distribution, and constructed the galaxy scaling relations. We found that star-forming blue early-type galaxies are bulge-dominated systems with axial ratio $>$ 0.5 and surface brightness profiles fitted by Sersic profiles with index ($n$) mostly $>$ 2. Twenty-three galaxies are found to have $n$ $<$ 2; these could be hosting a disc component. The residual images of the 32 galaxy surface brightness profile fits show structural features indicative of recent interactions. The star-forming blue elliptical galaxies follow the Kormendy relation and show the characteristics of normal elliptical galaxies as far as structural analysis is concerned. There is a general trend for high-luminosity galaxies to display interaction signatures and high star formation rates. The star-forming population of blue early-type galaxies at low redshifts could be normal ellipticals that might have undergone a recent gas-rich minor merger event. The star formation in these galaxies will shut down once the recently acquired fuel is consumed, following which the galaxy will evolve to a normal early-type galaxy.

Delayed triggering of radio active galactic nuclei in gas-rich minor mergers in the local Universe

We examine the processes triggering star formation and Active Galactic Nucleus (AGN) activity in a sample of 25 low redshift ($z<0.13$) gas-rich galaxy mergers observed at milli-arcsecond resolution with Very Long Baseline Interferometry as part of the mJy Imaging VLBA Exploration at 20cm (mJIVE-20) survey. The high ($>10^7$ K) brightness temperature required for an mJIVE-20 detection allows us to unambiguously identify the radio AGN in our sample. We find three such objects. Our VLBI AGN identifications are classified as Seyferts or LINERs in narrow line optical diagnostic plots; mid-infrared colours of our targets and the comparison of H$\alpha$ star formation rates with integrated radio luminosity are also consistent with the VLBI identifications. We reconstruct star formation histories in our galaxies using optical and UV photometry, and find that these radio AGN are not triggered promptly in the merger process, consistent with previous findings for non-VLBI samples of radio AGN. This delay can significantly limit the efficiency of feedback by radio AGN triggered in galaxy mergers. We find that radio AGN hosts have lower star formation rates than non-AGN radio-selected galaxies at the same starburst age. Conventional and VLBI radio imaging shows these AGN to be compact on arcsecond scales. Our modeling suggests that the actual sizes of AGN-inflated radio lobes may be much larger than this, but these are too faint to be detected in existing observations. Deep radio imaging is required to map out the true extent of the AGN, and to determine whether the low star formation rates in radio AGN hosts are a result of the special conditions required for radio jet triggering, or the effect of AGN feedback.

The SLUGGS survey: the assembly histories of individual early-type galaxies

Early-type (E and S0) galaxies may have assembled via a variety of different evolutionary pathways. Here we investigate these pathways by comparing the stellar kinematic properties of 24 early-type galaxies from the SLUGGS survey with the hydrodynamical simulations of Naab et al. (2014). In particular, we use the kinematics of starlight up to 4 effective radii (R$_e$) as diagnostics of galaxy inner and outer regions, and assign each galaxy to one of six Naab et al. assembly classes. The majority of our galaxies (14/24) have kinematic characteristics that indicate an assembly history dominated by gradual gas dissipation and accretion of many gas-rich minor mergers. Three galaxies, all S0s, indicate that they have experienced gas-rich major mergers in their more recent past. One additional elliptical galaxy is tentatively associated with a gas-rich merger which results in a remnant galaxy with low angular momentum. Pathways dominated by gas-poor (major or minor) mergers dominate the mass growth of six galaxies. Most SLUGGS galaxies appear to have grown in mass (and size) via the accretion of stars and gas from minor mergers, with late major mergers playing a much smaller role. We find that the fraction of accreted stars correlates with the stellar mean age and metallicity gradient, but not with the slope of the total mass density profile. We briefly mention future observational and modelling approaches that will enhance our ability to accurately reconstruct the assembly histories of individual present day galaxies.

VIMOS mosaic integral-field spectroscopy of the bulge and disc of the early-type galaxy NGC 4697

We present an integral field study of the internal structure, kinematics and stellar population of the almost edge-on, intermediate luminosity ($L_ {*}$) elliptical galaxy NGC 4697. We build extended 2-dimensional (2D) maps of the stellar kinematics and line-strengths of the galaxy up to $\sim 0.7 $ effective radii (R$_{eff}$) using a mosaic of 8 VIMOS (VIsible Multi-Objects Spectrograph on the VLT) integral-field unit pointings. We find clear evidence for a rotation-supported structure along the major axis from the 2D kinematical maps, confirming the previous classification of this system as a `fast-rotator'. We study the correlations between the third and fourth Gauss-Hermite moments of the line-of-sight velocity distribution (LOSVD) $h_3$ and $h_4$ with the rotation parameter ($V/\sigma$), and compare our findings to hydrodynamical simulations. We find remarkable similarities to predictions from gas-rich mergers. Based on photometry, we perform a bulge/disk decomposition and study the stellar population properties of the two components. The bulge and the disk show different stellar populations, with the stars in the bulge being older (age$_{\rm bulge}=13.5^{+1.4}_{-1.4}$ Gyr, age$_{\rm disk}=10.5^{+1.6}_{-2.0}$Gyr) and more metal-poor ($\mathrm{[M/H]_{bulge}} = -0.17^{+0.12}_{-0.1}$, $\mathrm{[M/H]_{disk}}=-0.03^{+0.02}_{-0.1}$). The evidence of a later-formed, more metal-rich disk embedded in an older, more metal-poor bulge, together with the LOSVD structure, supports a mass assembly scenario dominated by gas-rich minor mergers and possibly with a late gas-rich major merger that left a previously rapidly rotating system unchanged. The bulge and the disk do not show signs of different stellar Initial Mass Function slopes, and both match well with a Milky Way-like IMF.

Galaxy Zoo: dust lane early-type galaxies are tracers of recent, gas-rich minor mergers★

We present the second of two papers concerning the origin and evolution of local early-type galaxies exhibiting dust features. We use optical and radio data to examine the nature of active galactic nucleus (AGN) activity in these objects, and compare these with a carefully constructed control sample. We find that dust lane early-type galaxies are much more likely to host emission-line AGN than the control sample galaxies. Moreover, there is a strong correlation between radio and emission-line AGN activity in dust lane early-types, but not the control sample. Dust lane early-type galaxies show the same distribution of AGN properties in rich and poor environments, suggesting a similar triggering mechanism. By contrast, this is not the case for early-types with no dust features. These findings strongly suggest that dust lane early-type galaxies are starburst systems formed in gas-rich mergers. Further evidence in support of this scenario is provided by enhanced star formation and black hole accretion rates in these objects. Dust lane early-types therefore represent an evolutionary stage between starbursting and quiescent galaxies. In these objects, the AGN has already been triggered but has not as yet completely destroyed the gas reservoir required for star formation.

Dissecting the morphological and spectroscopic properties of galaxies in the local Universe

We revisit the scaling relations and star-forming histories of local elliptical galaxies using a novel selection method applied to the Sloan Digital Sky Survey DR7. We combine two probability-based automated spectroscopic and morphological classifications of about 600000 galaxies with z<0.25 to isolate true elliptical galaxies. Our sample selection method does not introduce artificial cuts in the parameters describing the galaxy but instead it associates to every object a weight measuring the probability of being in a given spectro-morphological class. Thus the sample minimizes the selection biases. We show that morphologically defined ellipticals are basically distributed in 3 spectral classes, which dominate at different stellar masses. The bulk of the population (about 50%) is formed by a well defined class of galaxies with old stellar populations that formed their stars at very early epochs in a short episode of star formation. They dominate the scaling relations of elliptical galaxies known from previous works and represent the canonical elliptical class. At the low mass end, we find a population of slightly larger ellipticals, with smaller velocity dispersions at fixed stellar mass, which seem to have experienced a more recent episode of star formation probably triggered by gas-rich minor mergers. The high mass end tends to be dominated by a third spectral class, slightly more metal rich and with more efficient stellar formation than the reference class. This third class contributes to the curvature of the mass-size relation at high masses reported in previous works. Our method is therefore able to isolate typical spectra of elliptical galaxies following different evolutive pathways.

A YOUNG MASSIVE STELLAR POPULATION AROUND THE INTERMEDIATE-MASS BLACK HOLE ESO 243-49 HLX-1

We present Hubble Space Telescope and simultaneous Swift X-ray telescope observations of the strongest candidate intermediate mass black hole ESO 243-49 HLX-1. Fitting the spectral energy distribution from X-ray to near-infrared wavelengths showed that the broadband spectrum is not consistent with simple and irradiated disc models, but is well described by a model comprised of an irradiated accretion disc plus a stellar population with a mass ~1E6 Msun. The age of the population cannot be uniquely constrained, with both very young and very old stellar populations allowed. However, the very old solution requires excessively high levels of disc reprocessing and an extremely small disc, leading us to favour the young solution with an age of ~13 Myr. In addition, the presence of dust lanes and the lack of any nuclear activity from X-ray observations of the host galaxy lead us to propose that a gas-rich minor merger may have taken place less than ~200 Myr ago. Such a merger event would explain the presence of the intermediate mass black hole and support a young stellar population.

Composite star formation histories of early-type galaxies from minor mergers: prospects for WFC3

The star formation history of nearby early-type galaxies is investigated via numerical modelling. Idealized hydrodynamical N-body simulations with a star formation prescription are used to study the minor merger process between a giant galaxy (host) and a less massive spiral galaxy (satellite) with reasonable assumptions for the ages and metallicities of the merger progenitors. We find that the evolution of the star formation rate is extended over several dynamical times and shows peaks which correspond to pericentre passages of the satellite. The newly formed stars are mainly located in the central part of the satellite remnant while the older stars of the initial disk are deposited at larger radii in shell-like structures. After the final plunge of the satellite, star formation in the central part of the remnant can continue for several Gyrs depending on the star formation efficiency. Although the mass fraction in new stars is small, we find that the half-mass radius differs from the half-light radius in the V and H bands. Moreover synthetic 2D images in J, H, NUV, Hb and V bands, using the characteristic filters of the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST), reveal that residual star formation induced by gas-rich minor mergers can be clearly observed during and after the final plunge, especially in the NUV band, for interacting systems at (z<0.023) over moderate numbers of orbits (~2 orbits correspond to typical exposure times of ~3600 sec). This suggests that WFC3 has the potential to resolve these substructures, characterize plausible past merger episodes, and give clues to the formation of early-type galaxies.

A near-infrared view of the 3CR: properties of hosts and nuclei†

AbstractThe 3CR catalogue provides a statistical sampling of the most powerful radio galaxies out to z ∼ 0.3. Over the decade and a half of Hubble observations we have amassed a major multi-wavelength dataset on these sources, discovering amongst other things, new jets, hotspots, dust disks, and faint point-like nuclei. We present here the results of our latest snapshot survey, a near-complete sampling of the 3CR host galaxies at z &lt; 0.3 in the near-IR (H-band). This un-extinguished view of the host galaxies has provided us with an accurate measure of the stellar/spheroid masses of the sources, and an unbiased view of their morphologies. We show that they exhibit an identical Kormendy relation to nearby QSO's and the massive Elliptical population, but are distinct from the Brightest Cluster Members, and mergers. We find that while a few sources exhibit signs of a recent or impending major merger, many more sources have remnants consistent with a gas-rich minor merger in their recent history. We detect unresolved nuclear sources in most (∼80%) of FRI, with their IR luminosities correlating linearly with radio core power. This implies that the IR nuclei are synchrotron radiation produced at the base of the relativistic jet, and confirms that no infrared (thermal) radiation in excess to synchrotron is present in FRIs, unlike in other classes of AGN.