Peculiar Galaxy Research Articles

Spectral observations of the systems with the disturbed spiral arms: Arp 42, Arp 82, and Arp 58

We study three Arp' systems of peculiar galaxies: Arp 42 (NGC 5829), Arp 82 (NGC 2535/36) and Arp 58 (UGC 4457), using the long-slit spectral observations carried out at the 6m telescope BTA of Special Astrophysical Observatory. Arp 82 and Arp 58 are the M51-type systems. In the third system - Arp 42, there are two extremely luminous kpc-sized clumps observed at the ends of the bifurcated spiral arm, however the source of perturbations remains unknown. From the emission line measurements we analyzed the distribution of gas line-of-sight (LOS) velocity, velocity dispersion and gas-phase metallicity along the slits estimated by different methods. A special attention is paid to the young regions of star formation at the peripheries of the galaxies and in tidal debris, their connection with gas kinematics and abundance. All three systems show the signs of velocity disturbances and the presence of regions of locally enhanced gas velocity dispersion, as well as the regions of diffuse ionized gas. A distribution of oxygen reveals shallow radial abundance gradients, typical for interacting systems. A faint spiral-like branches of tidal bridges which are observed in NGC 2535 and UGC 4457 may represent the remnants of pre-existing "old" mode of spiral waves.

Star formation in far-IR AGN and non-AGN galaxies in the green valley – II. Morphological analysis

This paper studies morphological properties of 103 green valley FIR active and 2609 non-active galaxies presented in Mahoro et al. 2017. The photometric data from the COSMOS survey were used, and the morphological parameters, such as Abraham and Conselice-Bershady concentration indices, Gini, M20 moment of light, and asymmetry, were analysed taking into account public catalogues. Furthermore, a visual classification of galaxies was performed. We found that the fraction of peculiar galaxies with clear signs of interactions and mergers is significantly higher in AGN (38%) than non-AGN (19%) green valley galaxies, while non-AGN galaxies from our sample are predominantly spirals (46%). We found that the largest fraction of our green valley galaxies is located on the main-sequence (MS) of star formation, independently on morphology, which is in contrast with most of previous studies carried out in optical. We also found that FIR AGN green valley galaxies have significantly higher star formation rates in all analysed morphological types. Therefore, our results suggest that interactions and mergers obtained in the high fraction of FIR AGN contribute significantly to high star formation rates observed in the selected sample, but are not the only mechanism responsible for enhancing star formation, and others such as AGN positive feedback could contribute as well. In future we will study in more details the possibility of AGN positive feedback through the spectroscopic analysis of public and our SALT data.

Merging cold front and AGN feedback in the peculiar galaxy cluster Abell 2626

This paper presents the analysis of a combined 134 ks {\it Chandra} data of a peculiar galaxy cluster Abell 2626. This study confirms the earlier detection of the east cavity at $\sim$13 kpc and reports detection of a new cavity at $\sim$39 kpc on the west of the X-ray peak. The average mechanical power injected by the AGN outburst ${\rm P_{cav} \sim 6.6 \times 10^{44}\, erg\, s^{-1}}$ is $\sim$29 times more than required to compensate the cooling luminosity ${\rm L_{cool} = 2.30 \pm 0.02 \times 10^{43} {\rm~erg\ s}^{-1}}$. The edges in the SB on the west and south-west at $\sim$36 kpc and 33 kpc, respectively, have the gas compressions of 1.57$\pm$0.08 and 2.06$\pm$0.44 and are spatially associated with the arcs in the temperature and metallicity maps due to the merging cold fronts. The systematic study of the nuclear sources exhibited dramatic changes over the span of ten years. The NE source that emitted mostly in the soft band in the past disappeared in the recent observations. Instead, an excess emission was seen at $2.2"$ on its west and required an unrealistic line of sight velocity of $\sim$ $675\times{}c$ if is due to its movement. The count rate analysis and spectral analysis exhibited a change in the state of the SW source from a soft state to the hard due to the change in the mass accretion rate. No such spectral change was noticed for the NE source.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

We observed the late-type peculiar galaxy NGC 4424 during the Virgo Environmental Survey Tracing Galaxy Evolution (VESTIGE), a blind narrow-band Hα+[NII] imaging survey of the Virgo cluster carried out with MegaCam at the Canada-French-Hawaii Telescope (CFHT). The presence of a ∼110 kpc (in projected distance) HI tail in the southern direction indicates that this galaxy is undergoing a ram pressure stripping event. The deep narrow-band image revealed a low surface brightness (Σ(Hα) ≃ 4 × 10−18 erg s−1 cm−2 arcsec−2) ionised gas tail ∼10 kpc in length extending from the centre of the galaxy to the north-west, thus in the direction opposite to the HI tail. Chandra and XMM X-rays data do not show a compact source in the nucleus or an extended tail of hot gas, while IFU spectroscopy (MUSE) indicates that the gas is photo-ionised in the inner regions and shock-ionised in the outer parts. Medium-resolution (MUSE) and high-resolution (Fabry-Perot) IFU spectroscopy confirms that the ionised gas is kinematically decoupled from the stellar component and indicates the presence of two kinematically distinct structures in the stellar disc. The analysis of the SED of the galaxy indicates that the activity of star formation was totally quenched in the outer disc ∼250–280 Myr ago, while only reduced by ∼80% in the central regions. All this observational evidence suggests that NGC 4424 is the remnant of an unequal-mass merger that occurred ≲500 Myr ago when the galaxy was already a member of the Virgo cluster, and is now undergoing a ram pressure stripping event that has removed the gas and quenched the activity of star formation in the outer disc. The tail of ionised gas probably results from the outflow produced by a central starburst fed by the collapse of gas induced by the merging episode. This outflow is sufficiently powerful to overcome the ram pressure induced by the intracluster medium on the disc of the galaxy crossing the cluster. This analysis thus suggests that feedback can participate in the quenching process of galaxies in high-density regions.

HELP: modelling the spectral energy distributions ofHerscheldetected galaxies in the ELAIS N1 field

Aims.TheHerschelExtragalactic Legacy Project (HELP) focuses on the data from ESA’sHerschelmission, which covered over 1300 deg2and is preparing to publish a multi-wavelength catalogue of millions of objects. Our main goal is to find the best approach to simultaneously fitting spectral energy distributions (SEDs) of millions of galaxies across a wide redshift range to obtain homogeneous estimates of the main physical parameters of detected infrared (IR) galaxies.Methods.We perform SED fitting on the ultraviolet(UV)/near-infrared(NIR) to far-infrared(FIR) emission of 42 047 galaxies from the pilot HELP field: ELAIS N1. To do this we use the latest release of CIGALE, a galaxy SED fitting code relying on energy balance, to deliver the main physical parameters such as stellar mass, star formation rate, and dust luminosity. We implement additional quality criteria to the fits by calculatingχ2values for the stellar and dust part of the spectra independently. These criteria allow us to identify the best fits and to identify peculiar galaxies. We perform the SED fitting of ELAIS N1 galaxies by assuming three different dust attenuation laws separately allowing us to test the impact of the assumed law on estimated physical parameters.Results.We implemented two additional quality value checks for the SED fitting method based on stellar mass estimation and energy budget. This method allows us to identify possible objects with incorrect matching in the catalogue and peculiar galaxies; we found 351 possible candidates of lensed galaxies using two complementaryχ2s criteria (stellar and infraredχ2s) and photometric redshifts calculated for the IR part of the spectrum only. We find that the attenuation law has an important impact on the stellar mass estimate (on average leading to disparities of a factor of two). We derive the relation between stellar mass estimates obtained by three different attenuation laws and we find the best recipe for our sample. We also make independent estimates of the total dust luminosity parameter from stellar emission by fitting the galaxies with and without IR data separately.

Gemini/GMOS Spectroscopy of Globular Clusters in the Merger Remnant Galaxy M85

Abstract M85 is a peculiar S0 galaxy in Virgo and a well-known merger remnant. We present the first spectroscopic study of globular clusters (GCs) in M85. We obtain spectra for 21 GC candidates and the nucleus of M85 using the Gemini Multi-Object Spectrograph on the Gemini North 8.1 m telescope. From their radial velocities, 20 of the GCs are found to be members of M85. We find a strong rotation signal of the M85 GC system with a rotation amplitude of 235 km s−1. The rotation axis of the GC system has a position angle of about 161°, which is 51.°5 larger than that of the stellar light. The rotation-corrected radial velocity dispersion of the GC system is estimated to be km s−1. The rotation parameter of the GC system is derived to be , which is one of the largest among known early-type galaxies. The ages and metallicities of the GCs, which show the same trend as the results based on Lick indices, are derived from full spectrum fitting (ULySS). About half of the GCs are an intermediate-age population whose mean age is ∼3.7 ± 1.9 Gyr, having a mean [Fe/H] value of −0.26. The other half are old and metal-poor. These results suggest that M85 experienced a wet merging event about 4 Gyr ago, forming a significant population of star clusters. The strong rotational feature of the GC system can be explained by an off-center major merging.

Star Formation Histories of the LEGUS Dwarf Galaxies. II. Spatially Resolved Star Formation History of the Magellanic Irregular NGC 4449∗

Abstract We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity (D = 3.82 ± 0.27 Mpc), we reach stars 3 mag fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history (SFH) spans the whole Hubble time, but due to the age–metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e., ∼3 Gyr. The most recent peak of star formation (SF) is around 10 Myr ago. The average surface density SF rate over the whole galaxy lifetime is 0.01 M ⊙ yr−1 kpc−2. From our study, it emerges that NGC 4449 has experienced a fairly continuous SF regime in the last 1 Gyr, with peaks and dips whose SF rates differ only by a factor of a few. The very complex and disturbed morphology of NGC 4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its SFH does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy.

VLBA polarimetric monitoring of 3C 111

Context.While studies of large samples of jets of active galactic nuclei (AGN) are important in order to establish a global picture, dedicated single-source studies are an invaluable tool for probing crucial processes within jets on parsec scales. These processes involve in particular the formation and geometry of the jet magnetic field as well as the flow itself.Aims.We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes.Methods.We analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. While electric vectors can probe the underlying magnetic field, other properties of the jet such as the variable (polarized) flux density, feature size, and brightness temperature, can give valuable insights into the flow itself. We complement the VLBA data with data from the IRAM 30-m Telescope as well as the SMA.Results.We observe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large rotation of ≳180∘ across a distance of about 20 pc. As opposed to this smooth swing, the EVPAs are strongly variable within the first parsecs of the jet. We find an overall tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The polarized flux density increases rapidly at that distance and eventually saturates towards the outermost observable regions. The transverse extent of the flow suddenly decreases simultaneously to a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradient of the feature size and particle density with distance steepens significantly at that region.Conclusions.We interpret the propagating polarized features as shocks and the observed local anomalies as the interaction of these shocks with a localized recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region.

Radio Galaxy Zoo: A Search for Hybrid Morphology Radio Galaxies

Abstract Hybrid morphology radio sources (HyMoRS) are a rare type of radio galaxy that display different Fanaroff–Riley classes on opposite sides of their nuclei. To enhance the statistical analysis of HyMoRS, we embarked on a large-scale search of these sources within the international citizen science project, Radio Galaxy Zoo (RGZ). Here, we present 25 new candidate hybrid morphology radio galaxies. Our selected candidates are moderate power radio galaxies ( W Hz−1 sr−1) at redshifts . Hosts of nine candidates have spectroscopic observations, of which six are classified as quasars, one as high- and two as low-excitation galaxies. Two candidate HyMoRS are giant ( Mpc) radio galaxies, one resides at the center of a galaxy cluster, and one is hosted by a rare green bean galaxy. Although the origin of the hybrid morphology radio galaxies is still unclear, this type of radio source starts depicting itself as a rather diverse class. We discuss hybrid radio morphology formation in terms of the radio source environment (nurture) and intrinsically occurring phenomena (nature; activity cessation and amplification), showing that these peculiar radio galaxies can be formed by both mechanisms. While high angular resolution follow-up observations are still necessary to confirm our candidates, we demonstrate the efficacy of the RGZ in the pre-selection of these sources from all-sky radio surveys, and report the reliability of citizen scientists in identifying and classifying complex radio sources.

Search for very high energy gamma-ray emission from the peculiar radio galaxy IC 310 with TACTIC during 2012 to 2015

Non-blazar active galactic nuclei like radio galaxies have emerged as a new class of γ-ray sources in the sky. Observations of very high energy (VHE) γ-rays from radio galaxies with misaligned jets offer a unique tool to understand the physical processes involved in these type of objects. In this work, we present the results of our observations of the nearby peculiar radio galaxy IC 310 (z = 0.0189) with TACTIC telescope for nearly 95.5 hours from 03 December, 2012 to 19 January, 2015 (MJD 56265 - 57041). Detailed analysis of the data reveals absence of a statistically significant γ-ray signal from the source direction (both on the overall period and on yearly basis). Our results suggest that the source was possibly in a low-TeV emission state (below the TACTIC sensitivity level) during the above mentioned observation period and the resulting 3σ upper limit on the integral flux above 850 GeV has been estimated to be 4.99×10−12phcm−2s−1 (23% of the Crab Nebula flux). Analysis of the contemporaneous data collected by Fermi-LAT in the 30 – 300 GeV energy range, also indicate the absence of a statistically significant γ-ray signal, therefore 2σ upper limit on the integral flux above 30 GeV has been estimated on yearly basis. We also report the results from dedicated optical observations in B, V and R bands from ARIES observatory carried out from December, 2014 to March, 2015.

A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674

The existence of binary supermassive black holes (SBHs) is predicted by models of hierarchical galaxy formation. To date, only a single binary SBH has been imaged, at a projected separation of 7.3 parsecs. Here we report the detection of a candidate dual SBH with projected separation of 0.35 pc in the gas-rich interacting spiral galaxy NGC 7674 (Mrk 533). This peculiar Seyfert galaxy possesses a $\sim$0.7 kpc Z-shaped radio jet; the leading model for the formation of such sources postulates the presence of an uncoalesced binary SBH created during the infall of a satellite galaxy. Using very long baseline interferometry (VLBI), we imaged the central region of Mrk 533 at radio frequencies of 2, 5, 8 and 15 GHz. Two, possibly inverted-spectrum radio cores were detected at 15 GHz only; the 8-15 GHz spectral indices of the two cores are $\ge-0.33$ and $\ge-0.38$ ($\pm 30\%$), consistent with accreting SBHs. We derive a jet speed $\sim0.28c$ from multi-epoch parsec-scale data of the hotspot region, and a source age $\ge8.2\times10^3$ yrs.

‘Zwicky's Nonet’: a compact merging ensemble of nine galaxies and 4C 35.06, a peculiar radio galaxy with dancing radio jets

We report the results of our radio, optical and infra-red studies of a peculiar radio source 4C~35.06, an extended radio-loud AGN at the center of galaxy cluster Abell 407 ($z=0.047$). The central region of this cluster hosts a remarkably tight ensemble of nine galaxies, the spectra of which resemble those of passive red ellipticals, embedded within a diffuse stellar halo of $\sim$1~arcmin size. This system (named the `Zwicky's Nonet') provides unique and compelling evidence for a multiple-nucleus cD galaxy precursor. Multifrequency radio observations of 4C~35.06 with the Giant Meterwave Radio Telescope (GMRT) at 610, 235 and 150 MHz reveal a system of 400~kpc scale helically twisted and kinked radio jets and outer diffuse lobes. The outer extremities of jets contain extremely steep spectrum (spectral index -1.7 to -2.5) relic/fossil radio plasma with a spectral age of a few$\,\times (10^7 - 10^8)$ yr. Such ultra-steep spectrum relic radio lobes without definitive hot-spots are rare, and they provide an opportunity to understand the life-cycle of relativistic jets and physics of black hole mergers in dense environments. We interpret our observations of this radio source in the context of the growth of its central black hole, triggering of its AGN activity and jet precession, all possibly caused by galaxy mergers in this dense galactic system. A slow conical precession of the jet axis due to gravitational perturbation between interacting black holes is invoked to explain the unusual jet morphology.

A direct localization of a fast radio burst and its host.

Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities that are orders of magnitude larger than those of all known short-duration radio transients. So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts and tidal disruption events. However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.

The 5 Gyr evolution of sub-M* galaxies

We gathered two complete samples of $M_{AB}(r)<-18$ ($M_{star} > 10^{9} M_{\odot}$) galaxies, which are representative of the present-day galaxies and their counterparts at 5 Gyr ago. We analysed their 2D luminosity profiles and carefully decomposed them into bulges, bars and discs. This was done in a very consistent way at the two epochs, by using the same image quality and same (red) filters at rest. We classified them into elliptical, lenticular, spiral, and peculiar galaxies on the basis of a morphological decision tree. We found that at $z=0$, sub-M* ($10^{9} M_{\odot} < M_{star} < 1.5 \times 10^{10} M_{\odot}$) galaxies follow a similar Hubble Sequence compared to their massive counterparts, though with a considerable larger number of (1) peculiar galaxies and (2) low surface brightness galaxies. These trends persist in the $z\sim0.5$ sample, suggesting that sub-M* galaxies have not reached yet a virialised state, conversely to their more massive counterparts. The fraction of peculiar galaxies is always high, consistent with a hierarchical scenario in which minor mergers could have played a more important role for sub-M* galaxies than for more massive galaxies. Interestingly, we also discovered that more than 10\% of the sub-M* galaxies at z=0.5 are low surface brightness galaxies with clumpy and perturbed features that suggest merging. Even more enigmatic is the fact that their disc scale-lengths are comparable to that of M31, while their stellar masses are similar to that of the LMC.

H i observations of galaxies in the southern filament of the Virgo Cluster with the Square Kilometre Array Pathfinder KAT-7 and the Westerbork Synthesis Radio Telescope

We map the H I distribution of galaxies in a ˜1.5 × 2.5° region located at the virial radius south of the Virgo Cluster using the KAT-7 and the Westerbork Synthesis Radio Telescope interferometers. Because of the different beam sizes of the two telescopes, a similar column density sensitivity of NH I ˜ 1 × 1018 atoms cm- 2 was reached with the two observations over 16.5 km s-1. We pioneer a new approach to combine the observations and take advantage of their sensitivity to both the large- and small-scale structures. Out to an unprecedented extent, we detect an H I tail of ˜60 kpc being stripped off NGC 4424, a peculiar spiral galaxy. The properties of the galaxy, together with the shape of the tail, suggest that NGC 4424 is a post-merger galaxy undergoing ram pressure stripping as it falls towards the centre of the Virgo Cluster. We detect a total of 14 galaxies and three H I clouds lacking optical counterparts. One of the clouds is a new detection with an H I mass of 7 × 107 M⊙ and a strong H I profile with W50 = 73 km s-1. We find that 10 out of the 14 galaxies present H I deficiencies not higher than those of the cluster's late spirals, suggesting that the environmental effects are not more pronounced in the region than elsewhere in the cluster.

BUDHIES – III: the fate of H i and the quenching of galaxies in evolving environments

In a hierarchical Universe clusters grow via the accretion of galaxies from the field, groups and even other clusters. As this happens, galaxies can lose their gas reservoirs via different mechanisms, eventually quenching their star-formation. We explore the diverse environmental histories of galaxies through a multi-wavelength study of the combined effect of ram-pressure stripping and group "processing" in Abell 963, a massive growing cluster at $z=0.2$ from the Blind Ultra Deep HI Environmental Survey (BUDHIES). We incorporate hundreds of new optical redshifts (giving a total of 566 cluster members), as well as Subaru and XMM-Newton data from LoCuSS, to identify substructures and evaluate galaxy morphology, star-formation activity, and HI content (via HI deficiencies and stacking) out to $3\times R_{200}$. We find that Abell 963 is being fed by at least 7 groups, that contribute to the large number of passive galaxies outside the cluster core. More massive groups have a higher fraction of passive and HI-poor galaxies, while low-mass groups host younger (often interacting) galaxies. For cluster galaxies not associated with groups we corroborate our previous finding that HI gas (if any) is significantly stripped via ram-pressure during their first passage through the intra-cluster medium, and find mild evidence for a starburst associated with this event. In addition, we find an overabundance of morphologically peculiar and/or star-forming galaxies near the cluster core. We speculate that these arise as groups pass through the cluster (post-processing). Our study highlights the importance of environmental quenching and the complexity added by evolving environments.

IGR J14488–4008: an X-ray peculiar giant radio galaxy discovered by INTEGRAL

We acknowledge financial support from ASI under contract INTEGRAL 2013.23.RO. We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.

Abell 1033: birth of a radio phoenix

Extended steep-spectrum radio emission in a galaxy cluster is usually associated with a recent merger. However, given the complex scenario of galaxy cluster mergers, many of the discovered sources hardly fit into the strict boundaries of a precise taxonomy. This is especially true for radio phoenixes that do not have very well defined observational criteria. Radio phoenixes are aged radio galaxy lobes whose emission is reactivated by compression or other mechanisms. Here, we present the detection of a radio phoenix close to the moment of its formation. The source is located in Abell 1033, a peculiar galaxy cluster which underwent a recent merger. To support our claim, we present unpublished Westerbork Synthesis Radio Telescope and Chandra observations together with archival data from the Very Large Array and the Sloan Digital Sky Survey. We discover the presence of two sub-clusters displaced along the N-S direction. The two sub-clusters probably underwent a recent merger which is the cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum extended radio source very close to an AGN is proposed to be a newly born radio phoenix: the AGN lobes have been displaced/compressed by shocks formed during the merger event. This scenario explains the source location, morphology, spectral index, and brightness. Finally, we show evidence of a density discontinuity close to the radio phoenix and discuss the consequences of its presence.

H i study of the environment around ESO 243−49, the host galaxy of an intermediate-mass black hole

The lenticular galaxy ESO 243−49 hosts the ultraluminous X-ray source HLX-1, the best candidate intermediate-mass black hole (IMBH) currently known. The environments of IMBHs remain unknown, however, the proposed candidates include the nuclei of dwarf galaxies or globular clusters. Evidence at optical wavelengths points at HLX-1 being the remnant of an accreted dwarf galaxy. Here we report the Australia Telescope Compact Array radio observations of H i emission in and around ESO 243−49 searching for signatures of a recent merger event. No H i line emission is detected in ESO 243−49 with a 5σ upper limit on the H i gas mass of a few 108 M⊙. A likely reason for this non-detection is the cluster environment depleting ESO 243−49's H i gas reservoir. The upper limit is consistent with an interpretation of HLX-1 as a dwarf satellite of ESO 243−49, however, more sensitive observations are required for a detection. We detect ∼5× 108 M⊙ of H i gas in the peculiar spiral galaxy AM 0108−462, located at a projected distance of ∼170 kpc from ESO 243−49. This amount of H i gas is ∼10 times less than in spiral galaxies with similar optical and near-infrared properties in the field, strengthening the conclusion that the cluster environment indeed depletes the H i gas reservoir of these two galaxies. Here we also report observations of AM 0108−462 in several optical and near-infrared bands using the Magellan 6.5-m telescopes, and archival X-ray and ultraviolet observations with XMM–Newton and Swift. These data combined with the H i line data suggest it is likely that AM 0108−462 is experiencing a merger event.

INTEGRAL-FIELD STELLAR AND IONIZED GAS KINEMATICS OF PECULIAR VIRGO CLUSTER SPIRAL GALAXIES

We present the stellar and ionized gas kinematics of 13 bright peculiar Virgo cluster galaxies observed with the DensePak Integral Field Unit at the WIYN 3.5-meter telescope, to seek kinematic evidence that these galaxies have experienced gravitational interactions or gas stripping. 2-Dimensional maps of the stellar velocity $V$, and stellar velocity dispersion $\sigma$ and the ionized gas velocity (H$\beta$ and/or [\ion{O}{3}]) are presented for galaxies in the sample. The stellar rotation curves and velocity dispersion profiles are determined for 13 galaxies, and the ionized gas rotation curves are determined for 6 galaxies. Misalignments between the optical and kinematical major axis are found in several galaxies. While in some cases this is due to a bar, in other cases it seems associated with a gravitational interaction or ongoing ram pressure stripping. Non-circular gas motions are found in nine galaxies, with various causes including bars, nuclear outflows, or gravitational disturbances. Several galaxies have signatures of kinematically distinct stellar components, which are likely signatures of accretion or mergers. We compute for all galaxies the angular momentum parameter $\lambda_{\rm R}$. An evaluation of the galaxies in the $\lambda_{\rm R}$-ellipticity plane shows that all but 2 of the galaxies have significant support from random stellar motions, and have likely experienced gravitational interactions. This includes some galaxies with very small bulges and truncated/compact H$\alpha$ morphologies, indicating that such galaxies cannot be fully explained by simple ram pressure stripping, but must have had significant gravitational encounters. Most of the sample galaxies show evidence for ICM-ISM stripping as well as gravitational interactions, indicating that the evolution of a significant fraction of cluster galaxies is likely strongly impacted by both effects.