Hickson 96: a physical compact group ?
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We analyze the morphology and dynamics of the galaxies of the Hickson 96 compact group by means of deep CCD images in the B, V and R photometric bands and long{slit spectroscopy. The four galaxies of this spiral rich group show signs of gravitational interaction. Two long tails come out from the con- tact region of the close pair H96ac. It is also there where both galaxies, bi-symmetrical in their inner parts, loose one of their arms. Moreover, both the photometric and kinematical center of H96a are displaced relative to the center of the disk. H96b, a giant elliptical galaxy, shows signicant deviations from a r 1=4 law at the inner parts and small rotation along its ma- jor axis. We nd at the centre of the galaxy an elongated com- ponent which is kinematically decoupled. A wide faint plume seems to emerge from this galaxy. H96d, the smallest galaxy of the group, seems to be influenced by the bigger members, as indicated by three prominent knots of recent bursts of star for- mation in its blue disk, and by hints of optical bridges joining H96d with H96a and b. Finallythegrouphasalowvelocitydispersion(160 km s 1 ) and is well isolated - no galaxies with comparable magnitude to H96a or H96b are found in its neighborhood. All these results lead us to conclude that Hickson 96 constitutes a real physical system.Keywords:
Peculiar galaxy
Center (category theory)
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We present a two-pronged approach to the formation of early-type galaxies, using a sample of 18 galaxies at 0.5 < z < 1 from the HST/ACS Ultra Deep Field and GRAPES surveys: 1) We combine slitless low resolution spectroscopy from the GRAPES dataset with simple models of galaxy formation to explore their star formation histories. 2) We also perform an analysis of their surface brightness distribution with the unprecedented details provided by the ACS superb angular resolution and photometric depth. Our spectroscopic analysis reveals that their stellar populations are rather homogeneous in age and metallicity and formed at redshifts z ~ 2-5. Evolving them passively, they become practically indistinguishable from ellipticals at z = 0. Also, their isophotal shapes appear very similar to those observed for nearby ellipticals, in that the percentages of disky and boxy galaxies at z ~ 1 are close to the values measured at z = 0. Moreover,we find that the isophotal structure of z ~ 1 early-type galaxies obeys the correlations already observed among nearby ellipticals, i.e. disky ellipticals have generally higher characteristic ellipticities, and boxy ellipticals have larger half-light radii and are brighter in the restframe B band. In this respect then, no significant structural differences are seen for ellipticals between z = 0 and 1.
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view Abstract Citations (434) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Neutral Hydrogen in Virgo Cluster Galaxies. I. The Atlas Cayatte, V. ; van Gorkom, J. H. ; Balkowski, C. ; Kotanyi, C. Abstract H I maps, velocity fields, position velocity plots, mosaics of the line channel, and H I profiles are presented for the 25 brightest spiral galaxies located in the center of the Virgo Cluster. A synthetic map presents the maps of all the individual galaxies. It confirms that the galaxies located within 3^deg^ of M87 have H I disks that are significantly smaller than the optical disks. Further away a number of galaxies have a very asymmetric H I distribution and a sharp edge on the side which points toward M87. The anemic galaxies usually show a ring structure and a patchy aspect. Several gas-rich dwarf galaxies have been found close to H I deficient galaxies and one has been found close to the cluster center. The galaxies located in the western part are larger than in the eastern part. Furthermore the most severely stripped spirals show signs of strong asymmetries in their gas distribution combined with enhanced star formation at the compressed side of the gas disk. This might suggest that we see in the Virgo Cluster a mild form of ram-pressure enhanced star formation, thought to be an important process in clusters at higher redshifts. Publication: The Astronomical Journal Pub Date: September 1990 DOI: 10.1086/115545 Bibcode: 1990AJ....100..604C Keywords: H I Regions; Hydrogen; Interstellar Gas; Very Large Array (Vla); Virgo Galactic Cluster; Red Shift; Spiral Galaxies; Velocity Distribution; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: INTERSTELLAR MATTER full text sources ADS | data products SIMBAD (26) NED (25)
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The present paper is a companion of two others dedicated one to the measurement of the line–strength indices (Longhetti et al. 1997a) and the second to trace back the star formation history of a sample of early–type galaxies by comparing observed indices to the predictions of new spectro–photometric models (Longhetti et al. 1997b). The sample of 51 early-type galaxies in low density environments is composed of two sub-sets of galaxies: 21 shell galaxies from the Malin & Carter (1983) catalogue (one of which shows double nucleous and has been considered as two separate objects) and 30 members of isolated interacting pairs from the Reduzzi & Rampazzo (1995) catalogue. Most of the objects show fine structures. The paper collects nuclear kinematic data together with the velocity and velocity dispersion curves of the stellar and gaseous components as a function of the distance from the galaxies centres. The galaxies heliocentric systemic velocity compares within km s-1 with RC3 data, while their central velocity dispersion compares within km s-1, km s-1 and km s-1 with Gonzalez (1993), Davies et al. (1987) and Carter et al. (1988) respectively. The detailed comparison between our velocity and velocity dispersion curves and those from several authors is discussed. 9 out of 22 shell galaxies nuclei show emission lines, 4 of which, using data in the literature, have line ratios characteristic of LINERs. 10 members of pairs out of 30 show emission lines. RR 331a has a Seyfert like nucleus, while for the remaining galaxies the ([O III] λ 5007)/Hβ ratio is characteristic of low ionization regions. In a small fraction of the objects the emission component is detectable outside the central value. None of the objects in the sample shows counter-rotation of the gaseous versus the stellar component. The two components appear associated, although, in two cases there is evidence that gas and stars lie on different planes. This latter phenomenon could be associated to accretion events. Emission lines in the central part of the RR 331a show a secondary component in the emission lines profile. E 2400100 has two nuclei embedded in the main body of the galaxy. The U-shape profile of the stellar velocity profile shows the ongoing interaction of the two nuclei. profile of shell galaxies is, finally, discussed in relation to the hypothesis of the accretion/merging origin of these galaxies.
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To understand the processes that build up galaxies we investigate the stellar structure and gas kinematics of spiral and irregular galaxies out to redshift 1. We target 92 galaxies in four cluster ( z = 0.3 & 0.5 ) fields to study the environmental influence. Their stellar masses derived from multiband VLT/FORS photometry are distributed around but mostly below the characteristic Schechter-fit mass. From HST/ACS images we determine morphologies and structural parameters like disk length, position angle and ellipticity. Combining the spectra of three slit positions per galaxy using the MXU mode of VLT/FORS2 we construct the two-dimensional velocity field from gas emission lines for 16 cluster members and 33 field galaxies. The kinematic position angle and flatness are derived by a Fourier expansion of elliptical velocity profiles. To trace possible interaction processes, we define three irregularity indicators based on an identical analysis of local galaxies from the SINGS project. Our distant sample displays a higher fraction of disturbed velocity fields with varying percentages (10%, 30% and 70%) because they trace different features. While we find far fewer candidates for major mergers than the SINS sample at z ˜ 2, our data are sensitive enough to trace less violent processes. Most irregular signatures are related to star formation events and less massive disks are affected more than Milky-Way type objects. We detect similarly high fractions of irregular objects both for the distant field and cluster galaxies with similar distributions. We conclude that we may witness the building-up of disk galaxies still at redshifts z ˜ 0.5 via minor mergers and gas accretion, while some cluster members may additionally experience stripping, evaporation or harassment interactions.
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We present B and R CCD images and optical spectroscopy in the range 3700–8900 Å for the galaxy NGC 5666, which until now was considered as a low-luminosity elliptical. The high-resolution images show a conspicuous spiral pattern which is limited to the inner region (, or 1/6 of the isophotal radius) and a faint (presumably tidal) outer feature dotted with small knots resembling star clusters or tidal dwarf galaxies. The patchy spiral structure and the luminosity profiles suggest a late-type morphological type, but the disk is of high central surface brightness and the bulge weak and very extended. The nuclear spectrum reveals typical emission lines found in late-type galaxies, but the underlying nuclear stellar population and continuum only match that of an elliptical galaxy combined with a young (<1 Gyr) stellar population. These and other properties of the galaxy suggest that this could be a minor merger, at an intermediate stage, between a gas-rich dwarf and a small early-type disk galaxy. If this is the case, the galaxy NGC 5666 will provide important constraints to the theory of hierarchical galaxy formation.
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Here we present H i line and 20-cm radio continuum data of the nearby galaxy pair NGC 1512/1510 as obtained with the Australia Telescope Compact Array (ATCA). These are complemented by GALEX (Galaxy Evolution Explorer) ultraviolet (UV)-, SINGG Hα- and Spitzer mid-infrared images, allowing us to compare the distribution and kinematics of the neutral atomic gas with the locations and ages of the stellar clusters within the system. For the barred, double-ring galaxy NGC 1512 we find a very large H i disc, ∼four times its optical diameter, with two pronounced spiral/tidal arms. Both its gas distribution and the distribution of the star-forming regions are affected by gravitational interaction with the neighbouring blue compact dwarf galaxy NGC 1510. While the inner disc of NGC 1512 shows quite regular rotation, deviations are visible along the outer arms and at the position of NGC 1510. From the H i rotation curve of NGC 1512 we estimate a dynamical mass of Mdyn≳ 3 × 1011 M⊙, compared to an H i mass of MH i= 5.7 × 109 M⊙ (∼2 per cent Mdyn). The two most distant H i clumps, at radii of ∼80 kpc, show signs of star formation (SF) and are likely tidal dwarf galaxies (TDGs). Both lie along an extrapolation of the eastern-most H i arm, with the most compact H i cloud located at the tip of the arm. The 20-cm radio continuum map indicates extended SF activity not only in the central regions of both galaxies but also in between them. SF in the outer disc of NGC 1512 is revealed by deep optical- and two-colour UV images. Using the latter we determine the properties of ≳200 stellar clusters and explore their correlation with dense H i clumps in the even larger 2X-H i disc. Outside the inner star-forming ring of NGC 1512, which must contain a large reservoir of molecular gas, H i turns out to be an excellent tracer of SF activity. The multiwavelength analysis of the NGC 1512/1510 system, which is probably in the first stages of a minor merger having started ∼400 Myr ago, links stellar and gaseous galaxy properties on scales from 1 to 100 kpc.
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We have used the ACS camera on HST to obtain (V, I) photometry for 5300 red giant stars in the halo of the dominant Leo group member NGC 3379, a galaxy usually regarded as a classic normal giant elliptical. We use this data to derive the metallicity distribution function (MDF) for its outer-halo field stars at a location centered 33 kpc from the galaxy center. In at least two ways the MDF is distinctly unlike all the other E galaxies for which similar data exist. First, the NGC 3379 MDF is extremely broad and flat, with many stars at every interval in [m/H]. Second, we see a metallicity gradient such that in ithe outermost parts of the field the low-metallicity stars ([m/H] < -0.7) begin to dominate and the higher metallicity stars are rapidly diminishing. We find that a distinct two-stage chemical evolution model is necessary to explain the MDF shape. Our target field is centered at a projected distance of 12Re, twice as far out in units of effective radius as in any of the other galaxies yet surveyed. If NGC 3379 is indeed representative of large E/S0 galaxies, we predict that other such galaxies will reveal diffuse low-metallicity subpopulations, but that photometry at radii r ≃ (10-15)Re will be necessary to get beyond the edge of the dominant metal-rich component.
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We present observations of the nuclear star cluster in the nearby edge-on spiral galaxy NGC 4244 using the Gemini Near-Infrared Integral Field Spectrograph (NIFS) with laser guide star adaptive optics. From a previous study of edge-on galaxies, this nuclear star cluster was found to be one of a sample of clusters that appear flattened along the plane of their host galaxies disks. Such clusters show evidence for multiple morphological components, with younger/bluer disk components and older/redder spheroidal components. Our new observations of NGC 4244 show clear rotation of 30 km/sec within the central 10 pc (0.5") of the cluster. The central velocity dispersion is found to be 28+/-2 km/sec. The multiple stellar populations inferred from the optical colors and spectra are seen as variations in the CO line strength in the NIFS spectra. The rotation is clearly detected even in the older, more spheroidal stellar component. We discuss evidence for similar structures and kinematics in the nuclear star clusters of other galaxies including M33 and the Milky Way. Our observations support two possible formation mechanisms: (1) episodic accretion of gas from the disk directly onto the nuclear star cluster, or (2) episodic accretion of young star clusters formed in the central part of the galaxy due to dynamical friction.
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