H. Bravo-Alfaro

The strong transformation of spiral galaxies infalling into massive clusters at z≈ 0.2

We describe two peculiar galaxies falling into the massive galaxy clusters Abell 1689 (Z ≈ 0.18) and Abell 2667 (Z ≈0.23), respectively. Hubble Space Telescope (HST) images show extraordinary trails composed of bright blue knots (-16.5 < M < -11.5 mag) and stellar streams associated with each of these systems. Combining optical, near- and mid-infrared and radio observations we prove that while both galaxies show similar extended trails of star-forming knots, their recent star formation histories are different. One (≈L*) is experiencing a strong burst of star formation, appearing as a rare example of a luminous infrared cluster galaxy. In comparison, the other (≈0.1L*) has recently ceased its star formation activity. Our model suggests that the morphologies and star formation in these galaxies have been influenced by the combined action of tidal interaction (likely with the cluster potential) and of ram pressure with the intracluster medium (ICM). These results can be used to gain more insights to the origin of SOs, dwarf and ultracompact dwarf (UCD) cluster galaxies.

The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies

Using a sample of 91 galaxies distributed over 27 compact groups (CGs) of galaxies, we define an index that allows us to quantify their level of activity due to an active galactic nucleus (AGN) or star formation. By combining the mean activity index with the mean morphological type of the galaxies in a group, we are able to quantify the evolutionary state of the groups. We find that they span an evolutionary sequence that correlates with the spatial configuration of the galaxies in the CG. We distinguish three main configuration types: A, B, and C. Type A CGs show predominantly low velocity dispersions and are rich in late-type spirals that show active star formation or harbor an AGN. Type B groups have intermediate velocity dispersions and contain a large fraction of interacting or merging galaxies. Type C comprises CGs with high velocity dispersions, which are dominated by elliptical galaxies that show no activity. We suggest that evolution proceeds A B C. Mapping the groups with different evolution levels in a diagram of radius versus velocity dispersion does not reveal the pattern expected based on the conventional fast merger model for CGs, which predicts a direct relation between these two parameters. Instead, we observe a trend contrary to expectation: the evolutionary state of a group increases with velocity dispersion. This trend seems to be related to the masses of the structures in which CGs are embedded. In general, the evolutionary state of a group increases with the mass of the structure. This suggests either that galaxies evolve more rapidly in massive structures or that the formation of CGs embedded in massive structures predated the formation of CGs associated with lower mass systems. Our observations are consistent with the structure formation predicted by the CDM model (or ΛCDM), only if the formation of galaxies is a biased process.

Probing evolutionary mechanisms in galaxy clusters: neutral atomic hydrogen in Abell 1367

The definitive article can be found at: http://www3.interscience.wiley.com/ Copyright Royal Astronomical Society

Two long HI tails in the outskirts of Abell 1367

We present VLA D{array Hi observations of the RSCG42 and FGC1287 galaxy groups, in the outskirts of the Abell 1367 cluster. These groups are projected 1.8 and 2.7 Mpc west from the cluster centre. The Arecibo Galaxy Environment survey provided evidence for Hi extending over as much as 200 kpc in both groups. Our new, higher resolution observations reveal that the complex Hi features detected by Arecibo are in reality two extraordinary long Hi tails extending for 160 and 250 kpc, respectively, i.e., among the longest Hi structures ever observed in groups of galaxies. Although in the case of RSCG42 the morphology and dynamics of the Hi tail, as well as the optical properties of the group members, support a low-velocity tidal interaction scenario, less clear is the origin of the unique features associated with FGC1287. This galaxy displays an exceptionally long ‘dog leg’ Hi tail and the large distance from the X{ray emitting region of Abell 1367 makes a ram-pressure stripping scenario highly unlikely. At the same time a low-velocity tidal interaction seems unable to explain the extraordinary length of the tail and the lack of any sign of disturbance in the optical properties of FGC1287. An intriguing possibility could be that this galaxy might have recently experienced a high{speed interaction with another member of the Coma{Abell 1367 Great Wall. We searched for the interloper responsible for this feature and, although we nd a possible candidate, we show that without additional observations it is impossible to settle this issue. While the mechanism responsible for this extraordinary Hi tail remains to be determined, our discovery highlights how little we know about environmental eects in galaxy groups.

Galaxy evolution in Abell 85: I. Cluster substructure and environmental effects on the blue galaxy population

In this series of papers we explore the evolution of late-type galaxies in the rich cluster Abell 85. In this first paper we revisit the complex dynamical state of A 85 by using independent methods. First, we analyze the galaxy redshift distribution towards A 85 in the whole range 0−40 000 km s −1 , and determine the mean redshifts of the background clusters A 87 and A 89, very close in projection to A 85. Then we search for substructures in A 85 by considering the 2D galaxy distribution of its members (13 000−20 000 km s −1 )a nd by applying the kinematical 3D Δ-test to both projected positions and radial velocities. This clearly reveals several substructures: one close to the cluster core and three more projected towards the southeast, along the region where an X-ray filament has been extensively studied. We also analyse the distribution of the brightest blue galaxies across a major fraction of the cluster volume, considering if they are gas-rich or poor. We report a very asymmetric distribution of the blue member galaxies, with most of them to the east and southeast, namely in the region joining the core of A 85 to its farthest substructure in this direction – dubbed the SE clump. By matching our sample of bright blue member galaxies with H i detections reported in the literature, we identify gas-rich and gas-poor ones. As expected, the H i-rich blue galaxies follow the same trend as the parent sample, with most of them projected on the eastern side of the cluster as well. Interestingly no blue objects have been detected in H i up to a projected radius of 2 Mpc in this zone. We finally estimate the ram pressure stripping exerted by the intra-cluster medium as a function of the projected distance from A 85, in order to quantify how important this mechanism might be in sweeping the gas out of the infalling spirals.

CO in late-type galaxies within the central region of Abell 1367

We present (CO)-C-12(J = 1-0) and (CO)-C-12(J = 2-1) spectra for 19 bright, late-type galaxies (spirals) in the central region of the galaxy cluster Abell 1367 (z = 0.02) from observations made with the IRAM 30-m telescope. All 19 spirals were observed at the position of their optical centre and for a subset, at multiple positions. For each spiral the integrated CO (J = 1-0) intensity from the central pointing, in few cases supplemented with intensities from offset pointings, was used to estimate its molecular hydrogen mass and H-2 deficiency. Accepting the considerable uncertainties involved in determining H-2 deficiencies, spirals previously identified by us to have redder colours and higher H I deficiencies as a result of environmental influence were found to be more H-2 deficient compared to members of the sample in less advanced evolutionary states. For eight of the observed spirals multiple pointing observations were made to investigate the distribution of their molecular gas. For these spirals we fitted Gaussians to the CO intensities projected in a line across the galaxy. In two cases, CGCG 097-079 and CGCG 097-102(N), the offset between the CO and optical intensity maxima was significantly larger than the pointing uncertainty, and the full width at half-maximum values of the fits were significantly greater than those of the other spirals, irrespective of optical size. Both signatures are indicators of an abnormal molecular gas distribution. In the case of CGCG 097-079, which is considered an archetype for ram-pressure stripping, our observations indicate that the CO intensity maximum lies similar to 15.6 +/- 8.5 arcsec (6 kpc) north-west of the optical centre at the same projected position as the HI intensity maximum.

HUBBLE SPACE TELESCOPE AND HI IMAGING OF STRONG RAM PRESSURE STRIPPING IN THE COMA SPIRAL NGC 4921: DENSE CLOUD DECOUPLING AND EVIDENCE FOR MAGNETIC BINDING IN THE ISM

Remarkable dust extinction features in the deep Hubble Space Telescope (HST) V and I images of the face-on Coma cluster spiral galaxy NGC 4921 show in unprecedented ways how ram pressure strips the ISM from the disk of a spiral galaxy. New VLA HI maps show a truncated and highly asymmetric HI disk with a compressed HI distribution in the NW, providing evidence for ram pressure acting from the NW. Where the HI distribution is truncated in the NW region, HST images show a well-defined, continuous front of dust that extends over 90? and 20 kpc. This dust front separates the dusty from dust-free regions of the galaxy, and we interpret it as galaxy ISM swept up near the leading side of the ICM?ISM interaction. We identify and characterize 100 pc?1 kpc scale substructure within this dust front caused by ram pressure, including head?tail filaments, C-shaped filaments, and long smooth dust fronts. The morphology of these features strongly suggests that dense gas clouds partially decouple from surrounding lower density gas during stripping, but decoupling is inhibited, possibly by magnetic fields that link and bind distant parts of the ISM.

Early Science with the Large Millimeter Telescope: Detection of Dust Emission in Multiple Images of a Normal Galaxy at z > 4 Lensed by a Frontier Fields Cluster

We directly detect dust emission in an optically detected, multiply imaged galaxy lensed by the Frontier Fields cluster MACSJ0717.5+3745. We detect two images of the same galaxy at 1.1 mm with the AzTEC camera on the Large Millimeter Telescope leaving no ambiguity in the counterpart identification. This galaxy, MACS0717_Az9, is at z > 4 and the strong lensing model (μ = 7.5) allows us to calculate an intrinsic IR luminosity of 9.7 × 10^(10) L_⊙ and an obscured star formation rate of 14.6 ± 4.5 M_⊙ yr^(−1). The unobscured star formation rate from the UV is only 4.1 ± 0.3 M_⊙ yr^(−1), which means the total star formation rate (18.7 ± 4.5 M_⊙ yr^(−1)) is dominated (75%–80%) by the obscured component. With an intrinsic stellar mass of only 6.9 × 10^9 M_⊙, MACS0717_Az9 is one of only a handful of z > 4 galaxies at these lower masses that is detected in dust emission. This galaxy lies close to the estimated star formation sequence at this epoch. However, it does not lie on the dust obscuration relation (IRX-β) for local starburst galaxies and is instead consistent with the Small Magellanic Cloud attenuation law. This remarkable lower mass galaxy, showing signs of both low metallicity and high dust content, may challenge our picture of dust production in the early universe.

Highly perturbed molecular gas in infalling cluster galaxies: the case of CGCG97-079

We report on CO (J = 2 -> 1) mapping with the IRAM 30-m HEtrodyne Receiver Array (HERA) of CGCG 97-079, an irregular galaxy in the merging galaxy cluster Abell 1367 (z = 0.022). We find that similar to 80 per cent of the detected CO (J = 2 -> 1) is projected within a 16 arcsec(2) (6.5 kpc(2)) region to the north and west of the optical/NIR centre, with the intensity maximum offset similar to 10 arcsec (4 kpc) NW of the optical/NIR centre and similar to 7 arcsec (3 kpc) south-east of the HI intensity maximum. Evolutionary synthesis models indicate CGCG 97-079 experienced a burst of star formation similar to 10(8) yr ago, most likely triggered by a tidal interaction with CGCG 97-073. For CGCG 97-079 we deduce an infall velocity to the cluster of similar to 1000 km s(-1) and moderate ram pressure (P-ram approximate to 10(-11) dyne cm(-2)). The observed offset in CGCG 97-079 of the highest density HI and CO (J = 2 -> 1) from the stellar components has not previously been observed in galaxies currently undergoing ram pressure stripping, although previous detailed studies of gas morphology and kinematics during ram pressure stripping were restricted to significantly more massive galaxies with deeper gravitational potential wells. We conclude the observed cold gas density maxima offsets are most likely the result of ram pressure and/or the high-speed tidal interaction with CGCG 97-073. However ram pressure stripping is likely to be playing a major role in the perturbation of lower density gas.

An Hi Survey of Clusters in the Local Universe

We outline the project of multifrequency observation of five clusters of galaxies spanning a redshift between 0.05 and 0.2. The core of the project is an HI survey of clusters accomplished with the VLA in its C configuration, and complemented with GMRT data. The 21 cm imaging is being combined with optical spectroscopy and deep NIR imaging with the aim to obtain a database on galaxy evolution in the nearby universe. We choose a sample of clusters with different degrees of dynamical evolution, some containing an important population of starburst or/and post–starburst galaxies, and a hot intracluster medium with emission in X–ray.