Neighbor Galaxy Research Articles

TRANSFORMATION OF GALAXY MORPHOLOGY AND LUMINOSITY

We studied the dependence of morphology and luminosity of the SDSS galaxies on the environmental factors. The environmental factors considered include the local density due to the nearest neighbor galaxy ρn, morphology of the nearest neighbor, and the large-scale background density. We found the local environment set up by the nearest neighbor galaxy gives strong effects on the galaxy morphology. The probability for a galaxy to have an early morphological type critically depends on whether or not ρn is above the virialization density. We conclude that the well-known morphology-density relation is basically due to the interactions between galaxy pairs. Dependence of galaxy morphology on the large-scale density is found only because there is a statistical correlation between the average pair separation and the large-scale background density. We also found that galaxy luminosity depends on ρn, and that, when the large-scale density is fixed, more isolated galaxies are more likely to be recent merger products. We propose a scenario that a series of morphology and luminosity transformation occur through a series of distant/close interactions and mergers, which results in the morphology-luminosity-local density relation.

Verifying the identity of high-redshift massive galaxies through the clustering of lower mass galaxies around them

Massive high-redshift galaxies form in over-dense regions where the probability of forming other galaxies is also strongly enhanced. Given an observed flux of a galaxy, the inferred mass of its host halo tends to be larger as its inferred redshift increases. As the mass and redshift of a galaxy halo increase, the expected clustering of other galaxies around it gets stronger. It is therefore possible to verify the high-redshift identity of a galaxy (prior to an unambiguous spectral identification) from the clustering of other galaxies around it. We illustrate this method for the massive galaxy suggested by Mobasher et al. (2005) to be at redshift z~6.5. If this galaxy were to exist at z~6.5, there should have been a mean of ~10 galaxies larger than a hundredth of its mass and having z-band magnitudes less than ~25 detected as i-dropouts in the HUDF. We calculate an approximate probability distribution for neighbor galaxies and determine that there is less than a ~0.3% chance of detecting no massive neighbor galaxies. The lack of other massive z~6.5 galaxies in the HUDF image argues that the Mobasher et al. (2005) galaxy is instead a low redshift interloper. We generalize our results to other galaxy masses and redshifts.

ASYMMETRY MEASURES FOR QUASI-STELLAR OBJECTS AND COMPANIONS

An asymmetry index is derived from ellipse fitting to galaxy images that gives weight to faint outer features and is not strongly redshift dependent. These measures are made on a sample of 13 Two Micron All Sky Survey quasi-stellar objects (QSOs) and their neighbor galaxies, and a control sample of field galaxies from the same wide-field imaging data. The QSO host-galaxy asymmetries correlate well with visual tidal interaction indices previously published. The companion galaxies have somewhat higher asymmetry than the control galaxy sample, and their asymmetry is inversely correlated with the distance from the QSO. The distribution of QSO-companion asymmetry indices is different from that for matched control field galaxies at the ~95% significance level. We present the data and discuss this evidence for tidal and other disturbances in the vicinity of QSOs.

NGC 1600: Cluster or Field Elliptical?

A study of the galaxy distribution in the field of the elliptical galaxy NGC 1600 has been undertaken. Although this galaxy is often classified as a member of a loose group, all the neighboring galaxies are much fainter and could be taken as satellites of NGC 1600. The number density profile of galaxies in the field of this galaxy shows a decline with radius, with evidence of a background at approximately 1.3 Mpc. The density and number density profile are consistent with that found for other isolated early-type galaxies. NGC 1600 appears as an extended source in X-rays, and the center of the X-ray emission seems not to coincide with the center of the galaxy. The velocity distribution of neighboring galaxies has been measured from optical spectroscopic observations and shows that the mean radial velocity is approximately 85 km s?1 less than that of NGC 1600, indicating that the center of mass could lie outside the galaxy. The velocity dispersion of the group is estimated at -->429 ? 57 km s?1. The inferred mass of the system is therefore of the order of -->1014 -->M?, a value that corresponds to a large group. NGC 1600 therefore shares some similarities, but is not identical to, the fossil clusters detected in X-ray surveys. Implications of this result for studies of isolated early-type galaxies are briefly discussed.

THE ARECIBO LEGACY FAST ALFA SURVEY. V. THE H I SOURCE CATALOG OF THE ANTI-VIRGO REGION AT δ = +27°

We present a second catalog of HI sources detected in the Arecibo Legacy Fast ALFA Survey. We report 488 detections over 135 deg 2 , within the region of the sky having 22h 6:5, where the reliability of the catalog is better than 95% or (b) 5:0 < S=N < 6:5 and a previously measured redshift that corroborates our detection. Of the 488 objects presented here, 49 are High Velocity Clouds or clumps thereof with negative heliocentric recession velocities. These clouds are mostly very compact and isolated, while some of them are associated with large features such as Wright’s Cloud or the northern extension of the Magellanic Stream. The remaining 439 candidate detections are identied as extragalactic objects and have all been matched with optical counterparts. Five of the six galaxies detected with MHI < 10 7:5 M are satellites of either the NGC672/IC1727 nearby galaxy pair or their neighboring dwarf irregular galaxy NGC784. The data of this catalog release include a slice through the Pisces-Perseus foreground void, a large nearby underdensity of galaxies. We report no detections within the void, where our catalog is complete for systems with HI masses of 10 8 M . Gas-rich, optically-dark galaxies do not seem to constitute an important void population, and therefore do not suce at producing a viable solution to the void phenomenon. Subject headings: galaxies: distances and redshifts | galaxies: spiral | galaxies: photometry | radio lines: galaxies | catalogs | surveys

NGC 2655: FROM INNER POLAR RING TO OUTER SHELLS AND TAILS

NGC 2655, a nearby bright S0/a galaxy in a loose group, has strongly asymmetric central dust lanes and an extended disk of neutral hydrogen gas. Here we present deep optical images showing tidal arms and regions of diffuse starlight well beyond the main galaxy, suggestive of a recent merger. Our maps in the 21 cm line show a layer of neutral hydrogen extending to at least 7.5' or 40 kpc from the center, with a broken streamer of gas trailing off toward the small neighbor galaxy UGC 4714. The global profile has sloping shoulders of extreme-velocity gas, usually a sign of interaction. The diffuse stellar light corresponds generally but not in detail with the distribution of H I. In particular, the stellar light of the main tidal arm extends beyond the H I layer, which is limb-brightened as if it had been compressed from outside. The pattern of gas velocities is generally bisymmetric, but the H I layer is strongly warped; it does not share the kinematics of the stellar disk. The gas orbits twist by at least 90° between 1' and 3' of the center, as the H I layer appears to warp through edge-on. The complex optical tails and H I dynamics suggest that the galaxy has undergone multiple mergers. Both at small and at large radii, the gas layer twists in a trailing sense relative to its orbital motion. If the twist results from differential precession in the galaxy's gravitational potential, this would indicate that the dark halo is prolate, elongated along the rotation axis of the disk.

The polar ring galaxy AM1934-563 revisited

We report long-slit spectroscopic observations of the dust-lane polar ring galaxy AM1934−563 obtained with the Southern African Large Telescope (SALT) during its performance verification phase. The observations target the spectral region of the Hα, [N ii] and [S ii] emission lines, but also show deep Na i absorption lines, that we interpret as being produced by stars in the galaxy. We derive rotation curves along the major axis of the galaxy that extend out to about 8 kpc from the centre for both the gaseous and the stellar components, using the emission and absorption lines. We derive similar rotation curves along the major axis of the polar ring and point out differences between these and the ones of the main galaxy. We identify a small diffuse object visible only in Hα emission and with a low velocity dispersion as a dwarf H ii galaxy and argue that it is probably metal poor. Its velocity indicates that it is a fourth member of the galaxy group in which AM1934−563 belongs. We discuss the observations in the context of the proposal that the object is the result of tidal mater transfer from a major neighbour galaxy and point out some observational discrepancies from this explanation. We argue that an alternative scenario that could better fit the observations may be the slow accretion of cold intergalactic gas, focused by a dense filament of galaxies in which this object is embedded. Given the pattern of rotation we found, with the asymptotic rotation of the gas in the ring being slower than that in the disc while both components have approximately the same extent, we point out that AM1934−563 may be a galaxy in which a dark matter halo is flattened along the galactic disc and the first object in which this predicted behaviour of polar ring galaxies in dark matter haloes is fulfilled.

A systematic search for novae in M 31 on a large set of digitized archival Schmidt plates

This paper reports on the detection of optical novae in our neighbour galaxy M31 based on digitized historical Tautenburg Schmidt plates. The accurate positions of the detected novae lead to a much larger database when searching for recurrent novae in M31. We conducted a systematic search for novae on 306 digitized Tautenburg Schmidt plates covering a time span of 36 years from 1960 to 1996. From the database of both ~ 300 000 light curves and about one million detections on only one plate per colour band, nova candidates were efficiently selected by automated algorithms and subsequently individually inspected by eye. We report the detection of 84 nova candidates. We found 55 nova candidates from the automated analysis of the light curves. Among these, 22 were previously unknown, 12 were known but not identified on Tautenburg Schmidt plates before, and 21 novae had been previously discovered on Tautenburg plates. An additional 29 known novae could be confirmed by the detailed investigation of single detections. One of our newly discovered nova candidates shows a high position coincidence with a nova detected about 30 years earlier. Therefore, this object is likely to be a recurrent nova. Furthermore, we re-investigated all 41 nova candidates previously found on Tautenburg plates and confirm all but two. Positions are given for all nova candidates with a typical accuracy of ~ 0.4 arcsec. We present light curves and finding charts as online material. The analysis of the plates has shown the wealth of information still buried in old plate archives. Extrapolating from this survey, digitization of other historical M31 plate archives (e.g. from the Mount Wilson or Asiago observatories) for a systematic nova search looks very promising.

Transformation of Morphology and Luminosity Classes of the SDSS Galaxies

We present a unified picture on the evolution of galaxy luminosity and morphology. Galaxy morphology is found to depend critically on the local environment set up by the nearest neighbor galaxy in addition to luminosity and the large scale density. When a galaxy is located farther than the virial radius from its closest neighbor, the probability for the galaxy to have an early morphological type is an increasing function only of luminosity and the local density due to the nearest neighbor ($\rho_n$). The tide produced by the nearest neighbor is thought to be responsible for the morphology transformation toward the early type at these separations. When the separation is less than the virial radius, i.e. when $\rho_n > \rho_{\rm virial}$, its morphology depends also on the neighbor's morphology and the large-scale background density over a few Mpc scales ($\rho_{20}$) in addition to luminosity and $\rho_n$. The early type probability keeps increasing as $\rho_n$ increases if its neighbor is an early type. But the probability decreases as $\rho_n$ increases when the neighbor is a late type. The cold gas streaming from the late type neighbor can be the reason for the morphology transformation toward late type. The overall early-type fraction increases as $\rho_{20}$ increases when $\rho_n > \rho_{\rm virial}$. This can be attributed to the hot halo gas of the neighbor which is confined by the pressure of the ambient medium held by the background mass. We have also found that galaxy luminosity depends on $\rho_n$, and that the isolated bright galaxies are more likely to be recent merger products. We propose a scenario that a series of morphology and luminosity transformation occur through distant interactions and mergers, which results in the morphology--luminosity--local density relation.

The isolated fossil group RX J1119.7+2126

Fossil groups are galaxy structures that probably underwent a nearly complete fusion of all intermediate magnitude galaxies into a single large central dominant galaxy. However, the formation and evolution processes of these structures are still not well understood. In order to test this scenario and its implications we studied the fossil group RXJ1119.7+2126, based on available spectroscopy of the galaxies in the low density large scale region around the fossil group and deep B and R band imaging of its close vicinity and three comparison fields. We used spectroscopic data to investigate the degree of isolation of RXJ1119.7+2126 in terms of bright neighbour galaxies. The imaging data were used to derive the color magnitude relation and select faint galaxies statistically belonging to this structure. The structure appears as a very isolated group exhibiting a red sequence in the color magnitude relation with characteristics close to the red sequences already observed for other fossil groups. All these results can be interpreted consistently in the framework of the building up process generally proposed for fossil groups.

The Anisotropy in the Galaxy Velocity Field Originated from the Gravitational Pancaking Effect

We analyze the Millennium run semi-analytic galaxy catalog to explore quantitatively the gravitational pancaking effect on the orientation of galaxy velocity field. We first calculate the probability density distribution of the cosine of the angle between the velocity of a field galaxy and the direction normal to a local pancake plane which is determined using two nearest neighbor field galaxies. A clear signal of alignment is detected for the case that the pancake scale is in the range of $5-8h^{-1}$ Mpc. The tendency of the velocity-pancake alignment is found to still exist when the pancakes are determined using three neighbor galaxies, indicating that it has a spatial coherence. The degree of the velocity-pancake alignment is shown to increase with the velocity magnitude and the local density, while it decreases with the separation distance from the galaxy to the pancake and disappears when the pancake has a filamentary shape. A final conclusion is that our work may provide another clue to understanding the large-scale structure in the universe.

The Faint Afterglow and Host Galaxy of the Short-Hard GRB 060121

We present optical and X-ray observations of the afterglow and host galaxy of the short-hard GRB 060121. The faint R-band afterglow is seen to decline as t-0.66±0.09 while the X-ray falls as t-1.18±0.04, indicating the presence of the cooling break between the two frequencies. However, the R-band afterglow is very faint compared to the predicted extrapolation of the X-ray afterglow to the optical regime (specifically, βOX ~ 0.2), while the K-band is consistent with this extrapolation (βKX ~ 0.6), demonstrating suppression of the optical flux. Late-time HST observations place stringent limits on the afterglow R-band flux, implying a break in the R-band light curve. They also show that the burst occurred at the edge of a faint red galaxy, presumably the host, which most likely lies at a significantly higher redshift than the previous optically identified short-duration bursts. Several neighboring galaxies also have very red colors that are similarly suggestive of higher redshift. The least extreme explanation for the faintness and color of the burst is that it occurred at moderately high redshift and was significantly obscured; however, it is also possible that it lies at z > 4.5, in which case the faintness of the R-band afterglow could be attributed to the Lyman break. We discuss the implications that either scenario would have for the nature of the progenitors of short bursts.

Properties of galaxies in Sloan Digital Sky Survey quasar environments at z &lt; 0.2

We analyse the environment of low redshift, z < 0.2, Sloan Digital Sky Survey (SDSS) quasars using the spectral and photometric information of galaxies from the Sloan Digital Sky Survey Third Data Release (SDSS-DR3). We compare quasar neighbourhoods with field and highdensity environments through an analysis on samples of typical galaxies and groups. We compute the surrounding surface number density of galaxies finding that quasar environments systematically avoid high-density regions. Their mean environments correspond to galaxy density enhancements similar to those of typical galaxies. We have also explored several galaxy properties in these environments, such as spectral types, specific star formation rates (SFRs), concentration indexes, colours and active nuclei activity. We find a higher relative fraction of blue galaxies in quasar environments compared to groups and typical galaxy neighbourhoods. Consistent with this picture, the distribution of the concentration index of these galaxies also indicate a larger fraction of late-type objects. By analysing the available information of galaxy spectra we have also studied the distribution of the SFRs of these neighbour galaxies finding that quasar environments are populated by objects with an enhanced star formation activity. An analysis of the relative flux ratios of [O III]λ5700/Hβ and [N II]λ6583/Hα of emission-line galaxies shows no excess of nuclei activity in quasar neighbourhood with respect to the environment of a typical galaxy. We conclude that low-redshift quasar neighbourhoods (r p < 1 h −1 Mpc, � V < 500 km s −1 ) are populated by bluer and more intense star forming galaxies of disc-type morphology than galaxies in groups and in the field. Although star formation activity is thought to be significantly triggered by interactions, we find that quasar fuelling may not require the presence of a close companion galaxy (r p < 100 h −1 kpc, � V < 350 km s −1 ). As a test of the unified active galactic nucleus (AGN) model, we have performed a similar analysis to the neighbours of a sample of active galaxies. The results indicate that these neighbourhoods are comparable to those of quasars giving further support to this unified scenario.

Alignment between luminous red galaxies and surrounding structures at z∼ 0.5

We analyse a high-redshift sample (0.4 < z < 0.5) of luminous red galaxies (LRGs) extracted from the Sloan Digital Sky Survey data release 4 and their surrounding structures to explore the presence of alignment effects of these bright galaxies with neighbour objects. In order to avoid projection effects, we compute photometric redshifts for galaxies within 3 h -1 Mpc in projection of LRGs and calculate the relative angle between the LRG major axis and the direction to neighbours within 1000 km s -1 . We find a clear signal of alignment between LRG orientations and the distribution of galaxies within 1.5 h -1 Mpc. The alignment effects are present only for the red population of tracers; LRG orientation is uncorrelated to the blue population of neighbour galaxies. These results add evidence to the alignment effects between primaries and satellites detected at low redshifts. We conclude that such alignments were already present at z ∼ 0.5.

Interacting E+A System SDSS J161330.18+510335.5. I. Spatially Extended Poststarburst Signatures and Age Gradient

An E+A galaxy and its neighbor were viewed with a long-slit spectrograph (FOCAS) mounted on the Subaru telescope. The galaxy is thought to be experiencing a dynamical interaction with its neighbor, and we have confirmed that the neighbor galaxy is at the same redshift. The neighbor galaxy shows no signs of current star formation or poststarburst activity. We investigated the spatial extent of the poststarburst signatures using long-slit spectroscopy of the E+A galaxy. The signatures extend over more than 5 kpc from the core to the tidal plume, which suggests that the starburst was a galaxy-wide phenomenon. We found a positive gradient in the age of young stars from the center to the outer regions of the plume.

Neighbourhoods of isolated star forming dwarf galaxies

We report the discovery of small groups of uncatalogued, compact, star forming (SF) dwarf galaxies (DGs) by Hα mapping of the neighbourhoods of apparently isolated, catalogued, SF DGs. Our sample consists of dwarf (M≥−18 mag) galaxies at least 2 Mpc away from any other catalogued galaxy. The galaxies were selected to exhibit Hα emission of any intensity, i.e. not selecting only strong starbursts, as an indicator of recent or on-going star formation with the goal of understanding why are they presently forming stars. We identified possible neighbours by imaging the galaxies and their surroundings through Hα filters centred at or near the redshift of the galaxy, and searching for localized Hα emission with the characteristics of the line emission from the sample galaxies. We identified 20 possible SF neighbour galaxies, 17 of them not previously catalogued, in three of the five search fields where we had good quality data, and present here their positions and, images and morphology, as well as some indications of binarity. The relatively large number of possible neighbour candidates, combined with their relative faintness, argue that it would be virtually impossible to identify truly isolated galaxies. It seems that the objects we selected as extremely isolated are probably the brightest members of sparse groups of galaxies, where the other members are also DGs that are presently forming stars. In order to enhance the confidence of this statement regular redshifts are required for our candidate neighbours.

M82, Starbursts, Star Clusters, and the Formation of Globular Clusters

We observed the nearby starburst galaxy M82 in CO in the higher frequency (2-1) transition to achieve an angular resolution below 1'' or 17 pc at the target. We resolved the molecular gas into a large number of compact clouds, with masses ranging from ~2 × 103 to 2 × 106 M☉. The mass spectrum scales as N(M) ∝ M-1.5±0.1, similar to the mass spectra of young massive star clusters, suggesting that individual molecular clouds are transformed in the starburst into individual star clusters. The larger clouds are surrounded by supernovae and H II regions, suggesting that star formation proceeds from the outside of the clouds and progresses inward, consistent with triggering by a sudden increase in external pressure. The clouds with internal star formation have velocity gradients and inverse P Cygni spectral line profiles indicating inward motions of 35 km s-1, consistent with shock-driven compression. Diffuse free-free radio emission and X-ray emission around the clouds provide evidence for superheated ionized gas sufficient to drive the compression. Clouds with spectral lines indicating expansion show little internal star formation, suggesting that the dynamics precedes and is responsible for the star formation rather than the inverse. M82 is known to be in interaction with the neighboring galaxy M81. The overall picture is consistent with the formation of massive star clusters from individual giant molecular clouds crushed by a sudden galactic-scale increase in external pressure generated by the changing dynamics that result from a near-collision with a neighboring galaxy. Present-day globular clusters may have formed in a similar fashion in primordial galaxies.

Massive Binaries in the Magellanic Clouds

We present results of our ongoing observing program on search and studies of massive stars (O and WR type) in binary systems in our neighbor galaxies, the Magellanic Clouds. Radial velocity orbits are presented for two new binaries, one in the Small Magellanic Cloud and another in the Large Magellanic Cloud, and improved orbits for previously known systems. We compare orbital parameters of selected binaries containing O and WR type components. We also discuss the present status of knowledge for massive binary stars in the Magellanic Clouds and the problems encountered in their orbital studies such as stellar winds the ubiquitous tendency to be born in multiple systems.

Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies

The globular cluster (GC) systems of the Milky Way and of our neighboring spiral galaxy, M31, comprise 2 distinct entities, differing in 3 respects. 1. M31 has young GCs, ages from ~100 Myr to 5 Gyr old, as well as old globular clusters. No such young GCs are known in the Milky Way. 2. We confirm that the oldest M31 GCs have much higher nitrogen abundances than do Galactic GCs at equivalent metallicities. 3. Morrison et al. found M31 has a subcomponent of GCs that follow closely the disk rotation curve of M31. Such a GC system in our own Galaxy has yet to be found. These data are interpreted in terms of the hierarchical-clustering-merging (HCM) paradigm for galaxy formation. We infer that M31 has absorbed more of its dwarf systems than has the Milky Way. This inference has 3 implications: 1. All spiral galaxies likely differ in their GC properties, depending on how many companions each galaxy has, and when the parent galaxy absorbs them. The the Milky Way ties down one end of this spectrum, as almost all of its GCs were absorbed 10-12 Gyr ago. 2. It suggests that young GCs are preferentially formed in the dwarf companions of parent galaxies, and then absorbed by the parent galaxy during mergers. 3. Young GCs seen in tidally-interacting galaxies might come from dwarf companions of these galaxies, rather than be made a-new in the tidal interaction. There is no ready explanation for the marked difference in nitrogen abundance for old M31 GCs relative to the oldest Galactic GCs. The predictions made by Li & Burstein regarding the origin of nitrogen abundance in globular clusters are consistent with what is found for the old M31 GCs compared to that for the two 5 Gyr-old M31 GCs.

H II emission line region in LEDA 212995, a small neighboring galaxy of Mrk 1040

AbstractWe present here new spectroscopic observations of Mrk 1040 and LEDA 212995 (Mrk 1040 companion) obtained with the Isaac Newton Telescope (INT). The intensity ratios and widths for the narrow emission lines found in LEDA 212995 are typical of H II regions. The red‐shift (0.0169 ± 0.00015) of the object derived from these emission lines is very close to the red‐shift of Mrk 1040 (z = 0.01665). The weak narrow and broad absorption lines were detected in the Hα wavelength band of LEDA 212995 spectra. These absorptions indicate that the companion might be at least partly obscured by Mrk 1040. Using this and previous observations we discuss the possible physical relationship between these two galaxies. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)