Close Galaxy Pairs Research Articles

Close galaxy pairs in low and medium density regions: The southern sky.

We extend to the southern hemisphere a continuing program of optical and HI observations of galaxy pairs (Chengalur \etal 1993, 1994, 1995 and Nordgren \etal 1997). These pairs are drawn from published redshift catalogs and represent a complete sample. We present new data of 15 pairs observed with the Palomar 5-meter telescope, Mount Stromlo Siding Spring 40-inch telescope, Australia Telescope Compact Array and VLA D synthesis array. These galaxy pairs are all defined as close pairs (projected separations < 100 kpc). HI companions are found near five of 15 pairs.

Evidence for Galaxy Interactions/Mergers from Medium Deep Survey WFPC2 Data

We examine the morphological and statistical properties of close galaxy pairs from two sets of 28 WFPC2 fields, acquired for the Medium Deep Survey (MDS) and the Groth-Westphal Survey (GWS) in F606W (V) and F814W (I) passbands. In the GWS sample all fields have uniform 95% completeness down to I 24.3 mag, whereas in the MDS sample, fields have varying 95% completeness limits in the range I 23.6-24.8 mag. In each field ~400 galaxies per 5 square arcmin field are detected. We exploit high-resolution WFPC2 images to systematically determine morphological classifications of galaxies as disk or bulge dominated down to I ≤ 23 mag (V 24 mag) and to differentiate galaxies from stars 1 magnitude fainter. Down to I ≤ 25 mag the number of galaxy pairs with separations θ ≤ 30 is consistent with a shortward extrapolation of the angular two-point correlation function ω(θ) θ-0.8 observed from the same data; the fraction of such pairs showing morphological evidence for physical association accounts for a third of the total numbers suggested by a shortward extrapolation of ω(θ). The latter result may not be too surprising given the low surface brightness of the tidal tails resulting from galaxy interactions; i.e., much of the evidence for interactions may fall below our detection limit. Moreover, we find no trend between apparent physical association (on the basis of morphology) and (V - I) color or I-magnitude difference between pair members of the θ 30 pair sample. We use recent galaxy redshift surveys to estimate the rate of galaxy merging occurring in the MDS and GWS galaxy pair samples. From this work we find that merging has a moderate dependence on redshift: we derive an estimate for the galaxy pair fraction Pf (1 + z)m, with m = 1.2 ± 0.4 for galaxies with I ≤ 25 mag (zmed 1-2). Two scenarios are consistent both with this low value of m and for the low correlation amplitude: (a) a low-density universe with strong clustering evolution parameterized by a clustering exponent e 1.0 such that galaxy cluster-scale structures shrink relative to the proper coordinate frame; and/or (b) a weakly clustered galaxy population, the majority of which fade or dissipate below zmed 0.5 (I 20 mag), thus mimicking the apparently strong evolution in the correlation amplitude, Aω. Although not directly observed using our data, the possible flattening of the slope of ω(θ) with increasing survey depth can explain the strong decline in Aω and allow for greater pair fraction evolution limited to m ≤ 1.6.

Close Pairs of Field Galaxies in the CNOC1 Redshift Survey

A redshift sample of 545 field galaxies is used to examine how the galaxy merger rate changes with redshift and how mergers affect the observed properties of galaxies in close pairs. Close pairs are defined as those with projected separations less than 20 h-1 kpc. At a mean redshift of 0.33, it is found that 7.1% ? 1.4% of galaxies are in close physical pairs, compared to 4.3% ? 0.4% locally. The merger rate is estimated to change with redshift as (1 + z)2.8?0.9. These results are shown to be consistent with previous close-pair studies and provide one of the strongest constraints to date on the redshift dependence of the merger rate. As with earlier studies, no significant differences between the mean properties of paired and isolated galaxies are detected. However, using the subset of confirmed close physical pairs, those which appear to be undergoing interactions or mergers (based on their images) are found to have galaxies with strong emission lines and very blue rest-frame colors. In addition, these pairs have low relative velocities, increasing the likelihood that the galaxies will merge. We interpret this as the first clear evidence of merger-induced star formation occurring in field galaxies at this redshift.

Counting Pairs of Faint Galaxies

view Abstract Citations (48) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Counting Pairs of Faint Galaxies Woods, David ; Fahlman, Gregory G. ; Richer, Harvey B. Abstract The number of close pairs of galaxies observed to faint magnitude limits, when compared to nearby samples, determines the interaction or merger rate as a function of redshift The prevalence of mergers at intermediate redshifts is fundamental to understanding how galaxies evolve and the relative population of galaxy types. Mergers have been used to explain the excess of galaxies in faint blue counts above the numbers expected from no-evolution models. Using deep CFHT (I ≤ 24) imaging of a "blank" field, we find a pair fraction which is consistent with the galaxies in our sample being randomly distributed with no significant excess of "physical" close pairs. This is contrary to the pair fraction of 34%±9% found by Burkey et al. for similar magnitude limits and using an identical approach to the pair analysis. Various reasons for this discrepancy are discussed. Colors and morphologies of our close pairs are consistent with the bulk of them being random superpositions although, as indicators of interaction, these criteria are uncertain as a result of contamination of field galaxies and difficulty in seeing structure at intermediate redshifts, respectively. This observed lack of close pairs is probably linked with the decreasing amplitude of the angular correlation function at faint limits. If our faint samples are comprised of galaxies which have a higher average redshift than brighter samples studied by other workers, then either the merger rate has been overestimated or there is a change in its behavior from what is observed at brighter magnitude limits. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176461 arXiv: arXiv:astro-ph/9506053 Bibcode: 1995ApJ...454...32W Keywords: GALAXIES: CLUSTERS: GENERAL; GALAXIES: INTERACTIONS; Astrophysics E-Print: 471 kb uuencoded, postscript file with figures included, 32 pages, To appear in the Nov., 1995 issue of Astrophysical Journal full text sources arXiv | ADS | data products SIMBAD (2)

Dynamics of Binary Galaxies. III. Details of the Close Pairs

In a previous paper [Chengalur et al. ApJ, 107, 1984(1994)] H I synthesis data and CCD images were presented for an objectively chosen sample of six close spiral-spiral galaxy pairs. In this paper we present a detailed analysis of the data. Although the presence of tidal features was not an explicit selection criterion, five of our six pairs show tidal tails and bridges. The remaining pair also shows signs of interaction: both galaxies in the pair have disturbed velocity fields and one of the two galaxies has a central concentration of molecular gas and is undergoing a starburst. The morphology of three of the six pairs implies that the galaxies are on high eccentricity, low total energy orbits, and for one of these pairs there is some evidence that the impact parameter is as small as ~1 disk radius. For the remaining three pairs the tidal features are not as pronounced, and the observations do not yield as much information on the orbit. However, even for these pairs, the observed morphology is consistent with the galaxies being on slow, small impact parameter orbits, provided the galaxies are at a relatively early phase of the orbit, i.e., prior to the first close encounter. We also find weak evidence for preferential alignment in the pairs (with the lack of preferential alignment being ruled out at the 97% level), in the sense that the major axis of a galaxy tends to point toward the companion galaxy. This sample of close pairs is complemented by a larger sample of wide pairs (Chengalur et al. 1993). The median velocity difference {DELTA}V of the sample of close pairs in low-density regions is not much different from that for the wide pairs sample, in particular, there is no evidence for positive energy impacts. The effect of dynamical friction on infall velocities is controversial and will be discussed in the following paper [Chengalur et al., in preparation (1995)] which combines the data on close and wide pairs.

Dynamics of binary galaxies. 2: Close pairs

view Abstract Citations (29) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamics of Binary Galaxies. II. Close Pairs Chengalur, Jayaram N. ; Salpeter, E. E. ; Terzian, Yervant Abstract As part of a comprehensive study of binary galaxies, broadband optical CCD images and high resolution H I maps are presented for six close pairs of spiral galaxies. A previous paper [Chengalur, et al., ApJ, 419,30 (1993)] presented the results of a study of binary galaxies with wide separations. The current sample of close pairs was chosen from the CfA redshift catalog using selection criteria that were objective and completely independent of any previously known morphological peculiarities. Nonetheless, all the galaxies show some sign of interaction, implying that the selection criteria were sufficiently well tailored to choose physically associated pairs. All the galaxies show kinematical disturbances and have asymmetric rotation curves. Tidal tails and bridges are clearly detected for some, and two pairs have a large common H I envelope. This common H I envelope also shows a large scale radial velocity gradient and gives the impression of being in rotation about a single kinematical axis. There is relatively good agreement between the optical and H I images, with similar tidal features being seen in both. The H I features, however, extend to a much larger galactocentric radius than the corresponding optical features. In at least one case, however, the stellar disk appears relatively normal, while the H I disk appears disturbed even towards the center of the galaxy. The systematic velocity difference for the galaxy pairs has been measured quite accurately either from the integrated H I spectral profile, or from a global fit to the galaxy velocity fields. The median velocity difference for the six galaxy pairs is very small, ~20 km s^-1^. Detailed analysis of the data will be presented in a separate paper. Publication: The Astronomical Journal Pub Date: June 1994 DOI: 10.1086/117010 Bibcode: 1994AJ....107.1984C Keywords: Angular Velocity; Galactic Structure; H I Regions; Interacting Galaxies; Spiral Galaxies; Two Body Problem; Charge Coupled Devices; Galactic Rotation; Red Shift; Astrophysics; GALAXIES: SPIRAL; GALAXIES: BINARY full text sources ADS | data products NED (27) SIMBAD (22) Related Materials (2) Part 1: 1993ApJ...419...30C Part 3: 1995AJ....110..167C

Mergers and clustering evolution

Merging of close pairs of galaxies diminishes the amplitude of clustering on both small and large scales. The power spectrum of the postmerger population at long waves is approximately the initial P(k) multiplied by (1−f mg ) 2 (1−k 2 R 2 mg ), valid for small kR mg , where R mg , is set by the characteristic initial separation of crossing orbits. The quantity f mg , is the fraction of galaxies merged, which can be estimated from a variety of observations. On small scales, less than ∼2 h −1 Mpc, the clustering of the merged population is sensitive to the details of the merger process, although continuing infall and orbital mixing will maintain density gradients of the galaxy population on small scales

Associations between quasi-stellar objects and galaxies

A table is presented here listing all close pairs of QSOs and galaxies that were found in a computer-aided search of catalogs of QSOs and bright galaxies and an extensive search of the literature. There is a large excess of pairs with separations of 2 arcmin or less, or about 60 kpc, over the numbers expected if the configurations were accidental. The angular separation for 392 pairs adds to the evidence for physical association, and it is shown that selection effects are not important. A general rule is stated that QSOs tend to lie in the vicinity of normal galaxies much more often than is expected by chance whether or not the galaxies and the QSOs have the same redshifts. It is emphasized that this rule cannot be explained in terms of gravitational microlensing, and it is concluded that some part of the redshift of all classes of active nuclei is not associated with the expansion of the universe. 120 refs.

On Leading Spiral Arms in Close Pairs of Galaxies

AbstractIt is explained why one can observe a leading spiral pattern in close pairs of galaxies where the direction of the orbital momentum of the satellite is opposite to the direction of the spin of the spiral galaxy.

Tidal distortions in pairs of early-type galaxies

We are conducting an imaging survey of pairs of elliptical galaxies which has already produced interesting results. Some pairs present a common pattern of distortion interpreted in terms of tidal effects (Davoust and Prugniel, 1988; Prugniel et al., 1989). Other examples drawn from the literature (Borne and Hoessel, 1988; Colina and Pérez-Fournon, 1990) share the same morphology. This Poster presents new cases and lists the characteristics of 24 such systems.Our pairs are drawn from a sample of binary and multiple galaxies which has in turn been extracted from the CGCG, UGC (Nilson, 1973) and VV (Vorontsov-Velyaminov, 1959) catalogues. This sample includes that of Karachentsev (1972). It contains 1800 pairs, among which 700 are S – S or mixed morphology pairs. We are working on the remainder to produce a sample of close physical pairs of elliptical galaxies (we also include bulge dominated S0’s since the morphological discrimination from ellipticals is often ambiguous, in particular for interacting galaxies).

Close pairs of galaxies in deep sky surveys

view Abstract Citations (135) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Close Pairs of Galaxies in Deep Sky Surveys Zepf, Stephen E. ; Koo, David C. Abstract A largely complete sample of 20 close pairs of faint (B <= 22) galaxies, separated by less than 4.5", is selected from catalogs based on UBVI photometry of 4 m plates taken in two regions of high galactic latitude (SA 57 and SA 68) which cover a total area of 0.5 deg^2^. The colors of the galaxy pairs indicate recently enhanced star formation in some systems, but generally the distribution of the colors and size of the galaxies in pairs is not significantly different from that of equally faint field galaxies. The number of pairs is roughly similar to the number expected from an extrapolation to small scales of the angular correlation function of the entire sample of faint galaxies, but there is some evidence, significant at about the 90% level, for an overabundance of pairs of galaxies separated by about 3.0". This rough agreement between the observed number of galaxy pairs and the extrapolation of the angular correlation function to separations as small as typical galactic radii is similar to that found in comparable studies of much brighter samples. A comparison of the number of close galaxy pairs in our faint sample to the number found in comparable nearby studies suggests that the frequency of interacting galaxies increases as (1 + z)^4.0 +/- 2.5^. This result lends observational support to the hypothesis that interactions were more common in the past than they are now, but its dependence on the assumption that the faint galaxy pairs are moderate-redshift analogs of nearby pairs with a distribution of absolute magnitudes not very different from equally faint field galaxies makes this conclusion tentative until further observations become available. Publication: The Astrophysical Journal Pub Date: February 1989 DOI: 10.1086/167085 Bibcode: 1989ApJ...337...34Z Keywords: Astronomical Photometry; Galactic Clusters; Angular Correlation; Color; Interacting Galaxies; Interactions; Magnitude; Ubv Spectra; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: INTERACTIONS; GALAXIES: PHOTOMETRY; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (4)

Star-formation rates, molecular clouds, and the origin of the far-infrared luminosity of isolated and interacting galaxies

The CO luminosities of 93 galaxies have been determined and are compared with their IRAS FIR luminosities. Strongly interacting/merging galaxies have L(FIR)/L(CO) substantially higher than that of isolated galaxies or galactic giant molecular clouds (GMCs). Galaxies with tidal tails/bridges are the most extreme type with L(FIR)/L(CO) nine times as high as isolated galaxies. Interactions between close pairs of galaxies do not have much effect on the molecular content and global star-formation rate. If the high ratio L(FIR)/L(CO) in strongly interacting galaxies is due to star formation then the efficiency of this process is higher than that of any galactic GMC. Isolated galaxies, distant pairs, and close pairs have an FIR/CO luminosity ratio which is within a factor of two of galactic GMCs with H II regions. The CO luminosities of FIR-luminous galaxies are among the highest observed for any spiral galaxies.

A Comment on the Seyfert Environment

The study on the Seyfert environment has been of considerable interest lately, since it is believed that the tidal force of a close companian may drive gas into the inner region and either fuel nuclear activity directly or lead to starbursts (Toomre et al., 1972, Byrd, 1987). Many authors noticed an excess of Seyfert among close pairs of galaxies (Adams, 1977, Kennicutt et al., 1984, Keel et al., 1984, Dahari, 1984, 1985 a, b). In a recent paper of ours (Chen and Zou, 1986), a neighborhood statistical study has been done for the galaxies in CfA sample (Huchra et al., 1983), and it is found that the fraction of Seyferts is 3 times higher than the average one, if the distances between them and its nearest neighbors are less than 20 kpc. The result would strengthen the opinion that the existance of close companian is a favourable environment to trigger Seyfert activities.

Some new ultraviolet galaxies with jets

On the basis of observations with the 2.6-m telescope at the Byurakan Observatory and the 6 m telescope at the Special Astrophysical Observatory, a morphological study has been made of four UV galaxies with jets, namely, Markaryan 423, 739, 773, and 984. Their parameters have been estimated. For one of them, Markaryan 984, a spectral study has been made of the nucleus and two condensations located in the jet on the basis of the spectrum obtained with the 6-m telescope. It has been found that the nucleus of the galaxy is a type Sy 2 object with some signs of liners. It has been specially noted that the first condensation also has broad emission lines and a flat Balmer decrement. Some characteristic properties of galaxies with jets are noted. The majority of galaxies with jets show Seyfert properties. They enter into clusters of galaxies and are, in many cases, their central objects, or they are components of close pairs of galaxies. The presence of strong radio emission is not essential for the existence of a jet, especially in the case of spiral galaxies.

H I observations of active and interacting galaxies

We present results of a survey of 21-cm H i emission from 18 Seyfert galaxies, 12 radio galaxies, and 9 close pairs of galaxies with radio emission. Nine of the Seyfert galaxies were detected, while none of the radio galaxies or close pairs had detectable H i emission. Neutral hydrogen masses for the detected Seyferts range from 7 x 10/sup 9/ M/sub sun/ to 3.4 x 10/sup 10/ M/sub sun/ (for H/sub 0/ = 50 km s/sup -1/ Mpc/sup -1/). The ratio M/sub Htsi/ /L/sub B/ for these detected Seyfert galaxies runs from 0.05 to 0.63 in solar units. Some galaxies with substantial H i have early, but peculiar, morphological types, consistent with results for non-Seyferts. The undetected radio galaxies and close pairs generally have H i upper limits consistent with their morphological type. We find no evidence for a correlation between integrated H i flux and 10.6-..mu..m nuclear flux density of the Seyfert galaxies.

Surface of zero relative velocity in the gravitational field of a pair of interacting galaxies

Zero relative velocity surfaces in the orbital plane are computed for close pairs of spherical and disk galaxies.

Three new remarkable pairs of spherical compact galaxies

AbstractOn plates taken with the 152 cm Ritchey‐Chrétien telescope of the Figl Observatory for Astrophysics three very close pairs of spherical compact galaxies were found. The separations of their components are so narrow that they must touch or even penetrate each other with their outer parts. On plates of the Tautenburg 134/200/400 cm Schmidt camera magnitudes in U, B, V, colour indices and the position in the two colour diagram could be determined. Distances, absolute magnitudes of the pairs and diameters and surface magnitudes for the single components were determined, too.