2007 (IPP)

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Astronomical Data Management
(2007-01-01) Norris, Ray; Andernach, Heinz; Eichhorn, Guenther; et al.
We present a summary of the major contributions to the Special Session on Data Management held at the IAU General Assembly in Prague in 2006. While recent years have seen enormous improvements in access to astronomical data, and the Virtual Observatory aims to provide astronomers with seamless access to on-line resources, more attention needs to be paid to ensuring the quality and completeness of those resources. For example, data produced by telescopes are not always made available to the astronomical community, and new instruments are sometimes designed and built with insuﬃcient planning for data management, while older but valuable legacy data often remain undigitised. Data and results published in journals do not always appear in the data centres, and astronomers in developing countries sometimes have inadequate access to on-line resources. To address these issues, an “Astronomers Data Manifesto” has been formulated with the aim of initiating a discussion that will lead to the development of a “code of best practice” in astronomical data management.
Virtual observatories and developing countries
(2007-01-04) Kembhavi, A.K.
I will discuss in this artical the emerging concept of Virtual Observatories, the efforts being made in various countries to set up these structures, and the relevance of the concept to astronomy in developing countries.
Evolution of transonicity in an accretion disc
(2007-01-10) Ray, Arnab K.; Bhattacharjee, Jayanta K.
For inviscid, rotational accretion ows driven by a general pseudo Newtonian potential on to a Schwarzschild black hole, the only possible xed points are saddle points and centre-type points. For the speci c choice of the Newtonian potential, the ow has only two critical points, of which the outer one is a saddle point while the inner one is a centre-type point. A restrictive upper bound is imposed on the admissible range of values of the angular momentum of sub-Keplerian ows through a saddle point. These ows are very unstable to any deviation from a necessarily precise boundary condition. The di culties against the physical realisability of a solution passing through the saddle point have been addressed through a temporal evolution of the ow, which gives a non-perturbative mechanism for selecting a transonic solution passing through the saddle point. An equation of motion for a real-time perturbation about the stationary ows reveals a very close correspondence with the metric of an acoustic black hole, which is also an indication of the primacy of transonicity
Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
(2007-01-15) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C´edric; et al.
Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z=2.1–3.1. Methods. We search for Ovi absorption and corresponding Si iv, Civ, and Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering Ovi at S/N>10. We then use optical depth proﬁle comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at <5 000 kms −1 from the QSO redshift) with detections of Ovi absorption. There are no clear Ovi non-detections, so the incidence of Ovi in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, Civ and Si iv are seen whenever data is available, and Nv is detected in 3 cases. Analysis of the line widths together with photoionization modelling suggests that two phases of DLA plasma exist: a hot, collisionally ionized phase (seen in broad Ovi components), and a warm, photoionized phase (seen just in narrow Civ and Si iv components). The presence of inﬂows and/or outﬂows is indicated by individual Ovi and Civ components displaced from the neutral gas (either blueshifted or redshifted) by up to 400 kms −1 . We ﬁnd tentative evidence (98% conﬁdence) for correlations between the DLA metallicity (measured in the neutral gas) and high-ion column density, and between the DLA metallicity and high-ion line width, as would be expected if supernova-driven galactic outﬂows rather than accretion produced the high ions. Using conservative ionization corrections, we ﬁnd lower limits to the total hydrogen column densities in the hot (Ovi-bearing) and warm (Civ-bearing) phases in the range logNHot H ii > 19.5 to > 21.1, and logNWarm H ii > 19.4 to > 20.9. On average, the hot and warm phases thus contain > ∼40% and > ∼20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the Ovi phase is ≈ 106 K and so fOvi = Ovi/O ≪ 0.2, the plasma can make a signiﬁcant contribution to the metal budget at high redshift. Additional searches for Ovi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.
Kinetic and magnetic alpha effects in nonlinear dynamo theory
(2007-01-19) Sur, Sharanya; Subramanian, Kandaswamy; Brandenburg, Axel
The backreaction of the Lorentz force on the α-effect is studied in the limit of small magnetic and ﬂuid Reynolds numbers, using the ﬁrst order smoothing approximation (FOSA) to solve both the induction and momentum equations. Both steady and time dependent forcings are considered. In the low Reynolds number limit, the velocity and magnetic ﬁelds can be expressed explicitly in terms of the forcing function. The nonlinear α-effect is then shown to be expressible in several equivalent forms in agreement with formalisms that are used in various closure schemes. On the one hand, one can express α completely in terms of the helical properties of the velocity ﬁeld as in traditional FOSA, or, alternatively, as the sum of two terms, a so-called kinetic α-effect and an oppositely signed term proportional to the helical part of the small scale magnetic ﬁeld. These results hold for both steady and time dependent forcing at arbitrary strength of the mean ﬁeld. In addition, the τ-approximation is considered in the limit of small ﬂuid and magnetic Reynolds numbers. In this limit, the τ closure term is absent and the viscous and resistive terms must be fully included. The underlying equations are then identical to those used under FOSA, but they reveal interesting differences between the steady and time dependent forcing. For steady forcing, the correlation between the forcing function and the small-scale magnetic ﬁeld turns out to contribute in a crucial manner to determine the net α-effect. However for delta-correlated time-dependent forcing, this force–ﬁeld correlation vanishes, enabling one to write α exactly as the sum of kinetic and magnetic α-effects, similar to what one obtains also in the large Reynolds number regime in theτ-approximation closure hypothesis. In the limit of strong imposed ﬁelds, B0, we ﬁnd α ∝ B−2 0 for delta-correlated forcing, in contrast to the well-known α ∝ B−3 0 behaviour for the case of a steady forcing. The analysis presented here is also shown to be in agreement with numerical simulations of steady as well as random helical ﬂows.
Composite Interstellar Grains
(2007-01-20) Vaidya, D.B.; Gupta, Ranjan; Snow, T. P.
A composite dust grain model which is consistent with the observed interstellar extinction and linear polarization is presented. The composite grain is made up of a host silicate spheroid and graphite inclusions. The extinction e ciencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA). The interstellar extinction curve is evaluated in the spectral region 3.40{0.10 musing the extinction e ciencies of the composite spheroidal grains. The model extinction curves are then compared with the average observed interstellar extinction curve.We also calculate the linear polarization for the spheroidal composite grains at three orientation angles and nd the wavelength of maximum polarization. Further, we estimate the volume extinction factor, an important parameter from the point of view of cosmic abundance, for the composite grain models that reproduce the average observed interstellar extinction. The estimated abundances derived from the composite grain models for both carbon and silicon are found to be lower than that are predicted by the bare silicate/graphite grain models but these values are still higher than that are implied from the recent ISM values
New Universal Local Feature in the Inflationary Perturbation Spectrum
(2007-01-28) Starobinsky, A. A.; Joy, Minu; Sahni, Varun
A model is developed in which the inflaton potential experiences a sudden small change in its second derivative (the effective mass of the inflaton). An exact treatment demonstrates that the resulting density perturbation has a quasi-flat power spectrum with a break in its slope (a step in ns). The step in the spectral index is modulated by characteristic oscillations and results in large running of the spectral index localised over a few e-folds of scales. A field-theoretic model giving rise to such behaviour of the inflationary potential is based on a fast phase transition experienced by a second scalar field weakly coupled to the inflaton. Such a transition is similar to that which terminates inflation in the hybrid inflationary scenario. This scenario suggests that the observed running of the spectral index in the WMAP data may be caused by a fast second order phase transition which occurred during inflation.
Detecting gravitional waves from inspiraling binaries with a network of detectors: coherent strategies by correlated detectors
(2007-02-03) Tagoshi, H.; Mukhopadhyay, Himan; Dhurandhar, Sanjeev; et al.
We discuss the coherent search strategy to detect gravitational waves from inspiraling compact binaries by a network of correlated laser interferometric detectors. From the maximum likelihood ratio statistic, we obtain a coherent statistic which is slightly diﬀerent from and generally better than what we obtained in our previous work. In the special case when the cross spectrum of two detectors normalized by the power spectrum density is constant, the new statistic agrees with the old one. The quantitative diﬀerence of the detection probability for a given false alarm rate is also evaluated in a simple case.
CMB power spectrum estimation with non-circular beam and incomplete sky coverage
(2007-02-05) Mitra, Sanjit; Sengupta, Anand; Souradeep, Tarun; et al.
Over the last decade, measurements of the CMB anisotropy has spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic eﬀects in the increasingly sensitive, high resolution, ‘full’ sky measurements from diﬀerent CMB experiments pose a stiﬀ challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but, the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the ﬁnal precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using Pseudo-Cl unavoidable for high resolution data. The debiasing of the Pseudo-Cl needs account for non-circular beams, together with non-uniform sky coverage. We provide an analytic framework for correcting the power spectrum for the eﬀect of beam noncircularity and non-uniform sky coverage (including incomplete/masked sky maps). The approach is perturbative in the distortion of the beam from non-circularity allowing for rapid computations when the beam is mildly non-circular. When non-circular beam eﬀect is important, we advocate that it is computationally advantageous to employ ‘soft’ azimuthally apodized masks whose spherical harmonic transform die down fast with m.
Axisymmetric black hole accretion in the Kerr metric as an autonomous dynamical system
(2007-02-07) Goswami, Sanghamitra; Khan, Saba Nashreen; Ray, Arnab K.; et al.
In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion ﬂow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the ﬂow as a ﬁrst-order autonomous dynamical system. The consequent linearised analysis of the critical points of the ﬂow leads to a comprehensive mathematical prescription for classifying these points, showing that the only possibilities are saddle points and centre-type points for all ranges of values of the ﬁxed ﬂow parameters. The spin parameter of the black hole inﬂuences the multitransonic character of the ﬂow, as well as some of its speciﬁc critical properties. The special case of a ﬂow in the space-time of a non-rotating black hole, characterised by the Schwarzschild metric, has also been studied for comparison and the conclusions are compatible with what has been seen for the Kerr geometric case.
Evidence for overdensity around z_em > 4 quasars from the proximity effect
(2007-02-14) Guimaraes, R.; Srianand, R.; Petitjean, Patrick; et al.
We study the density ﬁeld around zem > 4 quasars using high quality medium spectral resolution ESI-Keck spectra (R ∼ 4300, SNR > 25) of 45 high-redshift quasars selected from a total of 95 spectra. This large sample considerably increases the statistics compared to previous studies. The redshift evolution of the mean photo-ionization rate and the median optical depth of the intergalactic medium (IGM) are derived statistically from the observed transmitted ﬂux and the pixel optical depth probability distribution function respectively. This is used to study the so-called proximity effect, that is, the observed decrease of the median optical depth of the IGM in the vicinity of the quasar caused by enhanced photo-ionization rate due to photons emitted by the quasar. We show that the proximity effect is correlated with the luminosity of the quasars, as expected. By comparing the observed decrease of the median optical depth with the theoretical expectation we ﬁnd that the optical depth does not decrease as rapidly as expected when approaching the quasar if the gas in its vicinity is part of the standard IGM. We interpret this effect as revealing gaseous overdensities on scales as large as ∼15h−1 Mpc. The mean overdensity is of the order of two and ﬁve within, respectively, 10 and 3h−1 Mpc. If true, this would indicate that high redshift quasars are located in the center of overdense regions that could evolve with time into massive clusters of galaxies. The overdensity is correlated with luminosity: brighter quasars show higher overdensities.
Critical properties of spherically symmetric black hole accretion in Schwarzschild geometry
(2007-02-28) Mandal, Ipsita; Ray, Arnab K.; Das, Tapas K.
The stationary spherically symmetric accretion ﬂow in the Schwarzschild metric has been set up as an autonomous ﬁrst-order dynamical system, and it has been studied completely analytically. Of the three possible critical points in the ﬂow, the one that is physically realistic behaves like the saddle point of the standard Bondi accretion problem. One of the two remaining critical points exhibits the strange mathematical behaviour of being either a saddle point or a centre-type point, depending on the values of the ﬂow parameters. The third critical point is always unphysical and behaves like a centre-type point. The treatment has been extended to pseudo-Schwarzschild ﬂows for comparison with the general relativistic analysis.
Galactic dynamos supported by magnetic helicity fluxes
(2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
We present a simple semi-analytical model of nonlinear, mean-ﬁeld galactic dynamos and use it to study the effects of various magnetic helicity ﬂuxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity ﬂux. We discuss the implications and interplay of two types of magnetic helicity ﬂux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is signiﬁcant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We conﬁrm that the intensity of gas outﬂow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic ﬁeld supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity ﬂux can boost magnetic ﬁeld further to achieve energy equipartition with turbulence. For stronger outﬂows that may occur in starburst galaxies, the Vishniac-Cho ﬂux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic ﬁeld of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
Kinematical diagrams for conical relativistic jets
(2007-03-10) Gopal, Krishna; Sircar, Pronoy; Dhurde, Samir
We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (vapp = cfJapp) and the effective Doppler boosting factor (Oeff) of the relativistically moving radio knot. A novel aspect of these analytical computations of fJ-o diagrams is that they are made for parsecscale jets having a conical shape, with modest opening angles (w up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor, r), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis is surrounded by a slower moving sheath, possibly arising from a velocity shear.
Non-spherical gravitational collapse of strange quark matter
(2007-03-10) Patil, K. D.; Zade, S. S.; Mohod, A. N.
We study the non-spherical gravitational collapse of the strange quark null ﬂuid. The interesting feature which emerges is that the non-spherical collapse of charged strange quark matter leads to a naked singularity whereas the gravitational collapse of neutral quark matter proceeds to form a black hole. In the present brief report we extend the earlier work of T.Harko and K.S.Cheng to the non-spherical case.
Lenticular Galaxy Formation - Possible Luminosity Dependence
(2007-03-27) Barway, Sudhanshu; Kembhavi, A.K.; Wadadekar, Yogesh; et al.
We investigate the correlation between the bulge eﬀective radius (re) and disk scale length (rd), in the near-infrared K band for lenticular galaxies in the ﬁeld and in clusters. We ﬁnd markedly diﬀerent relations between the two parameters as a function of luminosity. Lenticulars with total absolute magnitude fainter than MT = −24.5 show a positive correlation, in line with predictions of secular formation processes for the pseudo bulges of late-type disk galaxies. But brighter lenticulars with MT < −24.5 show an anti-correlation, indicating that they formed through a diﬀerent mechanism. The available data are insuﬃcient to reliably determine the eﬀect of galaxy environment on this correlation.
Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011
(2007-03-29) Noterdaeme, P.; Srianand, R.; Petitjean, Patrick; et al.
Aims. We aim at deriving the physical conditions in the neutral gas associated with damped Lyman-α systems using observation and analysis of H2 and C  absorptions. Methods. We obtained a high-resolution VLT-UVES spectrum of the quasar Q2348−011 over a wavelength range that covers most of the prominent metal and molecular absorption lines from the log N(H ) = 20.50±0.10 damped Lyman-α system at zabs = 2.4263. We detected H2 in this system and measured column densities of H2, C , C ∗, C∗∗ , Si , P , S , Fe , and Ni . From the column density ratios and, in particular, the relative populations of H2 rotational and C  ﬁne-structure levels, we derived the physical conditions in the gas (relative abundances, dust-depletion, particle density, kinetic temperature, and ionising ﬂux) and discuss physical conditions in the neutral phase. Results. Molecular hydrogen was detected in seven components in the ﬁrst four rotational levels (J = 0-3) of the vibrational ground state. Absorption lines of H2 J = 4 (resp. J = 5) rotational levels are detected in six (resp. two) of these components. This leads to a total molecular fraction of log f ≃ −1.69+0.37 −0.58. Fourteen components are needed to reproduce the metal-line proﬁles. The overall metallicity is found to be −0.80, −0.62, −1.17±0.10 for, respectively, [Si/H], [S/H] and [Fe/H]. We conﬁrm the earlier ﬁndings that there is a correlation between log N(Fe )/N(S ) and log N(Si )/N(S ) from diﬀerent components indicative of a dust-depletion pattern. Surprisingly, however, the depletion of metals onto dust in the H2 components is not large in this system: [Fe/S] = −0.8 to −0.1. The gas in H2-bearing components is found to be cold but still hotter than similar gas in our Galaxy (T > 130 K, instead of typically 80 K) and dense (n ∼ 100 − 200 cm−3 ). There is an anti-correlation (R = −0.97) between the logarithm of the photo-absorption rate, log β0, and log N(H2)/N(C ) derived for each H2 component. We show that this is mostly due to shielding eﬀects and imply that the photo-absorption rate β0 is a good indicator of the physical conditions in the gas. We ﬁnd that the gas is immersed in an intense UV ﬁeld, about one order of magnitude higher than in the solar vicinity. These results suggest that the gas in H2-bearing DLAs is clumpy, and star-formation occurs in the associated object
Probing the star formation history using the redshift evolution of luminosity fuctions
(2007-03-30) Samui, Saumyadip; Srianand, R.; Subramanian, Kandaswamy
We present a self-consistent, semi-analytical ΛCDM model of star formation and reionization. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the electron scattering optical depth to reionization, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 ≤ z ≤ 6 for a reasonable range of model parameters. While simple photoionization feedback produces the correct shape of LF at z = 6, for z = 3 we need additional feedback that suppresses star formation activities in halos with 1010 . (M/M⊙) . 1011. Models with prolonged continuous star formation activities are preferred over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z ∼ 6. The halo number density evolution from the standard ΛCDMstructure formation model that ﬁts LF up to z = 6 is consistent with the upper limits on z ≃ 7 LF and source counts at 8 ≤ z ≤ 12 obtained fromthe Hubble Ultra Deep Field (HUDF) observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6 ≤ z ≤ 10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the initial mass function, reddening correction and the mode of star formation at z & 8. We show that low mass molecular cooled halos, which may be important for reionizing the universe, are not detectable in the present deep ﬁeld observations even if a considerable fraction of its baryonic mass goes through a star burst phase. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the luminosity function in the redshift range 6 ≤ z ≤ 12. In our model calculations, the contribution of low mass halos to global SFR density prior to reionization reveals itself in the form of second peak at z ≥ 6. However this peak will not be visible in the observed SFR density as a function of z as most of these galaxies have luminosity below the detection threshold of various ongoing deep ﬁeld surveys. Accurately measuring the LF at high redshifts can be used to understand the nature of star formation in the dark ages and probe the history of reionization.
Type I singularities and the Phantom Menace
(2007-04-15) Naskar, Tapan; Ward, John
We consider the future dynamics of a transient phantom dominated phase of the universe in LQC and in the RS braneworld, which both have a non-standard Friedmann equation. We ﬁnd that for a certain class of potentials, the Hubble parameter oscillates with simple harmonic motion in the LQC case and therefore avoids any future singularity. For more general potentials we ﬁnd that damping eﬀects eventually lead to the Hubble parameter becoming constant. On the other hand in the braneworld case we ﬁnd that although the type I singularity can be avoided, the scale factor still diverges at late times.
Unravelling the morphologies of Luminous Compact Galaxies using the HST/ACS GOODS survey
(2007-04-17) Rawat, A.; Kembhavi, A.K.; Barway, Sudhanshu; et al.
Context. Luminous Compact Galaxies (LCGs) (MB ≤ −20, R1/2 ≤ 4.5 kpc and EW0(OII) ≥15Å) constitute one of the most rapidly evolving galaxy populations over the last ∼8Gyr history of the universe. Due to their inherently compact sizes, any detailed quantitative analysis of their morphologies has proved to be diﬃcult in the past. Hence, the morphologies and thereby the local counterparts of these enigmatic sources have been hotly debated. Aims. Our aim is to use the high angular resolution, deep, multiband HST/ACS imaging data, from the HST/ACS GOODS survey, to study the quantitative morphology of a complete sample of LCGs in the redshift range 0.5 ≤ z ≤ 1.2. Methods. We have derived structural parameters for a representative sample of 39 LCGs selected from the GOODS-S HST/ACS ﬁeld, using full 2-dimensional surface brightness proﬁle ﬁtting of the galaxy images in each of the four ﬁlters available. B435W − z850LP color maps are constructed for the sample to aid in the morphological classiﬁcation. We then use the rest frame B band bulge ﬂux fraction (B/T) to determine the morphological class of galaxies which are well ﬁt by a ulge+disk two dimensional structure. Mergers were essentially identiﬁed visually by the presence of multiple maxima of comparable intensity in the rest frame B band images, aided by the color maps to distinguish them from HII regions. We also make use of the Spitzer 24 m source catalog of sources in the CDFS to derive the dust enshrouded star formation rates (SFR) for some of the sample LCGs Results. We derive the following morphological mix for our sample of intermediate redshift LCGs: Mergers: ∼36%, Disk dominated: ∼22%, S0: ∼20%, Early types: ∼7%, Irr/tadpole: ∼15%.We establish that our sample LCGs are intermediate mass objects with stellar mass ranging from 9.44 ≤ Log10(M/M⊙) ≤ 10.96, with a median mass of Log10(M/M⊙) = 10.32. We also derive SFR values ranging from a few to ∼ 65 M⊙/year as expected for this class of objects. We ﬁnd that LCGs account for ∼26% of the MB ≤ −20 galaxy population in the redshift range 0.5 ≤ z ≤ 1.2. We estimate a factor ∼11 fall in the comoving number density of blue LCGs from redshifts 0.5 ≤ z ≤ 1.2 to the current epoch, even though this number is subject to large uncertainities given the small sample size at zero redshift available from the literature. Conclusions. The strong redshift evolution exhibited by LCGs, and the fact that a signiﬁcant fraction of LCGs are in merging systems, seem to indicate that LCGs might be an important phase in the hierarchical evolution of galaxies. We envisage that some of the LCGs that are classiﬁed as merging systems, might go on to rebuild their disks and evolve into disk galaxies in the local universe.