Comparative Study of Asymmetry Origin of Galaxies in
II. Near-Infrared observations.
In this second paper of two analysis, we present near-infrared morphological and asymmetry studies perfomed in sample of 92 galaxies found in different density environments: galaxies in Compact Groups (HCGs), Isolated Pairs of Galaxies (KPGs), and Isolated Galaxies (KIGs). Both studies have proved useful to identify the effect of interactions on galaxies.
In the NIR, the properties of the galaxies in HCGs, KPGs and KIGs are more similar than in the optical. This is because the NIR band traces the older stellar populations, which formed earlier and are more relaxed than the younger populations. However, we found asymmetries related to interacions in both, KPG and HCG samples. In HCGs, the fraction of asymmetric galaxies is even higher than what we found in the optical.
In the KPGs, the interactions look like very recent events, while in the HCGs, galaxies are more morphologically evolved and show properties suggesting they suffered more frequent interactions. The key difference seems to be the absence of star formation in the HCGs: while interactions produce intense star formation in the KPGs we do not see this effect in the HCGs. This is consistent with the dry merger hypothesis (Coziol & Plauchu-Frayn, 2007): the interaction between galaxies in compact groups, (CG), are happening without the presence of gas. If the gas was spent in stellar formation (to build the bulge of the numerous early-type galaxies), then the HCGs possibly started interacting sometime before the KPGs. On the other hand, the dry interaction condition in CGs suggests the galaxies are on merging orbits, and consequently such system cannot be that much older either. Cosmologically speaking, the difference in formation time between pairs of galaxies and CGs may be relatively small. The two phenomena are typical of the formation of structures in low density environments. Their formation represent relatively recent events.
This paper presents the second of two analyses about the importance of environment on the formation and evolution of galaxies observed in the nearby universe. In the first paper (Plauchu-Frayn & Coziol 2010; hereafter Paper I), we have presented the results of our morphological and asymmetry study in the optical ( and filters) for a sample of 214 galaxies found in three different density environments: galaxies in Compact Groups (HCGs), Isolated Pairs of Galaxies (KPGs), and Isolated Galaxies (KIGs) (Karachentsev, 1972; Karachentseva, 1973; Hickson, 1982). We have also performed a comparative statistical analysis of the isophotal and asymmetry properties of these galaxies.
In the optical, we observed some clear differences in the properties of galaxies in the different environments. First, isolated galaxies tend to be more compact and symmetrical than galaxies in pairs or in compact groups (CGs). It suggests interactions produce stellar orbits with higher energies. Second, evidence for interactions seems more obvious for galaxies in pairs than for galaxies in CGs. Because the HCGs are richer in early-type galaxies than the KPGs, these differences suggest interactions played a more important role in the CGs in the past. Either because the high density environment of CGs favors more interactions and consequently the galaxies evolve more rapidly in such systems, or because galaxies in CGs started interacting earlier in the past than those in pairs.
The present paper extends our morphological and asymmetry study to the near-infrared, (NIR), using deep and images. Contrary to the optical, the NIR bands are sensible to the older, less massive but dominant stellar populations and trace better the distribution of the mass of galaxies (Frogel et al., 1978). Consequently, using NIR images allow us to compare interaction effects over different time scales than in the optical. This is important in CGs, where it is suspected that galaxies interact and merge under dry conditions (Coziol & Plauchu-Frayn, 2007).
Our analysis is similar to the one used in the optical (Paper I). We have applied independently two different methods: fitting of elliptical ellipses on the isophotal levels of the galaxies and determination of their asymmetry level. In Section 2, we present the properties and selection of the sub-samples observed in the NIR; in Section 3, we describe the conditions of observations and explain the reduction process; in Section 4, we present the results of our photometry and asymmetry analyses, and the comparative statistical studies in different environments; in Section 5, we compare the level of nuclear activity star formation or active galactic nucleus, (AGN) in the different environments using spectra extracted from SDSS111Sloan Digital Sky Survey http://www.sdss.org; in Section 6, we discuss our results, comparing them with those obtained in the optical; our conclusions are stated in Section 7.
2 Selection and properties of the observed galaxies
The sub-sample for our NIR analysis consists of 92 galaxies taken from our list of galaxies previously observed in the optical. A detailed description of the properties of these galaxies can be found in Paper I. Only 21 galaxies were part of our previous NIR analysis (Coziol & Plauchu-Frayn, 2007). The remaining 71 galaxies are new observations.
The galaxies observed in NIR form only 50% of those observed in the optical. This is because observations in NIR are more complicated and time expensive than in the optical. In particular, in the NIR, one has to move the telescope constantly forming a mosaic sequence. This technique is used to avoid ghost images (remnants of galaxy image on the pixels if the target is kept at the same position) and to allow proper sky subtraction. In setting up our sub-sample, therefore, a supplementary step consisted in selecting galaxies with a semi-major axis in the range 15” 90”, to allow the telescope to move in a cross or a square pattern.
The properties of the galaxies observed are reported in Table 1 for the KIGs, Table 2 for the KPGs, and Table 3 for the HCGs. As explained in Paper I, morphological types for the KIG sample were taken from Sulentic et al. (2006). For the KPG and HCG samples these were determined based on our own CCD images. Evidence for bars was added based on the optical images and/or our isophotal study. No new bars were added based on the NIR images.
In Figure 1, we compare the properties of the observed samples in the NIR with the properties of the galaxies in their respective catalogs. The and magnitudes (Paturel et al., 1994, 1997; Jarrett et al., 2000) and revised numerical code of the morphological type (de Vaucouleurs et al., 1991) were taken from the HyperLeda222HyperLeda database http://leda.univ-lyon1.fr and 2MASS3332 Micron All Sky Survey http://irsa.ipac.caltech.edu databases. Also, in Table 4 are compared the diameters in both magnitudes and the redshifts, as found in the HyperLeda and 2MASS databases. One can see that the observed samples are fairly representative of their parent samples. In Table 4, the results of the Mann-Whitney statistical tests are consistent with no difference in absolute magnitude for the observed KPGs. In the case of the KIGs, the observed sample is slightly more luminous than in the whole catalog. This is because they are located slightly nearer than the galaxies in the whole catalog. The HCGs are also brighter and bigger in the observed sample than in the whole catalog. These differences are consequence of the criteria used in preparing the NIR observations and will be taken into account during our analysis.
3 Observation and reduction
We have obtained new NIR images in (1.28 m) and (2.12 m) bands for 71 galaxies with the 2.1m telescope of the Observatorio Astronómico Nacional, located in the Sierra San Pedro Mártir, in Baja California, México. Five different runs were necessary (Table 5). The instrument used was CAMILA (Cruz–González et al., 1994). This camera is formed of four NICMOS3 detectors with 256256 pixels, which are sensitive over the range 1 to 2.5 . This instrument includes a diaphragm with a wheel of filters, which are cooled to reduce the background radiation level. The optical system, which consists of a mirror and a focal reducer designed for the f/4.5 secondary, gives a 3.63.6 field of view, corresponding to a plate scale of 0.85pixel.
Because the detector in the NIR saturates rapidly (due to the brightness of the sky), several short exposures were obtained and combined to form one final image. For each filter we have used the longest integration time possible that keeps the total counts within the linear range of the detector response. For the filter, we took for each galaxy 40 images of 80 seconds. For the filter, we took 80 images of 15 seconds.
As mentioned in the previous section, a mosaic technique was used in order to map properly sample the quickly varying sky background. The technique consists in moving the telescope between each short exposure. The way the telescope moves depends on the angular size of the target galaxy. Usually we form a cross or a square, but sometimes we must alternate between the target and the sky when the object covers the whole detector (for example, the whole CG). In general, the combination of a sequence of mosaic images yields a total integration time which is equal or below the observation time. This is due to the rejection of some bad images in a sequence. In Tables 13, the columns 6 and 7 list the final total exposure time in and K, respectively, for each galaxy. Note that to increase the S/N and/or to replace bad images, many galaxies were observed more than one night, but only during the same observing run.
The nights were clear and photometric. Usually, and as expected for NIR, the observations are not affected on full moon nights. However, in a few cases, where the moonlight was reflected by the dome we have discarded the images.
For photometry calibration several standard stars were observed during each observing run. These stars were taken from the UKIRT444 The United Kingdom Infrared Telescope is operated by the joint Astronomy Centre on behalf of the Science a Technology Facilities Council of U.K. (Hawarden et al., 2001) extended standards list, available at the WEB page of the observatory555 www.astrossp.unam.mx/estandar/standards/fsextended.html. The instrumental magnitudes for the standard stars were estimated by measuring the flux of each observed star after correcting for the atmospheric extinction. The calibration equations were calculated by fitting linear regressions on the observed values. For the photometric error we adopted the standard deviation between our estimated magnitude and the magnitude determined in the UKIRT standards list. Magnitudes for the observed galaxies have also been corrected for galactic extinction (Schlegel et al., 1998). Because the galaxies observed are at , no -correction has been applied. Details for the observations are given in Table 5. The calibration in flux was applied only after the different analyses (isophotal and asymmetry) were performed. This way of doing keeps the S/N in the different images as high as possible.
The reduction and calibration processes were standard and within IRAF666 IRAF is the Image Analysis and Reduction Facility made available to the astronomical community by the National Optical Astronomy Observatory, which is operated AURA, Inc., under contract with U.S. National Science Foundation.. One useful characteristic of CAMILA is that it subtracts a bias image from the target image after each exposure. This reduction step was consequently not needed. The next step consisted in trimming the individual images. This is done to remove the various bad lines and columns at the edges of the detector and to eliminate the effect due to vignetting. We subsequently applied a mask on all the images to remove the bad pixels. Normalized sky-flats in each filter were used to correct for the differences in quantum response. These normalized flats were obtained by fitting a 2D surface (with IMSURFIT) on combined sky-flats.
In the NIR, one important step of the reduction process is eliminate the sky contribution. To do so, we formed a median sky image by combining four adjacent exposures in the mosaic sequence and subtracted it from the corresponding exposure in the sequence. After aligning and trimming the images, the sequence of exposures was combined. First, the coordinates of the target in each image were measured using the circular aperture photometry option. These positions were used to determine the optimal shift needed to put the target in the different frames at exactly the same position, keeping the field of view as large as possible. These shifts were of the order of a tenth of a pixel. After shifting, we trimmed the individual images to the most optimum size. Before combining the exposures, most of the cosmic rays were removed using task. In some cases it was necessary remove by hand the remaining cosmic rays by using the task. An average image was then formed by combining the whole sequence of exposures.
4 Results of analysis
It is known, that during interactions some of the properties of galaxies are affected. Properties such as morphology, brightness, color and nuclear activity are changed as a result of the disruption of gas and stellar component during the encounter (Larson & Tinsley, 1978; Kennicutt & Keel, 1984; Lui & Kennicutt, 1995; Laurikainen & Salo, 2000; Patton et al., 2005; Woods & Geller, 2007; McIntosh et al., 2008).
To study further this, for each galaxy two different analysis were performed: fitting of ellipses on the NIR isophotes and estimation of the level of asymmetry. The application of these methods is similar to that used in the optical. A detailed explanation of the methods can be found in Paper I. For homogeneity sake, the analysis for the 21 HCG galaxies previously observed by Coziol & Plauchu-Frayn (2007) was completely redone. The results of our analysis are reported for individual galaxies in Tables 68 for galaxies in KIG, KPG and HCG samples, respectively.
By measuring these parameters at the inner and outer parts in the observed galaxies, we plan to study the effect interactions on galaxies in the three different environments. To do this, we have chosen to be a radius independent on the distribution of light inside and outside of which we will measure isophotal and asymmetry parameters, following the methods described in Paper I. Using the major axis at 20 mag arcsec in band (Jarrett et al., 2000) as given in 2MASS, we have determined the linear diameters in kiloparsec, , for galaxies in the HCG, KPG and KIG catalogs and determined the median of the diameters distribution. The median value obtained is 14 kpc. In our sample, a few galaxies (22% of the sample: 8 HCGs, 9 KPGs, and 3 KIGs) turned out to have a which is smaller than this value. Consequently, we have used two different , equal to 3.5 kpc (approximately ) for the normal size galaxies (approximately ) and half this value, 1.8 kpc, for the galaxies with smaller diameters ().
Based on the surface brightness profiles, of the galaxies, we have estimated the concentration index, . This is defined as (Paper I): and , inside and outside , respectively. is a measure of the light concentration of a galaxy profile, having high values for centrally concentrated light profiles. Early-type galaxies tend to have the most concentrate light profiles, while late-type galaxies have the least concentrated ones (Abraham et al., 1994; Shimasaku et al., 2001). On the other hand, interactions are expected to perturb the stellar material, changing in the process light profiles of galaxies and affecting their concentration indices.
Finally, to make our interpretation of the asymmetry study more straightforward, we have used a slightly different measure of asymmetry than that found in the literature (Schade et al., 1995; Conselice et al., 2000; Hutchings & Proulx, 2008). The level of asymmetry as a function of the semi-major axis is estimated by the following formula:
where is the intensity in the original image and is the intensity in the rotated image. This formula yields values between 1 (completely symmetric) and (completely asymmetric). We refer the reader to Paper I for a detailed description of this method.
In Figure 2, we show, as an example, the mosaic for one very asymmetric galaxy (the full sample of mosaic images is available in the online version of the journal). On the left of Figure 2, we present the isophotal profiles. The dashed vertical line marks the location of the half-radius, , adopted. On the right of the same figure, we present the -band image (or -band for galaxies that were observed only in this filter), displayed on a logarithmic scale. We also present the residual image from the asymmetry analysis (left bottom image). In all these images, the north is at the top and east is to the left.
4.1 Comparison of galaxies with same morphologies in different environments
We have divided our samples in three morphology groups: early-type (E–S0), intermediate-type (Sa–Sb), and late-type (Sbc–Im). The median values of the properties measured on the observed galaxies in the three different groups are reported in Tables 911 for early, intermediate and late types, respectively. We now discuss the variations on the properties encountered of each group depending on their environment. To check for the statistical significance of the observed variations, nonparametrical tests (Kruskal-Wallis for three samples or Mann-Whitney in case of only two samples) were also performed. All the tests were done at a level of significance of 95%, which is the standard for these kind of tests. The results for the statistical tests are reported in Tables 911.
4.1.1 Early-type (E–S0) galaxies
We present the variations of the isophotal parameters internal to in Figure 3 and external to in Figure 4. In each graph, the axis corresponds to the -band absolute magnitude of the galaxies as estimated inside . One observes a higher number of small mass galaxies in the HCG than in the other two samples.
In this morphology group only 2 galaxies belong to the KIGs. We have discarded them from our statistical tests. The only statistically significant differences encountered are that the HCGs tend to be less concentrated and have lower surface brightness inside than the KPG galaxies (see Table 9). This seems consistent with what we observed in the optical. However, since here we are observing in the NIR, the interpretation in terms of mass distribution is clearer. It seems that the orbits of the stars have higher energy in the HCGs than in the KPGs. In Paper I, we suggest that this effect was due to interactions. Thus, this difference would suggest more interactions in the HCGs than in the KPGs.
For all the other parameters we found no statistically significative differences (see Table 9). In general, we observe much less differences in the NIR than in the optical. Note however that some galaxies in the HCG and KPG samples are slightly more asymmetric than in the KIG one (see bottom panel in Figure 4).
As a product of the isophotal study, we have determined the isophotal shape and twist of the early-type galaxies. In Figure 5 we see that most of the galaxies present disky isophotes (): 19 out of 23 (83%) for the HCGs and 4 out of 9 (44%) of the KPGs. This confirms the trends observed in the optical. The fractions of disky galaxies are also consistent with what we found in the optical.
Also consistent with the optical, we found the ellipticity in these galaxies to be quite high. In Figure 6, we show the twists as a function of the absolute magnitudes in . The fraction of galaxies with large twists is in agreement with that found in the optical: 57% (13/23) of the HCGs, with a median value of 22, and 33% (3/9) of the KPGs, with a median value of 21.
In Figure 7, we show how the isophotal para maters and twists vary with the difference ellipticity . A positive value of indicates that a galaxy is rounder in its center than at the periphery. Most of the galaxies of our samples have such characteristic. Large values of together with large and suggest that galaxies were affected by interactions. Once again, the results are similar to those observed in the optical, while no other significant difference is observed between the HCG and the KPG samples.
4.1.2 Intermediate-type (Sa–Sb) galaxies
In Figures 8 and 9, we show for the Sa–Sb group the variations of the isophotal parameters internal and external to , respectively. The only statistically significant differences found are that the KPG galaxies tend to be slightly redder in their centers than the HCGs and more asymmetric than the KIGs outside (see Table 10). In the optical, differences between KPGs and HCGs were not significative. The difference in color in the NIR is puzzling. This cannot be due to extinction, because then we would have expected a difference also in the optical. Usually redder colors suggest older stellar populations. However, it may also suggest a difference in terms of star formation or AGN activity. For example, the presence of numerous supergiant stars due to a recent burst could also produce redder colors (Frogel et al., 1987). Alternatively, it could be that the KPGs are redder because of more intense AGN activity (Kotilainen & Ward, 1994) in some of these galaxies. Both effects would be consistent with interaction effects.
4.1.3 Late-type (Sbc–Im) galaxies
In Figure 10 and 11, we show the variations in the Late-type galaxies of the isophotal parameters internal and external to , respectively. In this morphological group we observe much more differences than in the other two groups (Table 11). The KPGs tend to be fainter in the -band than the HCGs and KIGs. The HCGs also have slightly higher surface brightness inside and are more concentrated outside this radius than the KPGs. These differences suggest variations in stellar populations and distributions. The lower luminosity for the KPGs and higher surface brightness for the HCGs suggest a larger number of old stars in the nucleus of in HCGs than that of KPGs. The higher concentration outside the radius for the HCGs is consistent with higher energy orbits.
In this morphology group, the KPGs and HCGs are also much more asymmetric than the KIG galaxies inside , while outside this radius the most asymmetric are the KPGs (see Table 11). This suggests that this difference is related to interaction effects, and not due to differences in internal processes, like a bar structure, stochastic star formation propagation or wave density.
4.2 Origin of the asymmetries in the NIR
In general, we found less differences among NIR properties of galaxies in different environments than that observed in the optical. This is as expected based on the sensitivity of the different filter-bands to different stellar populations. The NIR follows the distribution of the less massive, but dominant, stellar populations. Since these stars formed earlier, we expect their spatial distribution has already reached some sort of equilibrium within the potential well of the galaxies, and this is independent of galaxy environment.
On the other hand, we saw that in general, and as it already observed in the optical, the level of asymmetry appears slightly higher in the KPGs and the HCGs than in the KIGs, being more obvious in the KPGs than in the HCGs. However, this phenomenon in the NIR also seems to depend on the morphology: the early and intermediate type galaxies are more symmetric than the late-type ones. To isolate the effect of morphology on the asymmetry we must identify clearly the nature of the asymmetries in the NIR. Our method is similar to the one developed in the optical. It consists in classifying the galaxies according to different asymmetry types (see Paper I), with the difference that, since the NIR images are not sensible to dust extinction, no galaxies are classified as type 2.
In type 1, we have put all the “symmetric” galaxies or galaxies with “intrinsic” asymmetries, related to star formation clumps and/or spiral arms. Examples of galaxies with a type 1 asymmetry are shown in Figure 12. In type 3 we see the most obvious evidence of galaxy interactions, under the form of tidal tails, plumes, connecting bridges or common envelop between galaxies. Examples of galaxies with a type 3 asymmetry are presented in Figure 13. In type 4 we have put galaxies which are highly asymmetric, but where the cause is not obvious. In type 5 we have regrouped the cases where the asymmetry may be due to a smaller mass satellite galaxy. Finally, in type 6 we have regrouped the cases where the asymmetry is accompanied by a possible double nucleus. Galaxies with type 4 can be found in Figure 14a), those with type 5 in Figure 14b) and those with type 6 can be found in Figure 14c).
The distribution of asymmetry types in the different samples, as found in the NIR, is presented in Figure 15. In the KIG sample, 88% of the galaxies are of type 1, the rest (12%) are classified as type 4 and type 5. Consequently, the fraction of symmetric galaxies in NIR is higher than what it is found in the optical.
As in the optical, the KPGs present a higher fraction (66%) of asymmetries related to interactions: 54% are of type 3, 6% in types 4 and 5. The rest, 34% of KPGs, are symmetric. For the HCGs, the number of asymmetric galaxies is also quite high, reaching 67% of the galaxies of our sample: 44% are type 3, 15% type 4, 5% type 5, and 3% type 6.
In Table 12, we compare the fraction of galaxies belonging to each asymmetry type in the optical and NIR. We distinguish the same trends. Comparing the classification galaxy by galaxy, in general the asymmetry type is the same in both bands. The most notable difference is related to dust which does not affect the NIR images. Tidal tails and bridges seem as frequent in the NIR as in the optical. However, we do observe a higher fraction of asymmetric galaxies in the NIR than in the optical for the HCGs. Since we are not seeing this effect in the KPGs, this clearly states that something is different in the HCGs. Possibly the interactions in the HCGs are at a more advanced stage and are affecting the oldest stellar population. Or possibly, interactions are now occurring in the absence of the usual evidences of star formation or nuclear activity. This last possibility is consistent with the dry merger hypothesis.
4.3 Color gradients and blue cores
In normal early-type galaxies, color gradients make a galaxy core redder than the periphery (i.e. the gradient is negative; Peletier et al. 1990). These color gradients can be explained, in part by the concentration of older stellar population towards the center of the galaxies, and in other part, by an increase in stellar metallicity (Hinkley & Im, 2001). Galaxy formation models suggest that if an elliptical galaxy form rapidly by monolithic collapse and is undisturbed by interaction, a negative color gradients will form and stay unchanged for most of its life-time. However, some elliptical galaxies are known to present color gradients which are flat or positive, i.e., bluer to the inner part (Michard, 1999; Im et al., 2001; Yang et al., 2006). For these galaxies, models suggest that such features in color gradients can be the result of mergers or past interactions with gas-rich galaxies.
As in Coziol & Plauchu-Frayn (2007), we have search for NIR blue cores in the early-type galaxies of our sample. The color gradient is defined as . According to this definition, galaxies with blue cores have . For 29 early-type galaxies in the three samples we were able to estimate this gradient. In these galaxies we found colors consistent with blue cores or flat gradients in 10 out of 22 (45%) HCGs and 4 out of 6 (67%) KPGs. The only early-type galaxy in the KIG sample, where we were able to estimate this gradient, has . In Figure 16, we plot the gradients for early-type galaxies of our sample. The evidence for blue cores in early-type galaxies seems slightly higher in the KPGs than in the HCGs. This suggests slightly older ages for the interaction events in the CGs.
5 Evidence of star formation produced by interactions
In the HCGs, we have found more evidence of asymmetries in the NIR than in the optical. This last phenomenon is not observed in the KPGs. One possible interpretation is that in the KPGs the interactions involve a lot of gas, while in the HCGs these are occurring under dry conditions. To verify this assumption, we have search for evidence of star formation induced by interaction in the different samples. In CGs the star formation activity was already found to be low (Coziol et al. 1998; 2000; 2004; Martínez et al. 2010).
Using SDSS spectra together with the STARLIGHT777http://www.starlight.ufsc.br/ spectral synthesis code (Cid Fernandes et al., 2005), we have classified the nuclear activity type for 241 (26%) galaxies in the KPG catalog with available spectrum. In Table 13 we compare the activity type in the KPGs with those found by Martínez et al. (2010) for HCG sample. We clearly distinguish a larger number of emission line galaxies in the KPGs than in the HCGs. Also, the dominant type of activity in the KPG galaxies is different, being the majority of them star forming galaxies (SFG), while those in the HCGs are low luminosity AGNs. This comparison confirms that in the KPGs, interactions occur while galaxies are still rich in gas. This result also supports our interpretation that the color difference in terms of younger stellar populations in the KPGs is related to recent star formation events or AGN activity. For the 19 intermediate type galaxies in our observed sample of KPGs, we found a spectra for 7 of them. From these, we found that three are Sy 2 galaxies, one is LINER and two are starburst galaxies.
The fact that we observe as much evidence of interactions in the HCGs as in the KPGs, but no evidence of induced activity in the HCGs compared to the KPGs confirms that the interactions in the HCGs happen predominantly under dry conditions.
Comparing between the optical (Paper I) and NIR analyses, we saw that the properties of galaxies with different morphological types are much more similar in the NIR than in the optical and this is independent of the environment of the galaxies. This is consistent with older populations lying at lower energy levels in the gravitational potential well of their galaxies. Consequently, the asymmetry level induced by interactions is expected to be higher in the optical than in the NIR. This is because it requires much more energy to disrupt these stars.
On the other hand, asymmetric structures related to interactions seem as frequent in the NIR as in the optical. Moreover, there seems to be almost a one to one relation in the case of tidal tails and bridges. This confirms our interpretation that asymmetries are related to interactions: these correspond to real mass redistributions.
The fact that we find the HCGs to be less compact than in other environments is, consequently, quite revealing. This observation suggests the orbits of the stars in the HCGs are more energetic. Such an effect would be achieved by increasing the number of interactions: the larger the number of interactions and the higher the energy of the stars, and consequently the more energetic, or less compact, their orbits in equilibrium. A galaxy in isolation, on the other hand, would be expected to be much more compact, which seems consistent with our observations for the KIGs. For the CGs, the hypothesis of multiple interactions is also consistent with the numerous early-type galaxies found in this sample (Hickson et al., 1988). Our results agree with previous works in the sense that a considerable fractions of galaxies in CGs show perturbations related to interactions and/or mergers (Rubin et al., 1991; Mendes de Oliveira & Hickson, 1994; Verdes-Montenegro et al., 2001).
The question that remains for the CGs is what is the time scale of the interaction process? Is the evolution of galaxies accelerated by numerous interactions or are the galaxies in CGs more evolved morphologically because they began to interact at a earlier time? The multiple evidence of interactions in the CGs, both in the optical and NIR, suggest these galaxies are clearly not in equilibrium. Therefore, compact groups could not have formed that long ago in the past.
For the pairs of galaxies, we may easily assume that the interactions are relatively recent. These galaxies formed in low density environments and it took an Hubble time to two of them for meet and interact. This interpretation is consistent with our observations. In the KPGs, the stellar populations in the central region of the galaxies with different morphologies seem younger, in general, than in the HCGs. This is consistent with the spectroscopic evidence, which shows a higher level of star formation in the KPGs compared to the HCGs.
One key difference between the HCGs and KPGs seems to be that in the CGs, interactions are happening under dry conditions, confirming what we observed in Coziol & Plauchu-Frayn (2007). The fact that we do not see such phenomenon in the KPGs could be due to interactions in these systems are very recent (this is supported by spectroscopy). In the CGs, a first round of interactions would have produced the numerous early-type galaxies we now observe. Possibly when they formed, the CGs would have experienced a more active phase of star formation (and AGN). But now that the gas is exhausted, the galaxies in CGs, assuming merging orbits, can only interact under dry conditions. For the KPGs, we do not know what will be their future. Possibly those are systems with high energy orbits, that will interact again only after an extremely long time has passed. In the case of the CGs the evidence of dry interaction conditions would thus be an evidence that galaxies in these systems are now in merging orbits. Consequently, their formation cannot be that far in the past.
Our analysis suggests that pairs of galaxies are young structures: the galaxies in pairs formed and spent most of their life in relative isolation and are just beginning to interact. This behavior would be typical of low density environments, or what is found at the periphery of large scale structures.
On the other hand, galaxies in CGs are obviously more evolved morphologically and have suffered more interactions. However, based on the abundant evidence of interactions in both the NIR and the optical, these systems cannot be in equilibrium. In particular, the evidence for dry interactions in CGs is consistent with the hypothesis that galaxies are in merging orbits. Consequently, CGs cannot be extremely old.
Cosmologically speaking the difference in formation time between pairs and CGs may be relatively small. That is, the two phenomena are probably typical of the formation of structures in low density environments and consequently their respective formation represents relatively recent events compared to the formation of larger and more massive structures.
According to this interpretation, one would not expect systems like local CGs to exist at high redshifts. CGs may have formed in the past, but these would have been much more massive than what we find today and such systems would have be expected to merge with others to form cluster of galaxies (Coziol et al., 2009).
Note. – Columns: (1) catalog galaxy identification; (2) right ascension from HyperLeda (); (3) declination from HyperLeda (); (4) radial velocity from HyperLeda, corrected for infall of Local Group towards Virgo; (5) morphological type taken from Sulentic et al. (2006)– a star indicates morphology was determined in this work; (6)–(7): total exposure time in and bands, respectively;
|KPG 508B||7444||SBb pec||4100||1080|
Note. – Columns: (1) catalog galaxy identification; (2) right ascension from HyperLeda (); (3) declination from HyperLeda (); (4) radial velocity from HyperLeda, corrected for infall of Local Group towards Virgo; (5) morphological type as determined in this work; (6)–(7): total exposure time in and bands, respectively;
Note. – Columns: (1) catalog galaxy identification; (2) right ascension from HyperLeda (); (3) declination from HyperLeda (); (4) radial velocity from Hickson et al. (1992), corrected for infall of Local Group towards Virgo; (5) morphological type as determined in this work; (6)–(7): total exposure time in and bands, respectively;
Note. – Columns: (1) sample identification; (2) median absolute magnitude in , of observed/catalog galaxies; (3) probability for ; (4) median absolute magnitude in , ,of observed/catalog galaxies; (5) probability for ; (6) median diameter in optical, , of observed/catalog galaxies; (7) probability for ; (8) median diameter in NIR of the observed/catalog galaxies; (9) probability for ; (10) median redshift, , of observed/catalog galaxies; (11) probability for . values were obtained from Mann-Whitney tests; underlined values indicate significative differences between the samples.
Note. – Columns: (1) running number; (2) observation date; (3) average FWHM measured on standard stars used for focus; (4) Filters; (5)–(6): Calibration uncertainties for and K, respectively
Note. – Columns: (1) catalog galaxy identification; (2) concentration inside ; (3) concentration outside ; (4) color inside ; (5) color outside ; (6) asymmetry inside ; (7) asymmetry outside .; (8) asymmetry type.
Note. – Columns are the same as defined in Table 6
|HCG 74a*||0.3||1.4||1.13||1.02||1.01||1.02||Double nuclei|
|HCG 88b*||0.7||1.1||0.82||1.18||1.03||1.07||Tidal & satellite|
Note. – Columns are the same as defined in Table 6
Note. – Columns: (1) properties compared in each sample: absolute magnitude in (mag) inside , absolute magnitude in (mag) inside , surface brightness in (mag arcsec) inside and outside , color (mag) inside and outside , concentration index inside and outside , asymmetry level inside and outside ; (2) and (3) medians of galaxy properties in HCGs and KPGs, respectively; (4) values obtained from Mann-Whitney tests; underlined values values indicate significative differences.
Note. – Columns: (1) properties compared in each sample: absolute magnitude in (mag) inside , absolute magnitude in (mag) inside , average values of the surface brightness in (mag arcsec) inside and outside , average color (magnitude) inside and outside , concentration index inside and outside , average asymmetry level inside and outside ; (2)–(4): medians of galaxy properties in each sample: HCG, KPG, and KIG, respectively; (5)–(7): values obtained with Dunn’s post-tests; underlined value indicates a significative difference between the samples.
Note. – Columns are the same as defined in Table 10
|% / %||% / %||% / %|
|2 Dust & Inclination|
|3 Tidal & Bridges|
|6 Double nucleus|
Note. – Columns: (1) asymmetry type; (2)–(4): distribution of asymmetry types in the optical and in this NIR, for the HCG, KPG, and KIG galaxies, respectively.
|HCG||270||100 (37%)||170 (63%)||54 (32%)||39 (23%)||77 (45%)|
|KPG||241||43 (18%)||198 (82%)||111 (56%)||35 (18%)||52 (26%)|
Note. – Columns: (1) sample identification; (2) number of galaxies with available spectra; (3) fraction of galaxies with no emission lines; (4) fraction of galaxies showing emission lines; (5) fraction of star forming galaxies; (6) fraction of transition objects; (7) fraction of AGN (Sy2, LINER, and Sy1). Data for the HCG galaxies has been obtained from Martínez et al. (2010)
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