Arp found instances of quasars clumped around galaxies that he suggested were examples of this ejection mechanism. Simultaneously, Arp has argued these examples of quasar clumping around galaxies are evidence that quasar redshifts are not cosmological. This generally is based upon statistical grounds that quasars are unlikely to be seen clumped around a galaxy, if the galaxy and quasars merely are along the same line of sight but at much different distances. On the other hand, such clumping would be expected if quasars are associated with the galaxy despite their discordant redshifts.
Catalogue of Discordant Redshift Associations
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Using the publicly available outputs of the SAM of Henriques et al. (2015) run on the N-body Millennium simulation (Springel et al. 2005) re-scaled to the Planck cosmology1, we construct a mock lightcone galaxy catalogue following a procedure similar to that described in Zandivarez et al. (2014a); see subsection 2.3 for details). The lightcone was built using galaxies extracted from different redshift outputs of the simulation to include the evolution of structures and galaxy properties with time.
In this section we compare the two samples of CGs obtained using different algorithms. To start with, we note that the modified algorithm produces a sample that is 89% larger than the classic sample. We cross-correlated the two samples and considered that a group from one sample has been recovered in the other sample if the angular distance between their centres are less than the sum of the angular radii of the groups. By cross-correlating the samples, we found that 95% of the classic sample (647 mHCCGs) is included in the modified sample. This means that the new algorithm is able to recover most of the classic sample, but it also identifies almost twice the number of groups, improving the statistics significantly. We also analysed the reasons why the classic algorithm misses almost half of the mHMCGs. We found that 90% of the missing groups are discarded by the classic algorithm because there are discordant-velocity galaxies contaminating the isolation annulus in projection. In the remaining 10%, the membership was compromised: some of the true group galaxy members were identified in projection, but others were missing, and new discordant redshift galaxies were included as group galaxy members. These groups did not pass the posterior velocity filtering.
In Fig. 4, we show three examples of regions around HMCGs. Pictures were taken from the SDSS DR14 Image Tool. Galaxy members are shown as upward-pointing triangles combined with downward-pointing triangles (stars) within or on top of the inner circle. The left picture shows an HMCG without any objects with unknown redshift within the group radius or the isolation annulus. The picture in the centre shows an example where two objects without redshift lie within the isolation radius (3ΘG), but one of the objects is outside the magnitude range of the group members (open dashed circle), while the other has a photometric redshift that is clearly discordant with the group and therefore is classified as a non-contaminating galaxy (upward-pointing dashed triangle); the other objects that can be seen in the same field without symbols are either galaxies fainter than the apparent magnitude limit of the SDSS spectroscopic sample or stars. Finally, the picture on the right shows a CG that is surrounded by three objects without known red-shifts and in the same magnitude range of the group members, one galaxy in the isolation annulus that has been classified as non-contaminating based on the method described in Appendix C (light blue dashed star), and two others (one inside the group radius and the other in the isolation annulus) that we were not able to discard as potential contaminants (bright yellow stars).
For the sake of comparison with the results found from the mock catalogues, we applied the classic algorithm to the extended and corrected sample of galaxies from SDSS, and we obtained a sample of 218 HCCGs (see Table 1). Similar to the results found in the mock catalogue, the modified algorithm was able to recover 95% of these groups, while it nearly doubles the number of CGs identified. Also, the classic algorithm misses almost half of the HMCGs due to discordant-velocity galaxies contaminating the isolation annulus. In Fig. 2, we show the boxplots obtained for the properties of CGs identified using both algorithms, where the samples are labelled Hickson classic (HC) and Hickson modified (HM). From this comparison, we observe trends similar to those in the samples obtained from the mock catalogue: the median values of dij, H0 tcr, ΘG, and μr are significantly higher with the modified method than with the classic method. Moreover, the analyses of completeness as a function of different properties produced the same results found for the mock samples. The completeness of the sample of CGs has therefore been improved with the new implementation of the algorithm.
Similar to our work, Sohn et al. have also used the catalogue of spectroscopic galaxies from the SDSS DR12 (Alam et al. 2015), but they complemented the survey themselves adding spectroscopic information extracted from other surveys such as Hwang et al. (2010) and NED, especially for bright galaxies (r r z
The original definition of compact groups states that they are small, high density associations of bright galaxies that are relatively isolated in space (Hickson 1982). To identify such systems, groups must meet several criteria: limited population within a magnitude range, compactness, spatial isolation within a magnitude range, and velocity concordance of all of their galaxy members. While in principle the limiting values that can be adopted for each of the criteria are arbitrary, it is customary to adopt the definitions introduced by Hickson (1982) and Hickson et al. (1992). In this work, we followed these original ideas, and adopted the commonly used limits to present a new algorithm to identify CGs in redshift surveys.
There has been much fruitless discussion of what might appear a straightforward statistical problem - are there or are there not excess QSOs in the directions of bright galaxies? The difficulties lie in the fact that QSO searches are still quite inhomogeneous over the sky, and thus a search may be deep enough to tell us something but cover too little solid angle, or cover the whole sky with too few QSOs. For example, there are four close galaxy-QSO pairs in the 3C catalog (Burbidge, Burbidge, Solomon, and Strittmatter 1971 ApJ 170, 233). These are pretty famouspairs now - namely NGC 3067/3C 232, NGC 4651/3C 275.1, NGC 4138/3C 268.4, andNGC 5832/3C 309.1. But with only about 100 quasars over half the sky, the statistics were too sparse to do more. Perhaps large-scale automated surveys will be able to resolve this(using not only multicolor optical selection, but identifications ofROSAT survey sources).The methodology Arp has frequently adopted doesn't help - starting from a galaxy and searching outward until a quasar shows up, then if it's "interestingly" close keep on going outward. This is guaranteed to produce an apparent excess, on the "seek and ye shall find" principle. A final problem with a statistical analysis is that it is not always clear what it is whose likelihood we want to assess. Some papers talk about QSO-galaxy pairs, some about QSO pairs with discordant redshift, lines of quasars...Statistics after the fact has a bad reputation. As if to make things worse,we do expect an excess number of quasars in the directions of galaxiesat some redshift ranges from gravitational lensing, as long as the QSOcounts rise rapidly with magnitude, even ina boringly conventionalpicture.
Absorption-line systems again require that the QSO be beyond all the absorbing material unless all the intervening material has noncosmological redshifts as well. In this case, a strong coincidence is needed to make the redshift distributions of various kinds of absorber make any sense at all in a conventional model. Shaver has done aninteresting test of QSO pairs at different distances; when absorptionis seen at one QSO redshift, it's always the lower-redshift one against thehigh-z member. Much the same thing is found with associations ofQSOs and low-redshift galaxies, although the absorbing gas seems tobe patchy enough that some of the absorption lines are quite weak and one has to work hard to get a significant detection.
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View the references in ADS 2023.02.09-20:33:23 Université de Strasbourg/CNRS 2ff7e9595c
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