The large survey programs being performed nowadays, being the SDSS their
flagship, provide us with morphological parameters which allow for extraction
of large galaxy samples. We will analyze the methodology for obtaining an
AMIGA-like catalogue of isolated galaxies from the SDSS DR5 photometric
catalogue of galaxy objects, together with the roadblocks found in the process,
and suggested workarounds.Comment: 4 pages, 2 figures, to appear in the Proceedings of the International
  Conference CIG09: Galaxies in Isolation, held in May 19th in Granada, Spai

Santander-Vela, Juan de Dios

Verdes-Montenegro, Lourdes

Verley, Simon

English

arXiv

International conference “Galaxies in Isolation: Exploring Nature vs. Nurture” held in Granada (Spain), May 12–15, 2009.The large survey programs being performed nowadays, being the SDSS their flagship, provide us with morphological parameters which allow for extraction of large galaxy samples. We will analyze the methodology for obtaining an AMIGA-like catalogue of isolated galaxies from the SDSS DR5 photometric catalogue of galaxy objects, together with the roadblocks found in the process, and suggested workarounds.The authors wish to acknowledge the support from
DGI Grant AYA2008-06181-C02, and Junta de Andaluca (Spain) grants TIC-114 and P08-FQM-4205-PEX. In addition, SV is supported through the Spanish MICINN’s Juan de la Cierva post-doctoral program

Santander-Vela, J. D.

Verdes-Montenegro, L.

Repositorio Institucional Universidad de Granada

arXiv:0910.0661v1  [astro-ph.IM]  5 Oct 2009**FULL TITLE**ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION****NAMES OF EDITORS**Recovering isolated galaxies from large scale surveys:problems and strategiesJuan de Dios Santander-Vela,1,2,5 Simon Verley,3,4,5 and LourdesVerdes-Montenegro1,5Abstract. The large survey programs being performed nowadays, being theSDSS their flagship, provide us with morphological parameters which allow forextraction of large galaxy samples. We will analyze the methodology for obtain-ing an AMIGA-like catalogue of isolated galaxies from the SDSS DR5 photo-metric catalogue of galaxy objects, together with the roadblocks found in theprocess, and suggested workarounds.1. IntroductionThe Catalogue of Isolated Galaxies (CIG) by Karachentseva (1973), basis for theAMIGA sample (Verdes-Montenegro et al. 2005), was compiled using a visualsearch on photographic plates. In the era of the Virtual Observatory and largesurveys, however, the automated compilation of isolated galaxy samples not onlybecomes feasible, but potentially less biased and error prone.There are previous examples of automatic extraction of isolated galaxy sam-ples in the literature, some of them mimicking Karachentseva’s criterion, as isthe case of Allam et al. (2005). The search of compact groups uses similarconstraints, and a recent example of automated extraction of them in galaxycatalogues is shown by McConnachie et al. (2009).With automatically extracted samples, combined with the extra parametersderived by the SDSS pipeline, it is possible to estimate the degree of isolation ofthe selected galaxies, using for instance the tidal forces or local density estima-tions characterised in Verley et al. (2007a,b). Additionally, we gain the abilityto re-do the studies, and apply them to similar catalogues in the southern hemi-sphere, so that an all-sky sample of isolated galaxies can be finally assembled. Inthis proceeding we will focus on the automatic extraction of a CIG (or AMIGA)like sample of isolated galaxies.1Instituto de Astrof́ısica de Andalućıa-CSIC, Apdo. Correos 3004, E-18080 Granada, Spain2European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching-bei-München,Germany3University of Granada, Dept. F́ısica Teórica y del Cosmos, Edificio Mecenas, Campus Univer-sitario Fuentenueva, E-18071 Granada, Spain4LERMA-OBSPM, 61 Av. de l’Observatoire, F-75014 Paris, FRANCE5AMIGA Collaboration http://amiga.iaa.es/12 Santander-Vela, Verley, & Verdes-Montenegro2. Implementing Karachentseva’s isolation criterion on the SDSSA galaxy i, with angular diameter Di is considered isolated, following Karachent-seva’s criterion, if the projected distance between it and neighbor j, Pij , verifiesPij ≥ 20 × Dj (1)for all j galaxies with diameter Dj , also verifying14Di ≤ Dj ≤ 4 × Di (2)Allam et al. (2005) reformulates eq. 2, changing it for a relative magnitudedifference comparison:|gi − gj | > 3.0 (3)where gi is the g magnitude of galaxy i; both approaches are equivalent assum-ing constant surface brightness profiles. We tried both approximations in ourtesting.As the SDSS spectrometric catalogue is known to be complete only upto Petrosian magnitude r brighter than 17.7 (Adelman-McCarthy et al. 2007),we will make use of the SDSS photometric catalogue of objects identified asgalaxies. We targeted SDSS DR5, but as the SDSS table schemata for DR7have only added columns to specify clean objects, our procedure is valid also forSDSS DR7.Using CasJobs, we selected objects from the PhotoObj table view withtype = 3 (Galaxy), and g magnitudes between 10 and 15. For convenienceduring the testing, we applied a RA/Dec cut:206.6 < RA < 228.531.4 < Dec < 35.5which gave us a test field containing around 98000 objects, with 2 CIG galaxies(CIG 613 & CIG 642) among them.After downloading the test field data, a Python script was run to checkgalaxy isolation, marking galaxy i as not isolated if any galaxy within 80 timesDi in the test frame (the maximum relevant distance, giving the size comparison)which verified eq. 2 did not verify eq. 1. A second script used eqs. 3 and 1 instead.The two approaches resulted to be largely equivalent, but the results gavea lot more allegedly isolated galaxies than the two CIGs. If the factor of 20 ineq. 1 was increased, the results provided none of the two CIGs, and retainedsome objects in the center which did not appear to be isolated galaxies, plusobjects along the frame border (an artifact of the reduced sample).3. Problems found and strategies to avoid themIn order to understand why the two CIGs in the test frame could not be recoveredas isolated, we checked visually the frame and found several problems, mainlythe misclassification of stars as galaxies, and item blending.Recovering isolated galaxies from SDSS: problems & strategies 3Figure 1. Object positions, radii, and magnitudes for in subset of the testfield around CIG 613 (center object). Magnitude is shown in the color scale(darker is less bright), while radii are proportional to the Petrosian radius foreach object.As we intend to recover a catalogue of truly isolated galaxies, without re-sorting to visual inspection but at the latest stage, we need to find a way toobjectively distinguish stars which have been misclassified as galaxies by theoriginal SDSS pipeline1.Figure 1 shows how CIG 613 (the central, largest dark circle) is bigger thanthe surrounding objects, but less bright than most companion objects, whichexhibit a much higher surface brightness. This shows that there are parameters,or combinations of parameters, helpful to objectively improve SDSS’ star-galaxyseparation. In fact, such kind of star-galaxy separation had already been inde-pendently performed by Collister et al. (2007), disregarding SDSS type entirely,based also on additional information from the SDSS photometric catalogue it-self. However, their star-galaxy separation algorithm includes photometric red-shift estimation, which is not reliable in the the local universe, due to the stronginfluence of peculiar velocities.Of the large number of columns available for PhotoObj2, several provideinformation about the reliability of the SDSS pipeline’s star/galaxy separation,such as probPSF, lnLStar, plus other parameters such as lnLExp , lnLDeV, andmany others. However, many of them showed binary, non-continuous values,and where not useful for building a separation. The most promising singleparameter, able by itself to perform an effective separation, is lnLStar g. Thelowest absolute value of lnLStar g for a CIG galaxy was higher than 29100 (forCIG 299), with the rest of CIG’s having abs(lnLStar g) above 40000.This is shown in figure 2, where we plot the normalized distribution ofabs(lnLStar g) values across the SDSS DR5 galaxies of our test field (red line),and the CIG galaxies with SDSS DR5 counterpart (green). For comparison, wehave plotted also the histogram for SDSS star objects (type = 6; in green), and1The original SDSS photometric star-galaxy separation is done using the difference betweenmodeled profiles (linear combination of the best-fitting De Vaucouleurs and exponential pro-files) and PSF model magnitudes.2 http://cas.sdss.org/dr5/en/help/browser/description.asp?n=PhotoObj&t=V In particu-lar, probPSF is the probability of the object being a star; lnLStar is the ln of star fittinglikelihood; lnLExp and lnLDeV are the ln of the likelihood for the object fitting an exponentialdisk or a De Vaucouleurs profile. These parameters are available for each ugriz color.4 Santander-Vela, Verley, & Verdes-MontenegroFigure 2. Normalized cumulative frequency histogram for the absolutevalue of lnLStar g, both for SDSS galaxies with 10 ≤ magg ≤ 15 (red), CIGgalaxies with SDSS DR5 counterparts (green), and SDSS stars with the samemagnitude cut (blue).we can see that the absolute value of lnLStar g is below 10000 for more than95% of star objects.4. Conclusions and Future WorkWe have seen that in order to use the SDSS Photometric catalogue for retrievinggalaxy properties, and estimate their isolation degree, the basic star-galaxy sep-aration provided by the SDSS pipeline is not enough, and more work is needed inorder to be able to obtain meaningful results regarding the isolation of galaxies.The analysis of available parameters, however, has shown to be promisingin achieving a more accurate star-galaxy separation which, combined with datamining and clustering tools, may finally allow a reuse of SDSS photometric datafor this kind of studies.In the future, we will continue pursuing an automated isolated galaxy sam-ple builder system, using data mining techniques in order to find out the pa-rameters with the best star-galaxy separation properties, and be able to extendthe selection of isolated galaxies to similar catalogues in the future.Acknowledgments. The authors wish to acknowledge the support fromDGI Grant AYA2008-06181-C02, and Junta de Andaluca (Spain) grants TIC-114 and P08-FQM-4205-PEX. In addition, SV is supported through the SpanishMICINN’s Juan de la Cierva post-doctoral program.ReferencesAdelman-McCarthy, J. K., et al. 2007, ApJS, 172, 634Allam, S. S., Tucker, D. L., Lee, B. C., & Smith, J. A. 2005, AJ, 129, 2062Collister, A., et al. 2007, MNRAS, 375, 68Karachentseva, V. E. 1973, Astrofizicheskie Issledovaniia Izvestiya Spetsial’nojMcConnachie, A. W., Patton, D. R., Ellison, S. L., & Simard, L. 2009, MNRAS, 395,255Strauss, M. A., et al. 2002, AJ, 124, 1810Verdes-Montenegro, L., Sulentic, J., Lisenfeld, U., Leon, S., Espada, D., Garcia, E.,Sabater, J., & Verley, S. 2005, A&A, 436, 443Verley, S., et al. 2007a, A&A, 470, 505Verley, S., et al. 2007b, A&A, 472, 121

Recovering isolated galaxies from large scale surveys: problems and strategies

Verley, S.

Institutional Repository of the University of Granada

Recovering isolated galaxies from large scale surveys: problems and
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Recovering isolated galaxies from large scale surveys: problems and strategies

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