Se realizó un análisis de interacciones mayores y menores seleccionadas a partir del catálogo espectroscópico Sloan Digital Sky Survey (SDSSDR7). Se construyó una muestra de galaxias en pares cercanos aplicando el límite de separaciones proyectadas, rp By visual inspection of SDSS images we removed false identifications and we classify the interactions into three categories: pairs undergoing merging, M; pairs with evident tidal features, T; and non disturbed, N.We also divide the pair sample into minor and major interactions according to the luminosity ratio of the galaxy members. We conclude that the characteristics of the interactions and the ratio of luminosity galaxy pair members involved in a merger are important parameters in setting galaxy properties.Fil: Mesa, Valeria Alejandra. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Juan. Instituto de Ciencias Astronomicas de la Tierra y del Espacio; ArgentinaFil: Alonso Giraldes, Maria Sol. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Juan. Instituto de Ciencias Astronomicas de la Tierra y del Espacio; ArgentinaFil: Garcia Lambas, Diego Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: O'Mill, Ana Laura. Universidade de Sao Paulo; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Córdoba. Instituto de Astronomía Teórica y Experimental; Argentin

Alonso Giraldes, Maria Sol

Garcia Lambas, Diego Rodolfo

Mesa, Valeria Alejandra

O'Mill, Ana Laura

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Asociación Argentina de Astronomı́aBAAA, Vol. 54, 2011J.J. Clariá, P. Benaglia, R. Barba, A.E. Piatti & F.A. Bareilles, eds.PRESENTACIÓN ORALMajor and minor mergers: global star formation efficiencyValeria Mesa1,3, Sol Alonso1,3, Diego Garćıa Lambas2,3 & Ana LauraO’Mill4(1) Instituto de Ciencias Astronómicas de la Tierra y el Espacio(ICATE)(2) Instituto de Astronomı́a Teórica y Experimental (IATE-OAC)(3) CONICET(4) Departamento de Astronomia, Instituto de Astronomia, Geof́ısica eCiências Atmosféricas da USP, Rua do Matão 1226, CidadeUniversitária, 05508-090, São Paulo, Brazil.Abstract. We study galaxy pair samples selected from the Sloan DigitalSky Survey (SDSS-DR7) and we perform an analysis of minor and majormergers with the aim of investigating the dependence of galaxy propertieson interactions. We build a galaxy pair catalog requiring rp < 25 kpc h−1and ∆V < 350 km s−1 within redshift z < 0.1. By visual inspection ofSDSS images we removed false identifications and we classify the inter-actions into three categories: pairs undergoing merging, M ; pairs withevident tidal features, T ; and non disturbed, N . We also divide the pairsample into minor and major interactions according to the luminosity ra-tio of the galaxy members. We conclude that the characteristics of theinteractions and the ratio of luminosity galaxy pair members involved ina merger are important parameters in setting galaxy properties.Resumen. Se realizó un análisis de interacciones mayores y menoresseleccionadas a partir del catálogo espectroscópico Sloan Digital Sky Sur-vey (SDSS-DR7). Se construyó una muestra de galaxias en pares cercanosaplicando el ĺımite de separaciones proyectadas, rp < 25 kpc h−1 y ve-locidades radiales relativas, ∆V < 350 km s−1 dentro de un redshift z <0.1. Mediante inspección visual se removieron falsas identificaciones y secuantificó la intensidad de la interacción. Además, se dividió la muestrade galaxias en interacciones mayores y menores de acuerdo a la razónde luminosidad de sus galaxias miembro. A partir de este análisis con-cluimos que la intensidad de las interacciones y la razón de luminosidadde las galaxias involucradas en una fusión son parámetros importantes enestablecer las propiedades de las galaxias.1. IntroductionAccording to hierarchical structure formation models, galaxy-galaxy interac-tions play a critical role in the formation and evolution of galaxies as discussed12 V. Mesa, S. Alonso, D. G. Lambas & A. L. O’Millby Woods et al. 2007 and references there in. Simulations show that galaxiesgrow by accreting other galaxies, mostly minor companions. Although colli-sion of comparable galaxies are expected to be the most damaging, encountersbetween galaxies and minor companions should be the most common type ofinteraction because of the greater fractional abundance of low luminosity galax-ies. Numerical simulations show that in major interactions there is an importantredistribution of mass and a strong gravitational tidal torque causing gas an-gular momentum to be transferred outwards before the final merger. In minorinteractions the tidal action from the less massive companions can induce a non-axisymmetric structure in the disk of the main galaxy (Hernquist & Mihos 1995).The star formation activity in minor interactions depend on structural and or-bital parameters. In this work we focus on a statistical analysis of close galaxypairs and following Alonso et al. (2007) these pairs were classified accordingto the level of morphological disturbance associated to the interaction. In anattempt to explore the physical mechanisms that may affect the star formationactivity and galaxy colors. We use K-corrections of the publicly available codedescribed in Blanton & Roweis (2007) (k-correct_v4.2) as a calibration forour k-corrected magnitudes.2. The SampleWe build a Galaxy Pair Catalog (GPC) from the SDSS-DR7, following our pre-vious works (Alonso et al. 2007), requiring members to have relative projectedseparations, rp < 25 kpc h−1 and relative radial velocities, ∆V < 350 km s−1within redshifts z < 0.1. We classified all galaxies in the pair catalog takinginto account the eye-ball detection of features characteristics of interactions,using the photometric SDSS-DR7. We defined two categories: Disturbed andNon disturbed pairs. Disturbed pairs are sub-classified as merging (M , pairswith evidence of an ongoing merging process) and tidal (T , pairs with signs oftidal interactions but not necessarily merging). Non disturbed (N) pairs show-ing no evidence of distorted morphologies (see Fig. 1). We find that about 10 %of galaxy pairs are classified as M , 30 % as T and 60 % as N , these percentagesdo not depend on redshift.Figure 1. Examples of galaxy pair images with different classification: M(left panel), z=0.060; T (medium panel), z=0.027 and N , z=0.023(rightpanel); the scale, size in arcsec and the N-E direction can be seen in Fig-ure.Galaxy interactions in SDSS-DR7: Major and minor mergers 33. Major and minor interactionsFor the present analysis we have divided our sample in major and minor inter-action pairs according to the luminosity ratio of the galaxy members (Fig. 2,left), the usually adopted criterion for the classification into major or minorinteraction. In Fig. 2 (left, a) we show the distribution of the L2/L1 ratio,and the adopted threshold L2/L1 = 0.33 which gives 877 minor and 1082 majorinteractions.For comparison, we also construct a control sample for the pair catalog, de-fined by galaxies without a close companion within the adopted separation andvelocity thresholds. By using a Monte Carlo algorithm, for each galaxy pair, weselected two other galaxies without a companion within rp < 100 kpc h−1 andrelative radial velocities, ∆V < 350 km s−1. Moreover, these galaxies were alsorequired to match the observed redshift, luminosity and local density environ-ment, Σ5, distributions of the corresponding pair sample, to represent a robustcontrol sample (Perez et al. 2009) (see Fig. 2 right).z0 0.02 0.04 0.06 0.08 0.100.10.20.3Mr-24 -22 -20 -18 -16 -1400.10.20.3-2 -1 0 1 2 300.050.10.150.20.25Figure 2. Left: (a) Distribution of the luminosity ratios of galaxies in thepair sample. (b) and (c) show the Mr distributions of the most luminous (solidlines) and less luminous (dashed lines) galaxy member in minor and majorpair subsamples, respectively. Right: Distribution of z, Mr and log(Σ5) inpair galaxies (dashed lines) and in the control sample (solid lines)3.1. Global star formation efficiency in major and minor interactionsSince interacting galaxies may finally end in a single system, in this subsection weanalyze the efficiency of interactions to trigger the formation of stars in the pairconsidered as a whole. For this aim, we compute the sum of the stellar massesand the sum of the star formation rates for the two members of a given pairusing the data given in Brinchman et al (2004). Fig. 3 (left) shows the behaviorof the total star formation rate (SFR1+SFR2) as a function of the total stellarmass (M∗1+ M∗2). In a similar way, we performed an analysis computing theglobal index colors as a function of a total stellar mass. The results are displayedin Fig. 3 (right).4 V. Mesa, S. Alonso, D. G. Lambas & A. L. O’MillFigure 3. Total star formation rate < SFR1 + SFR2 > and Total <(u − r) > as a function of total stellar mass M∗1+ M∗2for major (dashed)and minor (solid) interactions classified as Disturbed (M and T ) and non-disturbed (N) (upper and lower panels, respectively). Dotted lines representthe control sample within uncertainties derived through the bootstrap re-sampling technique.4. ConclusionsWe have performed a statistical analysis of 1959 galaxy pairs (rp < 25 kpch−1 and ∆V < 350 km s−1) within z < 0.1 selected from SDSS-DR7 and wehave carried out an eye-ball classification of images according to the evidence ofinteraction through distorted morphologies and tidal features.We can summarize the main results in the following conclusions.• We classified 10% of the total pair sample as merging, 30% with tidalfeatures, and 60% as non disturbed.• We have also performed an analysis of the pairs considered as a singlesystem. We find that at a given total stellar mass, major interactionsare more efficient in forming new stars in comparison to minor pairs (by afactor ≈ 2). Nevertheless, in both, minor and major interactions, disturbedpairs (M and T systems) have a significantly higher total star formationrate than non-disturbed galaxies. In a similar way, at a given total stellarmass, disturbed pairs show blue global color with respect to non-disturbedsystems.ReferencesAlonso M. S., Lambas D. G., Tissera P., Coldwell G., 2007, MNRAS, 375, 1017.Blanton, M. & Roweis, S., AJ, 133,734.Brinchmann J., Chalot, S., White, S.D.M., Tremonti, C., Kauffmann, G. et al., 2004,MNRAS 351, 1151Hernquist L. & Mihos J.C., 1995, ApJ, 448, 41.Perez J., Tissera P., Padilla N, Alonso M S, Lambas G D, 2009, MNRAS, 399, 1157Woods D.; Geller M, 2007, AJ, 134, 527

English

Major and minor mergers: global star formation efficiency

CONICET Digital

Asociacio´n Argentina de Astronomı´aBAAA, Vol. 54, 2011J.J. Claria´, P. Benaglia, R. Barba, A.E. Piatti & F.A. Bareilles, eds.PRESENTACIO´N ORALMajor and minor mergers: global star formation efficiencyValeria Mesa1,3, Sol Alonso1,3, Diego Garc´ıa Lambas2,3 & Ana LauraO’Mill4(1) Instituto de Ciencias Astrono´micas de la Tierra y el Espacio(ICATE)(2) Instituto de Astronomı´a Teo´rica y Experimental (IATE-OAC)(3) CONICET(4) Departamento de Astronomia, Instituto de Astronomia, Geof´ısica eCieˆncias Atmosfe´ricas da USP, Rua do Mata˜o 1226, CidadeUniversita´ria, 05508-090, Sa˜o Paulo, Brazil.Abstract. We study galaxy pair samples selected from the Sloan DigitalSky Survey (SDSS-DR7) and we perform an analysis of minor and majormergers with the aim of investigating the dependence of galaxy propertieson interactions. We build a galaxy pair catalog requiring rp < 25 kpc h−1and ∆V < 350 km s−1 within redshift z < 0.1. By visual inspection ofSDSS images we removed false identifications and we classify the inter-actions into three categories: pairs undergoing merging, M ; pairs withevident tidal features, T ; and non disturbed, N . We also divide the pairsample into minor and major interactions according to the luminosity ra-tio of the galaxy members. We conclude that the characteristics of theinteractions and the ratio of luminosity galaxy pair members involved ina merger are important parameters in setting galaxy properties.Resumen. Se realizo´ un ana´lisis de interacciones mayores y menoresseleccionadas a partir del cata´logo espectrosco´pico Sloan Digital Sky Sur-vey (SDSS-DR7). Se construyo´ una muestra de galaxias en pares cercanosaplicando el l´ımite de separaciones proyectadas, rp < 25 kpc h−1 y ve-locidades radiales relativas, ∆V < 350 km s−1 dentro de un redshift z <0.1. Mediante inspeccio´n visual se removieron falsas identificaciones y secuantifico´ la intensidad de la interaccio´n. Adema´s, se dividio´ la muestrade galaxias en interacciones mayores y menores de acuerdo a la razo´nde luminosidad de sus galaxias miembro. A partir de este ana´lisis con-cluimos que la intensidad de las interacciones y la razo´n de luminosidadde las galaxias involucradas en una fusio´n son para´metros importantes enestablecer las propiedades de las galaxias.1. IntroductionAccording to hierarchical structure formation models, galaxy-galaxy interac-tions play a critical role in the formation and evolution of galaxies as discussed12 V. Mesa, S. Alonso, D. G. Lambas & A. L. O’Millby Woods et al. 2007 and references there in. Simulations show that galaxiesgrow by accreting other galaxies, mostly minor companions. Although colli-sion of comparable galaxies are expected to be the most damaging, encountersbetween galaxies and minor companions should be the most common type ofinteraction because of the greater fractional abundance of low luminosity galax-ies. Numerical simulations show that in major interactions there is an importantredistribution of mass and a strong gravitational tidal torque causing gas an-gular momentum to be transferred outwards before the final merger. In minorinteractions the tidal action from the less massive companions can induce a non-axisymmetric structure in the disk of the main galaxy (Hernquist & Mihos 1995).The star formation activity in minor interactions depend on structural and or-bital parameters. In this work we focus on a statistical analysis of close galaxypairs and following Alonso et al. (2007) these pairs were classified accordingto the level of morphological disturbance associated to the interaction. In anattempt to explore the physical mechanisms that may affect the star formationactivity and galaxy colors. We use K-corrections of the publicly available codedescribed in Blanton & Roweis (2007) (k-correct_v4.2) as a calibration forour k-corrected magnitudes.2. The SampleWe build a Galaxy Pair Catalog (GPC) from the SDSS-DR7, following our pre-vious works (Alonso et al. 2007), requiring members to have relative projectedseparations, rp < 25 kpc h−1 and relative radial velocities, ∆V < 350 km s−1within redshifts z < 0.1. We classified all galaxies in the pair catalog takinginto account the eye-ball detection of features characteristics of interactions,using the photometric SDSS-DR7. We defined two categories: Disturbed andNon disturbed pairs. Disturbed pairs are sub-classified as merging (M , pairswith evidence of an ongoing merging process) and tidal (T , pairs with signs oftidal interactions but not necessarily merging). Non disturbed (N) pairs show-ing no evidence of distorted morphologies (see Fig. 1). We find that about 10 %of galaxy pairs are classified as M , 30 % as T and 60 % as N , these percentagesdo not depend on redshift.Figure 1. Examples of galaxy pair images with different classification: M(left panel), z=0.060; T (medium panel), z=0.027 and N , z=0.023(rightpanel); the scale, size in arcsec and the N-E direction can be seen in Fig-ure.Galaxy interactions in SDSS-DR7: Major and minor mergers 33. Major and minor interactionsFor the present analysis we have divided our sample in major and minor inter-action pairs according to the luminosity ratio of the galaxy members (Fig. 2,left), the usually adopted criterion for the classification into major or minorinteraction. In Fig. 2 (left, a) we show the distribution of the L2/L1 ratio,and the adopted threshold L2/L1 = 0.33 which gives 877 minor and 1082 majorinteractions.For comparison, we also construct a control sample for the pair catalog, de-fined by galaxies without a close companion within the adopted separation andvelocity thresholds. By using a Monte Carlo algorithm, for each galaxy pair, weselected two other galaxies without a companion within rp < 100 kpc h−1 andrelative radial velocities, ∆V < 350 km s−1. Moreover, these galaxies were alsorequired to match the observed redshift, luminosity and local density environ-ment, Σ5, distributions of the corresponding pair sample, to represent a robustcontrol sample (Perez et al. 2009) (see Fig. 2 right).z0 0.02 0.04 0.06 0.08 0.100.10.20.3Mr-24 -22 -20 -18 -16 -1400.10.20.3-2 -1 0 1 2 300.050.10.150.20.25Figure 2. Left: (a) Distribution of the luminosity ratios of galaxies in thepair sample. (b) and (c) show theMr distributions of the most luminous (solidlines) and less luminous (dashed lines) galaxy member in minor and majorpair subsamples, respectively. Right: Distribution of z, Mr and log(Σ5) inpair galaxies (dashed lines) and in the control sample (solid lines)3.1. Global star formation efficiency in major and minor interactionsSince interacting galaxies may finally end in a single system, in this subsection weanalyze the efficiency of interactions to trigger the formation of stars in the pairconsidered as a whole. For this aim, we compute the sum of the stellar massesand the sum of the star formation rates for the two members of a given pairusing the data given in Brinchman et al (2004). Fig. 3 (left) shows the behaviorof the total star formation rate (SFR1+SFR2) as a function of the total stellarmass (M∗1+ M∗2). In a similar way, we performed an analysis computing theglobal index colors as a function of a total stellar mass. The results are displayedin Fig. 3 (right).4 V. Mesa, S. Alonso, D. G. Lambas & A. L. O’MillFigure 3. Total star formation rate < SFR1 + SFR2 > and Total <(u − r) > as a function of total stellar mass M∗1+ M∗2for major (dashed)and minor (solid) interactions classified as Disturbed (M and T ) and non-disturbed (N) (upper and lower panels, respectively). Dotted lines representthe control sample within uncertainties derived through the bootstrap re-sampling technique.4. ConclusionsWe have performed a statistical analysis of 1959 galaxy pairs (rp < 25 kpch−1 and ∆V < 350 km s−1) within z < 0.1 selected from SDSS-DR7 and wehave carried out an eye-ball classification of images according to the evidence ofinteraction through distorted morphologies and tidal features.We can summarize the main results in the following conclusions.• We classified 10% of the total pair sample as merging, 30% with tidalfeatures, and 60% as non disturbed.• We have also performed an analysis of the pairs considered as a singlesystem. We find that at a given total stellar mass, major interactionsare more efficient in forming new stars in comparison to minor pairs (by afactor ≈ 2). Nevertheless, in both, minor and major interactions, disturbedpairs (M and T systems) have a significantly higher total star formationrate than non-disturbed galaxies. In a similar way, at a given total stellarmass, disturbed pairs show blue global color with respect to non-disturbedsystems.ReferencesAlonso M. S., Lambas D. G., Tissera P., Coldwell G., 2007, MNRAS, 375, 1017.Blanton, M. & Roweis, S., AJ, 133,734.Brinchmann J., Chalot, S., White, S.D.M., Tremonti, C., Kauffmann, G. et al., 2004,MNRAS 351, 1151Hernquist L. & Mihos J.C., 1995, ApJ, 448, 41.Perez J., Tissera P., Padilla N, Alonso M S, Lambas G D, 2009, MNRAS, 399, 1157Woods D.; Geller M, 2007, AJ, 134, 527

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Major and minor mergers: global star formation efficiency

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