D. Espada

The AMIGA project - I. Optical characterization of the CIG catalog

The AMIGA project (Analysis of the Interstellar Medium of Isolated Galaxies) is compiling a multiwavelength database of isolated galaxies that includes optical (B and Hα), infrared (FIR and NIR) and radio (continuum plus HI and CO lines) properties. It involves a refinement of the pioneering Catalog of Isolated Galaxies. This paper is the first in a series and begins with analysis of the global properties of the nearly redshift-complete CIG with emphasis on the Optical Luminosity Function (OLF) which we compare with other recent estimates of the OLF for a variety of environments. The CIG redshift distribution for n = 956 galaxies re-enforces the evidence for a bimodal structure seen earlier in smaller samples. The peaks at redshift near 1500 and 6000 km s −1 , corresponding respectively to galaxies in the local supercluster and those in more distant large-scale components (particularly Perseus-Pisces). The two peaks in the redshift distribution are superimposed on 50% or more of the sample that is distributed in a much more homogeneous way. The CIG probably represents the most homogeneous local field example that has ever been compiled. Our derivation of the CIG OLF is consistent with other studies of the OLF for lower density environments. This comparison via the Schechter parameter formalization shows that: 1) M ∗ increases with galaxy surface density on the sky and 2) α shows a weaker tendency to do the same. The CIG represents the largest and most complete foundation for studies of isolated galaxies and is likely as close as we can come to a field sample.

The AMIGA sample of isolated galaxies. V. Quantification of the isolation

Context. The AMIGA project aims to build a well defined and statistically significant reference sample of isolated galaxies in order to estimate the environmental effects on the formation and evolution of galaxies. Aims. The goal of this paper is to provide a measure of the environment of the isolated galaxies in the AMIGA sample, quantifying the influence of the candidate neighbours identified in our previous work and their potential effects on the evolution of the primary galaxies. Here we provide a quantification of the isolation degree of the galaxies in this sample. Methods. Our starting sample is the Catalogue of Isolated Galaxies (CIG). We used two parameters to estimate the influence exerted by the neighbour galaxies on the CIG galaxy: the local number density of neighbour galaxies and the tidal strength affecting the CIG galaxy. We show that both parameters together provide a comprehensive picture of the environment. For comparison, those parameters have also been derived for galaxies in denser environments such as triplets, groups and clusters. Results. The CIG galaxies show a continuous spectrum of isolation, as quantified by the two parameters, from very isolated to interacting. The fraction of CIG galaxies whose properties are expected to be influenced by the environment is however low (159 out of 950 galaxies). The isolated parameters derived for the comparison samples gave higher values than for the CIG and we found clear differences for the average values of the 4 samples considered, proving the sensitivity of these parameters. Conclusions. The environment of the galaxies in the CIG has been characterised, using two complementary parameters quantifying the isolation degree, the local number density of the neighbour galaxies and the tidal forces affecting the isolated galaxies. A final catalogue of galaxies has been produced and the most isolated of these galaxies are consequently appropriate to serve as a reference sample for the AMIGA project.

The AMIGA sample of isolated galaxies - IX. Molecular gas properties

Aims. We characterize the molecular gas content (ISM cold phase) using CO emission of a redshift-limited subsample of isolated galaxies from the AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) project in order to provide a comparison sample for studies of galaxies in different environments. Methods. We present the 12 CO(1–0) data for 273 AMIGA galaxies, most of them (n = 186) from our own observations with the IRAM 30 m and the FCRAO 14 m telescopes and the rest from the literature. We constructed a redshift-limited sample containing galaxies with 1500 km s −1 <v< 5000 km s −1 and excluded objects with morphological evidence of possible interaction. This sample (n = 173) is the basis for our statistical analysis. It contains galaxies with molecular gas masses, MH2 , in the range of ∼10 8 −10 10 M� .I t is dominated, both in absolute number and in detection rate, by spiral galaxies of type T = 3–5 (Sb-Sc). Most galaxies were observed with a single pointing towards their centers. Therefore, we performed an extrapolation to the total molecular gas mass expected in the entire disk based on the assumption of an exponential distribution. We then studied the relationships between MH2 and other galactic properties (LB, D 2 , LK, LFIR ,a ndMHI). Results. We find correlations between MH2 and LB, D 2 , LK ,a ndLFIR. The tightest correlation of MH2 holds with LFIR and, for T = 3– 5, with LK, and the poorest with D 2 . The correlations with LFIR and LK are very close to linearity. The correlation with LB is nonlinear so that MH2 /LB increases with LB. The molecular and the atomic gas masses of our sample show no strong correlation. We find a low mean value, log(MH2 /MHI) = −0.7 (for T = 3–5), and a strong decrease in this ratio with morphological type. The molecular gas column density and the surface density of the star formation rate (the Kennicutt-Schmidt law) show a tight correlation with a rough unity slope. We compare the relations of MH2 with LB and LK found for AMIGA galaxies to samples of interacting galaxies from the literature and find an indication for an enhancement of the molecular gas in interacting galaxies of up to 0.2–0.3 dex.

The AMIGA sample of isolated galaxies III. IRAS data and infrared diagnostics

Aims. We describe the mid- (MIR) and far- (FIR) infrared properties of a large (∼1000) sample of the most isolated galaxies in the local Universe. This sample is intended as a “nurture-free” zero point against which more environmentally influenced samples can be compared. Methods. We reprocess IRAS MIR/FIR survey data using the ADDSCAN/SCANPI utility for 1030 out of 1050 galaxies from the Catalogue of Isolated Galaxies (CIG) as part of the AMIGA project. We focus on diagnostics (FIR luminosity LFIR, R = log(LFIR/LB), and IRAS colours) thought to be sensitive to effects of environment or interaction. Results. The distribution of log(LFIR) sharply peaks from 9.0–10.5, with very few (<2%) galaxies above 10.5. Review of available optical images of the most FIR luminous galaxies finds the majority likely to be interacting systems missed in our earlier morphological reevaluation. The optically normalised luminosity diagnostic R = log(LFIR/LB) shows a sharply peaked distribution between 0.0 and −1.0. These results were compared to the magnitude limited sample of the Center for Astrophysics that was selected without environmental discrimination. This modestly (e.g., compared to cluster, binary galaxy, and compact group samples) environmentally affected sample shows a significantly higher mean log(LFIR), and R, whereas the mean log(LB) is the same. Our sample shows a strong LFIR vs. LB correlation, with a slope steeper than one (LFIR ∝ L 1.41 B ). Interacting galaxies were found above this correlation, showing an enhancement in LFIR. With respect to the IRAS colours, we found higher F60/F100 values for ellipticals and late-type galaxies than for spirals, indicating a higher dust temperature. The mean value of F60/F100 was found to be lower than for interacting samples from the literature. Conclusions. The results indicate that the FIR emission is a variable enhanced by interaction, and that our sample probably shows the lowest possible mean value. This attests to the utility of our sample for defining a nurture-free zero point.

The AMIGA sample of isolated galaxies - II. Morphological refinement

Received / accepted Abstract. We present a complete refinement of the optical morphologies for galaxies in the Catalog of Isolated Galaxies (Karachentseva 1973) that forms the basis of the AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) project. Uniform reclassification using the digitized POSS II benefit ed from the high resolution and dynamic range of that sky survey. Comparison with independent classifications made for an SDS S overlap sample of more than 200 galaxies confirms the reli- ability of the early vs. late-type discrimination and the ac curacy of spiral subtypes withinT = 1-2. CCD images taken at the Observatorio de Sierra Nevada were also used to solve ambiguities in early versus late-type classifications. A consi derable number of galaxies in the catalog (n = 193) are flagged for the presence of nearby companions or sign s of distortion likely due to interaction. This most isolated sample of galaxies in the local Universe is dominated by two populations: 1) 82% spirals (Sa-Sd) with the bulk being luminous systems with small bulges (63% between types Sb-Sc) and 2) a significant population o f early-type E-S0 galaxies (14%). Most of the types later than Sd are low luminosity galaxies concentrated in the local superclus- ter where isolation is diffi cult to evaluate. The late-type spiral majority of the sampl e spans a luminosity range MB−corr =−18 to −22 mag. Few of the E/S0 population are more luminous than−21.0 marking an absence of, an often sought, super L ∗ merger (e.g. fossil elliptical) population. The rarity of high lum inosity systems results in a fainter derived M ∗ for this population com- pared to the spiral optical luminosity function (OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending how the sample is defined. This marks the AMIGA sample as almost unique among samples that compare early and late-type OLFs sep- arately. In other samples, which always involve galaxies in higher density environments, M ∗/S0 is almost always 0.3-0.5 mag brighter than M ∗, presumably reflecting a stronger correlation between M ∗ and environmental density for early-type galaxies.

The AMIGA sample of isolated galaxies IV. A catalogue of neighbours around isolated galaxies

Context. Studies of the effects of environment on galaxy properties and evolution require well defined control samples. Such isolated galaxy samples have up to now been small or poorly defined. The AMIGA project (Analysis of the interstellar Medium of Isolated GAlaxies) represents an attempt to define a statistically useful sample of the most isolated galaxies in the local (z ≤ 0.05) Universe. Aims. A suitable large sample for the AMIGA project already exists, the Catalogue of Isolated Galaxies (CIG, Karachentseva, 1973, Astrofizicheskie Issledovaniia Izvestiya Spetsial’noj Astrofizicheskoj Observatorii, 8, 3; 1050 galaxies), and we use this sample as a starting point to refine and perform a better quantification of its isolation properties. Methods. Digitised POSS-I E images were analysed out to a minimum projected radius R ≥ 0.5 Mpc around 950 CIG galaxies (those within Vr = 1500 km s −1 were excluded). We identified all galaxy candidates in each field brighter than B = 17.5 with a high degree of confidence using the LMORPHO software. We generated a catalogue of approximately 54 000 potential neighbours (redshifts exist for ≈30% of this sample). Results. Six hundred sixty-six galaxies pass and two hundred eighty-four fail the original CIG isolation criterion. The available redshift data confirm that our catalogue involves a largely background population rather than physically associated neighbours. We find that the exclusion of neighbours within a factor of four in size around each CIG galaxy, employed in the original isolation criterion, corresponds to ∆Vr ≈ 18 000 km s −1 indicating that it was a conservative limit. Conclusions. Galaxies in the CIG have been found to show different degrees of isolation. We conclude that a quantitative measure of this is mandatory. It will be the subject of future work based on the catalogue of neighbours obtained here.

On the molecular gas content and SFR in Hickson Compact Groups: enhanced or deficient? ⋆

Aims. We study the effect of the extreme environment in Hickson Compact groups (HCGs) on the molecular gas mass, MH2 , and the star formation rate (SFR) of galaxies as a function of atomic hydrogen (HI) content and evolutionary phase of the group. Methods. We have selected a redshift limited (D<100 Mpc) sample of 88 galaxies in 20 HCGs with available atomic hydrogen (HI) VLA maps, covering a wide range of HI deficiencies and evoluti onary phases of the groups, and containing at least one spiral galaxy. We derived the far-infrared (FIR) luminosity (LFIR) from IRAS data and used it as a tracer of the star formation rate (SFR). We calculated the HI mass (MHI), LFIR and MH2 deficiencies. Results. The mean deficiencies of LFIR and MH2 of spiral galaxies in HCGs are close to 0, indicating that the ir average SFR and molecular gas content are similar to those of isolated galaxies. However, there are indications of an excess in MH2 (∼ 50%) in spiral galaxies in HCGs which can be interpreted, assuming that there is no systematic difference in the CO-to-H2 conversion factor, as either an enhanced molecular gas content or as a higher concentration of the molecular component towards the center in comparison to galaxies in lower density environments. In contrast, the mean MHI of spiral galaxies in HCGs is only 12% of the expected value. The specific star formation rate (sSFR = SFR/stellar mass) tends to be lower for galaxies with a higher MH2 or MHI deficiency. This trend is not seen for the star formation effi ciency (SFE=SFR/MH2), which is very similar to isolated galaxies. We found tentative indications for an enhancement of MH2 in spiral galaxies in HCGs in an early evolutionary phase and a decrease in later phases. We suggest that this might be due to an enhancement of the conversion from atomic to molecular gas due to on-going tidal interactions in an early evolutionary phase, followed by HI stripping and a decrease of the molecular gas content because of lack of replenishment. Conclusions. The properties of MH2 and LFIR in galaxies in HCGs are surprisingly similar to those of isol ated galaxies, in spite of the much higher def(MHI) of the former. The trends of the sSFR and def(MH2 ) with def(MHI) and the evolutionary state indicate, however, that the ongoing interaction might have some effect on the molecular gas and SF.

The AMIGA sample of isolated galaxies VIII. The rate of asymmetric H i profiles in spiral galaxies

(abridged) Measures of the HI properties of a galaxy are among the most sensitive interaction diagnostic at our disposal. We report here on a study of HI profile asymmetries (e.g., lopsidedness) in a sample of some of the most isolated galaxies in the local Universe. This presents us with an excellent opportunity to quantify the range of intrinsic HI asymmetries and provides us with a zero-point calibration for evaluating these measurements in less isolated samples. We characterize the HI profile asymmetries and search for correlations between HI asymmetry and their environments, as well as their optical and far infrared (FIR) properties. We use high signal-to-noise global HI profiles for galaxies in the AMIGA project (this http URL). We restrict our study to N=166 galaxies with accurate measures of the HI shape properties. We quantify asymmetries using a flux ratio parameter. The asymmetry parameter distribution of our isolated sample is well described by a Gaussian model. The width of the distribution is sigma=0.13, and could be even smaller (sigma=0.11) if instrumental errors are reduced. Only 2% of our carefully vetted isolated galaxies sample show an asymmetry in excess of 3sigma. By using this sample we minimize environmental effects as confirmed by the lack of correlation between HI asymmetry and tidal force (one-on-one interactions) and neighbor galaxy number density. On the other hand, field galaxy samples show wider distributions and deviate from a Gaussian curve. As a result we find higher asymmetry rates (~10-20%) in such samples. We find evidence that the spiral arm strength is inversely correlated with the HI asymmetry. We also find an excess of FIR luminous galaxies with larger HI asymmetries that may be spirals associated with hidden accretion events. Our sample presents the smallest fraction of asymmetric HI profiles compared with any other yet studied.

The large asymmetric HI envelope of the isolated galaxy NGC 864 (CIG 96)

Received ; accepted Abstract. We present a HI synthesis imaging study of NGC 864 (CIG 96), a spiral galaxy well isolated from similarly sized companions, yet presenting an intriguing asymmetry in its integral HI spectrum. The asymmetry in the HI profile is associ ated with a strong kinematical perturbation in the gaseous envelope of the galaxy, where at one side the decay of the rotation curve is faster than keplerian. We detect a small (M(HI) = 5× 10 6 M⊙) galaxy with a faint optical counterpart at∼ 80 kpc projected distance from NGC 864. This galaxy is probably not massive enough to have caused the perturbations in NGC 864. We discuss alternatives, such as the accretion of a gaseous companion at a radial velocity lower than the maximum.

ISM of Galaxies in Extremely Different Environments: Isolated vs Compact Groups

1 Instituto de Astrofisica de Andalucia, CSIC, Apdo. 3004, 18080 Granada, Spain lourdes@iaa.es,daniel@iaa.es,gilles@iaa.es,garcia@iaa.es,jsm@iaa.es, simon@iaa.es 2 Department of Astronomy, University of Massachussetts, Amherst, MA 01003, USA myun@bonito.astro.umass.edu,sanchayeeta-lg@yahoo.com 3 National Research Institute of Astronomy and Geophysics, Cairo Egypt ibrahimselim@hotmail.com 4 Observatoire de Marseille, 2 Place le Verrier, 13248 Marseille Cedez 4, France lia@oamp.fr,bosma@oamp.fr 5 LERMA, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France Simon.Verley@obspm.fr 6 Max-Planck-Institut fur Radiostronomie, Auf dem Hugel 69, 53121 Bonn, Germany p083huc@mpifr-bonn.mpg.de 7 Departamento de Fisica Teorica y del Cosmos, Facultad de Ciencias, Universidad de Granada, Spain ute@ugr.es 8 sco@astro.as.utexas.edu 9 tjp@xun0.sr.bham.ac.uk,jesper@star.sr.bham.ac.uk 10 Department of Astronomy, Univ. of Alabama, AL 35487, USA giacomo@merlot.astr.ua.edu