arXiv subjects

J. Gracia-Carpio

Publications and source records attributed to J. Gracia-Carpio.

At least 163 records · Page 9Linked to original sources

arXiv2015

High-J CO SLEDs in nearby infrared bright galaxies observed by Herschel-PACS

We report the detection of far-infrared (FIR) CO rotational emission from nearby active galactic nuclei (AGN) and starburst galaxies, as well as several merging systems and Ultra-Luminous Infrared Galaxies (ULIRGs). Using Herschel-PACS, we have detected transitions in the J$_{upp}$ = 14 - 20 range ($λ\sim$ 130 - 185 $μ$m, $ν\sim$ 1612 - 2300 GHz) with upper limits on (and in two cases, detections of) CO line fluxes up to J$_{upp}$ = 30. The PACS CO data obtained here provide the first well-sampled FIR extragalactic CO SLEDs for this range, and will be an essential reference for future high redshift studies. We find a large range in the overall SLED shape, even amongst galaxies of similar type, demonstrating the uncertainties in relying solely on high-J CO diagnostics to characterize the excitation source of a galaxy. Combining our data with low-J line intensities taken from the literature, we present a CO ratio-ratio diagram and discuss its potential diagnostic value in distinguishing excitation sources and physical properties of the molecular gas. The position of a galaxy on such a diagram is less a signature of its excitation mechanism, than an indicator of the presence (or absence) of warm, dense molecular gas. We then quantitatively analyze the CO emission from a subset of the detected sources with Large Velocity Gradient (LVG) radiative transfer models to fit the CO SLEDs. Using both single-component and two-component LVG models to fit the kinetic temperature, velocity gradient, number density and column density of the gas, we derive the molecular gas mass and the corresponding CO-to-H$_2$ conversion factor, $α_{CO}$, for each respective source. For the ULIRGs we find $α$ values in the canonical range 0.4 - 5 M$_\odot$/(K kms$^{-1}$pc$^2$), while for the other objects, $α$ varies between 0.2 and 14.} Finally, we compare our best-fit LVG model ..

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arXiv2014

Combined CO & Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star Formation Rate and Stellar Mass

We combine molecular gas masses inferred from CO emission in 500 star forming galaxies (SFGs) between z=0 and 3, from the IRAM-COLDGASS, PHIBSS1/2 and other surveys, with gas masses derived from Herschel far-IR dust measurements in 512 galaxy stacks over the same stellar mass/redshift range. We constrain the scaling relations of molecular gas depletion time scale (tdepl) and gas to stellar mass ratio (Mmolgas/M*) of SFGs near the star formation main-sequence with redshift, specific star formation rate (sSFR) and stellar mass (M*). The CO- and dust-based scaling relations agree remarkably well. This suggests that the CO-H2 mass conversion factor varies little within 0.6dex of the main sequence (sSFR(ms,z,M*)), and less than 0.3dex throughout this redshift range. This study builds on and strengthens the results of earlier work. We find that tdepl scales as (1+z)^-0.3 *(sSFR/sSFR(ms,z,M*))^-0.5, with little dependence on M*. The resulting steep redshift dependence of Mmolgas/M* ~(1+z)^3 mirrors that of the sSFR and probably reflects the gas supply rate. The decreasing gas fractions at high M* are driven by the flattening of the SFR-M* relation. Throughout the redshift range probed a larger sSFR at constant M* is due to a combination of an increasing gas fraction and a decreasing depletion time scale. As a result galaxy integrated samples of the Mmolgas-SFR rate relation exhibit a super-linear slope, which increases with the range of sSFR. With these new relations it is now possible to determine Mmolgas with an accuracy of 0.1dex in relative terms, and 0.2dex including systematic uncertainties.

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arXiv2013

Herschel observations and a model for IRAS 08572+3915: a candidate for the most luminous infrared galaxy in the local (z < 0.2) Universe

We present Herschel photometry and spectroscopy, carried out as part of the Herschel ULIRG survey (HERUS), and a model for the infrared to submillimetre emission of the ultraluminous infrared galaxy IRAS 08572+3915. This source shows one of the deepest known silicate absorption features and no polycyclic aromatic hydrocarbon (PAH) emission. The model suggests that this object is powered by an active galactic nucleus (AGN) with a fairly smooth torus viewed almost edge-on and a very young starburst. According to our model the AGN contributes about 90% of the total luminosity of 1.1 x 10^13 Lo, which is about a factor of five higher than previous estimates. The large correction of the luminosity is due to the anisotropy of the emission of the best fit torus. Similar corrections may be necessary for other local and high-z analogs. This correction implies that IRAS 08572+3915 at a redshift of 0.05835 may be the nearest hyperluminous infrared galaxy and probably the most luminous infrared galaxy in the local (z < 0.2) Universe. IRAS 08572+3915 shows a low ratio of [CII] to IR luminosity (log L_[CII]/L_{IR} < -3.8) and a [OI]63um to [CII]158um line ratio of about 1 that supports the model presented in this letter.

astro-ph.CO↗

arXiv2013

Fast Molecular Outflows in Luminous Galaxy Mergers: Evidence for Quasar Feedback from Herschel

We report the results from a systematic search for molecular (OH-119 um) outflows with Herschel-PACS in a sample of 43 nearby (z < 0.3) galaxy mergers, mostly ultraluminous infrared galaxies (ULIRGs) and QSOs. We find that the character of the OH feature (strength of the absorption relative to the emission) correlates with that of the 9.7-um silicate feature, a measure of obscuration in ULIRGs. Unambiguous evidence for molecular outflows, based on the detection of OH absorption profiles with median velocities more blueshifted than -50 km/sec, is seen in 26 (70%) of the 37 OH-detected targets, suggesting a wide-angle (~145 degrees) outflow geometry. Conversely, unambiguous evidence for molecular inflows, based on the detection of OH absorption profiles with median velocities more redshifted than +50 km/sec, is seen in only 4 objects, suggesting a planar or filamentary geometry for the inflowing gas. Terminal outflow velocities of ~-1000 km/sec are measured in several objects, but median outflow velocities are typically ~-200 km s^{-1}. While the outflow velocities show no statistically significant dependence on the star formation rate, they are distinctly more blueshifted among systems with large AGN fractions and luminosities [log (L_AGN / L_sun) > 11.8 +/- 0.3]. The quasars in these systems play a dominant role in driving the molecular outflows. In contrast, the most AGN dominated systems, where OH is seen purely in emission, show relatively modest OH line widths, despite their large AGN luminosities, perhaps indicating that molecular outflows subside once the quasar has cleared a path through the obscuring material.

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arXiv2013

Towards a resolved Kennicutt-Schmidt law at high redshift

Massive galaxies in the distant Universe form stars at much higher rates than today. Although direct resolution of the star forming regions of these galaxies is still a challenge, recent molecular gas observations at the IRAM Plateau de Bure interferometer enable us to study the star formation efficiency on subgalactic scales around redshift z = 1.2. We present a method for obtaining the gas and star formation rate (SFR) surface densities of ensembles of clumps composing galaxies at this redshift, even though the corresponding scales are not resolved. This method is based on identifying these structures in position-velocity diagrams corresponding to slices within the galaxies. We use unique IRAM observations of the CO(3-2) rotational line and DEEP2 spectra of four massive star forming distant galaxies - EGS13003805, EGS13004291, EGS12007881, and EGS13019128 in the AEGIS terminology - to determine the gas and SFR surface densities of the identifiable ensembles of clumps that constitute them. The integrated CO line luminosity is assumed to be directly proportional to the total gas mass, and the SFR is deduced from the [OII] line. We identify the ensembles of clumps with the angular resolution available in both CO and [OII] spectroscopy; i.e., 1-1.5". SFR and gas surface densities are averaged in areas of this size, which is also the thickness of the DEEP2 slits and of the extracted IRAM slices, and we derive a spatially resolved Kennicutt-Schmidt (KS) relation on a scale of ~8 kpc. The data generally indicates an average depletion time of 1.9 Gyr, but with significant variations from point to point within the galaxies.

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arXiv2013

PHIBSS: molecular gas, extinction, star formation and kinematics in the z=1.5 star forming galaxy EGS13011166

We report matched resolution, imaging spectroscopy of the CO J=3-2 line (with the IRAM Plateau de Bure millimeter interferometer) and of the H-alpha line (with LUCI at the Large Binocular Telescope)in the massive z=1.53 main-sequence galaxy EGS 13011166, as part of the "Plateau de Bure high-z, blue sequence survey (PHIBSS). We combine these data with HST V-J-J-H-band maps to derive spatially resolved distributions of stellar surface density, star formation rate, molecular gas surface density, optical extinction and gas kinematics. The spatial distribution and kinematics of the ionized and molecular gas are remarkably similar and are well modeled by a turbulent, globally Toomre unstable rotating disk. The stellar surface density distribution is smoother than the clumpy rest-frame UV/optical light distribution, and peaks in an obscured, star forming massive bulge near the dynamical center. The molecular gas surface density and the effective optical screen extinction track each other and are well modeled by a 'mixed' extinction model. The inferred slope of the spatially resolved molecular gas to star formation rate relation depends strongly on the adopted extinction model and can vary from 0.8 to 1.7. For the preferred mixed dust-gas model we find a near linear slope.

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arXiv2012

PHIBSS: molecular gas content and scaling relations in z~1-3 normal star forming galaxies

We present PHIBSS, the IRAM Plateau de Bure high-z blue sequence CO 3-2 survey of the molecular gas properties in normal star forming galaxies (SFGs) near the cosmic star formation peak. PHIBSS provides 52 CO detections in two redshift slices at z~1.2 and 2.2, with log(M*(M_solar))>10.4 and log(SFR(M_solar/yr))>1.5. Including a correction for the incomplete coverage of the M*-SFR plane, we infer average gas fractions of ~0.33 at z~1.2 and ~0.47 at z~2.2. Gas fractions drop with stellar mass, in agreement with cosmological simulations including strong star formation feedback. Most of the z~1-3 SFGs are rotationally supported turbulent disks. The sizes of CO and UV/optical emission are comparable. The molecular gas - star formation relation for the z=1-3 SFGs is near-linear, with a ~0.7 Gyrs gas depletion timescale; changes in depletion time are only a secondary effect. Since this timescale is much less than the Hubble time in all SFGs between z~0 and 2, fresh gas must be supplied with a fairly high duty cycle over several billion years. At given z and M*, gas fractions correlate strongly with the specific star formation rate. The variation of specific star formation rate between z~0 and 3 is mainly controlled by the fraction of baryonic mass that resides in cold gas.

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arXiv2012

The impact of interactions, bars, bulges, and AGN on star formation efficiency in local massive galaxies

Using observations from the GASS and COLD GASS surveys and complementary data from SDSS and GALEX, we investigate the nature of variations in gas depletion time observed across the local massive galaxy population. The large and unbiased COLD GASS sample allows us to assess the relative importance of galaxy interactions, bar instabilities, morphologies and the presence of AGN in regulating star formation efficiency. Both the H2 mass fraction and depletion time vary as a function of the distance of a galaxy from the main sequence in the SFR-M* plane. The longest gas depletion times are found in below-main sequence bulge-dominated galaxies that are either gas-poor, or else on average less efficient than disk-dominated galaxy at converting into stars any cold gas they may have. We find no link between AGN and these long depletion times. The galaxies undergoing mergers or showing signs of morphological disruptions have the shortest molecular gas depletion times, while those hosting strong stellar bars have only marginally higher global star formation efficiencies as compared to matched control samples. Our interpretation is that depletion time variations are caused by changes in the ratio between the gas mass traced by the CO(1-0) observations, and the gas mass in high density star-forming cores, with interactions, mergers and bar instabilities able to locally increase pressure and raise the ratio of efficiently star-forming gas to CO-detected gas. Building a sample representative of the local massive galaxy population, we derive a global Kennicutt-Schmidt relation of slope 1.18+/-0.24, and observe structure within the scatter around this relation, with galaxies having low (high) stellar mass surface densities lying systematically above (below) the mean relation, suggesting that gas surface density is not the only parameter driving the global star formation ability of a galaxy.

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arXiv2012

Physical properties of dense molecular gas in centres of Seyfert galaxies

We present new ~1" resolution data of the dense molecular gas in the central 50-100 pc of four nearby Seyfert galaxies. PdBI observations of HCN and, in 2 of the 4 sources, simultaneously HCO+ allow us to carefully constrain the dynamical state of the dense gas surrounding the AGN. Analysis of the kinematics shows large line widths of 100-200 km/s FWHM that can only partially arise from beam smearing of the velocity gradient. The observed morphological and kinematic parameters (dimensions, major axis position angle, red and blue channel separation, and integrated line width) are well reproduced by a thick disk, where the emitting dense gas has a large intrinsic dispersion (20-40 km/s), implying that it exists at significant scale heights (25-30% of the disk radius). To put the observed kinematics in the context of the starburst and AGN evolution, we estimate the Toomre Q parameter. We find this is always greater than the critical value, i.e. Q is above the limit such that the gas is stable against rapid star formation. This is supported by the lack of direct evidence, in these 4 Seyfert galaxies, for on-going star formation close around the AGN. Instead, any current star formation tends to be located in a circumnuclear ring. We conclude that the physical conditions are indeed not suited to star formation within the central ~100 pc.

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arXiv2011

Star formation laws in Luminous Infrared Galaxies. New observational constraints on models

The observational study of star formation relations in galaxies is central to unraveling the physical processes at work on local and global scales. We wish to expand the sample of extreme starbursts, represented by local LIRGs and ULIRGs, with high quality observations in the 1-0 line of HCN. We study if a universal law can account for the star formation relations observed for the dense molecular gas in normal star forming galaxies and extreme starbursts. We have used the IRAM 30m telescope to observe a sample of 19 LIRGs in the 1-0 lines of CO, HCN and HCO+. The analysis of the new data proves that the efficiency of star formation in the dense molecular gas (SFE-dense) of extreme starbursts is a factor 3-4 higher compared to normal galaxies. We find a duality in Kennicutt-Schmidt (KS) laws that is reinforced if we account for the different conversion factor for HCN (alpha-HCN) in extreme starbursts and for the unobscured star formation rate in normal galaxies. This result extends to the higher molecular densities probed by HCN lines the more extreme bimodal behavior of star formation laws, derived from CO molecular lines by two recent surveys. We have confronted our observations with the predictions of theoretical models in which the efficiency of star formation is determined by the ratio of a constant star formation rate per free-fall time (SFR-ff) to the local free-fall time. We find that it is possible to fit the observed differences in the SFE-dense between normal galaxies and LIRGs/ULIRGs using a common constant SFR-ff and a set of physically acceptable HCN densities, but only if SFR-ff~0.005-0.01 and/or if alpha-HCN is a factor of a few lower than our favored values. Star formation recipes that explicitly depend on the galaxy global dynamical time scales do not significantly improve the fit to the new HCN data presented in this work.

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arXiv2011

The metallicity dependence of the CO {\rightarrow} H_2 conversion factor in z>1 star forming galaxies

We use the first systematic samples of CO millimeter emission in z>1 'main-sequence' star forming galaxies (SFGs) to study the metallicity dependence of the conversion factor αCO, from CO line luminosity to molecular gas mass. The molecular gas depletion rate inferred from the ratio of the star formation rate (SFR) to CO luminosity, is ~1 Gyr-1 for near-solar metallicity galaxies with stellar masses above M_S~1e11 M_sun. In this regime the depletion rate does not vary more than a factor of two to three as a function of molecular gas surface density, or redshift between z~0 and 2. Below M_S the depletion rate increases rapidly with decreasing metallicity. We argue that this trend is not caused by starburst events, by changes in the physical parameters of the molecular clouds, or by the impact of the fundamental metallicity-SFR-stellar mass relation. A more probable explanation is that the conversion factor is metallicity dependent and that star formation can occur in 'CO-dark' gas. The trend is also expected theoretically from the effect of enhanced photodissociation of CO by ultraviolet radiation at low metallicity. From the available z~0 and z~1-3 samples we constrain the slope of the log(αCO) -log (metallicity) relation to range between -1 and -2, fairly insensitive to the assumed slope of the gas-star formation rate relation. Because of the lower metallicities near the peak of the galaxy formation activity at z~1-2 compared to z~0, we suggest that molecular gas masses estimated from CO luminosities have to be substantially corrected upward for galaxies below M_S.

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arXiv2010

A Study of the Gas-Star Formation Relation over Cosmic Time

We use the first systematic data sets of CO molecular line emission in z~1-3 normal star forming galaxies for a comparison of the dependence of galaxy-averaged star formation rates on molecular gas masses at low and high redshifts, and in different galactic environments. Although the current high-z samples are still small and biased toward the luminous and massive tail of the actively star-forming 'main-sequence', a fairly clear picture is emerging. Independent of whether galaxy integrated quantities or surface densities are considered, low- and high-z SFG galaxy populations appear to follow similar molecular gas-star formation relations with slopes 1.1 to 1.2. The gas-depletion time scale in these SFGs grows from 0.5 Gyrs at z~2 to 1.5 Gyrs at z~0. Because star formation depletion times are significantly smaller than the Hubble time at all redshifts sampled, star formation rates and gas fractions are set by the balance between gas accretion from the halo and stellar feedback. In contrast, very luminous gas rich major mergers at both low-z and high-z produce on average 4 to10 times more far-infrared luminosity per unit gas mass. Only some fraction of this difference can be explained by uncertainties in gas-mass or luminosity estimators; much of it must be intrinsic. The most likely interpretation is that the star formation relation is driven by global dynamical effects. For a given mass, the more compact merger systems produce stars more rapidly because their gas clouds are more compressed with shorter dynamical times, so that they churn more quickly through the available gas reservoir than the typical normal disk galaxies. When the dependence on galactic dynamical time scale is explicitly included, disk galaxies and mergers appear to follow similar gas to star-formation relations. The mergers may be forming stars at slightly higher efficiencies than the disks.

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arXiv2010

High molecular gas fractions in normal massive star forming galaxies in the young Universe

Stars form from cold molecular interstellar gas. Since this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more gas rich. Molecular gas observations in the distant Universe have so far been largely restricted to very luminous, rare objects, including mergers and quasars. Here we report the results of a systematic survey of molecular gas in samples of typical massive star forming galaxies at ~1.2 and 2.3, when the Universe was 40% and 24% of its current age. Our measurements provide empirical evidence that distant star forming galaxies indeed were gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z= 2.3 and z=1.2 is ~44% and 34%, three to ten times higher than in today's massive spiral galaxies. The slow decrease between z~2 and 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

astro-ph.CO↗

arXiv2008

High-resolution molecular line observations of active galaxies

The study of the content, distribution and kinematics of interstellar gas is a key to understand the origin and maintenance of both starburst and nuclear (AGN) activity in galaxies. The processes involved in AGN fueling encompass a wide range of scales, both spatial and temporal, which have to be studied. Probing the gas flow from the outer disk down to the central engine of an AGN host, requires the use of specific tracers of the interstellar medium adapted to follow the change of phase of the gas as a function of radius. Current mm-interferometers can provide a sharp view of the distribution and kinematics of molecular gas in the circumnuclear disks of galaxies through extensive CO line mapping. As such, CO maps are an essential tool to study AGN feeding mechanisms in the local universe. This is the scientific driver of the NUclei of GAlaxies (NUGA) survey, whose latest results are here reviewed. On the other hand, the use of specific molecular tracers of the dense gas phase can probe the feedback influence of activity on the chemistry and energy balance/redistribution in the interstellar medium of nearby galaxies. Millimeter interferometers are able to unveil the strong chemical differentiation present in the molecular gas disks of nearby starbursts and AGNs. Nearby active galaxies can be used as local templates to address the study of more distant galaxies where both star formation and AGN activity are deeply embedded.

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arXiv2008

Molecular line probes of activity in galaxies

The use of specific tracers of the dense molecular gas phase can help to explore the feedback of activity on the interstellar medium (ISM) in galaxies. This information is a key to any quantitative assessment of the efficiency of the star formation process in galaxies. We present the results of a survey devoted to probe the feedback of activity through the study of the excitation and chemistry of the dense molecular gas in a sample of local universe starbursts and active galactic nuclei (AGNs). Our sample includes also 17 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007) we find the first clear observational evidence that the star formation efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting evidence of overabundant HCN in active environments would even reinforce the reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt law around L_FIR=10^11 L_sun. This result has major implications for the use of HCN as a tracer of the dense gas in local and high-redshift luminous infrared galaxies.

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arXiv2007

Evidence of enhanced star formation efficiency in luminous and ultraluminous infrared galaxies

We present new observations made with the IRAM 30m telescope of the J=1-0 and 3-2 lines of HCN and HCO^+ used to probe the dense molecular gas content in a sample of 17 local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These observations have allowed us to derive an updated version of the power law describing the correlation between the FIR luminosity (L_FIR) and the HCN(1-0) luminosity (L'_HCN(1-0)) of local and high-redshift galaxies. We present the first clear observational evidence that the star formation efficiency of the dense gas (SFE_dense), measured as the L_FIR/L'_HCN(1-0) ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies, a result that has also been found recently in high-redshift galaxies. This may imply a statistically significant turn upward in the Kennicutt-Schmidt law derived for the dense gas at L_FIR >= 10^11 L_sun. We have used a one-phase Large Velocity Gradient (LVG) radiative transfer code to fit the three independent line ratios derived from our observations. The results of this analysis indicate that the [HCN]/[HCO^+] abundance ratios could be up to one order of magnitude higher than normal in a significant number of LIRGs and ULIRGs of our sample. An overabundance of HCN at high L_FIR implies that the reported trend in the L_FIR/L'_HCN ratio as a function of L_FIR would be underestimating a potentially more dramatic change of the SFE_dense. Results obtained with two-phase LVG models corroborate that the L'_HCN(1-0)-to-M_dense conversion factor must be lowered at high L_FIR. We discuss the implications of these findings for the use of HCN as a tracer of the dense molecular gas in local and high-redshift luminous infrared galaxies.

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arXiv2007

A multi-transition HCN and HCO+ study of 12 nearby active galaxies: AGN versus SB environments

Recent studies have indicated that the HCN-to-CO(J=1-0) and HCO+-to-HCN(J=1-0) ratios are significantly different between galaxies with AGN (active galactic nucleus) and SB (starburst) signatures. In order to study the molecular gas properties in active galaxies and search for differences between AGN and SB environments, we observed the HCN(J=1-0), (J=2-1), (J=3-2), HCO+(J=1-0) and HCO+(J=3-2), emission with the IRAM 30m in the centre of 12 nearby active galaxies which either exhibit nuclear SB and/or AGN signatures. Consistent with previous results, we find a significant difference of the HCN(J=2-1)-to-HCN(J=1-0), HCN(J=3-2)-to-HCN(J=1-0), HCO+(J=3-2)-to-HCO+(J=3-2), and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and those with an additional or pure central SB: the HCN, HCO+ and HCO+-to-HCN intensity ratios tend to be higher in the galaxies of our sample with a central SB as opposed to the pure AGN cases which show rather low intensity ratios. Based on an LVG analysis of these data, i.e., assuming purely collisional excitation, the (average) molecular gas densities in the SB dominated sources of our sample seem to be systematically higher than in the AGN sources. The LVG analysis seems to further support systematically higher HCN and/or lower HCO+ abundances as well as similar or higher gas temperatures in AGN compared to the SB sources of our sample. Also, we find that the HCN-to-CO ratios decrease with increasing rotational number J for the AGN while they stay mostly constant for the SB sources.

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arXiv2007

Sub-arcsecond CO(1-0) and CO(2-1) observations of the ultraluminous infrared galaxy IRAS 10190+1322

We present the results of high resolution mapping of the CO(1-0) and CO(2-1) emission of the ultraluminous infrared galaxy (ULIRG) IRAS 10190+1322, with the IRAM interferometer, down to an angular resolution of ~0.3 arcsec. This object is composed of two interacting galaxies with a projected nuclear separation of 6 kpc, and was selected to analyze the physical and dynamical properties of the molecular gas in each galaxy in order to study the conditions that lead a galaxy pair to become ultraluminous in the infrared. With the exception of Arp 220, the closest ULIRG, this is the first time that the CO emission is morphologically and kinematically resolved in the two interacting galaxies of a ULIRG system. In one of the galaxies the molecular gas is highly concentrated, distributed in a circumnuclear disk of 1.7 kpc in size. The molecular gas in the presumably less infrared luminous galaxy is distributed in a more extended disk of 7.4 kpc. The molecular gas mass accounts for ~10% of the dynamical mass in each galaxy. Both objects are rich enough in molecular gas, Mgas ~ 4 10^9 Msun, as to experience an infrared ultraluminous phase.

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