arXiv subjects

S. García-Burillo

Publications and source records attributed to S. García-Burillo.

At least 73 records · Page 4Linked to original sources

arXiv2019

ALMA captures feeding and feedback from the active galactic nucleus in NGC613

We report ALMA observations of CO(3-2) emission in the Seyfert galaxy NGC613, at a spatial resolution of 17pc, as part of our NUclei of GAlaxies sample. Our aim is to investigate the morphology and dynamics of the gas inside the central kpc, and to probe nuclear fueling and feedback phenomena. The morphology of CO(3-2) line emission reveals a 2-arm trailing nuclear spiral at $r\lesssim$100pc and a circumnuclear ring at ~350pc radius, that is coincident with the star-forming ring seen in the optical images. The molecular gas in the galaxy disk is in a remarkably regular rotation, however, the kinematics in the nuclear region is very skewed. The nuclear spectrum of CO and dense gas tracers HCN(4-3), HCO+(4-3), and CS(7-6) show broad wings up to $\pm$300km/s, associated with a molecular outflow emanating from the nucleus (r~25pc). We derive a molecular outflow mass $M_{out}$=2x10$^6$M$_\odot$ and a mass outflow rate of $\dot{M}_{out}=$27$\rm M_\odot yr^{-1}$. The molecular outflow energetics exceed the values predicted by AGN feedback models: the kinetic power of the outflow corresponds to 20%L_AGN and the momentum rate is $\dot{M}_{out}v\sim400L_{AGN}/c$. The outflow is mainly boosted by the AGN through entrainment by the radio jet, but given the weak nuclear activity of NGC613, we might be witnessing a fossil outflow, resulted from a strong past AGN that now has already faded. The nuclear trailing spiral observed in CO emission is inside the ILR ring of the bar. We compute the gravitational torques exerted in the gas to estimate the efficiency of the angular momentum exchange. The gravity torques are negative from 25 to 100pc and the gas loses its angular momentum in a rotation period, providing evidence of a highly efficient inflow towards the center. This phenomenon shows that the massive central black hole has a significant dynamical influence on the gas, triggering its fueling.

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arXiv2019

Optical integral field spectroscopy of intermediate redshift infrared bright galaxies

The extreme infrared (IR) luminosity of local luminous and ultra-luminous IR galaxies (U/LIRGs; 11 < log LIR /Lsun < 12 and log LIR /Lsun > 12, respectively) is mainly powered by star-formation processes triggered by mergers or interactions. While U/LIRGs are rare locally, at z > 1, they become more common, they dominate the star-formation rate (SFR) density, and a fraction of them are found to be normal disk galaxies. Therefore, there must be an evolution of the mechanism triggering these intense starbursts with redshift. To investigate this evolution, we present new optical SWIFT integral field spectroscopic Hα+[NII] observations of a sample of 9 intermediate-z (0.2 < z < 0.4) U/LIRG systems selected from Herschel 250μm observations. The main results are the following: (a) the ratios between the velocity dispersion and the rotation curve amplitude indicate that 10-25% (1-2 out of 8) might be compatible with being isolated disks while the remaining objects are interacting/merging systems; (b) the ratio between un-obscured and obscured SFR traced by Hα and LIR, respectively, is similar in both local and these intermediate-z U/LIRGs; and (c) the ratio between 250μm and the total IR luminosities of these intermediate-z U/LIRGs is higher than that of local U/LIRGs with the same LIR . This indicates a reduced dust temperature in these intermediate-z U/LIRGs. This, together with their already measured enhanced molecular gas content, suggests that the interstellar medium conditions are different in our sample of intermediate-z galaxies when compared to local U/LIRGs.

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arXiv2019

Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100pc scales in M51

It remains unclear what sets the efficiency with which molecular gas transforms into stars. Here we present a new VLA map of the spiral galaxy M51 in 33GHz radio continuum, an extinction-free tracer of star formation, at 3" scales (~100pc). We combined this map with interferometric PdBI/NOEMA observations of CO(1-0) and HCN(1-0) at matched resolution for three regions in M51 (central molecular ring, northern and southern spiral arm segments). While our measurements roughly fall on the well-known correlation between total infrared and HCN luminosity, bridging the gap between Galactic and extragalactic observations, we find systematic offsets from that relation for different dynamical environments probed in M51, e.g. the southern arm segment is more quiescent due to low star formation efficiency (SFE) of the dense gas, despite having a high dense gas fraction. Combining our results with measurements from the literature at 100pc scales, we find that the SFE of the dense gas and the dense gas fraction anti-correlate and correlate, respectively, with the local stellar mass surface density. This is consistent with previous kpc-scale studies. In addition, we find a significant anti-correlation between the SFE and velocity dispersion of the dense gas. Finally, we confirm that a correlation also holds between star formation rate surface density and the dense gas fraction, but it is not stronger than the correlation with dense gas surface density. Our results are hard to reconcile with models relying on a universal gas density threshold for star formation and suggest that turbulence and galactic dynamics play a major role in setting how efficiently dense gas converts into stars.

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arXiv2019

Gas phase Elemental abundances in Molecular cloudS (GEMS) I. The prototypical dark cloud TMC 1

GEMS is an IRAM 30m Large Program whose aim is determining the elemental depletions and the ionization fraction in a set of prototypical star-forming regions. This paper presents the first results from the prototypical dark cloud TMC 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3mm and 2mm) and the 40m Yebes telescope (1.3cm and 7mm) to determine the fractional abundances of CO, HCO+, HCN, CS, SO, HCS+, and N2H+ in three cuts which intersect the dense filament at the well-known positions TMC 1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from Av~3 to ~20mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of (1-5)x10$^3$ cm$^{-3}$; and ii) the dense phase, located at Av>10mag, with molecular hydrogen densities >10$^4$ cm$^{-3}$. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C+/C/CO transition zone where C/H~8x10$^{-5}$ and C/O~0.8-1, until the beginning of the dense phase at Av~10 mag. This is consistent with the grain temperatures being below the CO evaporation temperature in this region. In the case of sulfur, a strong depletion should occur before the translucent phase where we estimate a S/H~(0.4 - 2.2) x10$^{-6}$, an abundance ~7-40 times lower than the solar value. A second strong depletion must be present during the formation of the thick icy mantles to achieve the values of S/H measured in the dense cold cores (S/H~8x10$^{-8}$). Based on our chemical modeling, we constrain the value of $ζ_{\rm H_2}$ to ~(0.5 - 1.8) x10$^{-16}$ s$^{-1}$ in the translucent cloud.

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arXiv2019

Hidden or missing outflows in highly obscured galaxy nuclei?

Understanding the nuclear growth and feedback processes in galaxies requires investigating their often obscured central regions. One way to do this is to use (sub)millimeter line emission from vibrationally excited HCN (HCN-vib). It has been suggested that the most intense HCN-vib emission from a galaxy is connected to a phase of nuclear growth that occurs before the nuclear feedback processes have been fully developed. We aim to investigate if there is a connection between the presence of strong HCN-vib emission and the development of feedback in (U)LIRGs. We collected literature and archival data to compare the luminosities of rotational lines of HCN-vib, normalized to the total infrared luminosity, to the median velocities of 119 μm OH absorption lines, potentially indicating outflows, in a total of 17 (U)LIRGs. The most HCN-vib luminous systems all lack signatures of significant molecular outflows in the far-infrared OH absorption lines. However, at least some of the systems with bright HCN-vib emission do have fast and collimated outflows that can be seen in spectral lines at longer wavelengths. We conclude that the galaxy nuclei with the highest L(HCN-vib)/L(IR) do not drive wide-angle outflows detectable using the median velocities of far-infrared OH absorption lines. It is possible that this is due to an orientation effect where sources which are oriented in such a way that their outflows are not along our line of sight also radiate a smaller proportion of their infrared luminosity in our direction. It could also be that massive wide-angle outflows destroy the deeply embedded regions responsible for bright HCN-vib emission, so that the two phenomena cannot coexist. This would strengthen the idea that vibrationally excited HCN traces a heavily obscured stage of evolution before nuclear feedback mechanisms are fully developed.

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arXiv2018

Molecular gas in the northern nucleus of Mrk273: Physical and chemical properties of the disk and its outflow

Aiming to characterise the properties of the molecular gas in the ultraluminous infrared galaxy Mrk273 and its outflow, we used the NOEMA interferometer to image the dense gas molecular tracers HCN, HCO+, HNC, HOC+ and HC3N at 86GHz and 256GHz with angular resolutions of 4.9x4.5 arcsec (3.7x3.4 kpc) and 0.61x0.55 arcsec (460x420 pc). We also modelled the flux of several H2O lines observed with Herschel using a radiative transfer code that includes excitation by collisions as well as by far-infrared photons. The disk of the Mrk273 north nucleus has two components with decoupled kinematics. The gas in the outer parts (1.5 kpc) rotates with a south-east to north-west direction, while in the inner disk (300 pc) follows a north-east to south-west rotation. The central 300 pc, which hosts a compact starburst region, is filled with dense and warm gas, contains a dynamical mass of (4-5)x10^9M_sun, a luminosity of L'_HCN=(3-4)x10^8 K km/s pc^2, and a dust temperature of 55 K. At the very centre, a compact core with R~50 pc has a luminosity of L_IR=4x10^11L_sun (30% of the total infrared luminosity), and a dust temperature of 95 K. The core is expanding at low velocities ~50-100 km/s, probably affected by the outflowing gas. We detect the blue-shifted component of the outflow, while the red-shifted counterpart remains undetected in our data. Its cold and dense phase reaches fast velocities up to ~1000 km/s, while the warm outflowing gas has more moderate maximum velocities of ~600 km/s. The outflow is detected as far as 460 pc from the centre in the northern direction, and has a mass of dense gas <8x10^8M_sun. The difference between the position angles of the inner disk (~70 degree) and the outflow (~10 degree) indicates that the outflow is likely powered by the AGN, and not by the starburst. Regarding the chemistry, we measure an extremely low HCO+/HOC+ ratio of 10+-5 in the inner disk of Mrk273.

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arXiv2018

Resolving the nuclear obscuring disk in the Compton-thick Seyfert galaxy NGC5643 with ALMA

We present ALMA Band 6 $^{12}$CO(2--1) line and rest-frame 232GHz continuum observations of the nearby Compton-thick Seyfert galaxy NGC5643 with angular resolutions 0.11-0.26arcsec (9-21pc). The CO(2--1) integrated line map reveals emission from the nuclear and circumnuclear region with a two-arm nuclear spiral extending 10arcsec on each side. The circumnuclear CO(2--1) kinematics can be fitted with a rotating disk, although there are regions with large residual velocities and/or velocity dispersions. The CO(2--1) line profiles of these regions show two different velocity components. One is ascribed to the circular component and the other to the interaction of the AGN outflow, as traced by the [O III]5007AA emission, with molecular gas in the disk a few hundred parsecs from the AGN. On nuclear scales, we detected an inclined CO(2--1) disk (diameter 26 pc, FWHM) oriented almost in a north-south direction. The CO(2--1) nuclear kinematics can be fitted with a rotating disk which appears to be tilted with respect to the large scale disk. There are strong non-circular motions in the central 0.2-0.3 arcsec with velocities of up to 110km/s. In the absence of a nuclear bar, these motions could be explained as radial outflows in the nuclear disk. We estimate a total molecular gas mass for the nuclear disk of $M({\rm H}_2)=1.1\times 10^7\,M_\odot$ and an H$_2$ column density toward the location of the AGN of $N({\rm H}_2)\sim 5 \times 10^{23}\,{\rm cm}^{-2}$, for a standard CO-to-H$_2$ conversion factor. We interpret this nuclear molecular gas disk as the obscuring torus of NGC5643 as well as the collimating structure of the ionization cone.

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arXiv2018

PHIBSS: Unified Scaling Relations of Gas Depletion Time and Molecular Gas Fractions

This paper provides an update of our previous scaling relations (Genzel et al.2015) between galaxy integrated molecular gas masses, stellar masses and star formation rates, in the framework of the star formation main-sequence (MS), with the main goal to test for possible systematic effects. For this purpose our new study combines three independent methods of determining molecular gas masses from CO line fluxes, far-infrared dust spectral energy distributions, and ~1mm dust photometry, in a large sample of 1444 star forming galaxies (SFGs) between z=0 and 4. The sample covers the stellar mass range log(M*/M_solar)=9.0-11.8, and star formation rates relative to that on the MS, delta_MS=SFR/SFR(MS), from 10^{-1.3} to 10^{2.2}. Our most important finding is that all data sets, despite the different techniques and analysis methods used, follow the same scaling trends, once method-to-method zero point offsets are minimized and uncertainties are properly taken into account. The molecular gas depletion time t_depl, defined as the ratio of molecular gas mass to star formation rate, scales as (1+z)^{-0.6}x(delta_MS)^{-0.44}, and is only weakly dependent on stellar mass. The ratio of molecular-to-stellar mass mu_gas depends on (1+z)^{2.5}x (delta_MS)^{0.52}x(M*)^{-0.36}, which tracks the evolution of the specific star formation rate. The redshift dependence of mu_gas requires a curvature term, as may the mass-dependences of t_depl and mu_gas. We find no or only weak correlations of t_depl and mu_gas with optical size R or surface density once one removes the above scalings, but we caution that optical sizes may not be appropriate for the high gas and dust columns at high-z.

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arXiv2017

First detection of the 448 GHz H2O transition in space

We present the first detection of the ortho-H2O 4_23-3_30 transition at 448 GHz in space. We observed this transition in the local (z = 0.010) luminous infrared (IR) galaxy ESO 320-G030 (IRAS F11506-3851) using the Atacama Large Millimeter/submillimeter Array (ALMA). The water 4_23-3_30 emission, which originates in the highly obscured nucleus of this galaxy, is spatially resolved over a region of ~65 pc in diameter and shows a regular rotation pattern compatible with the global molecular and ionized gas kinematics. The line profile is symmetric and well fitted by a Gaussian with an integrated flux of 37.0 +- 0.7 Jy km s-1 . Models predict this water transition as a potential collisionally excited maser transition. On the contrary, in this galaxy, we find that the 4_23-3_30 emission is primarily excited by the intense far-IR radiation field present in its nucleus. According to our modeling, this transition is a probe of deeply buried galaxy nuclei thanks to the high dust optical depths (tau_100μm > 1, N_H > 1e24 cm-2) required to efficiently excite it.

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arXiv2017

Sub-arcsecond imaging of the water emission in Arp 220

Extragalactic observations of water emission can provide valuable insights into the excitation of the interstellar medium. In addition, extragalactic megamasers are powerful probes of kinematics close to active nuclei. Therefore, it is paramount to determine the true origin of the water emission, whether it is excited by processes close to an AGN or in star-forming regions. We use ALMA Band 5 science verification observations to analyse the emission of the 183 GHz water line in Arp 220 on sub-arcsecond scales, in conjunction with new ALMA Band 7 data at 325 GHz. Specifically, the nature of the process leading to the excitation of emission at these water lines is studied in this context. Supplementary 22 GHz VLA observations are used to better constrain the parameter space in the excitation modelling of the water lines. We detect 183 GHz H2O and 325 GHz water emission towards the two compact nuclei at the center of Arp 220, being brighter in Arp 220 West. The emission at these two frequencies is compared to previous single-dish data and does not show evidence of variability. The 183 and 325 GHz lines show similar spectra and kinematics, but the 22 GHz profile is significantly different in both nuclei due to a blend with an NH3 absorption line. Our findings suggest that the most likely scenario to cause the observed water emission in Arp 220 is a large number of independent masers originating from numerous star-forming regions.

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arXiv2017

Luminous, pc-scale CO 6-5 emission in the obscured nucleus of NGC1377

High resolution submm observations are important in probing the morphology, column density and dynamics of obscured active galactic nuclei (AGNs). With high resolution (0.06 x 0.05) ALMA 690 GHz observations we have found bright (TB >80 K) and compact (FWHM 10x7 pc) CO 6-5 line emission in the nucleus of the extremely radio-quiet galaxy NGC1377. The integrated CO 6-5 intensity is aligned with the previously discovered jet/outflow of NGC1377 and is tracing the dense (n>1e4 cm-3), hot gas at the base of the outflow. The velocity structure is complex and shifts across the jet/outflow are discussed in terms of jet-rotation or separate, overlapping kinematical components. High velocity gas (deltaV +-145 km/s) is detected inside r<2-3 pc and we suggest that it is emerging from an inclined rotating disk or torus of position angle PA=140+-20 deg with a dynamical mass of approx 3e6 Msun. This mass is consistent with that of a supermassive black hole (SMBH), as inferred from the M-sigma relation. The gas mass of the proposed disk/torus constitutes <3% of the nuclear dynamical mass. In contrast to the intense CO 6-5 line emission, we do not detect dust continuum with an upper limit of S(690GHz)<2mJy. The corresponding, 5 pc, H2 column density is estimated to N(H2)<3e23 cm-2, which is inconsistent with a Compton Thick (CT) source. We discuss the possibility that CT obscuration may be occuring on small (subparsec) or larger scales. From SED fitting we suggest that half of the IR emission of NGC1377 is nuclear and the rest (mostly the far-infrared (FIR)) is more extended. The extreme radio quietness, and the lack of emission from other star formation tracers, raise questions on the origin of the FIR emission. We discuss the possibility that it is arising from the dissipation of shocks in the molecular jet/outflow or from irradiation by the nuclear source along the poles.

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arXiv2016

Molecular shock tracers in NGC 1068: SiO and HNCO

SiO(3-2) and HNCO(6-5) emission has been imaged in NGC 1068 with the Plateau de Bure Interferometer (PdBI). We perform an LTE and RADEX analysis to determine the column densities and physical characteristics of the gas emitting these two lines. We then use a chemical model to determine the origin of the emission. There is a strong SiO peak to the East of the AGN, with weak detections to the West. This distribution contrasts that of HNCO, which is detected more strongly to the West. The SiO emission peak in the East is similar to the peak of the molecular gas mass traced by CO. HNCO emission is offset from this peak by as much as 80 pc ( 1"). We compare velocity integrated line ratios in the East and West. We confirm that SiO emission strongly dominates in the East, while the reverse is true in the West. We use RADEX to analyse the possible gas conditions that could produce such emission. We find that, in both East and West, we cannot constrain a single temperature for the gas. We run a grid of chemical models of potential shock processes in the CND and find that SiO is significantly enhanced during a fast (60 km/s) shock but not during a slow (20 km/s) shock, nor in a gas not subjected to shocks at all. We find the inverse for HNCO, whose abundance increases during slow shocks and in warm non-shocked gas. High SiO and low HNCO indicated a fast shock, while high HNCO and low SiO indicates either a slow shock or warm, dense, non-shocked gas. The East Knot is therefore likely to contain gas that is heavily shocked. From chemical modelling, gas in the West Knot may be non-shocked, or maybe undergoing a much milder shock event. When taking into account RADEX results, the milder shock event is the more likely of the two scenarios.

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arXiv2016

High-velocity extended molecular outflow in the star-formation dominated luminous infrared galaxy ESO 320-G030

We analyze new high spatial resolution (~60 pc) ALMA CO(2-1) observations of the isolated luminous infrared galaxy ESO 320-G030 (d=48 Mpc) in combination with ancillary HST optical and near-IR imaging as well as VLT/SINFONI near-IR integral field spectroscopy. We detect a high-velocity (~450 km/s) spatially resolved (size~2.5 kpc; dynamical time ~3 Myr) massive (~10^7 Msun; mass rate~2-8 Msun/yr) molecular outflow originated in the central ~250 pc. We observe a clumpy structure in the outflowing cold molecular gas with clump sizes between 60 and 150 pc and masses between 10^5.5 and 10^6.4 Msun. The mass of the clumps decreases with increasing distance, while the velocity is approximately constant. Therefore, both the momentum and kinetic energy of the clumps decrease outwards. In the innermost (~100 pc) part of the outflow, we measure a hot-to-cold molecular gas ratio of 7x10^-5, which is similar to that measured in other resolved molecular outflows. We do not find evidence of an ionized phase in this outflow. The nuclear IR and radio properties are compatible with strong and highly obscured star-formation (A_k ~ 4.6 mag; SFR~15 Msun/yr). We do not find any evidence for the presence of an active galactic nucleus. We estimate that supernova explosions in the nuclear starburst (ν(SN) ~ 0.2 yr^-1) can power the observed molecular outflow. The kinetic energy and radial momentum of the cold molecular phase of the outflow correspond to about 2% and 20%, respectively, of the supernovae output. The cold molecular outflow velocity is lower than the escape velocity, so the gas will likely return to the galaxy disk. The mass loading factor is ~0.1-0.5, so the negative feedback due to this star-formation powered molecular outflow is probably limited.

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arXiv2016

AGN feedback in the nucleus of M51

AGN feedback is invoked as one of the most relevant mechanisms that shape the evolution of galaxies. Our goal is to understand the interplay between AGN feedback and the interstellar medium in M51, a nearby spiral galaxy with a modest AGN and a kpc-scale radio jet expanding through the disc of the galaxy. For that purpose, we combine molecular gas observations in the CO(1-0) and HCN(1-0) lines from the Plateau de Bure interferometer with archival radio, X-ray, and optical data. We show that there is a significant scarcity of CO emission in the ionisation cone, while molecular gas emission tends to accumulate towards the edges of the cone. The distribution and kinematics of CO and HCN line emission reveal AGN feedback effects out to r~500pc, covering the whole extent of the radio jet, with complex kinematics in the molecular gas which displays strong local variations. We propose that this is the result of the almost coplanar jet pushing on molecular gas in different directions as it expands; the effects are more pronounced in HCN than in CO emission, probably as the result of radiative shocks. Following previous interpretation of the redshifted molecular line in the central 5" as caused by a molecular outflow, we estimate the outflow rates to be Mdot_H2~0.9Msun/yr and Mdot_dense~0.6Msun/yr, which are comparable to the molecular inflow rates (~1Msun/yr); gas inflow and AGN feedback could be mutually regulated processes. The agreement with findings in other nearby radio galaxies suggests that this is not an isolated case, and probably the paradigm of AGN feedback through radio jets, at least for galaxies hosting low-luminosity active nuclei.

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arXiv2016

A precessing molecular jet signaling an obscured, growing supermassive black hole in NGC1377?

With high resolution (0."25 x 0."18) ALMA CO 3-2 observations of the nearby (D=21 Mpc, 1"=102 pc), extremely radio-quiet galaxy NGC1377, we have discovered a high-velocity, very collimated nuclear outflow which we interpret as a molecular jet with a projected length of +-150 pc. Along the jet axis we find strong velocity reversals where the projected velocity swings from -150 km/s to +150 km/s. A simple model of a molecular jet precessing around an axis close to the plane of the sky can reproduce the observations. The velocity of the outflowing gas is difficult to constrain due to the velocity reversals but we estimate it to be between 240 and 850 km/s and the jet to precess with a period P=0.3-1.1 Myr. The CO emission is clumpy along the jet and the total molecular mass in the high-velocity (+-(60 to 150 km/s)) gas lies between 2e6 Msun (light jet) and 2e7 Msun (massive jet). There is also CO emission extending along the minor axis of NGC1377. It holds >40% of the flux in NGC1377 and may be a slower, wide-angle molecular outflow which is partially entrained by the molecular jet. We discuss the driving mechanism of the molecular jet and suggest that it is either powered by a very faint radio jet or by an accretion disk-wind similar to those found towards protostars. The nucleus of NGC1377 harbours intense embedded activity and we detect emission from vibrationally excited HCN J=4-3 v_2=1f which is consistent with hot gas and dust. We find large columns of H2 in the centre of NGC1377 which may be a sign of a high rate of recent gas infall. The dynamical age of the molecular jet is short (<1 Myr), which could imply that it is young and consistent with the notion that NGC1377 is caught in a transient phase of its evolution. However, further studies are required to determine the age of the molecular jet, its mass and the role it is playing in the growth of the nucleus of NGC1377.

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arXiv2016

The unbearable opaqueness of Arp 220

We explore the potential of imaging vibrationally excited molecular emission at high angular resolution to better understand the morphology and physical structure of the dense gas in Arp~220 and to gain insight into the nature of the nuclear powering sources. Vibrationally excited emission of HCN is detected in both nuclei with a very high ratio relative to the total $L_{FIR}$, higher than in any other observed galaxy and well above what is observed in Galactic hot cores. HCN $v_2=1f$ is observed to be marginally resolved in $\sim60\times50$~pc regions inside the dusty $\sim100$~pc sized nuclear cores. Its emission is centered on our derived individual nuclear velocities based on HCO$^+$ emission ($V_{WN}=5342\pm4$ and $V_{EN}=5454\pm8$~\kms, for the western and eastern nucleus, respectively). With virial masses within $r\sim25-30$~pc based on the HCN~$v_2=1f$ line widths, we estimate gas surface densities (gas fraction $f_g=0.1$) of $3\pm0.3\times10^4~M_\odot~\rm pc^{-2}$ (WN) and $1.1\pm0.1\times10^4~M_\odot~\rm pc^{-2}$ (EN). The $4-3/3-2$ flux density ratio could be consistent with optically thick emission, which would further constrain the size of the emitting region to $>15$~pc (EN) and $>22$~pc (WN). The absorption systems that may hide up to $70\%$ of the HCN and HCO$^+$ emission are found at velocities of $-50$~\kms~(EN) and $6$, $-140$, and $-575$~\kms (WN) relative to velocities of the nuclei. Blueshifted absorptions are the evidence of outflowing motions from both nuclei. The bright vibrational emission implies the existence of a hot dust region radiatively pumping these transitions. We find evidence of a strong temperature gradient that would be responsible for both the HCN $v_2$ pumping and the absorbed profiles from the vibrational ground state as a result of both continuum and self-absorption by cooler foreground gas.

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arXiv2016

Gravitational torques imply molecular gas inflow towards the nucleus of M51

The transport of gas towards the centre of galaxies is critical for black hole feeding and, indirectly, it can control active galactic nucleus (AGN) feedback. We have quantified the molecular gas inflow in the central R<1kpc of M51 to be 1 Msun/yr, using a new gravitational torque map and the molecular gas traced by the PdBI Arcsecond Whirlpool Survey (PAWS). The nuclear stellar bar is responsible for this gas inflow. We have also used torque profiles to estimate the location of dynamical resonances, suggesting a corotation for the bar at R~20", and a corotation for the spiral at R~100". We demonstrate how important it is to correct 3.6um images for dust emission in order to compute gravitational torques, and we carefully examine further sources of uncertainty. Our observational measurement of gas inflow can be compared with nuclear molecular outflow rates and provide useful constraints for numerical simulations.

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arXiv2016

Sub-kpc star-formation law in the local luminous infrared galaxy IC 4687 as seen by ALMA

We analyze the spatially resolved (250 pc scales) and integrated star-formation (SF) law in the local luminous infrared galaxy (LIRG) IC4687. This is one of the first studies of the SF law on a starburst LIRG at these small spatial scales. We combined new interferometric ALMA CO(2-1) data with existing HST/NICMOS Pa$α$ narrow-band imaging and VLT/SINFONI near-IR integral field spectroscopy to obtain accurate extinction corrected SF rate (SFR) and cold molecular gas surface densities ($Σ_{gas}$ and $Σ_{SFR}$). We find that IC4687 forms stars very efficiently with an average depletion time ($t_{dep}$) of 160 Myr for the individual 250 pc regions. This is approximately one order of magnitude shorter than the $t_{dep}$ of local normal spirals and also shorter than that of main-sequence high-z objects, even when we use a Galactic $α_{CO}$ conversion factor. This result suggests a bimodal SF law in the $Σ_{SFR} \propto Σ_{gas}^{N}$ representation. A universal SF law is recovered if we normalize the $Σ_{gas}$ by the global dynamical time. However, at the spatial scales studied here, we find that the SF efficiency (or $t_{dep}$) does not depend on the local dynamical time for this object. Therefore, an alternative normalization (e.g., free-fall time) should be found if a universal SF law exists at these scales.

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