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Register a new userVALES V: A kinematic analysis of the molecular gas content in $H$-ATLAS galaxies at $zsim0.03-0.35$ using ALMA
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We present Atacama Large Millimeter/submillimeter Array (ALMA) resolved observations of molecular gas in galaxies up to $z=0.35$ to characterise the role of global galactic dynamics on the global interstellar medium (ISM) properties. These observations consist of a sub-sample of 39 galaxies taken from the Valparaiso ALMA Line Emission Survey (VALES). From the CO($J=1-0)$ emission line, we quantify the kinematic parameters by modelling the velocity fields. We find that the IR luminosity increases with the rotational to dispersion velocity ratio ($V_{rm rot}/sigma_v$, corrected for inclination). We find a dependence between $V_{rm rot}/sigma_v$ and the [CII]/IR ratio, suggesting that the so-called `[CII] deficit is related to the dynamical state of the galaxies. We find that global pressure support is needed to reconcile the dynamical mass estimates with the stellar masses in our systems with low $V_{rm rot}/sigma_v$ values. The star formation rate (SFR) is weakly correlated with the molecular gas fraction ($f_{rm H_2}$) in our sample, suggesting that the release of gravitational energy from cold gas may not be the main energy source of the turbulent motions seen in the VALES galaxies. By defining a proxy of the `star formation efficiency parameter as the SFR divided by the CO luminosity (SFE$equiv$ SFR/L$_{rm CO}$), we find a constant SFE$$ per crossing time ($t_{rm cross}$). We suggest that $t_{rm cross}$ may be the controlling timescale in which the star formation occurs in dusty $zsim0.03-0.35$ galaxies.
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We present an extragalactic survey using observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to characterise galaxy populations up to $z=0.35$: the Valparaiso ALMA Line Emission Survey (VALES). We use ALMA Band-3 CO(1--0) observations to study the molecular gas content in a sample of 67 dusty normal star-forming galaxies selected from the $Herschel$ Astrophysical Terahertz Large Area Survey ($H$-ATLAS). We have spectrally detected 49 galaxies at $>5sigma$ significance and 12 others are seen at low significance in stacked spectra. CO luminosities are in the range of $(0.03-1.31)times10^{10}$ K km s$^{-1}$ pc$^2$, equivalent to $log({rm M_{gas}/M_{odot}}) =8.9-10.9$ assuming an $alpha_{rm CO}$=4.6(K km s$^{-1}$ pc$^{2}$)$^{-1}$, which perfectly complements the parameter space previously explored with local and high-z normal galaxies. We compute the optical to CO size ratio for 21 galaxies resolved by ALMA at $sim 3$.$5$ resolution (6.5 kpc), finding that the molecular gas is on average $sim$ 0.6 times more compact than the stellar component. We obtain a global Schmidt-Kennicutt relation, given by $log [Sigma_{rm SFR}/({rm M_{odot} yr^{-1}kpc^{-2}})]=(1.26 pm 0.02) times log [Sigma_{rm M_{H2}}/({rm M_{odot},pc^{-2}})]-(3.6 pm 0.2)$. We find a significant fraction of galaxies lying at `intermediate efficiencies between a long-standing mode of star-formation activity and a starburst, specially at $rm L_{IR}=10^{11-12} L_{odot}$. Combining our observations with data taken from the literature, we propose that star formation efficiencies can be parameterised by $log [{rm SFR/M_{H2}}]=0.19 times {rm (log {L_{IR}}-11.45)}-8.26-0.41 times arctan[-4.84 (log {rm L_{IR}}-11.45) ]$. Within the redshift range we explore ($z<0.35$), we identify a rapid increase of the gas content as a function of redshift.
Shells are fine stellar structures identified by their arc-like shapes present around a galaxy and currently thought to be vestiges of galaxy interactions and/or mergers. The study of their number, geometry, stellar populations and gas content can help to derive the interaction/merger history of a galaxy. Numerical simulations have proposed a mechanism of shell formation through phase wrapping during a radial minor merger. Alternatively, there could be barely a space wrapping, when particles have not made any radial oscillation yet, but are bound by their radial expansion, or produce an edge-brightened feature. These can be distinguished, because they are expected to keep a high radial velocity. While shells are first a stellar phenomenon, HI and CO observations have revealed neutral gas associated with shells. Some of the gas, the most diffuse and dissipative, is expected to be driven quickly to the center if it is travelling on nearly radial orbits. Molecular gas, distributed in dense clumps, is less dissipative, and may be associated to shells, and determine their velocity, too difficult to obtain from stars. We present here a search for molecular gas in nine shell galaxies with the IRAM-30m telescope. Six of them are detected in their galaxy center, and in three galaxies, we clearly detect molecular gas in shells. The derived amount of molecular gas varies from 1.5 10$^8$ to 3.4 10$^9$ M$_odot$ in the shells. For two of them (Arp 10 and NGC 3656), the shells are characteristic of an oblate system. Their velocity is nearly systemic, and we conclude that these shells are phase-wrapped. For the third one (NGCB3934) the shells appear to participate to the rotation, and follow up with higher spatial resolution is required to conclude.
We present an analysis of the molecular gas properties, based on CO(2 - 1) emission, of twelve starburst galaxies at z~1.6 selected by having a boost (>~4x) in their star formation rate (SFR) above the average star-forming galaxy at an equivalent stellar mass. ALMA observations are acquired of six additional galaxies than previously reported through our effort. As a result of the larger statistical sample, we significantly detect, for the first time at high-z, a systematically lower L_CO/L_IR ratio in galaxies lying above the star-forming `main sequence (MS). Based on an estimate of alpha_CO (i.e., the ratio of molecular gas mass to L_CO(1-0)), we convert the observational quantities (e.g., L_CO/L_IR) to physical units (M_gas/SFR) that represent the gas depletion time or its inverse, the star formation efficiency. We interpret the results as indicative of the star formation efficiency increasing in a continuous fashion from the MS to the starburst regime, whereas the gas fractions remain comparable to those of MS galaxies. Although, the balance between an increase in star-formation efficiency or gas fraction depends on the adopted value of alpha_CO as discussed.
Context. Spatially resolved observations of the ionized and molecular gas are critical for understanding the physical processes that govern the interstellar medium (ISM) in galaxies. Aims. To study the morpho-kinematic properties of the ionized and molecular gas in three dusty starburst galaxies at $z = 0.12-0.17$ to explore the relation between molecular ISM gas phase dynamics and the star-formation activity. Methods. We analyse $sim$kpc-scale ALMA CO(1--0) and seeing limited SINFONI Paschen-$alpha$ observations. We use a dynamical mass model, which accounts for beam-smearing effects, to constrain the CO-to-H$_2$ conversion factor. Results. One starburst galaxy shows irregular morphology which may indicate a major merger, while the other two systems show disc-like morpho-kinematics. The two disc-like starbursts show molecular gas velocity dispersion values comparable with that seen in local LIRG/ULIRGs, but in an ISM with molecular gas fraction and surface density values consistent to that reported for local star-forming galaxies. These molecular gas velocity dispersion values can be explained by assuming vertical pressure equilibrium. The star-formation activity is correlated with the molecular gas content suggesting depletion times of the order of $sim 0.1-1$ Gyr. The star formation rate surface density ($Sigma_{rm SFR}$) correlates with the ISM pressure set by self-gravity ($P_{rm grav}$) following a power law with an exponent close to 0.8. Conclusions. In dusty disc-like starburst galaxies, our data support the scenario in which the molecular gas velocity dispersion values are driven by the ISM pressure set by self-gravity, responsible to maintain the vertical pressure balance. The correlation between $Sigma_{rm SFR}$ and $P_{rm grav}$ suggests that, in these dusty starbursts galaxies, the star formation activity arises as a consequence of the ISM pressure balance.
We present a detailed study of the molecular gas content and stellar population properties of three massive galaxies at 1 < z < 1.3 that are in different stages of quenching. The galaxies were selected to have a quiescent optical/near-infrared spectral energy distribution and a relatively bright emission at 24 micron, and show remarkably diverse properties. CO emission from each of the three galaxies is detected in deep NOEMA observations, allowing us to derive molecular gas fractions Mgas/Mstar of 13-23%. We also reconstruct the star formation histories by fitting models to the observed photometry and optical spectroscopy, finding evidence for recent rejuvenation in one object, slow quenching in another, and rapid quenching in the third system. To better constrain the quenching mechanism we explore the depletion times for our sample and other similar samples at z~0.7 from the literature. We find that the depletion times are highly dependent on the method adopted to measure the star formation rate: using the UV+IR luminosity we obtain depletion times about 6 times shorter than those derived using dust-corrected [OII] emission. When adopting the star formation rates from spectral fitting, which are arguably more robust, we find that recently quenched galaxies and star-forming galaxies have similar depletion times, while older quiescent systems have longer depletion times. These results offer new, important constraints for physical models of galaxy quenching.
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