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Register a new userResolving the ISM at the peak of cosmic star formation with ALMA - The distribution of CO and dust continuum in z~2.5 sub-millimetre galaxies
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Gabriela Calistro Rivera
Publication date
2018
fields
Physics
and research's language is
English
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We use ALMA observations of four sub-millimetre galaxies (SMGs) at $zsim2-3$ to investigate the spatially resolved properties of the inter-stellar medium (ISM) at scales of 1--5 kpc (0.1--0.6$$). The velocity fields of our sources, traced by the $^{12}$CO($J$=3-2) emission, are consistent with disk rotation to first order, implying average dynamical masses of $sim$3$times10^{11}$M$_{odot}$ within two half-light radii. Through a Bayesian approach we investigate the uncertainties inherent to dynamically constraining total gas masses. We explore the covariance between the stellar mass-to-light ratio and CO-to-H$_{2}$ conversion factor, $alpha_{rm CO}$, finding values of $alpha_{rm CO}=1.1^{+0.8}_{-0.7}$ for dark matter fractions of 15 %. We show that the resolved spatial distribution of the gas and dust continuum can be uncorrelated to the stellar emission, challenging energy balance assumptions in global SED fitting. Through a stacking analysis of the resolved radial profiles of the CO(3-2), stellar and dust continuum emission in SMG samples, we find that the cool molecular gas emission in these sources (radii $sim$5--14 kpc) is clearly more extended than the rest-frame $sim$250 $mu$m dust continuum by a factor $>2$. We propose that assuming a constant dust-to-gas ratio, this apparent difference in sizes can be explained by temperature and optical-depth gradients alone. Our results suggest that caution must be exercised when extrapolating morphological properties of dust continuum observations to conclusions about the molecular gas phase of the ISM.
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ALMA Cycle 2 observations of the long wavelength dust emission in 180 star-forming (SF) galaxies are used to investigate the evolution of ISM masses at z = 1 to 6.4. The ISM masses exhibit strong increases from z = 0 to $rm <z>$ = 1.15 and further to $rm <z>$ = 2.2 and 4.8, particularly amongst galaxies above the SF galaxy main sequence (MS). The galaxies with highest SFRs at $rm <z>$ = 2.2 and 4.8 have gas masses 100 times that of the Milky Way and gas mass fractions reaching 50 to 80%, i.e. gas masses 1 - 4$times$ their stellar masses. For the full sample of galaxies, we find a single, very simple SF law: $rm SFR propto M_{rm ISM}^{0.9}$, i.e. a `linear dependence on the ISM mass -- on and above the MS. Thus, the galaxies above the MS are converting their larger ISM masses into stars on a timescale similar to those on the MS. At z $> 1$, the entire population of star-forming galaxies has $sim$5 - 10$times$ shorter gas depletion times ($sim0.2$ Gyr) than galaxies at low redshift. These {bf shorter depletion times are due to a different, dominant mode of SF in the early universe} -- dynamically driven by compressive, high dispersion gas motions and/or galaxy interactions. The dispersive gas motions are a natural consequence of the extraordinarily high gas accretion rates which must occur to maintain the prodigious SF.
ALMA Cycle 2 observations of the long wavelength dust emission in 145 star-forming galaxies are used to probe the evolution of star-forming ISM. We also develop the physical basis and empirical calibration (with 72 low-z and z ~ 2 galaxies) for using the dust continuum as a quantitative probe of interstellar medium (ISM) masses. The galaxies with highest star formation rates (SFRs) at <z> = 2.2 and 4.4 have gas masses up to 100 times that of the Milky Way and gas mass fractions reaching 50 to 80%, i.e. gas masses 1 - 4 times their stellar masses. We find a single high-z star formation law: SFR = 35 M_ mol^0.89 x (1+z)_{z=2}^0.95 x (sSFR)_{MS}^0.23 msun yr^-1 -- an approximately linear dependence on the ISM mass and an increased star formation efficiency per unit gas mass at higher redshift. Galaxies above the Main Sequence (MS) have larger gas masses but are converting their ISM into stars on a timescale only slightly shorter than those on the MS -- thus these starbursts are largely the result of having greatly increased gas masses rather than and increased efficiency for converting gas to stars. At z $> 1$, the entire population of star-forming galaxies has $sim$ 2 - 5 times shorter gas depletion times than low-z galaxies. These shorter depletion times indicate a different mode of star formation in the early universe -- most likely dynamically driven by compressive, high-dispersion gas motions -- a natural consequence of the high gas accretion rates.
We present the ALMA view of 11 main-sequence DSFGs, (sub-)millimeter selected in the GOODS-S field, and spectroscopically confirmed to be at the peak of Cosmic SFH (z = 2-3). Our study combines the analysis of galaxy SED with ALMA continuum and CO spectral emission, by using ALMA Science Archive products at the highest spatial resolution currently available for our sample (< 1 arcsec). We include galaxy multi-band images and photometry (in the optical, radio and X-rays) to investigate the interlink between dusty, gaseous and stellar components and the eventual presence of AGN. We use multi-band sizes and morphologies to gain an insight on the processes that lead galaxy evolution, e.g. gas condensation, star formation, AGN feedback. The 11 DSFGs are very compact in the (sub-)millimeter (median r(ALMA) = 1.15 kpc), while the optical emission extends tolarger radii (median r(H)/r(ALMA) = 2.05). CO lines reveal the presence of a rotating disc of molecular gas, but we can not exclude either the presence of interactions and/or molecular outflows. Images at higher (spectral and spatial) resolution are needed to disentangle from the possible scenarios. Most of the galaxies are caught in the compaction phase, when gas cools and falls into galaxy centre, fuelling the dusty burst of star formation and the growing nucleus. We expect these DSFGs to be the high-zstar-forming counterparts of massive quiescent galaxies. Some features of CO emission in three galaxies are suggestive of forthcoming/ongoing AGN feedback, that is thought to trigger the morphological transition from star-forming disks to ETGs.
We present spatially-resolved Atacama Large Millimeter/sub-millimeter Array (ALMA) 870 $mu$m dust continuum maps of six massive, compact, dusty star-forming galaxies (SFGs) at $zsim2.5$. These galaxies are selected for their small rest-frame optical sizes ($r_{rm e, F160W}sim1.6$ kpc) and high stellar-mass densities that suggest that they are direct progenitors of compact quiescent galaxies at $zsim2$. The deep observations yield high far-infrared (FIR) luminosities of L$_{rm IR}=10^{12.3-12.8}$ L$_{odot}$ and star formation rates (SFRs) of SFR$=200-700$ M$_{odot}$yr$^{-1}$, consistent with those of typical star-forming main sequence galaxies. The high-spatial resolution (FWHM$sim$0.12-0.18) ALMA and HST photometry are combined to construct deconvolved, mean radial profiles of their stellar mass and (UV+IR) SFR. We find that the dusty, nuclear IR-SFR overwhelmingly dominates the bolometric SFR up to $rsim5$ kpc, by a factor of over 100$times$ from the unobscured UV-SFR. Furthermore, the effective radius of the mean SFR profile ($r_{rm e, SFR}sim1$ kpc) is $sim$30% smaller than that of the stellar mass profile. The implied structural evolution, if such nuclear starburst last for the estimated gas depletion time of $Delta t=pm100$ Myr, is a 4$times$ increase of the stellar mass density within the central 1 kpc and a 1.6$times$ decrease of the half-mass radius. This structural evolution fully supports dissipation-driven, formation scenarios in which strong nuclear starbursts transform larger, star-forming progenitors into compact quiescent galaxies.
We present an analysis of the dust attenuation of star forming galaxies at $z=2.5-4.0$ through the relationship between the UV spectral slope ($beta$), stellar mass ($M_{ast}$) and the infrared excess (IRX$=L_{rm{IR}}/L_{rm{UV}}$) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A$^3$COSMOS team, which includes an unprecedented sample of $sim1500$ galaxies at $zsim3$ as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 $rm{mu Jy/beam}$ (1$sigma$). The detection rate is highly mass dependent, decreasing drastically below $log (M_{ast}/M_{odot})=10.5$. The detected galaxies show that the IRX-$beta$ relationship of massive ($log M_{ast}/M_{odot} > 10$) main sequence galaxies at $z=2.5-4.0$ is consistent with that of local galaxies, while starbursts are generally offset by $sim0.5,{rm dex}$ to larger IRX values. At the low mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX-$M_{ast}$ relation at $rm{log,(M_{ast}/M_{odot})>9}$ exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at $log M_{ast}/M_{odot}<10$. However, our results are consistent with early measurements at $zsim5.5$, indicating a potential redshift evolution between $zsim2$ and $zsim6$. Deeper observations targeting low mass galaxies will be required to confirm this finding.
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