Kennicutt-Schmidt Research Articles

The ATLAS3D Project – XXVIII. Dynamically driven star formation suppression in early-type galaxies

We present measurements of the star formation rate (SFR) in the early-type galaxies (ETGs) of the ATLAS3D sample, based on Wide-field Infrared Survey Explorer (WISE) 22um and Galaxy Evolution Explorer (GALEX) far-ultraviolet emission. We combine these with gas masses estimated from 12CO and HI data in order to investigate the star formation efficiency (SFE) in a larger sample of ETGs than previously available. We first recalibrate (based on WISE data) the relation between old stellar populations (traced at Ks-band) and 22um luminosity, allowing us to remove the contribution of 22um emission from circumstellar dust. We then go on to investigate the position of ETGs on the Kennicutt-Schmidt (KS) relation. Molecular gas-rich ETGs have comparable star formation surface densities to normal spiral galaxy centres, but they lie systematically offset from the KS relation, having lower star formation efficiencies by a factor of ~2.5 (in agreement with other authors). This effect is driven by galaxies where a substantial fraction of the molecular material is in the rising part of the rotation curve, and shear is high. We show here for the first time that although the number of stars formed per unit gas mass per unit time is lower in ETGs, it seems that the amount of stars formed per free-fall time is approximately constant. The scatter around this dynamical relation still correlates with galaxy properties such as the shape of the potential in the inner regions. This leads us to suggest that dynamical properties (such as shear or the global stability of the gas) may be important second parameters that regulate star formation and cause much of the scatter around star-formation relations.

Interpreting the sub-linear Kennicutt–Schmidt relationship: the case for diffuse molecular gas

Recent statistical analysis of two extragalactic observational surveys strongly indicate a sublinear Kennicutt-Schmidt (KS) relationship between the star formation rate (Sigsfr) and molecular gas surface density (Sigmol). Here, we consider the consequences of these results in the context of common assumptions, as well as observational support for a linear relationship between Sigsfr and the surface density of dense gas. If the CO traced gas depletion time (tau_mol) is constant, and if CO only traces star forming giant molecular clouds (GMCs), then the physical properties of each GMC must vary, such as the volume densities or star formation rates. Another possibility is that the conversion between CO luminosity and Sigmol, the XCO factor, differs from cloud-to-cloud. A more straightforward explanation is that CO permeates the hierarchical ISM, including the filaments and lower density regions within which GMCs are embedded. A number of independent observational results support this description, with the diffuse gas comprising at least 30% of the total molecular content. The CO bright diffuse gas can explain the sublinear KS relationship, and consequently leads to an increasing tau_mol with Sigmol. If Sigsfr linearly correlates with the dense gas surface density, a sublinear KS relationship indicates that the fraction of diffuse gas fdiff grows with Sigmol. In galaxies where Sigmol falls towards the outer disk, this description suggests that fdiff also decreases radially.

CO map and steep Kennicutt-Schmidt relation in the extended UV disk of M 63

Results from the UV satellite GALEX revealed large extensions of disks in some nearby spiral galaxies, extending out to 3 to 4 times the isophotal radius, r25. M63 is a remarkable example of a spiral galaxy with one of the most extended UV disks, so it offers the opportunity to search for the molecular gas and characterize the star formation in outer disk regions as revealed by the UV emission. We obtained deep CO(1-0) and CO(2-1) observations on the IRAM 30 m telescope along the major axis of the M63 disk from the center out to the galactocentric radius rgal = 1.6 r25 and over a bright UV region at rgal = 1.36 r25. CO(1-0) is detected all along the M63 major axis out to r25, and CO(2-1) is confined to rgal = 0.68 r25, which may betray lower excitation temperatures in the outer disk. CO(1-0) is also detected in the external bright UV region of M63. The radial profiles of the CO emission and of the Halpha, 24 micron, NUV and FUV star formation tracers and HI taken from the literature show a severe drop with the galactocentric radius, such that beyond r25 they are all absent with the exception of a faint UV emission and HI. The CO emission detection in the external UV region, where the UV flux is higher than the UV flux observed beyond r25, highlights a tight correlation between the CO and UV fluxes, namely the amount of molecular gas and the intensity of star formation. This external UV region is dominated by the atomic gas, suggesting that HI is more likely the precursor of H2 rather than the product of UV photodissociation. A broken power law needs to be invoked to describe the Kennicutt-Schmidt (K-S) relation of M63 from the center of the galaxy out to rgal = 1.36 r25. While all along the major axis out to r25 the K-S relation is almost linear, in the external UV region the SFR regime is highly nonlinear and characterized by a steep K-S relation and very low star formation efficiency.

Starbursts triggered by intergalactic tides andinterstellar compressive turbulence

AbstractUsing parsec-resolution simulations of a typical galaxy merger, we study the triggering of starbursts by connecting the (inter-)galactic dynamics to the structure of the interstellar medium. The gravitational encounter between two galaxies enhances tidal compression over large volumes, which increases and modifies the turbulence, in particular its compressive mode with respect to the solenoidal one. This generates an excess of dense gas leading to intense star formation activity. Along the interaction, the compressive turbulence modifies the efficiency of gas-to-star conversion which, in the Schmidt–Kennicutt diagram, drives the galaxies from the sequence of discs to that of starbursts.

THE INTERSTELLAR MEDIUM AND STAR FORMATION IN LOCAL GALAXIES: VARIATIONS OF THE STAR FORMATION LAW IN SIMULATIONS

We use the Adaptive Mesh Refinement code Enzo to model the interstellar medium in isolated local disk galaxies. The simulation includes a treatment for star formation and stellar feedback. We get a highly supersonic turbulent disk, which is fragmented at multiple scales and characterized by a multi-phase interstellar medium. We show that a Kennicutt-Schmidt (KS) relation only holds when averaging over large scales. However, values of star formation rates and gas surface densities lie close in the plot for any averaging size. This suggests an intrinsic relation between stars and gas at cell-size scales, which dominates over the global dynamical evolution. To investigate this effect, we develop a method to simulate the creation of stars based on the density field from the snapshots, without running the code again. We also investigate how the star formation law is affected by the characteristic star formation timescale, the density threshold and the efficiency considered in the recipe. We find that the slope of the law might vary from ~1.4 for a free-fall timescale, to ~1.0 for a constant depletion timescale. We further demonstrate that a power-law is recovered just by assuming that the mass of the new stars is a fraction of the mass of the cell $m_\star=\epsilon\rho_{\rm gas}\Delta x^3$, with no other physical criteria required. We show that both efficiency and density threshold do not affect the slope, but the right combination of them can adjust the normalization of the relation, which in turn could explain a possible bi-modality in the law.

The evolution of the dust and gas content in galaxies

We use deep Herschel PACS and SPIRE observations in GOODSS, GOODSN and COSMOS to estimate the average dust mass (Mdust) of galaxies on a redshift-stellar mass (Mstar)-SFR grid. We study the scaling relations between Mdust, Mstar and SFR at z<=2.5. No clear evolution of Mdust is observed at fixed SFR and Mstar. We find a tight correlation between SFR and Mdust, likely a consequence of the Schmidt-Kennicutt (S-K) law. The Mstar-Mdust correlation observed by previous works flattens or sometimes disappears when fixing the SFR. Most of it likely derives from the combination of the Mdust-SFR and Mstar-SFR correlations. We then investigate the gas content as inferred by converting Mdust by assuming that the dust/gas ratio scales linearly with the gas metallicity. All galaxies in the sample follow, within uncertainties, the same SFR-Mgas relation (integrated S-K law), which broadly agrees with CO-based results for the bulk of the population, despite the completely different approaches. The majority of galaxies at z~2 form stars with an efficiency (SFE=SFR/Mgas) ~5 times higher than at z~0. It is not clear what fraction of such variation is an intrinsic redshift evolution and what fraction arises from selection effects. The gas fraction (fgas) decreases with Mstar and increases with SFR, and does not evolve with z at fixed Mstar and SFR. We explain these trends by introducing a universal relation between fgas, Mstar and SFR, non-evolving out to z~2.5. Galaxies move across this relation as their gas content evolves in time. We use the 3D fundamental fgas-Mstar-SFR relation and the redshift evolution of the Main Sequence to estimate the evolution of fgas in the average population of galaxies as a function of z and Mstar, and we find evidence a downsizing scenario.

MOLECULAR HYDROGEN REGULATED STAR FORMATION IN COSMOLOGICAL SMOOTHED PARTICLE HYDRODYNAMICS SIMULATIONS

It has been shown observationally that star formation (SF) correlates tightly with the presence of molecular hydrogen (H$_2$). Therefore it would be important to investigate its implication on galaxy formation in a cosmological context. In the present work, we track the H$_2$ mass fraction within our cosmological smoothed particle hydrodynamics (SPH) code GADGET-3 using an equilibrium analytic model by Krumholz et al. This model allows us to regulate the star formation in our simulation by the local abundance of H$_2$ rather than the total cold gas density, and naturally introduce the dependence of star formation on metallicity. We investigate implications of the equilibrium H$_2$-based SF model on galaxy population properties, such as the stellar-to-halo mass ratio (SHMR), baryon fraction, cosmic star formation rate density (SFRD), galaxy specific SFR, galaxy stellar mass functions (GSMF), and Kennicutt-Schmidt (KS) relationship. The advantage of our work over the previous ones is having a large sample of simulated galaxies in a cosmological volume from high-redshift to $z=0$. We find that low-mass halos with M$_{DM}<10^{10.5}$ M$_\odot$ are less efficient in producing stars in the H$_2$-based SF model at $z\geq6$, which brings the simulations to a better agreement with observational estimates of SHMR and GSMF at the low-mass end. This is particularly evident by a reduction in the number of low-mass galaxies at M$_\star\leq10^{8}$ M$_\odot$ in the GSMF. The overall SFRD is also reduced at high-$z$ in the H$_2$ run, which results in slightly higher SFRD at low-redshift due to more abundant gas available for star formation at later times. This new H$_2$ model is able to reproduce the empirical KS relationship at $z=0$ naturally without the need for setting its normalization by hand, and overall it seems to have more advantages than the previous pressure-based SF model.

Indications of a sub-linear and non-universal Kennicutt–Schmidt relationship

Abstract We estimate the parameters of the Kennicutt–Schmidt (KS) relationship, linking the star formation rate (ΣSFR) to the molecular gas surface density (Σmol), in the Survey Toward Infrared-Bright Nearby Galaxies sample of nearby disc galaxies using a hierarchical Bayesian method. This method rigorously treats measurement uncertainties, and provides accurate parameter estimates for both individual galaxies and the entire population. Assuming standard conversion factors to estimate ΣSFR and Σmol from the observations, we find that the KS parameters vary between galaxies, indicating that no universal relationship holds for all galaxies. The KS slope of the whole population is 0.76, with the 2σ range extending from 0.58 to 0.94. These results imply that the molecular gas depletion time is not constant, but varies from galaxy-to-galaxy, and increases with the molecular gas surface density. Therefore, other galactic properties besides just Σmol affect ΣSFR, such as the gas fraction or stellar mass. The non-universality of the KS relationship indicates that a comprehensive theory of star formation must take into account additional physical processes that may vary from galaxy to galaxy.

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.

THE STAR FORMATION LAWS OF EDDINGTON-LIMITED STAR-FORMING DISKS

Two important avenues into understanding the formation and evolution of galaxies are the Kennicutt-Schmidt (KS) and Elmegreen-Silk (ES) laws. These relations connect the surface densities of gas and star formation (\sigmagas\ and \sigmastar, respectively) in a galaxy. To elucidate the KS and ES laws for disks where \sigmagas >~ 10^4 Msun pc-2}, we compute 132 Eddington-limited star-forming disk models with radii spanning tens to hundreds of parsecs. The theoretically expected slopes (approx. 1 for the KS law and approx. 0.5 for the ES relation) are relatively robust to spatial averaging over the disks. However, the star formation laws exhibit a strong dependence on opacity that separates the models by the dust-to-gas ratio that may lead to the appearance of a erroneously large slope. The total infrared luminosity (L_TIR) and multiple carbon monoxide (CO) line intensities were computed for each model. While L_TIR can yield an estimate of the average \sigmastar\ that is correct to within a factor of 2, the velocity-integrated CO line intensity is a poor proxy for the average \sigmagas\ for these warm and dense disks, making the CO conversion factor (\alpha_CO) all but useless. Thus, observationally derived KS and ES laws at these values of \sigmagas\ that uses any transition of CO will provide a poor measurement of the underlying star formation relation. Studies of the star formation laws of Eddington-limited disks will require a high-J transition of a high density molecular tracer, as well as a sample of galaxies with known metallicity estimates.

Evidence for a non-universal Kennicutt–Schmidt relationship using hierarchical Bayesian linear regression

For investigating the relationship between the star formation rate and gas surface density, we develop a Bayesian linear regression method that rigorously treats measurement uncertainties and accounts for hierarchical data structure. The hierarchical Bayesian method simultaneously estimates the intercept, slope and scatter about the regression line of each individual subject (e.g. a galaxy) and the population (e.g. an ensemble of galaxies). Using synthetic data sets, we demonstrate that the method accurately recovers the underlying parameters of both the individuals and the population, especially when compared to commonly employed ordinary least squares techniques, such as the bisector fit. We apply the hierarchical Bayesian method to estimate the Kennicutt–Schmidt (KS) parameters of a sample of spiral galaxies compiled by Bigiel et al. We find significant variation in the KS parameters, indicating that no single KS relationship holds for all galaxies. This suggests that the relationship between molecular gas and star formation differs from galaxy to galaxy, possibly due to the influence of other physical properties within a given galaxy, such as metallicity, molecular gas fraction, stellar mass and/or magnetic fields. In four of the seven galaxies the slope estimates are sublinear, especially for M51, where unity is excluded at the 2σ level. We estimate the mean index of the KS relationship for the population to be 0.84, with 2σ range [0.63, 1.0]. For the galaxies with sublinear KS relationships, a possible interpretation is that CO emission is tracing some molecular gas that is not directly associated with star formation. Equivalently, a sublinear KS relationship may be indicative of an increasing gas depletion time at higher surface densities, as traced by CO emission. The hierarchical Bayesian method can account for all sources of uncertainties, including variations in the conversion of observed intensities to star formation rates and gas surface densities (e.g. the XCO factor), and is therefore well suited for a thorough statistical analysis of the KS relationship.

Resolved Schmidt-Kennicutt Relation for Star Forming Regions in the Galaxy and Magellanic Clouds

AbstractThe relationship between star formation rate (SFR) and the gas surface density (Σgas) is one of the most critical links between star formation and galaxy evolution. The observed SFR- Σgas relation, the “Schmidt-Kennicutt (S-K) law”, is tight when properties are averaged over kpc, but breaks down at the scale of giant molecular clouds (GMCs). To understand the physics governing the variations at GMC scales and the tight correlation at kpc scales, spatially and temporally resolved data covering a wide range of linear scale are needed. We have used the Spitzer surveys of the Large Magellanic Cloud and Magellanic Bridge to identify massive young stellar objects (YSOs), estimate “instantaneous” SFRs, and compare them to the S-K relation. These instantaneous SFRs are further compared to that estimated from integrated Hα and 24 μm luminosities to examine how SFRs vary on 10 Myr timescales. We have also used SINFONI near-IR integral field spectra of two Galactic mini-starbursts W31 and W43 to determine their underlying massive stellar content, estimate the SFRs, and compare to the S-K relation. To investigate evironmental effects on star formation, we have used complete YSO samples in the LMC and the Bridge to estimate global star formation efficiencies (SFE) in these two systems.

The Schmidt-Kennicutt Law of Matched-Age Star Forming Regions

AbstractWe show that the dispersion in the Schmidt-Kennicutt (SK) law in galaxies is affected significantly by the evolutionary stage of star forming molecular gas, using narrow band Paα imaging of Taffy I, an interacting pair of galaxies. Star forming regions in the system show very uniform ages except for the bridge region, and the SK law of regions at the same age show a exceptionally tight SK law.

Schmidt-Kennicutt relations in SPH simulations of disc galaxies with effective thermal feedback from supernovae

We study several versions of the Schmidt-Kennicutt (SK) relation obtained for isolated spiral galaxies in TreeSPH simulations run with the GADGET3 code including the novel MUlti-Phase Particle Integrator (MUPPI) algorithm for star formation and stellar feedback. [...] The standard SK relation between surface densities of cold (neutral+molecular) gas and star formation rate of simulated galaxies shows a steepening at low gas surface densities, starting from a knee whose position depends on disc gas fraction: for more gas-rich discs the steepening takes place at higher surface densities. Because gas fraction and metallicity are typically related, this environmental dependence mimics the predictions of models where the formation of H2 is modulated by metallicity. The cold gas surface density at which HI and molecular gas surface densities equate can range from ~10 up to 34 Msun/pc^2. As expected, the SK relation obtained using molecular gas shows much smaller variations among simulations. We find that disc pressure is not well represented by the classical external pressure of a disc in vertical hydrostatic equilibrium. Instead is well fit by the expression P_fit = Sigma_cold sigma_cold kappa / 6, where the three quantities on the right-hand side are cold gas surface density, vertical velocity dispersion and epicyclic frequency. When the "dynamical" SK relation, i.e. the relation that uses gas surface density divided by orbital time, is considered, we find that all of our simulations stay on the same relation. We interpret this as a manifestation of the equilibrium between energy injection and dissipation in stationary galaxy discs, when energetic feedback is effective and pressure is represented by the expression given above. These findings further support the idea that a realistic model of the structure of galaxy discs should take into account energy injection by SNe. [Abridged]

Star-formation laws in luminous infrared galaxies

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.

STAR FORMATION FROM DLA GAS IN THE OUTSKIRTS OF LYMAN BREAK GALAXIES ATz∼ 3

We present evidence for spatially extended low surface brightness emission around Lyman break galaxies (LBGs) in the V-band image of the Hubble Ultra Deep Field, corresponding to the z ∼ 3 rest-frame far-UV (FUV) light, which is a sensitive measure of star formation rates (SFRs). We find that the covering fraction of molecular gas at z ∼ 3 is not adequate to explain the emission in the outskirts of LBGs, while the covering fraction of neutral atomic-dominated hydrogen gas at high redshift is sufficient. We develop a theoretical framework to connect this emission around LBGs to the expected emission from neutral H i gas, i.e., damped Lyα systems (DLAs), using the Kennicutt–Schmidt (KS) relation. Working under the hypothesis that the observed FUV emission in the outskirts of LBGs is from in situ star formation in atomic-dominated hydrogen gas, the results suggest that the SFR efficiency in such gas at z ∼ 3 is between factors of 10 and 50 lower than predictions based on the local KS relation. The total SFR density in atomic-dominated gas at z ∼ 3 is constrained to be ∼10% of that observed from the inner regions of LBGs. In addition, the metals produced by in situ star formation in the outskirts of LBGs yield metallicities comparable to those of DLAs, which is a possible solution to the "Missing Metals" problem for DLAs. Finally, the atomic-dominated gas in the outskirts of galaxies at both high and low redshifts has similar reduced SFR efficiencies and is consistent with the same power law.

ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT-SCHMIDT RELATION IN GALAXIES

We present a detailed description of a phenomenological H2 formation model and local star formation prescription based on the density of molecular (rather than total) gas. Such approach allows us to avoid the arbitrary density and temperature thresholds typically used in star formation recipes. We present results of the model based on realistic cosmological simulations of high-z galaxy formation for a grid of numerical models with varied dust-to-gas ratios and interstellar far UV (FUV) fluxes. Our results show that both the atomic-to-molecular transition on small, ~10 pc scales and the Kennicutt-Schmidt (KS) relation on ~kpc scales are sensititive to the dust-to-gas ratio and the FUV flux. The atomic-to-molecular transition as a function of gas density or column density has a large scatter but is rather sharp and shifts to higher densities with decreasing dust-to-gas ratio and/or increasing FUV flux. Consequently, star formation is concentrated to higher gas surface density regions, resulting in steeper slope and lower amplitude of the KS relation at a given gas surface density, in less dusty and/or higher FUV flux environments. These trends should have a particularly strong effect on the evolution of low-mass, low surface brightness galaxies which typically have low dust content and anemic star formation, but are also likely to be important for evolution of the Milky Way-sized systems. We parameterize the dependencies observed in our simulations in convenient fitting formulae, which can be used to model the dependence of the KS relation on the dust-to-gas ratio and FUV flux in semi-analytic models and in cosmological simulations that do not include radiative transfer and H2 formation.

The Impact of ISM Turbulence, Clustered Star Formation and Feedback on Galaxy Mass Assembly through Cold Flows and Mergers

AbstractTwo of the dominant channels for galaxy mass assembly are cold flows (cold gas supplied via the filaments of the cosmic web) and mergers. How these processes combine in a cosmological setting, at both low and high redshift, to produce the whole zoo of galaxies we observe is largely unknown. Indeed there is still much to understand about the detailed physics of each process in isolation. While these formation channels have been studied using hydrodynamical simulations, here we study their impact on gas properties and star formation (SF) with some of the first from simulations that capture the multiphase, cloudy nature of the interstellar medium (ISM), by virtue of their high spatial resolution (and corresponding low temperature threshold). In this regime, we examine the competition between cold flows and a supernovae(SNe)-driven outflow in a very high-redshift galaxy (z ≈ 9) and study the evolution of equal-mass galaxy mergers at low and high redshift, focusing on the induced SF. We find that SNe-driven outflows cannot reduce the cold accretion at z ≈ 9 and that SF is actually enhanced due to the ensuing metal enrichment. We demonstrate how several recent observational results on galaxy populations (e.g. enhanced HCN/CO ratios in ULIRGs, a separate Kennicutt Schmidt (KS) sequence for starbursts and the population of compact early type galaxies (ETGs) at high redshift) can be explained with mechanisms captured in galaxy merger simulations, provided that the multiphase nature of the ISM is resolved.

Molecular gas and star formation in early-type galaxies

We present new mm interferometric and optical integral-field unit (IFU) observations and construct a sample of 12 elliptical (E) and lenticular (S0) galaxies with molecular gas which have both CO and optical maps. The galaxies contain 2 × 107 to 5 × 109 M⊙ of molecular gas distributed primarily in central discs or rings (radii 0.5–4 kpc). The molecular gas distributions are always coincident with distributions of optically obscuring dust that reveal tightly wound spiral structures in many cases. The ionized gas always approximately corotates with the molecular gas, evidencing a link between these two gas components, yet star formation is not always the dominant ionization source. The galaxies with less molecular gas tend to have [O iii]/Hβ emission-line ratios at high values not expected for star formation. Most E/S0s with molecular gas have young or intermediate-age stellar populations based on optical colours, ultraviolet colours and absorption linestrengths. The few that appear purely old lie close to the limit where such populations would be undetectable based on the mass fractions of expected young to observed old stars. The 8 μm polycyclic aromatic hydrocarbon (PAH) and 24 μm emission yield similar star formation rate (SFR) estimates of E/S0s, but the total infrared overpredicts the rate due to a contribution to dust heating from older stars. The radio–far-infrared relation also has much more scatter than for other star-forming galaxies. However, despite these biases and additional scatter, the derived star formation rates locate the E/S0 galaxies within the large range of the Schmidt–Kennicutt and constant efficiency star formation laws. Thus, the star formation process in E/S0s is not overwhelmingly different than in other star-forming galaxies, although one of the more reliable tracers (24 μm) points to a possible lower star formation efficiency at a given gas surface density.

DEVIATIONS FROM THE SCHMIDT-KENNICUTT RELATIONS DURING EARLY GALAXY EVOLUTION

We utilize detailed time-varying models of the coupled evolution of stars and the HI, H_2, and CO-bright H_2 gas phases in galaxy-sized numerical simulations to explore the evolution of gas-rich and/or metal-poor systems, expected to be numerous in the Early Universe. The inclusion of the CO-bright H_2 gas phase, and the realistic rendering of star formation as an H_2-regulated process (and the new feedback processes that this entails) allows the most realistic tracking of strongly evolving galaxies, and much better comparison with observations. We find that while galaxies eventually settle into states conforming to Schmidt-Kennicutt (S-K) relations, significant and systematic deviations of their star formation rates (SFRs) from the latter occur, especially pronounced and prolonged for ... ...This indicates potentially serious limitations of (S-K)-type relations as reliable sub-grid elements of star formation physics in simulations of structure formation in the Early Universe. We anticipate that galaxies with marked deviations from the S-K relations will be found at high redshifts as unbiased inventories of total gas mass become possible with ALMA and the EVLA.