We analyze the physical conditions in the interstellar gas of 11 actively star-forming galaxies at z~2, based on integral-field spectroscopy from the ESO-VLT and Hubble Space Telescope/Near Infrared ...Camera and Multi-Object Spectrometer imaging. We concentrate on the high Halpha surface brightnesses, large line widths, line ratios, and the clumpy nature of these galaxies. We show that photoionization calculations and emission line diagnostics imply gas pressures and densities that are similar to the most intense nearby star-forming regions at z = 0 but over much larger scales (10-20 kpc). A relationship between surface brightness and velocity dispersion can be explained through simple energy injection arguments and a scaling set by nearby galaxies with no free parameters. The high velocity dispersions are a natural consequence of intense star formation thus regions of high velocity dispersion are not evidence for mass concentrations such as bulges or rings. External mechanisms such as cosmological gas accretion generally do not have enough energy to sustain the high velocity dispersions. In some cases, the high pressures and low gas metallicites may make it difficult to robustly distinguish between active galactic nucleus ionization cones and star formation, as we show for BzK-15504 at z = 2.38. We construct a picture where the early stages of galaxy evolution are driven by self-gravity which powers strong turbulence until the velocity dispersion is high. Then massive, dense, gas-rich clumps collapse, triggering star formation with high efficiencies and intensities as observed. At this stage, the intense star formation is likely self-regulated by the mechanical energy output of massive stars.
The apparent correlation between the specific star formation rate (sSFR) and total stellar mass (M⋆) of galaxies is a fundamental relationship indicating how they formed their stellar populations. To ...attempt to understand this relation, we hypothesize that the relation and its evolution is regulated by the increase in the stellar and gas mass surface density in galaxies with redshift, which is itself governed by the angular momentum of the accreted gas, the amount of available gas, and by self-regulation of star formation. With our model, we can reproduce the specific SFR − M⋆ relations at z ~ 1–2 by assuming gas fractions and gas mass surface densities similar to those observed for z = 1–2 galaxies. We further argue that it is the increasing angular momentum with cosmic time that causes a decrease in the surface density of accreted gas. The gas mass surface densities in galaxies are controlled by the centrifugal support (i.e., angular momentum), and the sSFR is predicted to increase as, sSFR(z) = (1 + z)3/tH0, as observed (where tH0 is the Hubble time and no free parameters are necessary). In addition, the simple evolution for the star-formation intensity we propose is in agreement with observations of Milky Way-like galaxies selected through abundance matching. At z ≳ 2, we argue that star formation is self-regulated by high pressures generated by the intense star formation itself. The star formation intensity must be high enough to either balance the hydrostatic pressure (a rather extreme assumption) or to generate high turbulent pressure in the molecular medium which maintains galaxies near the line of instability (i.e. Toomre Q ~ 1). We provide simple prescriptions for understanding these self-regulation mechanisms based on solid relationships verified through extensive study. In all cases, the most important factor is the increase in stellar and gas mass surface density with redshift, which allows distant galaxies to maintain high levels of sSFR. Without a strong feedback from massive stars, such galaxies would likely reach very high sSFR levels, have high star formation efficiencies, and because strong feedback drives outflows, ultimately have an excess of stellar baryons.
We have analyzed the properties of the Hα and Niiλ6583 rest-frame optical emission lines of a sample of 53 intensely star forming galaxies at z = 1.3 to 2.7 observed with SINFONI on the ESO-VLT. ...Similar to previous authors, we find large velocity dispersions in the lines, σ = few 10−250 km s-1. Our data agree well with simulations where we applied beam-smearing and assumed a scaling relation of the form: velocity dispersion is proportional to the square root of the star-formation intensity (star-formation rate per unit surface area). We conclude that the dispersions are primarily driven by star formation. To explain the high surface brightness and optical line ratios, high thermal pressures in the warm ionized medium, WIM, are required (P/k ~ > 106−107 K cm-3). Such thermal pressures in the WIM are similar to those observed in nearby starburst galaxies, but occur over much larger physical scales. Moreover, the relatively low ionization parameters necessary to fit the high surface brightnesses and optical line ratios suggest that the gas is not only directly associated with regions of star formation, but is wide spread throughout the general interstellar medium (ISM). Thus the optical emission line gas is a tracer of the large scale dynamics of the bulk of the ISM. We present a simple model for the energy input from young stars in an accreting galaxy, to argue that the intense star-formation is supporting high turbulent pressure, which roughly balances the gravitational pressure and thus enables distant gas accreting disks to maintain a Toomre disk instability parameter Q ~ 1. For a star formation efficiency of 3%, only 5−15% of the mechanical energy from young stars that is deposited in the ISM is needed to support the level of turbulence required for maintaining this balance. Since this balance is maintained by energy injected into the ISM by the young stars themselves, this suggests that star formation in high redshift galaxies is self-regulating.
We investigate the properties of a clump-cluster galaxy at redshift 1.57. In optical observations, the morphology of this galaxy is dominated by eight star-forming clumps, and its photometric ...properties are typical of most clump-cluster and chain galaxies. Its complex asymmetrical morphology has led to the suggestion that this system is a group merger of several initially separate proto-galaxies. We performed H\alpha integral field spectroscopy of this system using SINFONI on VLT UT4. These observations reveal a large-scale velocity gradient throughout the system, but with large local kinematic disturbances. Using a numerical model of gas-rich disk fragmentation, we find that clump interactions and migration can explain the observed disturbed rotation. On the other hand, the global rotation would not be expected for a multiply merging system. We also find that this system follows the relations of stellar mass versus metallicity, star formation rate, and size that are expected for a disk at this redshift. Furthermore, the galaxy exhibits a disk-like radial metallicity gradient. A formation scenario of internal disk fragmentation is therefore the most likely one. A red and metallic central concentration appears to be a bulge in this proto-spiral clumpy galaxy. A chain galaxy at redshift 2.07 in the same field also shows disk-like rotation. Such systems are likely progenitors of present-day bright spiral galaxies, which shape their exponential disks through clump migration and disruption, a process that in turn fuels their bulges. Our results show that disturbed morphologies and kinematics are not necessarily signs of galaxy mergers and interactions, but may instead be produced by the internal evolution of primordial disks.
An important property of star-forming galaxies at z ~ 1−2 is the high local star-formation intensities they maintain over tens of kiloparsecs at levels that are only observed in the nearby Universe ...in the most powerful nuclear starbursts. To investigate how these high star-formation intensities affect the warm ionized medium, we present an analysis of the average spectra of about 50 such galaxies at z ≈ 1.2−2.6 and of subsamples selected according to their local and global star-formation intensity. Stacking allows us to probe relatively weak lines like Siiλλ6716,6731 and Oiλ6300, which are tracers of the conditions of the ISM and are undetectable in most individual targets. We find higher gas densities (hence pressures) in intensely star-forming regions compared to fainter diffuse gas and, overall, values that are comparable to starburst regions and the diffuse ISM in nearby galaxies. By modeling the Hα surface brightnesses and Sii/Hα line ratios with the Cloudy photoionization code, we find that our galaxies continue trends observed in local galaxies, where gas pressures scale with star-formation intensity. We discuss these results in the context of models of self-regulated star formation, where star formation determines the average thermal and turbulent pressure in the ISM, which in turn determines the rate at which stars can form, finding good agreement with our data. We also confirm the detection of broad, faint lines underlying Hα and Nii, which have previously been considered evidence of either outflows or active galactic nuclei. Finding that the broad component is only significantly detected in stacks with the highest average local and global star-formation intensities strongly supports the outflow interpretation, and further emphasizes the importance of star-formation feedback and self-regulation in the early Universe.
In previous studies, it has been shown that the large line widths observed in high surface brightness Hα emitters at low and high redshifts are likely due to the mechanical energy injected by intense ...star formation. Here we discuss the possibility that the high surface brightnesses observed are not due to star formation, but due to cosmological gas accretion. We assume that all of the accretion energy is dissipated as shocks from the accreting gas. We show that in order to explain the high surface brightnesses both the mass accretion rate and energy would have to be much higher than expected from simulations or from equating the star formation with the accretion rate. We also investigate scaling relations between the surface brightness expected from accretion and for star formation through mechanical heating and photo-ionization, trying to identify a regime where such accretion may become evident in galaxies. Unfortunately, the surface brightness necessary to detect the gas in optical line emission is about an order of magnitude lower than what has currently been achieved with near-infrared observations of distant galaxies.
We analyze the physical conditions in the interstellar gas of 11 actively star-forming galaxies at z{approx}2, based on integral-field spectroscopy from the ESO-VLT and Hubble Space Telescope/Near ...Infrared Camera and Multi-Object Spectrometer imaging. We concentrate on the high H{alpha} surface brightnesses, large line widths, line ratios, and the clumpy nature of these galaxies. We show that photoionization calculations and emission line diagnostics imply gas pressures and densities that are similar to the most intense nearby star-forming regions at z = 0 but over much larger scales (10-20 kpc). A relationship between surface brightness and velocity dispersion can be explained through simple energy injection arguments and a scaling set by nearby galaxies with no free parameters. The high velocity dispersions are a natural consequence of intense star formation thus regions of high velocity dispersion are not evidence for mass concentrations such as bulges or rings. External mechanisms such as cosmological gas accretion generally do not have enough energy to sustain the high velocity dispersions. In some cases, the high pressures and low gas metallicites may make it difficult to robustly distinguish between active galactic nucleus ionization cones and star formation, as we show for BzK-15504 at z = 2.38. We construct a picture where the early stages of galaxy evolution are driven by self-gravity which powers strong turbulence until the velocity dispersion is high. Then massive, dense, gas-rich clumps collapse, triggering star formation with high efficiencies and intensities as observed. At this stage, the intense star formation is likely self-regulated by the mechanical energy output of massive stars.
We investigate the properties of a clump-cluster galaxy at redshift 1.57. In optical observations, the morphology of this galaxy is dominated by eight star-forming clumps, and its photometric ...properties are typical of most clump-cluster and chain galaxies. Its complex asymmetrical morphology has led to the suggestion that this system is a group merger of several initially separate proto-galaxies. We performed Hα integral field spectroscopy of this system using SINFONI on VLT UT4. These observations reveal a large-scale velocity gradient throughout the system, but with large local kinematic disturbances. Using a numerical model of gas-rich disk fragmentation, we find that clump interactions and migration can explain the observed disturbed rotation. On the other hand, the global rotation would not be expected for a multiply merging system. We also find that this system follows the relations of stellar mass versus metallicity, star formation rate, and size that are expected for a disk at this redshift. Furthermore, the galaxy exhibits a disk-like radial metallicity gradient. A formation scenario of internal disk fragmentation is therefore the most likely one. A red and metallic central concentration appears to be a bulge in this proto-spiral clumpy galaxy. A chain galaxy at redshift 2.07 in the same field also shows disk-like rotation. Such systems are likely progenitors of present-day bright spiral galaxies, which shape their exponential disks through clump migration and disruption, a process that in turn fuels their bulges. Our results show that disturbed morphologies and kinematics are not necessarily signs of galaxy mergers and interactions, but may instead be produced by the internal evolution of primordial disks.
The apparent correlation between the specific star formation rate (sSFR) and total stellar mass (M_star) of galaxies is a fundamental relationship indicating how they formed their stellar ...populations. To attempt to understand this relation, we hypothesize that the relation and its evolution is regulated by the increase in the stellar and gas mass surface density in galaxies with redshift, which is itself governed by the angular momentum of the accreted gas, the amount of available gas, and by self-regulation of star formation. With our model, we can reproduce the specific SFR-M_star relations at z~1-2 by assuming gas fractions and gas mass surface densities similar to those observed for z=1-2 galaxies. We further argue that it is the increasing angular momentum with cosmic time that causes a decrease in the surface density of accreted gas. The gas mass surface densities in galaxies are controlled by the centrifugal support (i.e., angular momentum), and the sSFR is predicted to increase as, sSFR(z)=(1+z)^3/t_H0, as observed (where t_H0 is the Hubble time and no free parameters are necessary). At z>~2, we argue that star formation is self-regulated by high pressures generated by the intense star formation itself. The star formation intensity must be high enough to either balance the hydrostatic pressure (a rather extreme assumption) or to generate high turbulent pressure in the molecular medium which maintains galaxies near the line of instability (i.e. Toomre Q~1). The most important factor is the increase in stellar and gas mass surface density with redshift, which allows distant galaxies to maintain high levels of sSFR. Without a strong feedback from massive stars, such galaxies would likely reach very high sSFR levels, have high star formation efficiencies, and because strong feedback drives outflows, ultimately have an excess of stellar baryons (abridged).
(abridged) We have analyzed the properties of the rest-frame optical emission lines of a sample of 53 intensely star forming galaxies at z=1.3 to 2.7 observed with SINFONI on the ESO-VLT. We find ...large velocity dispersions in the lines, sigma=30-250 km/s. Our data agree well with simulations where we applied beam-smearing and assumed a scaling relation of the form: velocity dispersion is proportional to the square root of the star-formation intensity (star-formation rate per unit area). We conclude that the dispersions are primarily driven by star formation. To explain the high surface brightness and optical line ratios, high thermal pressures in the warm ionized medium, WIM, are required (log P/k (K/cm^3)>~6-7). Such thermal pressures in the WIM are similar to those observed in nearby starburst galaxies, but occur over much larger physical scales. Moreover, the relatively low ionization parameters necessary to fit the high surface brightnesses and optical line ratios suggest that the gas is not only directly associated with regions of star formation, but is wide spread throughout the general ISM. Thus the optical emission line gas is a tracer of the large scale dynamics of the bulk of the ISM. We present a simple model for the energy input from young stars in an accreting galaxy, to argue that the intense star-formation is supporting high turbulent pressure, which roughly balances the gravitational pressure and thus enables distant gas accreting disks to maintain a Toomre disk instability parameter Q~1. For a star formation efficiency of 3%, only 5-15% of the mechanical energy from young stars that is deposited in the ISM is needed to support the level of turbulence required for maintaining this balance. Since this balance is maintained by energy injected into the ISM by the young stars themselves, this suggests that star formation in high redshift galaxies is self-regulating.
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