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Register a new userOn the Elevation and Suppression of Star Formation within Galaxies
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To understand star formation in galaxies, we investigate the star formation rate (SFR) surface density ($Sigma_{rm SFR}$) profiles for galaxies, based on a well-defined sample of 976 star-forming MaNGA galaxies. We find that the typical $Sigma_{rm SFR}$ profiles within 1.5Re of normal SF galaxies can be well described by an exponential function for different stellar mass intervals, while the sSFR profile shows positive gradients, especially for more massive SF galaxies. This is due to the more pronounced central cores or bulges rather than the onset of a `quenching process. While galaxies that lie significantly above (or below) the star formation main sequence (SFMS) show overall an elevation (or suppression) of $Sigma_{rm SFR}$ at all radii, this central elevation (or suppression) is more pronounced in more massive galaxies. The degree of central enhancement and suppression is quite symmetric, suggesting that both the elevation and suppression of star formation are following the same physical processes. Furthermore, we find that the dispersion in $Sigma_{rm SFR}$ within and across the population is found to be tightly correlated with the inferred gas depletion time, whether based on the stellar surface mass density or the orbital dynamical time. This suggests that we are seeing the response of a simple gas-regulator system to variations in the accretion rate. This is explored using a heuristic model that can quantitatively explain the dependence of $sigma(Sigma_{rm SFR})$ on gas depletion timescale. Variations in accretion rate are progressively more damped out in regions of low star-formation efficiency leading to a reduced amplitude of variations in star-formation.
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NGC1266 is a nearby lenticular galaxy that harbors a massive outflow of molecular gas powered by the mechanical energy of an active galactic nucleus (AGN). It has been speculated that such outflows hinder star formation (SF) in their host galaxies, providing a form of feedback to the process of galaxy formation. Previous studies, however, indicated that only jets from extremely rare, high power quasars or radio galaxies could impart significant feedback on their hosts. Here we present detailed observations of the gas and dust continuum of NGC1266 at millimeter wavelengths. Our observations show that molecular gas is being driven out of the nuclear region at $dot{M}_{rm out} approx 110 M_odot$ yr$^{-1}$, of which the vast majority cannot escape the nucleus. Only 2 $M_odot$ yr$^{-1}$ is actually capable of escaping the galaxy. Most of the molecular gas that remains is very inefficient at forming stars. The far-infrared emission is dominated by an ultra-compact ($lesssim50$pc) source that could either be powered by an AGN or by an ultra-compact starburst. The ratio of the SF surface density ($Sigma_{rm SFR}$) to the gas surface density ($Sigma_{rm H_2}$) indicates that SF is suppressed by a factor of $approx 50$ compared to normal star-forming galaxies if all gas is forming stars, and $approx$150 for the outskirt (98%) dense molecular gas if the central region is is powered by an ultra-compact starburst. The AGN-driven bulk outflow could account for this extreme suppression by hindering the fragmentation and gravitational collapse necessary to form stars through a process of turbulent injection. This result suggests that even relatively common, low-power AGNs are able to alter the evolution of their host galaxies as their black holes grow onto the M-$sigma$ relation.
We present CO(1-0) maps of 12 warm H$_2$-selected Hickson Compact Groups (HCGs), covering 14 individually imaged warm H$_2$ bright galaxies, with CARMA. We found a variety of molecular gas distributions within the HCGs, including regularly rotating disks, bars, rings, tidal tails, and possibly nuclear outflows, though the molecular gas morphologies are more consistent with spirals and early-type galaxies than mergers and interacting systems. Our CO-imaged HCG galaxies show star formation suppression of $langle$S$rangle$=10$pm$5, distributed bimodally, with five objects exhibiting suppressions of S$gtrsim$10 and depletion timescales $gtrsim$10Gyr. This star formation inefficiency is also seen in the efficiency per freefall time. We investigate the gas-to-dust ratios of these galaxies to determine if an incorrect conversion caused the apparent suppression and find that HCGs have normal ratios. It is likely that the cause of the suppression in these objects is associated with shocks injecting turbulence into the molecular gas. Galaxies with high star formation suppression (S$gtrsim$10) also appear to be those in the most advanced stages of transition across optical and infrared color space. This supports the idea that some galaxies in HCGs are transitioning objects, where a disruption of the existing molecular gas in the system suppresses star formation by inhibiting the molecular gas from collapsing and forming stars efficiently. These observations, combined with recent work on poststarburst galaxies with molecular reservoirs, indicates that galaxies do not need to expel their molecular reservoirs prior to quenching star formation and transitioning from blue spirals to red early-type galaxies. This may imply that star formation quenching can occur without the need to starve a galaxy of cold gas first.
We develop a simple analytical criterion to investigate the role of the environment on the onset of star formation. We will consider the main external agents that influence the star formation (i.e. ram pressure, tidal interaction, Rayleigh-Taylor and Kelvin-Helmholtz instabilities) in a spherical galaxy moving through an external environment. The theoretical framework developed here has direct applications to the cases of dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy. We develop an analytic formalism to solve the fluid dynamics equations in a non-inertial reference frame mapped with spherical coordinates. The two-fluids instability at the interface between a stellar system and its surrounding hotter and less dense environment is related to the star formation processes through a set of differential equations. The solution presented here is quite general, allowing us to investigate most kinds of orbits allowed in a gravitationally bound system of stars in interaction with a major massive companion. We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system (as a dwarf galaxy or a globular cluster) on its surrounding environment useful in theoretical interpretations of numerical results as well as observational applications. We show how spherical coordinates naturally enlighten the interpretation of the two-fluids instability in a geometry that directly applies to astrophysical case. This criterion predicts the threshold value for the onset of star formation in a mass vs. size space for any orbit of interest. Moreover, we show for the first time the theoretical dependencies of the different instability phenomena acting on a system in a fully analytical way.
The interstellar medium is a key ingredient that governs star formation in galaxies. We present a detailed study of the infrared (~ 1-500 micron) spectral energy distributions of a large sample of 193 nearby (z ~ 0.088) luminous infrared galaxies (LIRGs) covering a wide range of evolutionary stages along the merger sequence. The entire sample has been observed uniformly by 2MASS, WISE, Spitzer, and Herschel. We perform multi-component decomposition of the spectra to derive physical parameters of the interstellar medium, including the intensity of the interstellar radiation field and the mass and luminosity of the dust. We also constrain the presence and strength of nuclear dust heated by active galactic nuclei. The radiation field of LIRGs tends to have much higher intensity than in quiescent galaxies, and it increases toward advanced merger stages as a result of central concentration of the interstellar medium and star formation. The total gas mass is derived from the dust mass and the galaxy stellar mass. We find that the gas fraction of LIRGs is on average ~ 0.3 dex higher than that of main-sequence star-forming galaxies, rising moderately toward advanced merger stages. All LIRGs have star formation rates that place them above the galaxy star formation main sequence. Consistent with recent observations and numerical simulations, the global star formation efficiency of the sample spans a wide range, filling the gap between normal star-forming galaxies and extreme starburst systems.
Using data obtained for twelve galaxies as part of the {it Spitzer} Infrared Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope (WSRT)-SINGS radio continuum survey, we study how star formation activity affects the far-infrared (FIR)--radio correlation {it within} galaxies by testing a phenomenological model, which describes the radio image as a smeared version of the FIR image. The physical basis of this description is that cosmic-ray (CR) electrons will diffuse measurably farther than the mean free path of dust-heating photons before decaying by synchrotron radiation. This description works well in general. Galaxies with higher infrared surface brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs, substantially shorter than those for lower surface brightness galaxies. We interpret this result to suggest that galaxies with higher disk averaged star formation rates have had a recent episode of enhanced star formation and are characterized by a higher fraction of young CR electrons that have traveled only a few hundred parsecs from their acceleration sites in supernova remnants compared to galaxies with lower star formation activity.
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