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Register a new userGoulds Belt to Starburst Galaxies: The IMF of Extreme Star Formation
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Recent results indicate the stellar initial mass function is not a strong function of star-forming environment or ``initial conditions (e.g. Meyer et al. 2000). Some studies suggest that a universal IMF may extend to sub-stellar masses (see however Briceno et al. 2002). Yet most of this work is confined to star-forming environments within 1 kpc of the Sun. In order to probe the universality of the IMF over a wider range of parameter space (metalicity, ambient pressure, magnetic field strength) new techniques are required. We begin by summarizing our approach to deriving the sub-stellar IMF down to the opacity-limit for fragmentation using NGC 1333 as an example. Next, we describe results from simulations using the observed point-spread function of the new 6.5m MMT adaptive optics system and examine the confusion-limited sensitivity to low mass stars in rich star-forming clusters out to 0.5 Mpc. We also present preliminary results from observations with this system of the W51 star-forming complex. Finally, we outline a new technique to estimate the ratio of high to low mass stars in unresolved stellar populations, such as the massive star clusters observed in interacting galaxies (e.g. Mengel et al. 2002). While evidence for variations in the IMF remains inconclusive, new studies are required to rule them out and determine whether or not the IMF is universal over the range of parameter space relevant to star-forming galaxies over cosmic time.
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Very Long Baseline Interferometry (VLBI) observations can provide the position of compact radio sources with an accuracy of order 50 micro-arcseconds. This is sufficient to measure the trigonometric parallax and proper motions of any object within 500 pc of the Sun to better than a few percent. Because they are magnetically active, young stars are often associated with compact radio emission detectable using VLBI techniques. Here we will show how VLBI observations have already constrained the distance to the most often studied nearby regions of star-formation (Taurus, Ophiuchus, Orion, etc.) and have started to provide information on their internal structure and kinematics. We will then briefly describe a large project (called The Goulds Belt Distance Survey) designed to provide a detailed view of star-formation in the Solar neighborhood using VLBI observations.
Very Long Baseline Interferometry (VLBI) observations can provide the position of compact radio sources with an accuracy of order 50 micro-arcseconds. This is sufficient to measure the trigonometric parallax and proper motions of any object within 500 pc of the Sun to better than a few percent. Because they are magnetically active, young stars are often associated with compact radio emission detectable using VLBI techniques. Here we will show how VLBI observations have already constrained the distance to the most often studied nearby regions of star-formation (Taurus, Ophiuchus, Orion, etc.) and have started to provide information on their internal structure and kinematics. We will then briefly describe a large project (called The Goulds Belt Distances Survey) designed to provide a detailed view of star-formation in the Solar neighborhood using VLBI observations.
The frequency and properties of multiple star systems offer powerful tests of star formation models. Multiplicity surveys over the past decade have shown that binary properties vary strongly with mass, but the functional forms and the interplay between frequency and semimajor axis remain largely unconstrained. We present the results of a large-scale survey of multiplicity at the bottom of the IMF in several nearby young associations, encompassing 78 very low mass members observed with Keck laser guide star adaptive optics. Our survey confirms the overall trend observed in the field for lower-mass binary systems to be less frequent and more compact, including a null detection for any substellar binary systems with separations wider than ~7 AU. Combined with a Bayesian re-analysis of existing surveys, our results demonstrate that the binary frequency and binary separations decline smoothly between masses of 0.5 Msun and 0.02 Msun, though we can not distinguish the functional form of this decline due to a degeneracy between the total binary frequency and the mean binary separation. We also show that the mass ratio distribution becomes progressively more concentrated at q~1 for declining masses, though a small number of systems appear to have unusually wide separations and low mass ratios for their mass. Finally, we compare our results to synthetic binary populations generated by smoothed particle hydrodynamic simulations, noting the similarities and discussing possible explanations for the differences.
We use two catalogues, a Herschel catalogue selected at 500 mu (HerMES) and an IRAS catalogue selected at 60 mu (RIFSCz), to contrast the sky at these two wavelengths. Both surveys demonstrate the existence of extreme starbursts, with star-formation rates (SFRs) > 5000 Msun/yr. The maximum intrinsic star-formation rate appears to be ~30,000 Msun/yr. The sources with apparent SFR estimates higher than this are in all cases either lensed systems, blazars, or erroneous photometric redshifts. At redshifts of 3 to 5, the time-scale for the Herschel galaxies to make their current mass of stars at their present rate of formation ~ 10^8 yrs, so these galaxies are making a significant fraction of their stars in the current star-formation episode. Using dust mass as a proxy for gas mass, the Herschel galaxies at redshift 3 to 5 have gas masses comparable to their mass in stars. Of the 38 extreme starbursts in our Herschel survey for which we have more complete SED information, over 50% show evidence for QSO-like optical emission, or exhibit AGN dust tori in the mid-infrared SEDs. In all cases however the infrared luminosity is dominated by a starburst component. We derive a mean covering factor for AGN dust as a function of redshift and derive black hole masses and black hole accretion rates. There is a universal ratio of black-hole mass to stellar mass, ~ 10^{-3}, driven by the strong period of star-formation and black-hole growth at z = 1-5.
I present a model for the star formation properties of z~2 starburst galaxies. Here, I discuss models for the formation of high-z Submillimeter Galaxies, as well as the CO-H2 conversion factor for these systems. I then apply these models to literature observations. I show that when using a functional form for XCO that varies smoothly with the physical properties in galaxies, galaxies at both local and high-z lie on a unimodal Kennicutt-Schmidt star formation law, with power-law index of ~2. The inferred gas fractions of these galaxies are large (fgas ~ 0.2-0.4), though a factor ~2 lower than most literature estimates that utilize locally-calibrated CO-H2 conversion factors.
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