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Register a new userConstraining the IMF in Extreme Environments: Detecting Young Low Mass Stars in Unresolved Starbursts
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We demonstrate the feasibility of detecting directly low mass stars in unresolved super-star clusters with ages < 10 Myr using near-infrared spectroscopy at modest resolution (R ~ 1000). Such measurements could constrain the ratio of high to low mass stars in these extreme star-forming events, providing a direct test on the universal nature of the initial mass function (IMF) compared to the disk of the Milky Way (Chabrier, 2003). We compute the integrated light of super-star clusters with masses of 10^6 Msun drawn from the Salpeter (1955) and Chabrier (2003) IMFs for clusters aged 1, 3, and 10 Myr. We combine, for the first time, results from Starburst99 (Leitherer et al. 1999) for the main sequence and post-main sequence population (including nebular emission) with pre-main sequence (PMS) evolutionary models (Siess et al. 2000) for the low mass stars as a function of age. We show that ~ 4-12 % of the integrated light observed at 2.2 microns comes from low mass PMS stars with late-type stellar absorption features at ages < 3 Myr. This light is discernable using high signal-to-noise spectra (> 100) at R=1000 placing constraints on the ratio of high to low mass stars contributing to the integrated light of the cluster.
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We investigate the low-mass population of the young cluster IC348 down to the deuterium-burning limit, a fiducial boundary between brown dwarf and planetary mass objects, using a new and innovative method for the spectral classification of late-type objects. Using photometric indices, constructed from HST/NICMOS narrow-band imaging, that measure the strength of the 1.9 micron water band, we determine the spectral type and reddening for every M-type star in the field, thereby separating cluster members from the interloper population. Due to the efficiency of our spectral classification technique, our study is complete from approx 0.7 Msun to 0.015 Msun. The mass function derived for the cluster in this interval, dN/dlogM propto M^{0.5}, is similar to that obtained for the Pleiades, but appears significantly more abundant in brown dwarfs than the mass function for companions to nearby sun-like stars. This provides compelling observational evidence for different formation and evolutionary histories for substellar objects formed in isolation vs. as companions. Because our determination of the IMF is complete to very low masses, we can place interesting constraints on the role of physical processes such as fragmentation in the star and planet formation process and the fraction of dark matter in the Galactic halo that resides in substellar objects.
We investigate the physical and chemical conditions necessary for low-mass star formation in extragalactic environments by calculating various characteristic timescales associated with star formation for a range of initial conditions. The balance of these timescales indicates whether low-mass star formation is enhanced or inhibited under certain physical conditions. In this study, we consider timescales for free-fall, cooling, freeze-out, desorption, chemistry and ambipolar diffusion and their variations with changes in the gas density, metallicity, cosmic ray ionisation rate and FUV radiation field strength. We find that extragalactic systems with high FUV radiation field strengths and high cosmic ray fluxes considered at a range of metallicities, are likely to have enhanced low-mass star formation unless the magnetic pressure is sufficient to halt collapse. Our results indicate that this is only likely to be the case for high-redshift galaxies approaching solar metallicities. Unless this is true for all high-redshift sources, this study finds little evidence for a high-mass biased IMF at high redshifts.
Strong bursts of star formation in galaxies may be triggered either by internal or external mechanisms. We study the distribution and kinematics of the HI gas in the outer regions of 18 nearby starburst dwarf galaxies, that have accurate star-formation histories from HST observations of resolved stellar populations. We find that starburst dwarfs show a variety of HI morphologies, ranging from heavily disturbed HI distributions with major asymmetries, long filaments, and/or HI-stellar offsets, to lopsided HI distributions with minor asymmetries. We quantify the outer HI asymmetry for both our sample and a control sample of typical dwarf irregulars. Starburst dwarfs have more asymmetric outer HI morphologies than typical irregulars, suggesting that some external mechanism triggered the starburst. Moreover, galaxies hosting an old burst (>100 Myr) have more symmetric HI morphologies than galaxies hosting a young one (<100 Myr), indicating that the former ones probably had enough time to regularize their outer HI distribution since the onset of the burst. We also investigate the nearby environment of these starburst dwarfs and find that most of them ($sim$80$%$) have at least one potential perturber at a projected distance <200 kpc. Our results suggest that the starburst is triggered either by past interactions/mergers between gas-rich dwarfs or by direct gas infall from the IGM.
We derive a semi-empirical galactic initial mass function (IMF) from observational constraints. We assume that the star formation rate in a galaxy can be expressed as the product of the IMF, $psi (m)$, which is a smooth function of mass $m$ (in units of msun), and a time- and space-dependent total rate of star formation per unit area of galactic disk. The mass dependence of the proposed IMF is determined by five parameters: the low-mass slope $gamma$, the high-mass slope $-Gamma$, the characteristic mass $m_{ch}$ (which is close to the mass $m_{rm peak}$ at which the IMF turns over), and the lower and upper limits on the mass, $m_l$ (taken to be 0.004) and $m_u$ (taken to be 120). The star formation rate in terms of number of stars per unit area of galactic disk per unit logarithmic mass interval, is proportional to $m^{-Gamma} left{1-expleft[{-(m/m_{ch})^{gamma +Gamma}}right]right}$, where $cal N_*$ is the number of stars, $m_l<m<m_u$ is the range of stellar masses. The values of $gamma$ and $emch$ are derived from two integral constraints: i) the ratio of the number density of stars in the range $m=0.1-0.6$ to that in the range $m=0.6-0.8$ as inferred from the mass distribution of field stars in the local neighborhood, and ii) the ratio of the number of stars in the range $m=0.08 - 1$ to the number of brown dwarfs in the range $m=0.03-0.08$ in young clusters. The IMF satisfying the above constraints is characterized by the parameters $gamma=0.51$ and $emch=0.35$ (which corresponds to $m_{rm peak}=0.27$). This IMF agrees quite well with the Chabrier (2005) IMF for the entire mass range over which we have compared with data, but predicts significantly more stars with masses $< 0.03, M_odot$; we also compare with other IMFs in current use.
The Extreme starbursts in the local universe workshop was held at the Insituto de Astrofisica de Andalucia in Granada, Spain on 21-25 June 2010. Bearing in mind the advent of a new generation of facilities such as JWST, Herschel, ALMA, eVLA and eMerlin, the aim of the workshop was to bring together observers and theorists to review the latest results. The purpose of the workshop was to address the following issues: what are the main modes of triggering extreme starbursts in the local Universe? How efficiently are stars formed in extreme starbursts? What are the star formation histories of local starburst galaxies? How well do the theoretical simulations model the observations? What can we learn about starbursts in the distant Universe through studies of their local counterparts? How important is the role of extreme starbursts in the hierarchical assembly of galaxies? How are extreme starbursts related to the triggering of AGN in the nuclei of galaxies? Overall, 41 talks and 4 posters with their corresponding 10 minutes short talks were presented during the workshop. In addition, the workshop was designed with emphasis on discussions, and therefore, there were 6 discussion sessions of up to one hour during the workshop. Here is presented a summary of the purposes of the workshop as well as a compilation of the abstracts corresponding to each of the presentations. The summary and conclusions of the workshop along with a description of the future prospects by Sylvain Veilleux can be found in the last section of this document. A photo of the assistants is included.
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