Top-heavy Stellar Initial Mass Function Research Articles

Z∼ 4 Hα EMITTERS IN THE GREAT OBSERVATORIES ORIGINS DEEP SURVEY: TRACING THE DOMINANT MODE FOR GROWTH OF GALAXIES

We present evidence for unusually strong Halpha emission in galaxies with spectroscopic redshifts in the range of 3.8<z<5.0, over the Great Observatories Origins Deep Survey (GOODS) fields. Among 74 galaxies detected in the Spitzer IRAC 3.6 and 4.5um bands, more than 70% of the galaxies show clear excess at 3.6um compared to the expected flux density from stellar continuum only. We provide evidence that this 3.6um excess is due to Halpha emission redshifted into the 3.6um band, and classify these 3.6um excess galaxies to be Halpha emitter (HAE) candidates. The selection of HAE candidates using an excess in broad-band filters is sensitive to objects whose rest-frame H$\alpha$ equivalent width is larger than 350A, star formation rates of 20-500 Msun/yr. The Halpha-to-UV luminosity ratio of HAEs is on average larger than that of local starbursts. Possible reasons for such strong Halpha emission in these galaxies include different dust extinction properties, young stellar population age, continuous star formation history, low metallicity, and top-heavy stellar initial mass function. Although the correlation between UV slope beta and L(Ha)/L(UV) raises the possibility that HAEs prefer a dust extinction curve which is steeper in the UV, the most dominant factor that results in strong Halpha emission appears to be star formation history. The Halpha equivalent widths of HAEs are large despite their relatively old stellar population ages constrained by SED fitting, suggesting that at least 60% of HAEs produce stars at a constant rate. Under the assumption that the cold gas supply is sustained, HAEs are able to produce ~50% of the observed stellar mass density at z~3 that is caught in massive (M*>10^11 Msun) galaxies. This 'strong Halpha phase' of star formation plays a dominant role in early galaxy growth, being a feasible progenitors of massive red galaxies at lower redshifts.

Mass loss and expansion of ultra compact dwarf galaxies through gas expulsion and stellar evolution for top-heavy stellar initial mass functions

(abridged) The dynamical V-band mass-to-light ratios of ultra compact dwarf galaxies (UCDs) are higher than predicted by simple stellar population models with the canonical stellar initial mass function (IMF). One way to explain this finding is a top-heavy IMF, so that the unseen mass is provided by additional remnants of high-mass stars. A possible explanation for why the IMF in UCDs could be top-heavy while this is not the case in less massive stellar systems is that encounters between proto-stars and stars become probable in forming massive systems. However, the required number of additional stellar remnants proves to be rather high, which raises the question of how their progenitors would affect the early evolution of a UCD. We have therefore calculated the first 200 Myr of the evolution of the UCDs, using the particle-mesh code Superbox. It is assumed that the stellar populations of UCDs were created in an initial starburst, which implies heavy mass loss during the following approximately 40 Myr due to primordial gas expulsion and supernova explosions. We find at the end of the simulations for various initial conditions and (tabulated) mass-loss histories objects that roughly resemble UCDs. Thus, the existence of UCDs does not contradict the notion that their stellar populations formed rapidly and with a top-heavy IMF. We find tentative evidence that the UCDs may have had densities as high as 10^8 M_sun/pc^3 at birth.

THE WARM MOLECULAR GAS AROUND THE CLOVERLEAF QUASAR

We present the first broadband lambda = 1 mm spectrum toward the z=2.56 Cloverleaf Quasar, obtained with Z-Spec, a 1-mm grating spectrograph on the 10.4-meter Caltech Submillimeter Observatory. The 190-305 GHz observation band corresponds to rest-frame 272 to 444 microns, and we measure the dust continuum as well as all four transitions of carbon monoxide (CO) lying in this range. The power-law dust emission, F_nu = 14 mJy (nu/240GHz)^3.9 is consistent with the published continuum measurements. The CO J=6->5, J=8->7, and J=9->8 measurements are the first, and now provide the highest-J CO information in this source. Our measured CO intensities are very close to the previously-published interferometric measurements of J=7->6, and we use all available transitions and our 13CO upper limits to constrain the physical conditions in the Cloverleaf molecular gas disk. We find a large mass (2-50x10^9 Msun) of highly-excited gas with thermal pressure nT > 10^6 Kcm^-3. The ratio of the total CO cooling to the far-IR dust emission exceeds that in the local dusty galaxies, and we investigate the potential heating sources for this bulk of warm molecular gas. We conclude that both UV photons and X-rays likely contribute, and discuss implications for a top-heavy stellar initial mass function arising in the X-ray-irradiated starburst. Finally we present tentative identifications of other species in the spectrum, including a possible detection of the H20 2_0,2->1_1,1 transition at lambda_rest = 303 microns.

TRENDS IN MOLECULAR EMISSION FROM DIFFERENT EXTRAGALACTIC STELLAR INITIAL MASS FUNCTIONS

Banerji et al. (2009) suggested that top-heavy stellar Initial Mass Functions (IMFs) in galaxies may arise when the interstellar physical conditions inhibit low-mass star formation, and they determined the physical conditions under which this suppression may or may not occur. In this work, we explore the sensitivity of the chemistry of interstellar gas under a wide range of conditions. We use these results to predict the relative velocity-integrated antenna temperatures of the CO rotational spectrum for several models of high redshift active galaxies which may produce both top-heavy and unbiased IMFs. We find that while active galaxies with solar metallicity (and top-heavy IMFs) produce higher antenna temperatures than those with sub-solar metallicity (and unbiased IMFs) the actual rotational distribution is similar. The high-J to peak CO ratio however may be used to roughly infer the metallicity of a galaxy provided we know whether it is active or quiescent. The metallicity strongly influences the shape of the IMF. High order CO transitions are also found to provide a good diagnostic for high far-UV intensity and low metallicity counterparts of Milky Way type systems both of which show some evidence for having top-heavy IMFs. We also compute the relative abundances of molecules known to be effective tracers of high density gas in these galaxy models. We find that the molecules CO and CS may be used to distinguish between solar and sub-solar metallicity in active galaxies at high redshift whereas HCN, HNC and CN are found to be relatively insensitive to the IMF shape at the large visual magnitudes typically associated with extragalactic sources.

A top-heavy stellar initial mass function in starbursts as an explanation for the high mass-to-light ratios of ultra-compact dwarf galaxies

It has been shown recently that the dynamical V-band mass-to-light ratios of compact stellar systems with masses from 10^6 to 10^8 Solar masses are not consistent with the predictions from simple stellar population (SSP) models. Top-heavy stellar initial mass functions (IMFs) in these so-called ultra compact dwarf galaxies (UCDs) offer an attractive explanation for this finding, the stellar remnants and retained stellar envelopes providing the unseen mass. We therefore construct a model which quantifies by how much the IMFs of UCDs would have to deviate in the intermediate-mass and high-mass range from the canonical IMF in order to account for the enhanced M/L_V ratio of the UCDs. The deduced high-mass IMF in the UCDs depends on the age of the UCDs and the number of faint products of stellar evolution retained by them. Assuming that the IMF in the UCDs is a three-part power-law equal to the canonical IMF in the low-mass range and taking 20% as a plausible choice for the fraction of the remnants of high-mass stars retained by UCDs, the model suggests the exponent of the high-mass IMF to be approximately 1.6 if the UCDs are 13 Gyr old (i.e. almost as old as the Universe) or approximately 1.0 if the UCDs are 7 Gyr old, in contrast to 2.3 for the Salpeter-Massey IMF. If the IMF was as top-heavy as suggested here, the stability of the UCDs might have been threatened by heavy mass loss induced by the radiation and evolution of massive stars. The central densities of UCDs must have been in the range 10^6 to 10^7 Solar masses per cubic parsec when they formed with star formation rates of 10 to 100 Solar masses per year.

THE CHEMICAL ABUNDANCES IN THE GALACTIC CENTER FROM THE ATMOSPHERES OF RED SUPERGIANTS

The Galactic Centre (GC) has experienced a high degree of recent star-forming activity, as evidenced by the large number of massive stars currently residing there. The relative abundances of chemical elements in the GC may provide insights into the origins of this activity. Here, we present high-resolution $H$-band spectra of two Red Supergiants in the GC (IRS~7 and VR~5-7), and in combination with spectral synthesis we derive abundances for Fe and C, as well as other $\alpha$-elements Ca, Si, Mg Ti and O. We find that the C-depletion in VR~5-7 is consistent with the predictions of evolutionary models of RSGs, while the heavy depletion of C and O in IRS~7's atmosphere is indicative of deep mixing, possibly due to fast initial rotation and/or enhanced mass-loss. Our results indicate that the {\it current} surface Fe/H content of each star is slightly above Solar. However, comparisons to evolutionary models indicate that the {\it initial} Fe/H ratio was likely closer to Solar, and has been driven higher by H-depletion at the stars' surface. Overall, we find $\alpha$/Fe ratios for both stars which are consistent with the thin Galactic disk. These results are consistent with other chemical studies of the GC, given the precision to which abundances can currently be determined. We argue that the GC abundances are consistent with a scenario in which the recent star-forming activity in the GC was fuelled by either material travelling down the Bar from the inner disk, or from the winds of stars in the inner Bulge -- with no need to invoke top-heavy stellar Initial Mass Functions to explain anomalous abundance ratios.

A detailed study of the enigmatic cluster M82F

We present a detailed study of the stellar cluster M82F, using multiband high-resolution Hubble Space Telescope (HST) imaging and deep ground-based optical slit and integral field spectroscopy. Using the imaging, we create colour maps of the cluster and surrounding region in order to search for substructure. We find a large amount of substructure, which we interpret as the result of differential extinction across the projected face of the cluster. With this interpretation, we are able to construct a spatially resolved extinction map across the cluster which is used to derive the intrinsic flux distribution. Fitting cluster profiles (King and Elson, Fall & Freeman) to the intrinsic images, we find that the cluster is 15-30 per cent larger than previous estimates, and that no strong evidence of mass segregation in this cluster exists. Using the optical spectra, we find that the age of M82F is 60-80 Myr and from its velocity conclude that the cluster is not physically associated with a large H II region that it is projected upon, both in agreement with previous studies. The reconstructed integral field maps show that that majority of the line emission comes from a nearby H II region. The spatial dependence of the line widths (implying the presence of multiple components) measured corresponds to the extinction map derived from photometry, indicating that the gas/dust clouds responsible for the extinction are also partially ionized. Even with the wealth of observations presented here, we do not find a conclusive solution to the problem of the high light-to-mass ratio previously found for this cluster and its possible top-heavy stellar initial mass function.

Simulating galaxy clusters - I. Thermal and chemical properties of the intracluster medium

We have performed a series of N-body/hydrodynamical (TREESPH) simulations of clusters and groups of galaxies, selected from a cosmological volume within a Lambda cold dark matter (� CDM) framework: these objects have been resimulated at higher resolution to z = 0, in order to follow also the dynamical, thermal and chemical input on to the intracluster medium (ICM) from stellar populations within galaxies. The simulations include metallicitydependent radiative cooling, star formation according to different initial mass functions (IMFs), energy feedback as strong starburst-driven galactic superwinds, chemical evolution with noninstantaneous recycling of gas and heavy elements, effects of a metagalactic ultraviolet (UV) field and thermal conduction in the ICM. In this paper, the first in a series of three, we derive results, mainly at z = 0, on the temperature and entropy profiles of the ICM, its X-ray luminosity, the cluster cold components [cold fraction as well as mass-to-light ratio (MLR)] and the metal distribution between ICM and stars. In general, models with efficient superwinds (produced by the action of supernovae and, in some simulations, of active galactic nuclei (AGNs), along with a top-heavy stellar IMF, are able to reproduce fairly well the observed LX‐T relation, the entropy profiles and the cold fraction: both features are found to be needed in order to remove high-density and low-entropy cold gas at core scales, although additional alternative feedback mechanisms would still be required to prevent late-time central cooling flows, and subsequent overproduction of stars and heavy elements at the centre. Observed radial ICM temperature profiles can be matched, except for the gradual decline in temperature inside r ∼ 0.1Rvir. Metal enrichment of the ICM gives rise to somewhat steep inner iron gradients; yet, the global level of enrichment compares well to observational estimates when a top-heavy IMF is adopted, and after correcting for the stars formed at late times at the base of the cooling flows, the metal partition between stars and ICM gets into good agreement with observations. The overall abundance and profile of iron in the ICM is found essentially unchanged from z = 1 to present time. Finally, the α/Fe of the gas is found to increase steadily with radius, decreasing over time.

X-ray pre-ionization powered by accretion on the first black holes - I. A model for theWMAPpolarization measurement

In this paper we investigate the possibility that there is a first phase of partial ionization due to X-rays produced by black hole (BH) accretion in small-mass galaxies at redshifts 7 < z < 20. This is followed by complete reionization by stellar sources at z ≃ 7. This scenario is motivated by the large optical depth to Thomson scattering, τ e ≃ 0.17 ± 0.04, recently measured by the Wilkinson Microwave Anisotropy Probe (WMAP). However, it is also consistent with the observed Gunn-Peterson trough in the spectra of quasars at z ∼ 5-6. We use a semi-analytic code to explore models with different BH accretion histories and cosmological parameters. We find that 'pre-ionization' by X-rays can increase the intergalactic medium (IGM) optical depth from τ e 0.06 given by stellar sources only, to 0.1 < τ e < 0.2, if a fraction of baryons 10 -5 < ω ac < 10 -4 is accreted on to seed BHs produced in the collapse of low-metallicity, high-mass stars before z ≃ 15. To be effective, pre-ionization requires a non-negligible star formation in the first small-mass galaxies in which seed BHs are formed. By z ∼ 20-25 the IGM is reheated to 10 000 K and the ionization fraction is about 20 per cent. The increase of the IGM Jeans mass is effective in reducing star formation in the smaller mass haloes. Large values of τ e are obtained in models with top-heavy stellar initial mass function only if pair-instability supernovae (SNe) are not important. Seed BHs are assumed to accrete at near the Eddington limit with a duty cycle that decreases slowly with increasing time. Alternatively, a moderate fraction of the black holes must be ejected from the host galaxy or exist without merging into the supermassive BHs in galactic centres. The model predicts that dwarf spheroidal galaxies, if they are preserved fossils of the first galaxies, may host a mass in BHs that is 5-40 per cent of their stellar mass. The redshifted X-ray background produced by this early epoch of BH accretion constitutes about 5-10 per cent of the hard X-ray background in the 2-50 keV bands and roughly half of the currently estimated BH mass density was formed at early times. Moreover, in most models, the photons from the redshifted background are sufficient to fully reionize He II at redshift z ∼ 3 without any additional contribution from quasars at lower redshifts and the temperature of the mean density IGM remains close to 10 4 K down to redshift z ∼ 1.

Early enrichment of quasars by the first stars

Studies of the broad emission-line regions (BLRs) in quasars have revealed solar or higher enrichment levels up to the highest redshifts. In combination with the presence of large amounts of dust in QSOs at z∼ 6, this implies that substantial amounts of star formation and nucleosynthesis took place at significantly earlier epochs. Here, we examine whether a top-heavy stellar initial mass function (IMF) is indicated by current data, by modelling the contributions from different regions of the IMF, including Type Ia/II and pair instability supernovae, to the metal synthesis in BLRs. We find that, in order to reproduce the observations of roughly solar values of N/C and Fe/Mg in these objects, (i) stars with a present-day IMF are sufficient, regardless of their metallicity, (ii) zero-metallicity stars with a top-heavy IMF severely underproduce N/C, and (iii) the contribution of Type Ia SNe is not strongly required by the data. Therefore, stars of mass ∼1–40 M⊙ must have existed at z∼ 10–20, possibly coeval with any hypothesized stars of masses ≳ 100 M⊙ at these epochs. This is in agreement with the nucleosynthetic abundance pattern detected in extremely metal-poor stars in the galactic halo.

Reionization, chemical enrichment and seed black holes from the first stars: is Population III important?

We investigate the effects of a top-heavy stellar initial mass function on the reionization history of the intergalactic medium (IGM). We use cosmological simulations that include self-consistently the feedback from ionizing radiation, H 2 dissociating radiation and supernova (SN) explosions. We run a set of simulations to check the numerical convergence and the effect of mechanical energy input from SNe. In agreement with other studies, we find that it is difficult to reionize the IGM at z rei > 10 with stellar sources even after making extreme assumptions. If star formation in 10 9 M ○. galaxies is not suppressed by SN explosions, the optical depth to Thomson scattering is τ e ≤ 0.13. If we allow for the normal energy input from SNe or if pair-instability SNe are dominant, we find τ e ≤ 0.09. Assuming normal yields for the first stars (Population III), the mean metallicity of the IGM is already Z/Z ○. = 2 x 10 -3 (10 -3 260 M ○. , they collapse directly on to black holes without exploding as SNe. If metal-poor stars are initially important and if collapse to black holes is the typical outcome, then the secondary emission of ionizing radiation from accretion on SN-induced seed black holes might be more important than the primary emission. We also develop a semi-analytic code to study how τ e is sensitive to cosmological parameters, finding essentially the same results. Neglecting feedback effects, we find simple relationships for τ e as a function of the power spectrum spectral index and the emission efficiency of ionizing radiation for cold dark matter and warm dark matter cosmologies. Surprisingly, we estimate that a warm dark matter scenario (with particle mass of 1.25 keV) reduces τ e by only approximately 10 per cent.

Shedding Light on Baryonic Dark Matter

Halo dark matter, if it is baryonic, may plausibly consist of compact stellar remnants. Jeans mass clouds containing 10(6) to 10(8) solar masses could have efficiently formed stars in the early universe and could plausibly have generated, for a suitably top-heavy stellar initial mass function, a high abundance of neutron stars as well as a small admixture of long-lived low mass stars. Within the resulting clusters of dark remnants, which eventually are tidally disrupted when halos eventually form, captures of neutron stars by non-degenerate stars resulted in formation of close binaries. These evolve to produce, by the present epoch, an observable x-ray signal associated with dark matter aggregations in galaxy halos and galaxy cluster cores.