Star formation near an obscured AGN: Variations in the initial mass function

The conditions that affect the formation of stars in radiatively and mechanically active environments are quite different than the conditions that apply to our local interstellar neighborhood. In such galactic environments, a variety of feedback processes can play a significant role in shaping the initial mass function (IMF). Here, we present a numerical study on the effects of an accreting black hole and the influence of nearby massive stars to a collapsing, 800 M_sun, molecular cloud at 10 pc distance from the black hole. We parametrize and study radiative feedback effects of hard X-rays emanating from the black hole broad line region, increased cosmic ray rates due to supernovae in starbursts, and strong UV radiation produced by nearby massive stars. We also investigate the importance of shear from the supermassive, 10^6-10^8 M_sun, black hole as the star-forming cloud orbits around it. We find that thermal pressure from X-rays compresses the cloud, which induces a high star formation rate early on, but reduces the overall star formation efficiency to about 7% due to gas depletion by evaporation. We see that the turn-over mass of the IMF increases up to a factor of 2.3, M_turn = 1-1.5 M_sun, for the model with the highest X-ray flux (160 erg s^-1 cm^-2), while the high-mass slope of the IMF becomes Gamma > -1. This results in more high mass stars and a non-Salpeter IMF. Cosmic rays penetrate deeply into the cloud and increase the gas temperature (50-200 K), which leads to a reduced formation efficiency of low mass stars. High cosmic ray rates increase the average mass of stars, thereby shifting the turn-over mass to higher values, i.e., up to several solar masses. Due to this process, the onset of star formation is also delayed. We conclude that the IMF inside active galaxies is different than the one obtained from local environments.Comment: 25 pages, 17 figure

The impact of freeze-out on collapsing molecular clouds

Atoms and molecules, and in particular CO, are important coolants during the evolution of interstellar star-forming gas clouds. The presence of dust grains, which allow many chemical reactions to occur on their surfaces, strongly impacts the chemical composition of a cloud. At low temperatures, dust grains can lock-up species from the gas phase which freeze out and form ices. In this sense, dust can deplete important coolants. Our aim is to understand the effects of freeze-out on the thermal balance and the evolution of a gravitationally bound molecular cloud. For this purpose, we perform 3D hydrodynamical simulations with the adaptive mesh code FLASH. We simulate a gravitationally unstable cloud under two different conditions, with and without grain surface chemistry. We let the cloud evolve until one free-fall time is reached and track the thermal evolution and the abundances of species during this time. We see that at a number density of 10$^4$ cm$^{-3}$ most of the CO molecules are frozen on dust grains in the run with grain surface chemistry, thereby depriving the most important coolant. As a consequence, we find that the temperature of the gas rises up to $\sim$25 K. The temperature drops once again due to gas-grain collisional cooling when the density reaches a few$\times$10$^4$ cm$^{-3}$. We conclude that grain surface chemistry not only affects the chemical abundances in the gas phase, but also leaves a distinct imprint in the thermal evolution that impacts the fragmentation of a star-forming cloud. As a final step, we present the equation of state of a collapsing molecular cloud that has grain surface chemistry included.Comment: Increased the number of significant digits in EQ 2. It mattered. Accepted for publication in MNRAS letter

CO+ in M 82: A Consequence of Irradiation by X-rays

Based on its strong CO+ emission it is argued that the M 82 star-burst galaxy is exposed to a combination of FUV and X-ray radiation. The latter is likely to be the result of the star-burst superwind, which leads to diffuse thermal emission at ~0.7 keV, and a compact hard, 2-10 keV, source (but not an AGN). Although a photon-dominated region (FUV) component is clearly present in the nucleus of M 82, and capable of forming CO+, only X-ray irradiated gas of density 10^3-10^5 cm^-3 can reproduce the large, ~(1-4)x10^13 cm^-2, columns of CO+ that are observed toward the proto-typical star-burst M 82. The total X-ray luminosity produced by M 82 is weak, ~10^41 erg s^-1, but this is sufficient to drive the formation of CO+.Comment: added discussion on more recent X-ray observation

Gravoturbulent Star Formation: Effects of the Equation of State on Stellar Masses

Stars form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars and star clusters. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamic behavior of parental gas clouds, which governs when and where protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material to build up stars. The equation of state plays a pivotal role in the fragmentation process. Under typical cloud conditions, massive stars form as part of dense clusters following the "normal" mass function observed, e.g. in the solar neighborhood. However, for gas with an effective polytropic index greater than unity star formation becomes biased towards isolated massive stars. This is relevant for understanding the properties of zero-metallicity stars (Population III) or stars that form under extreme environmental conditions like in the Galactic center or in luminous starbursts.Comment: 9 pages, 4 figure, to be published in the Proceedings of the IAU Colloquium No. 227, 2005, "Massive Star Birth: A Crossroads of Astrophysics

Probing high-redshift quasars with ALMA. I. Expected observables and potential number of sources

(abridged) We explore how ALMA observations can probe high-redshift galaxies in unprecedented detail. We discuss the main observables that are excited by the large-scale starburst, and formulate expectations for the chemistry and the fluxes in the center of active galaxies, where chemistry may be driven by the absorption of X-ray photons. We show that such X-ray dominated regions (XDRs) should be large enough to be resolved with ALMA, and predict the expected amount of emission in CO and various fine-structure lines. We discuss how such XDRs can be distinguished from a strong starburst on the same spatial scales based on the CO line SED. Our models are compared to known sources like NGC 1068 and APM 08279. We also analyze the properties of the z=6.42 quasar SDSS J114816.64+525150.3, and find that the observed emission in CO, [CII] and [CI] requires a dense warm and a low-density cold gas component. We estimate the expected number of sources at redshifts higher than 6, finding that one could expect one black hole with $10^6$ solar masses per arcmin$^2$.Comment: 15 pages, 17 figures, accepted by A&

The nature of the ISM in galaxies during the star-formation activity peak of the Universe

We combine a semi-analytic model of galaxy formation, tracking atomic and molecular phases of cold gas, with a three-dimensional radiative-transfer and line tracing code to study the sub-mm emission from atomic and molecular species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that drives the formation of stars at the epoch of peak star formation (SF) in the Universe (z = 2.0) with that in local galaxies. We find that normal star-forming galaxies at high redshift have much higher CO-excitation peaks than their local counterparts and that CO cooling takes place at higher excitation levels. CO line ratios increase with redshift as a function of galaxy star-formation rate, but are well correlated with H2 surface density independent of redshift. We find an increase in the [OI]/[CII] line ratio in typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to counterparts at z = 0. Our model results suggest that typical star-forming galaxies at high redshift consist of much denser and warmer star-forming clouds than their local counterparts. Galaxies belonging to the tail of the SF activity peak at z = 1.2 are already less dense and cooler than counterparts during the actual peak of SF activity (z = 2.0). We use our results to discuss how future ALMA surveys can best confront our predictions and constrain models of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA

The formation of massive primordial stars in the presence of moderate UV backgrounds

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.Comment: Submitted to APJ, comments are welcome. High resolution copy is available at http://www.astro.physik.uni-goettingen.de/~mlatif/IMBHs_apj.pd

How realistic UV spectra and X-rays suppress the abundance of direct collapse black holes

Observations of high redshift quasars at $z>6$ indicate that they harbor supermassive black holes (SMBHs) of a billion solar masses. The direct collapse scenario has emerged as the most plausible way to assemble SMBHs. The nurseries for the direct collapse black holes are massive primordial halos illuminated with an intense UV flux emitted by population II (Pop II) stars. In this study, we compute the critical value of such a flux ($J_{21}^{\rm crit}$) for realistic spectra of Pop II stars through three-dimensional cosmological simulations. We derive the dependence of $J_{21}^{\rm crit}$ on the radiation spectra, on variations from halo to halo, and on the impact of X-ray ionization. Our findings show that the value of $J_{21}^{\rm crit}$ is a few times $\rm 10^4$ and only weakly depends on the adopted radiation spectra in the range between $T_{\rm rad}=2 \times 10^4-10^5$ K. For three simulated halos of a few times $\rm 10^{7}$~M$_{\odot}$, $J_{21}^{\rm crit}$ varies from $\rm 2 \times 10^4 - 5 \times 10^4$. The impact of X-ray ionization is almost negligible and within the expected scatter of $J_{21}^{\rm crit}$ for background fluxes of $J_{\rm X,21} \leq 0.1$. The computed estimates of $J_{21}^{\rm crit}$ have profound implications for the quasar abundance at $z=10$ as it lowers the number density of black holes forming through an isothermal direct collapse by a few orders of magnitude below the observed black holes density. However, the sites with moderate amounts of $\rm H_2$ cooling may still form massive objects sufficient to be compatible with observations.Comment: Accepted for publication in MNRAS, comments are welcom

Interstellar ices as witnesses of star formation: selective deuteration of water and organic molecules unveiled

Observations of star forming environments revealed that the abundances of some deuterated interstellar molecules are markedly larger than the cosmic D/H ratio of 10-5. Possible reasons for this pointed to grain surface chemistry. How- ever, organic molecules and water, which are both ice constituents, do not enjoy the same deuteration. For example, deuterated formaldehyde is very abundant in comets and star forming regions, while deuterated water rarely is. In this article, we explain this selective deuteration by following the formation of ices (using the rate equation method) in translucent clouds, as well as their evolu- tion as the cloud collapses to form a star. Ices start with the deposition of gas phase CO and O onto dust grains. While reaction of oxygen with atoms (H or D) or molecules (H2) yields H2O (HDO), CO only reacts with atoms (H and D) to form H2CO (HDCO, D2CO). As a result, the deuteration of formaldehyde is sensitive to the gas D/H ratio as the cloud undergoes gravitational collapse, while the deuteration of water strongly depends on the dust temperature at the time of ice formation. These results reproduce well the deuterium fractionation of formaldehyde observed in comets and star forming regions and can explain the wide spread of deuterium fractionation of water observed in these environments.Comment: 4 pages, 3 figures, Accepted in ApJ letter; Astrophysical Journal LET26536R1 201

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies: An ALMA mm-wave spectral scan of NGC 4418

We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418, spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined local thermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order to identify the molecular species and derive column densities and excitation temperatures. We derive molecular abundances and compare them with other Galactic and extragalactic sources by means of a principal component analysis. We detect 317 emission lines from a total of 45 molecular species, including 15 isotopic substitutions and six vibrationally excited variants. Our LTE/NLTE fit find kinetic temperatures from 20 to 350 K, and densities between 10$^5$ and 10$^7$ cm$^{-3}$. The spectrum is dominated by vibrationally excited HC$_3$N, HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find high abundances of HC$_3$N, SiO, H$_2$S, and c-HCCCH and a low CH$_3$OH abundance. A principal component analysis shows that NGC 4418 and Arp 220 share very similar molecular abundances and excitation, which clearly set them apart from other Galactic and extragalactic environments. The similar molecular abundances observed towards NCG 4418 and Arp 220 are consistent with a hot gas-phase chemistry, with the relative abundances of SiO and CH$_3$OH being regulated by shocks and X-ray driven dissociation. The bright emission from vibrationally excited species confirms the presence of a compact IR source, with an effective diameter $$350 K. The molecular abundances and the vibrationally excited spectrum are consistent with a young AGN/starburst system. We suggest that NGC 4418 may be a template for a new kind of chemistry and excitation, typical of compact obscured nuclei (CON). Because of the narrow line widths and bright molecular emission, NGC 4418 is the ideal target for further studies of the chemistry in CONs.Comment: accepted by A&A on 29/06/201