arXiv subjects

Mika Juvela

Publications and source records attributed to Mika Juvela.

At least 127 records · Page 7Linked to original sources

arXiv2009

A far-infrared survey at the North Galactic Pole I: Nearby star-forming galaxies and the effect of confused sources on source counts

We present follow-up observations of the far-infrared (FIR) sources at 90, 150 and 180 mu detected as part of the ISOPHOT EBL project, which has recently measured the absolute surface brightness of the cosmic infrared background radiation (CIRB) for the first time independently from COBE data. We have observed the fields at the North Galactic Pole region in the optical and near-IR, and complement these data with SDSS photometry, and spectroscopy where available, and present identifications of the 25 FIR sources which reach down to ~150 mJy in all three ISOPHOT bands. Identifications are done by means of full spectral energy density fitting to all sources in the FIR error circle areas. Approximately 80 per cent are identified as star-forming or star-bursting galaxies at z<0.3. We also find that more than half of the counterparts have disturbed morphologies, with signs of past or present interactions. However, only 20 per cent of all the sources are uniquely matched with a single galaxy -- 40 per cent are blends of two or more of these nearby star-forming galaxies, while another 20 per cent are likely blends of nearby and fainter galaxies. The final 20 per cent are likely to be more luminous IR galaxies at higher redshifts. The blended sources have an effect on the FIR source counts. In particular, taking into account realistic confusion or blending of sources, the differential FIR counts move down by a factor of ~1.5 and steepen in the 100 to 400 mJy range.

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arXiv2009

A Corona Australis cloud filament seen in NIR scattered light II: Comparison with sub-millimeter data

We study a northern part of the Corona Australis molecular cloud that consists of a filament and a dense sub-millimetre core inside the filament. Our aim is to measure dust temperature and sub-mm emissivity within the region. We also look for confirmation that near-infrared (NIR) surface brightness can be used to study the structure of even very dense clouds. We extend our previous NIR mapping south of the filament. The dust colour temperatures are estimated using Spitzer 160um and APEX/Laboca 870um maps. The column densities derived based on the reddening of background stars, NIR surface brightness, and thermal sub-mm dust emission are compared. A three dimensional toy model of the filament is used to study the effect of anisotropic illumination on near-infrared surface brightness and the reliability of dust temperature determination. Relative to visual extinction, the estimated emissivity at 870um is kappa(870) = (1.3 +- 0.4) x 10^{-5} 1/mag. This is similar to the values found in diffuse medium. A significant increase in the sub-millimetre emissivity seems to be excluded. In spite of saturation, NIR surface brightness was able to accurately pinpoint, and better than measurements of the colour excesses of background stars, the exact location of the column density maximum. Both near- and far-infrared data show that the intensity of the radiation field is higher south of the filament.

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arXiv2009

The Super-Alfvénic Model of Molecular Clouds: Predictions for Mass-to-Flux and Turbulent-to-Magnetic Energy Ratios

Recent measurements of the Zeeman effect in dark-cloud cores provide important tests for theories of cloud dynamics and prestellar core formation. In this Letter we report results of simulated Zeeman measurements, based on radiative transfer calculations through a snapshot of a simulation of supersonic and super-Alfvénic turbulence. We have previously shown that the same simulation yields a relative mass-to-flux ratio (core versus envelope) in agreement with the observations (and in contradiction with the ambipolar-drift model of core formation). Here we show that the mass-to-flux and turbulent-to-magnetic-energy ratios in the simulated cores agree with observed values as well. The mean magnetic field strength in the simulation is very low, \bar{B}=0.34 \muG, presumably lower than the mean field in molecular clouds. Nonetheless, high magnetic field values are found in dense cores, in agreement with the observations (the rms field, amplified by the turbulence, is B_{rms}=3.05 \muG). We conclude that a strong large-scale mean magnetic field is not required by Zeeman effect measurements to date, although it is not ruled out by this work.

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arXiv2008

The Super-Alfvénic Model of Molecular Clouds: Predictions for Zeeman Splitting Measurements

We present synthetic OH Zeeman splitting measurements of a super-Alfvenic model of molecular clouds. We select dense cores from synthetic 13CO maps computed from the largest simulation to date of supersonic and super-Alfvenic turbulence. The synthetic Zeeman splitting measurements in the cores yield a relation between the magnetic field strength, B, and the column density, N, in good agreement with the observations. The large scatter in B at a fixed value of N is partly due to intrinsic variations in the magnetic field strength from core to core. We also compute the relative mass-to-flux ratio between the center of the cores and their envelopes, ${\cal R}_μ$, and show that super-Alfvenic turbulence produces a significant scatter also in ${\cal R}_μ$, including negative values (field reversal between core center and envelope). We find ${\cal R}_μ<1$ for 70% of the cores, and ${\cal R}_μ<0$ for 12%. Of the cores with $|B_{\rm LOS}|>10$ \muG, 81% have ${\cal R}_μ<1$. These predictions of the super-Alfvenic model are in stark contrast to the ambipolar drift model of core formation, where only ${\cal R}_μ>1$ is allowed.

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arXiv2008

Luminous Infrared Galaxies with the Submillimeter Array: I. Survey Overview and the Central Gas to Dust Ratio

We present new data obtained with the Submillimeter Array for a sample of fourteen nearby luminous and ultraluminous infrared galaxies. The galaxies were selected to have luminosity distances D < 200 Mpc and far-infrared luminosities log(L_FIR) > 11.4. The galaxies were observed with spatial resolutions of order 1 kpc in the CO J=3-2, CO J=2-1, 13CO J=2-1, and HCO+ J=4-3 lines as well as the continuum at 880 microns and 1.3 mm. We have combined our CO and continuum data to measure an average gas-to-dust mass ratio of 120 +/- 28 (rms deviation 109) in the central regions of these galaxies, very similar to the value of 150 determined for the Milky Way. This similarity is interesting given the more intense heating from the starburst and possibly accretion activity in the luminous infrared galaxies compared to the Milky Way. We find that the peak H_2 surface density correlates with the far-infrared luminosity, which suggests that galaxies with higher gas surface densities inside the central kiloparsec have a higher star formation rate. The lack of a significant correlation between total H_2 mass and far-infrared luminosity in our sample suggests that the increased star formation rate is due to the increased availability of molecular gas as fuel for star formation in the central regions. In contrast to previous analyses by other authors, we do not find a significant correlation between central gas surface density and the star formation efficiency, as trace by the ratio of far-infrared luminosity to nuclear gas mass. Our data show that it is the star formation rate, not the star formation efficiency, that increases with increasing central gas surface density in these galaxies.

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arXiv2008

Near-infrared reddening of extra-galactic GMCs in a face-on geometry

[Abridged] We describe the near-infrared reddening signature of giant molecular clouds (GMCs) in external galaxies. In particular, we examine the E(J-H) and E(H-K) color-excesses, and the effective extinction law observed in discrete GMC regions. We also study the effect of the relative scale height of the GMC distribution to the color-excesses, and to the observed mass function of GMCs. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions, resembling a face-on geometry. The scattered light is included in the models, and near-infrared color maps are calculated from the simulated data. The effective near-infrared reddening law, i.e. the ratio E(J-H)/E(H-K), has a value close to unity in GMC regions. The ratio depends on the relative scale height of GMCs, xi, and for xi values 0.1...0.75 we find the typical ratios of 0.6...1.1. The effective extinction law turns out to be very flat in GMC regions. We find the ratios of apparent extinctions of A(H)/A(K)=1.35...1.55 and A(J)/A(H)=1.15. The effect of the scattered flux on the effective reddening law, as well as on the effective extinction law, is significant. Regarding the GMC mass function, we find no correlation between the input and observed slopes of the mass functions. Rather, the observed slope reflects the parameter $ξ$ and the dynamical range of the mass function. We estimate that only a fraction of 10...20 % of the total mass of GMCs is recovered, if the observed color-excess values are transformed to masses using the Galactic reddening law. In the case of individual clouds the fraction can vary between ~0...50 %.

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arXiv2007

Determination of the mass function of extra-galactic GMCs via NIR color maps. Testing the method in a disk-like geometry

[Abridged] The giant molecular clouds (GMCs) of external galaxies can be mapped with sub-arcsecond resolution using multiband observations in the near-infrared. However, the interpretation of the observed features, and their transformation into physical quantities, is greatly hampered by the effects arising from the unknown geometry and the scattering of light by dust particles. We examine the relation between the observed NIR reddening and the column density of the dust clouds. In this paper we particularly assess the feasibility of deriving the mass function of GMCs from NIR color-excess data. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions. We include the scattered light in the models and calculate NIR color maps from the simulated data. We extract clumps from the color maps and compare the observed mass function to the true mass function. For the physical configuration chosen in this study, essentially a face-on geometry, the observed mass function is a non-trivial function of the true mass function. The dynamical range of the observed mass function is confined to ~10^3.5... 10^5.5 M_\odot regardless of the dynamical range of the true mass function. The color maps are more sensitive in detecting the high-mass end of the mass function, and on average the masses of clouds are underestimated by a factor of ~10. The simulations show that the cloud mass function derived from JHK color excess data using simple foreground screening geometry cannot be regarded as a one-to-one tracer of the underlying mass function.

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arXiv2006

The Power Spectrum of Supersonic Turbulence in Perseus

We test a method of estimating the power spectrum of turbulence in molecular clouds based on the comparison of power spectra of integrated intensity maps and single-velocity-channel maps, suggested by Lazarian and Pogosyan. We use synthetic 13CO data from non-LTE radiative transfer calculations based on density and velocity fields of a simulation of supersonic hydrodynamic turbulence. We find that the method yields the correct power spectrum with good accuracy. We then apply the method to the Five College Radio Astronomy Observatory 13CO map of the Perseus region, from the COMPLETE website. We find a power law power spectrum with slope beta=1.81+-0.10. The values of beta as a function of velocity resolution are also confirmed using the lower resolution map of the same region obtained with the AT&T Bell Laboratories antenna. Because of its small uncertainty, this result provides a useful constraint for numerical codes used to simulate molecular cloud turbulence.

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arXiv2006

Can We Trust the Dust? Evidence of Dust Segregation in Molecular Clouds

Maps of estimated dust column density in molecular clouds are usually assumed to reliably trace the total gas column density structure. In this work we present results showing a clear discrepancy between the dust and the gas distribution in the Taurus molecular cloud complex. We compute the power spectrum of a 2MASS extinction map of the Taurus region and find it is much shallower than the power spectrum of a 13CO map of the same region previously analyzed. This discrepancy may be explained as the effect of grain growth on the grain extinction efficiency. However, this would require a wide range of maximum grain sizes, which is ruled out based on constraints from the extinction curve and the available grain models. We show that major effects due to CO formation and depletion are also ruled out. Our result may therefore suggest the existence of intrinsic spatial fluctuations of the dust to gas ratio, with amplitude increasing toward smaller scales. Preliminary results of numerical simulations of trajectories of inertial particles in turbulent flows illustrate how the process of clustering of dust grains by the cloud turbulence may lead to observable effects. However, these results cannot be directly applied to large scale supersonic and magnetized turbulence at present.

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arXiv2005

High Resolution Mapping of Interstellar Clouds by Near--IR Scattering

We discuss the possibility of mapping interstellar clouds at unprecedentedly high spatial resolution by means of near-IR imaging of their scattered light. We calculate the scattering of the interstellar radiation field by a cloud model obtained from the simulation of a supersonic turbulent flow. Synthetic maps of scattered light are computed in the J, H and K bands and are found to allow an accurate estimate of column density, in the range of visual extinction between 1 and 20 magnitudes. We provide a formalism to convert the intensity of scattered light at these near-IR bands into a total gas column density. We also show that this new method of mapping interstellar clouds is within the capability of existing near-IR facilities, which can achieve a spatial resolution of up to ~ 0.1 arcsec. This opens new perspectives in the study of interstellar dust and gas structure on very small scales. The validity of the method has been recently demonstrated by the extraordinary images of the Perseus region obtained by Foster & Goodman (2005), which motivated this investigation.

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arXiv2004

Electron Abundance in Protostellar Cores

The determination of the fractional electron abundance, Xe, in protostellar cores relies on observations of molecules, such as DCO+, H13CO+ and CO, and on chemical models to interpret their abundance. Studies of protostellar cores have revealed significant variations of Xe from core to core within a range 10^-8<Xe<10^-6. The physical origin of these large variations in Xe is not well understood, unless unlikely variations in the cosmic ray ionization rate or ad hoc values of metal depletion are assumed. In this work we explore other potential causes of these variations in Xe, including core age, extinction and density. We compute numerically the intensity of the radiation field within a density distribution generated by supersonic turbulence. Taking into account the lines of sight in all directions, the effective visual extinction in dense regions is found to be always much lower than the extinction derived from the column density along a fixed line of sight. Dense cores with volume and column densities comparable to observed protostellar cores have relatively low mass-averaged visual extinction, 2mag <= A_V <= 5mag, such that photo-ionization can sometimes be as important as cosmic ray ionization. Chemical models, including gas-grain chemistry and time dependent gas depletion and desorption, are computed for values of visual extinction in the range 2mag <= A_V <= 6mag, and for a hydrogen gas density of 10^4cm^-3$, typical of protostellar cores. The models presented here can reproduce some of the observed variations of ion abundance from core to core as the combined effect of visual extinction and age variations. The range of electron abundances predicted by the models is relatively insenstive to density over 10^4 to 10^6 cm^{-3}.

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arXiv2004

The Average Magnetic Field Strength in Molecular Clouds: New Evidence of Super-Alfvenic Turbulence

The magnetic field strength in molecular clouds is a fundamental quantity for theories of star formation. It is estimated by Zeeman splitting measurements in a few dense molecular cores, but its volume--averaged value within large molecular clouds (over several parsecs) is still uncertain. In this work we provide a new method to constrain the average magnetic field strength in molecular clouds. We compare the power spectrum of gas density of molecular clouds with that of two $350^3$ numerical simulations of supersonic MHD turbulence. The numerical simulation with approximate equipartition of kinetic and magnetic energies (model A) yields the column density power spectrum $P(k)\propto k^{-2.25\pm 0.01}$, the super--Alfvénic simulation (model B) $P(k)\propto k^{-2.71\pm 0.01}$. The column density power spectrum of the Perseus, Taurus and Rosetta molecular cloud complexes is found to be well approximated by a power law, $P_{\rm o}(k)\propto k^{-a}$, with $a=2.74\pm 0.07$, $2.74\pm 0.08$ and $2.76\pm 0.08$ respectively. We conclude that the observations are consistent with the presence of super--Alfvénic turbulence in molecular clouds (model B) while model A is inconsistent (more than 99% confidence) with the observations.

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arXiv2003

The Spectral Correlation Function of Molecular Clouds: A Statistical Test for Theoretical Models

We compute the spectral correlation function (SCF) of 13CO J=1--0 maps of molecular cloud complexes. The SCF is a power law over approximately an order of magnitude in spatial separation in every map. The power law slope of the SCF, alpha, its normalization, S_0(1pc), and the spectral line width averaged over the whole map, sigma_v, are computed for all the observational maps. The values of alpha, S_0(1pc) and sigma_v are combined to obtain empirical correlations to be used as tests for theoretical models of molecular clouds. Synthetic spectral maps are computed from different theoretical models, including solutions of the magneto-hydrodynamic (MHD) equations with different values of the rms Mach number of the flow and stochastic models with different power spectra of the velocity field. In order to compute the radiative transfer from the MHD models it is necessary to assign the models a physical scale and a physical density. When these assignments are made according to Larson type relations the best fit to the observational correlations is obtained. Unphysical stochastic models are instead ruled out by the empirical correlations. MHD models with equipartition of magnetic and kinetic energy of turbulence do not reproduce the observational data when their average magnetic field is oriented approximately parallel to the line of sight.

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arXiv2001

Theoretical Models of Polarized Dust Emission from Protostellar Cores

We model the polarized thermal dust emission from protostellar cores that are assembled by super-sonic turbulent flows in molecular clouds. Self-gravitating cores are selected from a three dimensional simulation of super-sonic and super-Alfvenic magneto-hydrodynamic (MHD) turbulence. The polarization is computed in two ways. In model A it is assumed that dust properties and grain alignment efficiency are uniform; in model B it is assumed that grains are not aligned at visual extinction larger than 3 mag. The main results of this work are: i) Values of the degree of polarization P between 1 and 10% are typical, despite the super-Alfvenic nature of the turbulence; ii) A steep decrease of P with increasing values of the sub-mm dust continuum intensity I is always found in self--gravitating cores selected from the MHD simulations, if grains are not aligned above a certain value of visual extinction (model B); iii) The same behavior is hard to reproduce if grains are aligned independently of visual extinction (model A); iv) The Chandrasekhar-Fermi formula, corrected by a factor f=0.4, provides an approximate estimate of the average magnetic field strength in the cores. Sub-mm dust continuum polarization maps of quiescent protostellar cores and Bok globules always show a decrease in P with increasing value of I consistent with the predictions of our model B. We therefore conclude that sub-mm polarization maps of quiescent cores do not map the magnetic field inside the cores at visual extinction larger than approximately 3 mag. There is no inconsistency between the results from optical and near-IR polarized absorption of background stars, and the observed polarization of sub-mm dust continuum from quiescent cores. In both cases, grains at large visual extinction appear to be virtually unaligned.

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arXiv2000

The Turbulent Shock Origin of Proto--Stellar Cores

The fragmentation of molecular clouds (MC) into protostellar cores is a central aspect of the process of star formation. Because of the turbulent nature of super-sonic motions in MCs, it has been suggested that dense structures such as filaments and clumps are formed by shocks in a turbulent flow. In this work we present strong evidence in favor of the turbulent origin of the fragmentation of MCs. The most generic result of turbulent fragmentation is that dense post shock gas traces a gas component with a smaller velocity dispersion than lower density gas, since shocks correspond to regions of converging flows, where the kinetic energy of the turbulent motion is dissipated. Using synthetic maps of spectra of molecular transitions, computed from the results of numerical simulations of super--sonic turbulence, we show that the dependence of velocity dispersion on gas density generates an observable relation between the rms velocity centroid and the integrated intensity (column density), Sigma(V_0)-I, which is indeed found in the observational data. The comparison between the theoretical model (maps of synthetic 13CO spectra), with 13CO maps from the Perseus, Rosette and Taurus MC complexes, shows excellent agreement in the Sigma(V_0)-I relation. The Sigma(V_0)-I relation of different observational maps with the same total rms velocity are remarkably similar, which is a strong indication of their origin from a very general property of the fluid equations, such as the turbulent fragmentation process.

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arXiv1999

Proto-Galactic Starbursts at High Redshift

We have computed the evolving ultraviolet-millimeter spectral energy distribution (SED) produced by proto-galactic starbursts at high redshift, incorporating the chemical evolution of the interstellar medium in a consistent manner. Dust extinction is calculated in a novel way, that is not based on empirical calibrations of extinction curves, but rather on the lifetime of molecular clouds which delays the emergence of each successive generation of stars at ultraviolet wavelengths by typically 15 Myr. The predicted rest-frame far-infrared to millimeter-wave emission includes the calculation of molecular emission-line luminosities ($^{12}$CO and O$_{2}$ among other molecules) consistent with the evolving chemical abundances. Here we present details of this new model, along with the results of comparing its predictions with several high-redshift observables, namely: the ultraviolet SEDs of Lyman-limit galaxies, the high-redshift radiogalaxies 4C41.17 and 8C1435, the SCUBA sub-millimeter survey of the Hubble Deep Field (HDF), and the SEDs of intermediate-redshift elliptical galaxies. With our new reddening method, we are able to fit the spectrum of the Lyman-limit galaxy 1512-cB58, and we find an extinction of about 1.9 magnitudes at 1600 Å. The model also predicts that most Lyman-limit galaxies should have a value of $α$ inside that range, as it is observed. The 850 $μ$m flux density of a typical Lyman-limit galaxy is expected to be only $\simeq 0.5$ mJy, and therefore the optical counterparts of the most luminous sub-mm sources in the HDF (or any other currently feasible sub-mm survey) are unlikely to be Lyman-break galaxies.

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arXiv1999

Super-Sonic Turbulence in the Perseus Molecular Cloud

We compare the statistical properties of J=1-0 13CO spectra observed in the Perseus Molecular Cloud with synthetic J=1-0 13CO spectra, computed solving the non-LTE radiative transfer problem for a model cloud obtained as solutions of the three dimensional magneto-hydrodynamic (MHD) equations. The model cloud is a randomly forced super-Alfvenic and highly super-sonic turbulent isothermal flow. The purpose of the present work is to test if idealized turbulent flows, without self-gravity, stellar radiation, stellar outflows, or any other effect of star formation, are inconsistent or not with statistical properties of star forming molecular clouds. We present several statistical results that demonstrate remarkable similarity between real data and the synthetic cloud. Statistical properties of molecular clouds like Perseus are appropriately described by random super-sonic and super-Alfvenic MHD flows. Although the description of gravity and stellar radiation are essential to understand the formation of single protostars and the effects of star formation in the cloud dynamics, the overall description of the cloud and of the initial conditions for star formation can apparently be provided on intermediate scales without accounting for gravity, stellar radiation, and a detailed modeling of stellar outflows. We also show that the relation between equivalent line width and integrated antenna temperature indicates the presence of a relatively strong magnetic field in the core B1, in agreement with Zeeman splitting measurements.

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arXiv1997

Synthetic Molecular Clouds from Supersonic MHD and Non-LTE Radiative Transfer Calculations

The dynamics of molecular clouds is characterized by supersonic random motions in the presence of a magnetic field. We study this situation using numerical solutions of the three-dimensional compressible magneto-hydrodynamic (MHD) equations in a regime of highly supersonic random motions. The non-LTE radiative transfer calculations are performed through the complex density and velocity fields obtained as solutions of the MHD equations, and more than 5x10^5 synthetic molecular spectra are obtained. We use a numerical flow without gravity or external forcing. The flow is super-Alfvenic and corresponds to model A of Padoan and Nordlund (1997). Synthetic data consist of sets of 90x90 synthetic spectra with 60 velocity channels, in five molecular transitions: J=1-0 and J=2-1 for 12CO and 13CO, and J=1-0 for CS. Though we do not consider the effects of stellar radiation, gravity, or mechanical energy input from discrete sources, our models do contain the basic physics of magneto-fluid dynamics and non-LTE radiation transfer and are therefore more realistic than previous calculations. As a result, these synthetic maps and spectra bear a remarkable resemblance to the corresponding observations of real clouds.

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