Luminosity Correlation Research Articles

CAIXA: a catalogue of AGN in the XMM-Newton archive

We presented CAIXA, a Catalogue of AGN in the XMM-Newton Archive, in a companion paper. Here, a systematic search for correlations between the X-ray spectral properties and the multiwavelength data was performed for the sources in CAIXA. All the significant (>99.9% confidence level) correlations are discussed along with their physical implications on current models of AGN. Two main correlations are discussed in this paper: a) a very strong anti-correlation between the FWHM of the H$\beta$ optical line and the ratio between the soft and the hard X-ray luminosity. Although similar anti-correlations between optical line width and X-ray spectral steepness have already been discussed in the literature (see e.g., Laor et al. 1994, Boller et al. 1996, Brandt et al. 1997), we consider the formulation we present in this paper is more fundamental, as it links model-independent quantities. Coupled with a strong anti-correlation between the V to hard X-ray flux ratio and the H$\beta$ FHWM, it supports scenarios for the origin of the soft excess in AGN, which require strong suppression of the hard X-ray emission; b) a strong (and expected) correlation between the X-ray luminosity and the black hole mass. Its slope, flatter than 1, is consistent with Eddington ratio-dependent bolometric corrections, such as that recently proposed by Vasudevan & Fabian (2009). Moreover, we critically review through various statistical tests the role that distance biases play in the strong radio to X-ray luminosity correlation found in CAIXA and elsewhere; we conclude that only complete, unbiased samples (such as that recently published by Behar & Laor, 2008) should be used to draw observational constraints on the origin of radio emission in radio-quiet AGN.

AN OBJECTIVE SURVEY OF Mpc-SCALE RADIO EMISSION IN 0.03 <z< 0.3 BRIGHT X-RAY CLUSTERS

We have performed the the first census of Mpc-scale radio emission to include control fields and quantifiable upper limits for bright X-ray clusters in the range 0.03<z<0.3 . Through reprocessing radio images from the WENSS survey, we detect diffuse emission from approximately 30% of the sample. We find a correlation similar to the well-studied relationship between radio halo and X-ray luminosities of the host cluster, but also find that large scale radio galaxy detections follow a similar trend to that for radio halos. With this quantitative study, we thus confirm the upper envelope to the radio luminosities for X-ray selected clusters, and the higher detection rates for diffuse radio emission (including halos, relics and radio galaxies) at X-ray luminosities above approximately 10**45 erg/s. We can neither confirm nor refute the claims for a tight correlation between these radio halo and X-ray luminosities, i.e. whether the halo luminosity function of X-ray clusters is bimodal (having high/on and low/off states), or whether the radio luminosity can take on a wide range of values up to a maximum at each cluster X-ray luminosity. The resolution of this issue may provide a unique diagnostic for the timescales over which relativistic particles can be accelerated following cluster mergers. We discuss several important selection effects on radio vs. X-ray luminosity correlations, including surface brightness thresholds and non-X-ray-selected diffuse radio sources. We also report several new detections of diffuse emission, including a Mpc-scale relic in RXJ1053.7+5450, a possible halo/relic combination in Abell 2061, a serendipitous diffuse X-ray source associated with poor clusters in the Abell 781 field, and confirmation of very weak diffuse emission patches outside of Abell~2255.

Velocity dispersion measurements of dwarf galaxies in the Coma cluster - implications for the structure of the fundamental plane

We present intermediate-resolution spectroscopic data for a set of dwarf and giant galaxies in the Coma Cluster, with -20.6 < M_R < -15.7. The photometric and kinematic properties of the brighter galaxies can be cast in terms of parameters which present little scatter with respect to a set of scaling relations known as the Fundamental Plane. To determine the form of these fundamental scaling relations at lower luminosities, we have measured velocity dispersions for a sample comprising 69 galaxies on the border of the dwarf and giant regime. Combining these data with our photometric survey, we find a tight correlation of luminosity and velocity dispersion, L \propto \sigma^{2.0}, substantially flatter than the Faber-Jackson relation characterising giant elliptical galaxies. In addition, the variation of mass-to-light ratio with velocity dispersion is quite weak in our dwarf sample: M/L \propto \sigma^{0.2}. Our overall results are consistent with theoretical models invoking large-scale mass removal and subsequent structural readjustment, e.g., as a result of galactic winds.

Resolving the mid-infrared cores of local Seyferts

We present new photometry of 16 local Seyferts including 6 Compton-thick sources in <i>N<i/>-band filters around 12-<i>μ<i/>m, obtained with the VISIR instrument on the 8-m Very Large Telescope. The near-diffraction-limited imaging provides the least-contaminated core fluxes for these sources to date. Augmenting these with our previous observations and with published intrinsic X-ray fluxes, we form a total sample of 42 sources for which we find a strong mid-infrared:X-ray (12.3 <i>μ<i/>m:2–10 keV) luminosity correlation. Performing a physically-motivated subselection of sources in which the Seyfert torus is likely to be best-resolved results in the correlation <i>L<i/><sub>MIR<sub/> <i>L<i/><sub>X<sub/>, with a reduction of the scatter in luminosities as compared to the full sample. Consideration of systematics suggests a range of 1.02–1.21 for the correlation slope. The mean 2-keV:12.3-<i>μ<i/>m spectral index () is found to be -1.100.01, largely independent of luminosity. Indirectly-computed 12-<i>μ<i/>m bolometric corrections range over <i>≈<i/>10–30 if a known luminosity trend of X-ray bolometric corrections is assumed. Comparison with <i>ISO<i/> data spanning a wider luminosity range suggests that our correlation can be extended into the quasar regime. That unobscured, obscured, and Compton-thick sources all closely follow the same luminosity correlation has important implications for the structures of Seyfert cores. The typical resolution-limit of our imaging corresponds to ~70 pc at a median , and we use the tightness of the correlation to place constraints on the dominance of any residual emission sources within these physical scales. An upper-limit for any contaminating star formation of <i>≈<i/>40% of the unresolved flux is inferred, on average. We suggest that uncontaminated mid-IR continuum imaging of AGN is an accurate proxy for their intrinsic power.

Investigation of gamma-ray bursts with known redshifts: Statistical analysis of parameters

Observational parameters of the optical and gamma-ray emissions from 58 gamma-ray bursts (GRBs) with discovered afterglows and known redshifts are analyzed. The distributions of these parameters and pair correlations between them are studied. Approximately half of the objects exhibit a relatively slow decrease in the optical flux at initial afterglow phases (with a power-law index in the decay law α < 1). Correlations have been found between the luminosities, energies, and durations of the optical and gamma-ray emissions, which can be explained by the presence of universal features in the light curves. A correlation of the peak luminosity for afterglows with the redshift and an anticorrelation of their durations with the redshift have been found for the first time. Against the background of a weak z dependence of the total afterglow energy, this effect can be explained by cosmological evolution of the GRB environment, which determines the rate of optical energy release.

A time–luminosity correlation for γ-ray bursts in the X-rays

Abstract γ-ray bursts (GRBs) have recently attracted much attention as a possible way to extend the Hubble diagram to a very high redshift. However, the large scatter in their intrinsic properties prevents directly using them as a distance indicator so that the hunt is open for a relation involving an observable property to standardize GRBs in the same way as the Phillips law makes it possible to use Type Ia supernovae as standardizable candles. We use here the data on the X-ray decay curve and spectral index of a sample of GRBs observed with the Swift satellite. These data are used as input to a Bayesian statistical analysis looking for a correlation between the X-ray luminosity LX(Ta) and the time constant Ta of the afterglow curve. We find a linear relation between log [LX(Ta)] and log [Ta/(1 +z)] with an intrinsic scatter σint= 0.33 comparable to previously reported relations. Remarkably, both the slope and the intrinsic scatter are almost independent on the matter density ΩM and the constant equation of state w of the dark energy component thus suggesting that the circularity problem is alleviated for the LX–Ta relation.

Difference in Narrow-Emission-Line Spectra of Seyfert 1 and 2 Galaxies

In the unification scheme of Seyfert galaxies, a dusty torus blocks the continuum source and broad line region in Seyfert 2 galaxies. However it is not clear whether or not and to what extent the torus affects the narrow line spectra. In this paper, we show that Seyfert 1 and Seyfert 2 galaxies have different distributions on the [OIII]/H$\beta $ vs [NII]/H$\alpha$ diagram (BPT diagram) for narrow lines. Seyfert 2 galaxies display a clear left boundary on the BPT diagram and only 7.3% of them lie on the left. By contrast, Seyfert 1 galaxies do not show such a cutoff and 33.0% of them stand on the left side of the boundary. Among Seyfert 1 galaxies, the distribution varies with the extinction to broad lines. As the extinction increases, the distribution on BPT diagram moves to larger [NII]/H$\alpha$ value. We interpret this as an evidence for the obscuration of inner dense narrow line region by the dusty torus. We also demonstrate that the [OIII] and broad line luminosity correlation depends on the extinction of broad lines in the way that high extinction objects have lower uncorrected [OIII] luminosities, suggesting that [OIII] is partially obscured in these objects. Therefore, using [OIII] as an indicator for the nuclear luminosity will systematically under-estimate the nuclear luminosity of Seyfert 2 galaxies.

Improving the calibration of Type Ia supernovae using late-time light curves

The use of Type la supernovae (SNe Ia) as cosmological standard candles is a key to solving the mystery of dark energy. Improving the calibration of SNe Ia increases their power as cosmological standard candles. We find tentative evidence for a correlation between the late-time light-curve slope and the peak luminosity of SNe Ia in the B band; brighter SNe Ia seem to have shallower light-curve slopes between 100 and 150 d from maximum light. Using a Markov Chain Monte Carlo (MCMC) analysis in calibrating SNe Ia, we are able to simultaneously take into consideration the effect of dust extinction, the luminosity and light-curve width correlation (parametrized by Δm 15 ), and the luminosity and late-time light-curve slope correlation. For the available sample of 11 SNe Ia with well-measured late-time light curves, we find that correcting for the correlation between luminosity and late-time light-curve slope of the SNe Ia leads to an intrinsic dispersion of 0.12 mag in the Hubble diagram. Our results have significant implications for future supernova surveys aimed to illuminate the nature of dark energy.

Peak energy of the prompt emission of long gamma-ray bursts versus their fluence and peak flux

The spectral-energy (and luminosity) correlations in long gamma-ray bursts are being hotly debated to establish, first of all, their reality against possible selection effects. These are best studied in the observer planes, namely the peak energy E obs peak versus the fluence F or the peak flux P. In a recent paper, we have started to investigate this problem considering all bursts with known redshift and spectral properties. Here, we consider instead all bursts with known E obs peak , irrespective of redshift, adding to those a sample of 100 faint BATSE bursts representative of a larger population of 1000 objects. This allows us to construct a complete, fluence-limited, sample, tailored to study the selection/instrumental effects we consider. We found that the fainter BATSE bursts have smaller E obs peak than those of bright events. As a consequence, the E obs peak of these bursts is correlated with the fluence, though with a slope flatter than that defined by bursts with z. Selection effects, which are present, are shown not to be responsible for the existence of such a correlation. About six per cent of these bursts are surely outliers of the E peak -E iso correlation (updated in this paper to include 83 bursts), since they are inconsistent with it for any redshift. E obs peak also correlates with the peak flux, with a slope similar to the Ep eak -L iso correlation. In this case, there is only one sure outlier. The scatter of the E obs peak -P correlation defined by the BATSE bursts of our sample is significantly smaller than the E obs peak -F correlation of the same bursts, while for the bursts with known redshift the E peak -E iso correlation is tighter than the E peak -L iso one. Once a very large number of bursts with E obs peak and redshift will be available, we thus expect that the E peak -L iso correlation will be similar to that currently found, whereas it is very likely that the E peak -E iso correlation will become flatter and with a larger scatter.

Synoptic studies of 17 blazars detected in very high-energy γ-rays

Since 2002, the number of detected blazars at γ-ray energies above 100 GeV has more than doubled. I study 17 blazars currently known to emit E > 100 GeV γ-rays. Their intrinsic energy spectra are reconstructed by removing extragalactic background light attenuation effects. Luminosity and spectral slope in the E > 100 GeV region are then compared and correlated among each other, with X-ray, optical and radio data, and with the estimated black hole (BH) masses of the respective host galaxies. According to expectations from synchrotron self-Compton emission models, a correlation on the 3.6σ significance level between γ-ray and X-ray fluxes is found, while correlations between γ-ray and optical/radio fluxes are less pronounced. Further, a general hardening of the blazar spectra in the E > 100 GeV region with increasing γ-ray luminosity is observed, both for the full 17-source sample and for those sources which have been detected at distinct flux levels. This goes in line with a correlation of the γ-ray luminosity and the synchrotron peak frequency, which is also seen. Tests for possible selection effects reveal a hardening of the spectra with increasing redshift. The blazar γ-ray emission might depend on the mass of the central BH. The blazars under study do, however, show no correlation of the BH masses with the spectral index and the luminosity in the E > 100 GeV region. I also consider temporal properties of the X-ray and E > 100 GeV γ-ray flux. No general trends are found, except for the observation that the blazars with the most massive BHs do not show particularly high duty cycles. These blazars include Mkn 501 and PKS 2155−304, for which recently very fast flares have been reported. In general, very high-energy flare time-scales are not found to scale with the BH mass. As a specific application of the luminosity study, a constraint for the still undetermined redshift of the blazar PG 1553+113 is discussed.

X‐Ray Emission from Active Galactic Nuclei with Intermediate‐Mass Black Holes

We present a systematic X-ray study of eight AGNs with intermediate mass black holes (M_BH 8-95x10^4 Msun) based on 12 XMM-Newton observations. The sample includes the two prototype AGNs in this class - NGC4395 and POX52 and six other AGNs discovered with the SDSS. These AGNs show some of the strongest X-ray variability with the normalized excess variances being the largest and the power density break time scales being the shortest observed among radio-quiet AGNs. The excess variance -- luminosity correlation appears to depend on both the BH mass and the Eddington luminosity ratio. The break time scale -- black hole mass relations for AGN with IMBHs are consistent with that observed for massive AGNs. We find that the FWHM of the Hbeta or Halpha line is uncorrelated with the BH mass, but shows strong anticorrelation with the Eddington luminosity ratio. Four AGNs show clear evidence for soft X-ray excess emission (kT_in~150-200eV). X-ray spectra of three other AGNs are consistent with the presence of the soft excess emission. NGC4395 with lowest L/L_Edd lacks the soft excess emission. Evidently small black mass is not the primary driver of strong soft X-ray excess emission from AGNs. The X-ray spectral properties and optical-to-X-ray spectral energy distributions of these AGNs are similar to those of Seyfert 1 galaxies. The observed X-ray/UV properties of AGNs with IMBHs are consistent with these AGNs being low mass extension of more massive AGNs; those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1s while those with low $L/L_{Edd}$ looking more like broad-line Seyfert 1s.

Multiwavelength Study of Massive Galaxies atz∼2. I. Star Formation and Galaxy Growth

Examining a sample of massive galaxies at 1.4<z<2.5 with K_{Vega}<22 from the Great Observatories Origins Deep Survey, we compare photometry from Spitzer at mid- and far-IR, to submillimeter, radio and rest-frame ultraviolet wavelengths, to test the agreement between different tracers of star formation rates (SFRs) and to explore the implications for galaxy assembly. For z~2 galaxies with moderate luminosities(L_{8um}<10^{11}L_sun), we find that the SFR can be estimated consistently from the multiwavelength data based on local luminosity correlations. However,20--30% of massive galaxies, and nearly all those with L_{8um}>10^{11}L_sun, show a mid-IR excess which is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24um-detected galaxies. For a given mass, the SFR at z=2 was larger by a factor of ~4 and ~30 relative to that in star forming galaxies at z=1 and z=0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z=2 are relatively 'transparent' to ultraviolet light, and their activity is long lived (~400 Myr), unlike that in local ULIRGs and high redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z=2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity.Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4, and the space density of ULIRGs by an orderof magnitude, but give better agreement for z>1.4 quiescent galaxies.

Blazar γ-Rays, Shock Acceleration, and the Extragalactic Background Light

The observed spectra of blazars, their intrinsic emission, and the underlying populations of radiating particles are intimately related. The use of these sources as probes of the extragalactic infrared background, a prospect propelled by recent advances in TeV-band telescopes, soon to be augmented by observations by NASA's upcoming Gamma-Ray Large Area Space Telescope, has been a topic of great recent interest. Here, it is demonstrated that if particles in blazar jets are accelerated at relativistic shocks, then γ-ray spectra with indices less than 1.5 can be produced. This, in turn, loosens the upper limits on the near-infrared extragalactic background radiation previously proposed. We also show evidence hinting that TeV blazars with flatter spectra have higher intrinsic TeV γ-ray luminosities, and we indicate that there may be a correlation of flatness and luminosity with redshift.

Suppressed radio emission in supercluster galaxies: enhanced ram pressure in merging clusters?

Context. Continuum radio emission of galaxies is related to AGN activity and to starbursts, both of which require a supply of gas, respectively to the central black hole and to molecular clouds. Aims. The environmental influence on the 21 cm (1.4 GHz) continuum radio emission of galaxies is analyzed in a 600 deg 2 region of the local Universe containing the Shapley Supercluster (SSC), whose core is thought to be the site of cluster-cluster merging. Methods. Galaxies in the FLASH and 6dFGS optical/NIR redshift surveys are cross-identified with NVSS radio sources, selected in a subsample doubly complete in volume and luminosity, and classified as starbursts or AGN according to their radio luminosity. We study radio luminosities as well as radio loudness (luminosities normalized by stellar mass) RK . Environmental effects are studied through a smoothed density field (normalized to that obtained from random catalogs with the same survey edges and redshift selection function) and through the projected distance to the nearest cluster (in units of its virial radius, whose relation to the aperture velocity dispersion is quantified). Results. The fraction of high RK galaxies in the dense 10 Mpc Abell 3558 cluster complex at the core of the SSC (SSC-CR) is half as large than elsewhere. Moreover, radio loudness in the SSC-CR is anti-correlated with the density of the large-scale environment and correlated with clustercentric radius: central brightest cluster galaxies (BCGs) in the SSC-CR are an order of magnitude less radioloud than BCGs elsewhere, with signs of suppressed radio loudness also present beyond the BCGs, out to at least 0.3r200. The gradual suppression of radio loudness from inner cluster regions to the cluster centers highlights a significant correlation of radio loudness with clustercentric radius, not seen outside the SSC-CR. This correlation is nearly as strong as the tight correlation of K-band luminosity, LK, with clustercentric radius (K-luminosity segregation), inside the SSC-CR, with a mild K-luminosity segregation outside the SSC-CR. Conclusions. The suppression of radio loudness in SSC-CR BCGs can be attributed to major cluster-cluster mergers that destroy the cool core and thus the supply of gas to the central AGN. We analytically demonstrate that the low radio loudness of non-BCG galaxies within SSC-CR clusters cannot be explained by direct major galaxy mergers or rapid galaxy flyby collisions, but by the loss of gas supply through the enhanced ram pressure felt when these galaxies cross the shock front between the two merging clusters and are afterwards subjected to the stronger wind from the second cluster.

Testing the gamma-ray burst variability/peak luminosity correlation on a Swift homogeneous sample

We test the gamma-ray burst (GRB) correlation between temporal variability and peak luminosity of the γ -ray profile on a homogeneous sample of 36 Swift/Burst Alert Telescope (BAT) GRBs with firm redshift determination. This is the first time that this correlation can be tested on a homogeneous data sample. The correlation is confirmed, as long as the six GRBs with low luminosity (<5 × 10 50 erg s −1 in the rest-frame 100‐1000 keV energy band) are ignored. We confirm that the considerable scatter of the correlation already known is not due to the combination of data from different instruments with different energy bands, but it is intrinsic to the correlation itself. Thanks to the unprecedented sensitivity of Swift/BAT, the variability/peak luminosity correlation is tested on low-luminosity GRBs. Our results show that these GRBs are definite outliers.

The Relationship between Molecular Gas Tracers and Kennicutt‐Schmidt Laws

We provide a model for how Kennicutt-Schmidt (KS) laws, which describe the correlation between star formation rate and gas surface or volume density, depend on the molecular line chosen to trace the gas. We show that, for lines that can be excited at low temperatures, the KS law depends on how the line critical density compares to the median density in a galaxy's star-forming molecular clouds. High critical density lines trace regions with similar physical properties across galaxy types, and this produces a linear correlation between line luminosity and star formation rate. Low critical density lines probe regions whose properties vary across galaxies, leading to a star formation rate that varies superlinearly with line luminosity. We show that a simple model in which molecular clouds are treated as isothermal and homogenous can quantitatively reproduce the observed correlations between galactic luminosities in far-infrared and in the CO(1→0) and HCN(1→0) lines, and naturally explains why these correlations have different slopes. We predict that IR-line luminosity correlations should change slope for galaxies in which the median density is close to the line critical density. This prediction may be tested by observations of lines such as HCO+(1→0) with intermediate critical densities, or by HCN(1 → 0) observations of intensely star-forming high-redshift galaxies with very high densities. Recent observations by Gao et al. hint at just such a change in slope. We argue that deviations from linearity in the HCN(1→0)-IR correlation at high luminosity are consistent with the assumption of a constant star formation efficiency.

Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution

We present a comprehensive study of the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced with a modest dispersion, consistent with observations, to take into account the radio detectability, which is important for blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but here we introduce the luminosity-dependent density evolution (LDDE) model, which is favored on the basis of the evolution of the X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses of the observed redshift and gamma-ray flux distributions of the EGRET blazars. Interestingly, we find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars and that the jet activity is universally correlated with the accretion history of AGNs. We then find that only 25–50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with XLF data. Therefore, unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer (by a factor of more than 3) than a previous estimate.

A Search for Dense Gas in Luminous Submillimeter Galaxies with the 100 m Green Bank Telescope

We report deep K-band (18-27GHz) observations with the 100-m Green Bank Telescope of HCN(1-0) line emission towards the two submillimeter-selected galaxies (SMGs) SMMJ02399-0136 (z=2.81) and SMMJ16359+6612 (z=2.52). For both sources we have obtained spectra with channel-to-channel rms noise of <=0.5mJy, resulting in velocity-integrated line fluxes better than < 0.1 Jy km/s, although we do not detect either source. Such sensitive observations -- aided by gravitational lensing of the sources -- permit us to put upper limits of L_HCN(1-0) < 2x10^10 K km/s pc^2 on the intrinsic HCN(1-0) line luminosities of the two SMGs. The far-infrared (FIR) luminosities for all three SMGs with sensitive HCN(1-0) observations to date are found to be consistent with the tight FIR-HCN luminosity correlation observed in Galactic molecular clouds, quiescent spirals and (ultra) luminous infrared galaxies in the local Universe. Thus, the observed HCN luminosities remain in accordance with what is expected from the universal star formation efficiency per dense molecular gas mass implied by the aforementioned correlation, and more sensitive observations with today's large aperture radio telescopes hold the promise of detecting HCN(1-0) emission in similar objects in the distant Universe.

Slow Star Formation in Dense Gas: Evidence and Implications

It has been known for more than 30 years that star formation in giant molecular clouds (GMCs) is slow, in the sense that only ~1% of the gas forms stars every free-fall time. This result is entirely independent of any particular model of molecular cloud lifetime or evolution. Here we survey observational data on higher density objects in the interstellar medium, including infrared dark clouds and dense molecular clumps, to determine if these objects form stars slowly like GMCs, or rapidly, converting a significant fraction of their mass into stars in one free-fall time. We find no evidence for a transition from slow to rapid star formation in structures covering three orders of magnitude in density. This has important implications for models of star formation, since competing models make differing predictions for the characteristic density at which star formation should transition from slow to rapid. The data are inconsistent with models that predict that star clusters form rapidly and in free-fall collapse. Magnetic- and turbulence-regulated star formation models can reproduce the observations qualitatively, and the turbulence-regulated star formation model of Krumholz & McKee quantitatively reproduces the infrared-HCN luminosity correlation recently reported by Gao & Solomon. Slow star formation also implies that the process of star cluster formation cannot be one of global collapse, but must instead proceed over many free-fall times. This suggests that turbulence in star-forming clumps must be driven, and that the competitive accretion mechanism does not operate in typical cluster-forming molecular clumps.

The XMM-Newton extended survey of the Taurus molecular cloud (XEST)

The Taurus Molecular Cloud (TMC) is the nearest large star-forming region, prototypical for the distributed mode of low-mass star formation. Pre-main sequence stars are luminous X-ray sources, probably mostly owing to magnetic energy release. Aims. The XMM-Newton Extended Survey of the Taurus Molecular Cloud (EST) presented in this paper surveys the most populated =5 square degrees of the TMC, using the XMM-Newton X-ray observatory to study the thermal structure, variability, and long-term evolution of hot plasma, to investigate the magnetic dynamo, and to search for new potential members of the association. Many targets are also studied in the optical, and high-resolution X-ray grating spectroscopy has been obtained for selected bright sources. Methods. The X-ray spectra have been coherently analyzed with two different thermal models (2-component thermal model, and a continuous emission measure distribution model). We present overall correlations with fundamental stellar parameters that were derived from the previous literature. A few detections from Chandra observations have been added. Results. The present overview paper introduces the project and provides the basic results from the X-ray analysis of all sources detected in the XEST survey. Comprehensive tables summarize the stellar properties of all targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus by about an order of magnitude and for the first time systematically accesses very faint and strongly absorbed TMC objects. We find a detection rate of 85% and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169 surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios, and show evidence for lower X-ray luminosities in CTTS compared to WTTS. Detailed analysis (e.g., variability, rotation-activity relations, influence of accretion on X-rays) will be discussed in a series of accompanying papers.