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At least 955 records · Page 53
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DOE OSTI2015

A new black hole mass estimate for obscured active galactic nuclei

We propose a new method for estimating the mass of a supermassive black hole, applicable to obscured active galactic nuclei (AGNs). This method estimates the black hole mass using the width of the narrow core of the neutral FeKα emission line in X-rays and the distance of its emitting region from the black hole based on the isotropic luminosity indicator via the luminosity scaling relation. Assuming the virial relation between the locations and the velocity widths of the neutral FeKα line core and the broad Hβ emission line, the luminosity scaling relation of the neutral FeKα line core emitting region is estimated. We find that the velocity width of the neutral FeKα line core falls between that of the broad Balmer emission lines and the corresponding value at the dust reverberation radius for most of the target AGNs. The black hole mass M{sub BH,FeKα} estimated with this method is then compared with other black hole mass estimates, such as the broad emission-line reverberation mass M{sub BH,rev} for type 1 AGNs, the mass M{sub BH,H{sub 2O}} based on the H{sub 2}O maser, and the single-epoch mass estimate M{sub BH,pol} based on the polarized broad Balmer lines for type 2 AGNs. We find that M{sub BH,FeKα} is consistent with M{sub BH,rev} and M{sub BH,pol}, and find that M{sub BH,FeKα} correlates well with M{sub BH,H{sub 2O}}. These results suggest that M{sub BH,FeKα} is a potential indicator of the black hole mass for obscured AGNs. In contrast, M{sub BH,FeKα} is systematically larger than M{sub BH,H{sub 2O}} by about a factor of 5, and the possible origins are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Metal-poor, strongly star-forming galaxies in the DEEP2 Survey: The relationship between stellar mass, temperature-based metallicity, and star formation rate

We report on the discovery of 28 z≈0.8 metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [O iii] λ4363 emission line, which provides a “direct” measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFR with stellar mass estimates from modeling the optical photometry. We find that these galaxies are 1.05 ± 0.61 dex above the z∼1 stellar mass–SFR relation and 0.23 ± 0.23 dex below the local mass–metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains (0.29 dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation, and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of ≈100{sub −75}{sup +310} Myr, which suggests stochastic star formation. Combining our sample with other z∼1 metal-poor galaxies, we find a weak positive SFR–metallicity dependence (at fixed stellar mass) that is significant at 94.4% confidence. We interpret this positive correlation as recent star formation that has enriched the gas but has not had time to drive the metal-enriched gas out with feedback mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A physical approach to the identification of high-Z mergers: Morphological classification in the stellar mass domain

At z ≳ 1, the distinction between merging and “normal” star-forming galaxies based on single band morphology is often hampered by the presence of large clumps which result in a disturbed, merger-like appearance even in rotationally supported disks. In this paper we discuss how a classification based on canonical, non-parametric structural indices measured on resolved stellar mass maps, rather than on single-band images, reduces the misclassification of clumpy but not merging galaxies. We calibrate the mass-based selection of mergers using the MIRAGE hydrodynamical numerical simulations of isolated and merging galaxies which span a stellar mass range of 10{sup 9.8}–10{sup 10.6} M{sub ⊙} and merger ratios between 1:1–1:6.3. These simulations are processed to reproduce the typical depth and spatial resolution of observed Hubble Ultra Deep Field (HUDF) data. We test our approach on a sample of real z≃2 galaxies with kinematic classification into disks or mergers and on ∼100 galaxies in the HUDF field with photometric/spectroscopic redshift between 1.5 ≤ z ≤ 3 and M > 10{sup 9.4} M{sub ⊙}. We find that a combination of the asymmetry A{sub MASS} and M{sub 20, MASS} indices measured on the stellar mass maps can efficiently identify real (major) mergers with ≲20% contamination from clumpy disks in the merger sample. This mass-based classification cannot be reproduced in star-forming galaxies by H-band measurements alone, which instead result in a contamination from clumpy galaxies which can be as high as 50%. Moreover, we find that the mass-based classification always results in a lower contamination from clumpy galaxies than an H-band classification, regardless of the depth of the imaging used (e.g., CANDELS versus HUDF).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Dependency of dynamical ejections of O stars on the masses of very young star clusters

Massive stars can be efficiently ejected from their birth star clusters through encounters with other massive stars. We study how the dynamical ejection fraction of O star systems varies with the masses of very young star clusters, M{sub ecl}, by means of direct N-body calculations. We include diverse initial conditions by varying the half-mass radius, initial mass segregation, initial binary fraction, and orbital parameters of the massive binaries. The results show robustly that the ejection fraction of O star systems exhibits a maximum at a cluster mass of 10{sup 3.5} M{sub ⊙} for all models, even though the number of ejected systems increases with cluster mass. We show that lower mass clusters (M{sub ecl}≈400 M{sub ⊙}) are the dominant sources for populating the Galactic field with O stars by dynamical ejections, considering the mass function of embedded clusters. About 15% (up to ≈38%, depending on the cluster models) of O stars of which a significant fraction are binaries, and which would have formed in a ≈10 Myr epoch of star formation in a distribution of embedded clusters, will be dynamically ejected to the field. Individual clusters may eject 100% of their original O star content. A large fraction of such O stars have velocities up to only 10 km s{sup −1}. Synthesising a young star cluster mass function, it follows, given the stellar-dynamical results presented here, that the observed fractions of field and runaway O stars, and the binary fractions among them, can be well understood theoretically if all O stars form in embedded clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE EFFECTS ON SUPERNOVA SHOCK BREAKOUT AND SWIFT LIGHT CURVES DUE TO THE MASS OF THE HYDROGEN-RICH ENVELOPE

Mass loss remains one of the primary uncertainties in stellar evolution. In the most massive stars, mass loss dictates the circumstellar medium and can significantly alter the fate of the star. Mass loss is caused by a variety of wind mechanisms and also through binary interactions. Supernovae (SNe) are excellent probes of this mass loss, both the circumstellar material and the reduced mass of the hydrogen-rich envelope. In this paper, we focus on the effects of reducing the hydrogen-envelope mass on the SN light curve, studying both the shock breakout and peak light-curve emission for a wide variety of mass-loss scenarios. Even though the trends of this mass loss will be masked somewhat by variations caused by different progenitors, explosion energies, and circumstellar media, these trends have significant effects on the SN light curves that should be seen in SN surveys. We conclude with a comparison of our results to a few key observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE MASS DISTRIBUTION FUNCTION OF PLANETS

The distribution of orbital period ratios of adjacent planets in extrasolar planetary systems discovered by the Kepler space telescope exhibits a peak near ∼1.5–2, a long tail of larger period ratios, and a steep drop-off in the number of systems with period ratios below ∼1.5. We find from these data that the dimensionless orbital separations have an approximately log-normal distribution. Using Hill’s criterion for the dynamical stability of two planets, we find an upper bound on planet masses such that the most common planet mass does not exceed 10{sup −3.2}m{sub ∗}, or about two-thirds of Jupiter’s mass for solar-mass stars. Assuming that the mass ratio and the dynamical separation (orbital spacings in units of mutual Hill radius) of adjacent planets are independent random variates, and adopting empirical distributions for these, we use Hill’s criterion in a statistical way to estimate the planet mass distribution function from the observed distribution of orbital separations. We find that the planet mass function is peaked in logarithm of mass, with a peak value and standard deviation of logm/M{sub ⊕} of ∼(0.6−1.0) and ∼(1.1−1.2), respectively.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Measuring the mass of the central black hole in the bulgeless galaxy NGC 4395 from gas dynamical modeling

NGC 4395 is a bulgeless spiral galaxy, harboring one of the nearest known type 1 Seyfert nuclei. Although there is no consensus on the mass of its central engine, several estimates suggest it is one of the lightest massive black holes (MBHs) known. We present the first direct dynamical measurement of the mass of this MBH from a combination of two-dimensional gas kinematic data, obtained with the adaptive optics assisted near-infrared integral field spectrograph Gemini/NIFS and high-resolution multiband photometric data from Hubble Space Telescope's Wide Field Camera 3. We use the photometric data to model the shape and stellar mass-to-light ratio of the nuclear star cluster (NSC). From the Gemini/NIFS observations, we derive the kinematics of warm molecular hydrogen gas as traced by emission through the H{sub 2} 1–0 S(1) transition. These kinematics show a clear rotational signal, with a position angle orthogonal to NGC 4395's radio jet. Our best-fitting tilted ring models of the kinematics of the molecular hydrogen gas contain a black hole with mass M=4{sub −3}{sup +8}×10{sup 5} M{sub ⊙} (3σ uncertainties) embedded in an NSC of mass M=2×10{sup 6} M{sub ⊙}. Our black hole mass measurement is in excellent agreement with the reverberation mapping mass estimate of Peterson et al. but shows some tension with other mass measurement methods based on accretion signals.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

CONNECTING FLARES AND TRANSIENT MASS-LOSS EVENTS IN MAGNETICALLY ACTIVE STARS

We explore the ramification of associating the energetics of extreme magnetic reconnection events with transient mass-loss in a stellar analogy with solar eruptive events. We establish energy partitions relative to the total bolometric radiated flare energy for different observed components of stellar flares and show that there is rough agreement for these values with solar flares. We apply an equipartition between the bolometric radiated flare energy and kinetic energy in an accompanying mass ejection, seen in solar eruptive events and expected from reconnection. This allows an integrated flare rate in a particular waveband to be used to estimate the amount of associated transient mass-loss. This approach is supported by a good correspondence between observational flare signatures on high flaring rate stars and the Sun, which suggests a common physical origin. If the frequent and extreme flares that young solar-like stars and low-mass stars experience are accompanied by transient mass-loss in the form of coronal mass ejections, then the cumulative effect of this mass-loss could be large. We find that for young solar-like stars and active M dwarfs, the total mass lost due to transient magnetic eruptions could have significant impacts on disk evolution, and thus planet formation, and also exoplanet habitability.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FORMALDEHYDE MASERS: EXCLUSIVE TRACERS OF HIGH-MASS STAR FORMATION

The detection of four formaldehyde (H{sub 2}CO) maser regions toward young high-mass stellar objects in the last decade, in addition to the three previously known regions, calls for an investigation of whether H{sub 2}CO masers are an exclusive tracer of young high-mass stellar objects. We report the first survey specifically focused on the search for 6 cm H{sub 2}CO masers toward non high-mass star-forming regions (non HMSFRs). The observations were conducted with the 305 m Arecibo Telescope toward 25 low-mass star-forming regions, 15 planetary nebulae and post-AGB stars, and 31 late-type stars. We detected no H{sub 2}CO emission in our sample of non HMSFRs. To check for the association between high-mass star formation and H{sub 2}CO masers, we also conducted a survey toward 22 high-mass star-forming regions from a Hi-GAL (Herschel infrared Galactic Plane Survey) sample known to harbor 6.7 GHz CH{sub 3}OH masers. We detected a new 6 cm H{sub 2}CO emission line in G32.74−0.07. This work provides further evidence that supports an exclusive association between H{sub 2}CO masers and young regions of high-mass star formation. Furthermore, we detected H{sub 2}CO absorption toward all Hi-GAL sources, and toward 24 low-mass star-forming regions. We also conducted a simultaneous survey for OH (4660, 4750, 4765 MHz), H110α (4874 MHz), HCOOH (4916 MHz), CH{sub 3}OH (5005 MHz), and CH{sub 2}NH (5289 MHz) toward 68 of the sources in our sample of non HMSFRs. With the exception of the detection of a 4765 MHz OH line toward a pre-planetary nebula (IRAS 04395+3601), we detected no other spectral line to an upper limit of 15 mJy for most sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

SHAPING THE DUST MASS-STAR-FORMATION RATE RELATION

There is a remarkably tight relation between the observationally inferred dust masses and star-formation rates (SFRs) of Sloan Digital Sky Survey galaxies, M {sub dust} ∝ SFR{sup 1.11}. Here we extend the M {sub dust}-SFR relation to the high end and show that it bends over at very large SFRs (i.e., dust masses are lower than predicted for a given SFR). We identify several distinct evolutionary processes in the diagram: (1) a star-bursting phase in which dust builds up rapidly at early times. The maximum attainable dust mass in this process is the cause of the bend-over of the relation. A high dust-formation efficiency, a bottom-light initial mass function, and negligible supernova shock dust destruction are required to produce sufficiently high dust masses. (2) A quiescent star-forming phase in which the subsequent parallel decline in dust mass and SFR gives rise to the M {sub dust}-SFR relation, through astration and dust destruction. The dust-to-gas ratio is approximately constant along the relation. We show that the power-law slope of the M {sub dust}-SFR relation is inversely proportional to the global Schmidt-Kennicutt law exponent (i.e., ∼0.9) in simple chemical evolution models. (3) A quenching phase which causes star formation to drop while the dust mass stays roughly constant or drops proportionally. Combined with merging, these processes, as well as the range in total baryonic mass, give rise to a complex population of the diagram which adds significant scatter to the original M {sub dust}-SFR relation. (4) At very high redshifts, a population of galaxies located significantly below the local relation is predicted.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

TESTING THE ASTEROSEISMIC MASS SCALE USING METAL-POOR STARS CHARACTERIZED WITH APOGEE AND KEPLER

Fundamental stellar properties, such as mass, radius, and age, can be inferred using asteroseismology. Cool stars with convective envelopes have turbulent motions that can stochastically drive and damp pulsations. The properties of the oscillation frequency power spectrum can be tied to mass and radius through solar-scaled asteroseismic relations. Stellar properties derived using these scaling relations need verification over a range of metallicities. Because the age and mass of halo stars are well-constrained by astrophysical priors, they provide an independent, empirical check on asteroseismic mass estimates in the low-metallicity regime. We identify nine metal-poor red giants (including six stars that are kinematically associated with the halo) from a sample observed by both the Kepler space telescope and the Sloan Digital Sky Survey-III APOGEE spectroscopic survey. We compare masses inferred using asteroseismology to those expected for halo and thick-disk stars. Although our sample is small, standard scaling relations, combined with asteroseismic parameters from the APOKASC Catalog, produce masses that are systematically higher (<ΔM > =0.17 ± 0.05 M {sub ☉}) than astrophysical expectations. The magnitude of the mass discrepancy is reduced by known theoretical corrections to the measured large frequency separation scaling relationship. Using alternative methods for measuring asteroseismic parameters induces systematic shifts at the 0.04 M {sub ☉} level. We also compare published asteroseismic analyses with scaling relationship masses to examine the impact of using the frequency of maximum power as a constraint. Upcoming APOKASC observations will provide a larger sample of ∼100 metal-poor stars, important for detailed asteroseismic characterization of Galactic stellar populations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

LOW-MASS AGNs AND THEIR RELATION TO THE FUNDAMENTAL PLANE OF BLACK HOLE ACCRETION

We put active galactic nuclei (AGNs) with low-mass black holes on the fundamental plane of black hole accretion—the plane that relates X-ray emission, radio emission, and mass of an accreting black hole—to test whether or not the relation is universal for both stellar-mass and supermassive black holes. We use new Chandra X-ray and Very Large Array radio observations of a sample of black holes with masses less than 10{sup 6.3} M {sub ☉}, which have the best leverage for determining whether supermassive black holes and stellar-mass black holes belong on the same plane. Our results suggest that the two different classes of black holes both belong on the same relation. These results allow us to conclude that the fundamental plane is suitable for use in estimating supermassive black hole masses smaller than ∼10{sup 7} M {sub ☉}, in testing for intermediate-mass black holes, and in estimating masses at high accretion rates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE MASS-DEPENDENCE OF ANGULAR MOMENTUM EVOLUTION IN SUN-LIKE STARS

To better understand the observed distributions of the rotation rate and magnetic activity of Sun-like and low-mass stars, we derive a physically motivated scaling for the dependence of the stellar wind torque on the Rossby number. The torque also contains an empirically derived scaling with stellar mass (and radius), which provides new insight into the mass-dependence of stellar magnetic and wind properties. We demonstrate that this new formulation explains why the lowest mass stars are observed to maintain rapid rotation for much longer than solar-mass stars, and simultaneously why older populations exhibit a sequence of slowly rotating stars, in which the low-mass stars rotate more slowly than solar-mass stars. The model also reproduces some previously unexplained features in the period-mass diagram for the Kepler field, notably: the particular shape of the ''upper envelope'' of the distribution, suggesting that ∼95% of Kepler field stars with measured rotation periods are younger than ∼4 Gyr; and the shape of the ''lower envelope'', corresponding to the location where stars transition between magnetically saturated and unsaturated regimes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

First Penning trap mass measurement of 36 Ca

Background: Isobaric quintets provide the best test of the isobaric multiplet mass equation (IMME) and can uniquely identify higher order corrections suggestive of isospin symmetry breaking effects in the nuclear Hamilto nian. The Generalized IMME (GIMME) is a novel microscopic interaction theory that predicts an extension to the quadratic form of the IMME. Only the A = 20, 32 T = 2 quintets have the exotic T z = –2 member ground state mass determined to high-precision by Penning trap mass spectrometry. Purpose: To establish A = 36 as the third high-precision T = 2 isobaric quintet with the T z = –2 member ground state mass measured by Penning trap mass spectrometry and provide the first test of the predictive power of the GIMME. Method: Here, a radioactive beam of neutron-deficient 36 Ca was produced by projectile fragmentation at the National Superconducting Cyclotron Laboratory. The beam was thermalized and the mass of 36 Ca + and 36 Ca 2+ measured by the Time of Flight - Ion Cyclotron Resonance method in the LEBIT 9.4 T Penning trap. Results: We measure the mass excess of 36 Ca to be ME= –6483.6(56) keV, an improvement in precision by a factor of 6 over the literature value. The new datum is considered together with evaluated nuclear data on the A = 36, T = 2 quintet. We find agreement with the quadratic form of the IMME given by isospin symmetry, but only coarse qualitative agreement with predictions of the GIMME. Conclusion: A total of three isobaric quintets have their most exotic members measured by Penning trap mass spectrometry. The GIMME predictions in the T = 2 quintet appear to break down for A = 32 and greater.

20 ≤ A ≤ 38↗
DOE OSTI2015

Neutron-proton effective mass splitting in terms of symmetry energy and its density slope

Using a simple density-dependent finite-range effective interaction having Yukawa form, the density dependence of isoscalar and isovector effective masses is studied. The isovector effective mass is found to be different for different pairs of like and unlike nucleons. Using HVH theorem, the neutron-proton effective mass splitting is represented in terms of symmetry energy and its density slope. It is again observed that the neutron-proton effective mass splitting has got a positive value when isoscalar effective mass is greater than the isovector effective mass and has a negative value for the opposite case. Furthermore, the neutron-proton effective mass splitting is found to have a linear dependence on asymmetry β. The second-order symmetry potential has a vital role in the determination of density slope of symmetry energy but it does not have any contribution on neutron-proton effective mass splitting. The finite-range effective interaction is compared with the SLy2, SKM, f{sub −}, f{sub 0}, and f{sub +} forms of interactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Nucleon Resonance Electroexcitation Amplitudes and Emergent Hadron Mass

Understanding the strong interaction dynamics that govern the emergence of hadron mass (EHM) represents a challenging open problem in the Standard Model. In this paper we describe new opportunities for gaining insight into EHM from results on nucleon resonance (N*) electroexcitation amplitudes (i.e., γvpN* electrocouplings) in the mass range up to 1.8 GeV for virtual photon four-momentum squared (i.e., photon virtualities Q2) up to 7.5 GeV2 available from exclusive meson electroproduction data acquired during the 6-GeV era of experiments at Jefferson Laboratory (JLab). These results, combined with achievements in the use of continuum Schwinger function methods (CSMs), offer new opportunities for charting the momentum dependence of the dressed quark mass from results on the Q2-evolution of the γvpN* electrocouplings. This mass function is one of the three pillars of EHM and its behavior expresses influences of the other two, viz. the running gluon mass and momentum-dependent effective charge. A successful description of the Δ(1232)3/2+ and N(1440)1/2+ electrocouplings has been achieved using CSMs with, in both cases, common momentum-dependent mass functions for the dressed quarks, for the gluons, and the same momentum-dependent strong coupling. The properties of these functions have been inferred from nonperturbative studies of QCD and confirmed, e.g., in the description of nucleon and pion elastic electromagnetic form factors. Parameter-free CSM predictions for the electrocouplings of the Δ(1600)3/2+ became available in 2019. The experimental results obtained in the first half of 2022 have confirmed the CSM predictions. We also discuss prospects for these studies during the 12-GeV era at JLab using the CLAS12 detector, with experiments that are currently in progress, and canvass the physics motivation for continued studies in this area with a possible increase of the JLab electron beam energy up to 22 GeV. Such an upgrade would finally enable mapping of the dressed quark mass over the full range of distances (i.e., quark momenta) where the dominant part of hadron mass and N* structure emerge in the transition from the strongly coupled to perturbative QCD regimes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

ASAS J083241+2332.4: A NEW EXTREME LOW MASS RATIO OVERCONTACT BINARY SYSTEM

We present the R- and V-band CCD photometry and Hα line studies of an overcontact binary ASAS J083241+2332.4. The light curves exhibit totality along with a trace of the O’Connell effect. The photometric solution indicates that this system falls into the category of extreme low-mass ratio overcontact binaries with a mass ratio, q ∼ 0.06. Although a trace of the O’ Connell effect is observed, constancy of the Hα line along various phases suggest that a relatively higher magnetic activity is needed for it to show a prominent fill-in effect. The study of O–C variations reveals that the period of the binary shows a secular increase at the rate of dP/dt ∼ 0.0765 s years{sup −1}, which is superimposed by a low, but significant, sinusoidal modulation with a period of ∼8.25 years. Assuming that the sinusoidal variation is due to the presence of a third body, orbital elements have been derived. There exist three other similar systems, SX Crv, V857 Her, and E53, which have extremely low mass ratios and we conclude that ASAS J083241+2332.4 resembles SX Crv in many respects. Theoretical studies indicate that at a critical mass ratio range, q{sub critical} = 0.07–0.09, overcontact binaries should merge and form a fast rotating star, but it has been suggested that q{sub critical} can continue to fall up to 0.05 depending on the primary's mass and structure. Moreover, the obtained fill-out factors (50%–70%) indicate that mass loss is considerable and hydrodynamical simulations advocate that mass loss from L{sub 2} is mandatory for a successful merging process. Comprehensively, the results indicate that ASAS J083241+2332.4 is at a stage of merger. The pivotal role played by the subtle nature of the derived mass ratio in forming a rapidly rotating star has been discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A population of intermediate-mass black holes in dwarf starburst galaxies up to redshift = 1.5

We study a sample of ∼50,000 dwarf starburst and late-type galaxies drawn from the COSMOS survey with the aim of investigating the presence of nuclear accreting black holes (BHs) as those seed BHs from which supermassive BHs could grow in the early universe. We divide the sample into five complete redshift bins up to z = 1.5 and perform an X-ray stacking analysis using the Chandra COSMOS-Legacy survey data. After removing the contribution from X-ray binaries and hot gas to the stacked X-ray emission, we still find an X-ray excess in the five redshift bins that can be explained by nuclear accreting BHs. This X-ray excess is more significant for z<0.5. At higher redshifts, these active galactic nuclei could suffer mild obscuration, as indicated by the analysis of their hardness ratios. The average nuclear X-ray luminosities in the soft band are in the range 10{sup 39}–10{sup 40} erg s{sup −1}. Assuming that the sources accrete at ≥1% the Eddington rate, their BH masses would be ≤10{sup 5} M{sub ⊙}, thus in the intermediate-mass BH regime, but their mass would be smaller than the one predicted by the BH-stellar mass relation. If instead the sources follow the correlation between BH mass and stellar mass, they would have sub-Eddington accreting rates of ∼10{sup −3} and BH masses 1–9 × 10{sup 5} M{sub ⊙}. We thus conclude that a population of intermediate-mass BHs exists in dwarf starburst galaxies, at least up to z = 1.5, though their detection beyond the local universe is challenging due to their low luminosity and mild obscuration unless deep surveys are employed.

79 ASTRONOMY AND ASTROPHYSICS↗