Luminosity Function Of Sources Research Articles

Neutrino follow-up with the Zwicky transient facility: results from the first 24 campaigns

ABSTRACT The Zwicky Transient Facility (ZTF) performs a systematic neutrino follow-up programme, searching for optical counterparts to high-energy neutrinos with dedicated Target-of-Opportunity (ToO) observations. Since first light in March 2018, ZTF has taken prompt observations for 24 high-quality neutrino alerts from the IceCube Neutrino Observatory, with a median latency of 12.2 h from initial neutrino detection. From two of these campaigns, we have already reported tidal disruption event (TDE) AT 2019dsg and likely TDE AT 2019fdr as probable counterparts, suggesting that TDEs contribute >7.8 per cent of the astrophysical neutrino flux. We here present the full results of our programme through to December 2021. No additional candidate neutrino sources were identified by our programme, allowing us to place the first constraints on the underlying optical luminosity function of astrophysical neutrino sources. Transients with optical absolutes magnitudes brighter that −21 can contribute no more than 87 per cent of the total, while transients brighter than −22 can contribute no more than 58 per cent of the total, neglecting the effect of extinction and assuming they follow the star formation rate. These are the first observational constraints on the neutrino emission of bright populations such as superluminous supernovae. None of the neutrinos were coincident with bright optical AGN flares comparable to that observed for TXS 0506+056/IC170922A, with such optical blazar flares producing no more than 26 per cent of the total neutrino flux. We highlight the outlook for electromagnetic neutrino follow-up programmes, including the expected potential for the Rubin Observatory.

AGN Selection and Demographics in GOODS-S/HUDF from X-Ray to Radio

We present a comprehensive census of the active galactic nuclei (AGNs) in the GOODS-S/HUDF region from the X-ray to the radio, covering both the obscured and unobscured populations. This work includes a robust analysis of the source optical to mid-IR spectral energy distributions (SEDs) featuring (semi)empirical AGN and galaxy dust emission models and Bayesian fitting techniques, ultra-deep Very Large Array 3 and 6 GHz observations, and an integrated analysis of various AGN selection techniques, including X-ray properties, UV to mid-IR SED analysis, optical spectral features, mid-IR colors, radio-loudness and spectral slope, and AGN variability. In total, we report ∼900 AGNs over the ∼170 arcmin2 3D-HST GOODS-S footprint, which has doubled the AGN number identified in the previous X-ray sample, with ∼26% of our sample undetected in the deepest Chandra image. With a summary of AGN demographics from different selection methods, we find that no one single band or technique comes close to selecting a complete AGN sample despite the great depth of the data in GOODS-S/HUDF. We estimate the yields of various approaches and explore the reasons for incompleteness. We characterize the statistical properties, such as source number density, obscuration fraction, and luminosity function of the AGN sample in this field, and discuss their immediate implications. We also provide some qualitative predictions of the AGN sample that might be discovered by the upcoming JWST surveys.

ASAS-SN follow-up of IceCube high-energy neutrino alerts

ABSTRACT We report on the search for optical counterparts to IceCube neutrino alerts released between 2016 April and 2021 August with the All-Sky Automated Survey for SuperNovae (ASAS-SN). Despite the discovery of a diffuse astrophysical high-energy neutrino flux in 2013, the source of those neutrinos remains largely unknown. Since 2016, IceCube has published likely astrophysical neutrinos as public real-time alerts. Through a combination of normal survey and triggered target-of-opportunity observations, ASAS-SN obtained images within 1 h of the neutrino detection for 20 per cent (11) of all observable IceCube alerts and within one day for another 57 per cent (32). For all observable alerts, we obtained images within at least two weeks from the neutrino alert. ASAS-SN provides the only optical follow-up for about 17 per cent of IceCube’s neutrino alerts. We recover the two previously claimed counterparts to neutrino alerts, the flaring-blazar TXS 0506 + 056 and the tidal disruption event AT2019dsg. We investigate the light curves of previously detected transients in the alert footprints, but do not identify any further candidate neutrino sources. We also analysed the optical light curves of Fermi 4FGL sources coincident with high-energy neutrino alerts, but do not identify any contemporaneous flaring activity. Finally, we derive constraints on the luminosity functions of neutrino sources for a range of assumed evolution models.

Further support for a trio of mass-to-light deviations in Abell 370: free-form grale lens inversion using BUFFALO strong lensing data

ABSTRACT We use the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) strong lensing image catalogue of the merging galaxy cluster Abell 370 to obtain a mass model using free-form lens inversion algorithm grale. The improvement of the strong lensing data quality results in a lens plane rms of only 0.45 arcsec, about a factor of two lower than that of our existing HFF v4 reconstruction. We attribute the improvement to spectroscopic data and use of the full reprocessed HST mosaics. In our reconstructed mass model, we found indications of three distinct mass features in Abell 370: (i) a ∼35 kpc offset between the northern BCG and the nearest mass peak, (ii) a ∼100 kpc mass concentration of roughly critical density ∼250 kpc east of the main cluster, and (iii) a probable filament-like structure passing N-S through the cluster. While (i) is present in some form in most publicly available reconstructions spanning the range of modelling techniques: parametric, hybrid, and free-form, (ii) and (iii) are recovered by only about half of the reconstructions. We tested our hypothesis on the presence of the filament-like structure by creating a synthetic cluster – Irtysh IIIc – mocking the situation of a cluster with external mass. We also computed the source plane magnification distributions. Using them, we estimated the probabilities of magnifications in the source plane, and scrutinized their redshift dependence. Finally, we explored the lensing effects of Abell 370 on the luminosity functions of sources at zs = 9.0, finding it consistent with published results.

Modelled 3D distribution of OH/IR stars in the Galactic disc

ABSTRACT We have modelled the 3D distribution of OH/IR stars in the Galactic plane, traced by 1612 MHz OH maser sources with classic double horned spectral profiles. We statistically analysed over 700 maser sources detected by the HI/OH/Recombination line survey of the Milky Way (THOR) and the Australia Telescope Compact Array interferometric follow-up observations of the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH). With a simple model constructed from a classical density distribution of stars and luminosity functions of OH maser sources in the Galaxy, we estimate the scale height, or the half thickness of the OH/IR star distribution along the Galactic disc to be 90–290 pc. The simple model also implies that there are ∼4000 OH/IR stars hosting 1612 MHz OH masers along the Galactic Plane. Therefore, next generation telescopes such as the Australian Square Kilometre Array Pathfinder (ASKAP) and SKA Phase 1 will detect about 80 per cent of such OH/IR stars in the Galaxy at a 10 mJy detection limit. Comparing the data of previously detected circumstellar 1612 MHz OH maser sources with those of THOR and SPLASH, the maser source lifetime is estimated to be ∼300 yr. This is likely a lower limit, since non-detections of masers in some cases could be affected by the flux variation of the maser source.

AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124

ABSTRACT Swift J0243.6+6124, the first Galactic ultraluminous X-ray pulsar, was observed during its 2017–2018 outburst with AstroSat at both sub- and super-Eddington levels of accretion with X-ray luminosities of LX ∼ 7 × 1037 and 6 × 1038 erg s−1, respectively. Our broad-band timing and spectral observations show that X-ray pulsations at ${\sim}9.85~\rm {s}$ have been detected up to 150 keV when the source was accreting at the super-Eddington level. The pulse profiles are a strong function of both energy and source luminosity, showing a double-peaked profile with pulse fraction increasing from ∼$10{{{\ \rm per\ cent}}}$ at $1.65~\rm {keV}$ to 40–80 ${{\ \rm per\ cent}}$ at $70~\rm {keV}$. The continuum X-ray spectra are well modelled with a high-energy cut-off power law (Γ ∼ 0.6–0.7) and one or two blackbody components with temperatures of ∼0.35 and $1.2~\rm {keV}$, depending on the accretion level. No iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9 keV is observed at the super-Eddington level, along with a blackbody radius ($121\!-\!142~\rm {km}$) that indicates the presence of optically thick outflows.

New constraints on the 1.4 GHz source number counts and luminosity functions in the Lockman Hole field

ABSTRACT We present a study of the 1173 sources brighter than $S_{1.4\, \rm GHz}= 120\, \mu$Jy detected over an area of $\simeq 1.4\, \hbox{deg}^{2}$ in the Lockman Hole field. Exploiting the multiband information available in this field for ∼79 per cent of the sample, sources have been classified into radio loud (RL) active galactic nuclei (AGNs), star-forming galaxies (SFGs), and radio quiet (RQ) AGNs, using a variety of diagnostics available in the literature. Exploiting the observed tight anticorrelations between IRAC band 1 or band 2 and the source redshift we could assign a redshift to 177 sources missing a spectroscopic measurement or a reliable photometric estimate. A Monte Carlo approach was used to take into account the spread around the mean relation. The derived differential number counts and luminosity functions at several redshifts of each population show a good consistency with models and with earlier estimates made using data from different surveys and applying different approaches. Our results confirm that below $\sim 300\, \mu$Jy SFGs+RQ AGNs overtake RL AGNs that dominate at brighter flux densities. We also confirm earlier indications of a similar evolution of RQ AGNs and SFGs. Finally, we discuss the angular correlation function of our sources and highlight its sensitivity to the criteria used for the classification.

Characterizing the nature of the unresolved point sources in the Galactic Center: An assessment of systematic uncertainties

The Galactic Center Excess (GCE) of GeV gamma rays can be explained as a signal of annihilating dark matter or of emission from unresolved astrophysical sources, such as millisecond pulsars. Evidence for the latter is provided by a statistical procedure---referred to as Non-Poissonian Template Fitting (NPTF)---that distinguishes the smooth distribution of photons expected for dark matter annihilation from a "clumpy" photon distribution expected for point sources. In this paper, we perform an extensive study of the NPTF on simulated data, exploring its ability to recover the flux and luminosity function of unresolved sources at the Galactic Center. When astrophysical background emission is perfectly modeled, we find that the NPTF successfully distinguishes between the dark matter and point source hypotheses when either component makes up the entirety of the GCE. When the GCE is a mixture of dark matter and point sources, the NPTF may fail to reconstruct the correct contribution of each component. We further study the impact of mismodeling the Galactic diffuse backgrounds, finding that while a dark matter signal could be attributed to point sources in some outlying cases for the scenarios we consider, the significance of a true point source signal remains robust. Our work enables us to comment on a recent study by Leane and Slatyer (2019) that questions prior NPTF conclusions because the method does not recover an artificial dark matter signal injected on actual Fermi data. We demonstrate that the failure of the NPTF to extract an artificial dark matter signal can be natural when point sources are present in the data---with the effect further exacerbated by the presence of diffuse mismodeling---and does not on its own invalidate the conclusions of the NPTF analysis in the Inner Galaxy.

Optimally Mapping Large-scale Structures with Luminous Sources

Abstract Intensity mapping has emerged as a promising tool to probe the three-dimensional structure of the universe. The traditional approach of galaxy redshift surveys is based on individual galaxy detection, typically performed by thresholding and digitizing large-scale intensity maps. By contrast, intensity mapping uses the integrated emission from all sources in a 3D pixel (or voxel) as an analog tracer of large-scale structure. In this work, we develop a formalism to quantify the performance of both approaches when measuring large-scale structures. We compute the Fisher information of an arbitrary observable, derive the optimal estimator, and study its performance as a function of source luminosity function, survey resolution, instrument sensitivity, and other survey parameters. We identify regimes where each approach is advantageous and discuss optimal strategies for different scenarios. To determine the best strategy for any given survey, we develop a metric that is easy to compute from the source luminosity function and the survey sensitivity, and we demonstrate the application with several planned intensity mapping surveys.

The population of SNe/SNRs in the starburst galaxy Arp 220

Context. The nearby ultra-luminous infrared galaxy (ULIRG) Arp 220 is an excellent laboratory for studies of extreme astrophysical environments. For 20 years, Very Long Baseline Interferometry (VLBI) has been used to monitor a population of compact sources thought to be supernovae (SNe), supernova remnants (SNRs), and possibly active galactic nuclei (AGNs). SNe and SNRs are thought to be the sites of relativistic particle acceleration powering star formation induced radio emission in galaxies, and are hence important for studies of for example the origin of the FIR–radio correlation. Aims. In this work we aim for a self-consistent analysis of a large collection of Arp 220 continuum VLBI data sets. With more data and improved consistency in calibration and imaging, we aim to detect more sources and improve source classifications with respect to previous studies. Furthermore, we aim to increase the number of sources with robust size estimates, to analyse the compact source luminosity function (LF), and to search for a luminosity–diameter (LD) relation within Arp 220. Methods. Using new and archival VLBI data spanning 20 years, we obtained 23 high-resolution radio images of Arp 220 at wavelengths from 18 cm to 2 cm. From model-fitting to the images we obtained estimates of flux densities and sizes of detected sources. The sources were classified in groups according to their observed lightcurves, spectra and sizes. We fitted a multi-frequency supernova light-curve model to the object brightest at 6 cm to estimate explosion properties for this object. Results. We detect radio continuum emission from 97 compact sources and present flux densities and sizes for all analysed observation epochs. The positions of the sources trace the star forming disks of the two nuclei known from lower-resolution studies. We find evidence for a LD-relation within Arp 220, with larger sources being less luminous. We find a compact source LF n(L)∝Lβ with β = −2.19 ± 0.15, similar to SNRs in normal galaxies, and we argue that there are many relatively large and weak sources below our detection threshold. The brightest (at 6 cm) object 0.2195+0.492 is modelled as a radio SN with an unusually long 6 cm rise time of 17 years. Conclusions. The observations can be explained by a mixed population of SNe and SNRs, where the former expand in a dense circumstellar medium (CSM) and the latter interact with the surrounding interstellar medium (ISM). Nine sources are likely luminous SNe, for example type IIn, and correspond to few percent of the total number of SNe in Arp 220. Assuming all IIns reach these luminosities, and no confusion with other SNe types, our data are consistent with a total SN-rate of 4 yr−1 as inferred from the total radio emission given a normal stellar initial mass function (IMF). Based on the fitted luminosity function, we argue that emission from all compact sources, also below our detection threshold, make up at most 20% of the total radio emission at GHz frequencies. However, colliding SN shocks and the production of secondary electrons through cosmic ray (CR) protons colliding with the dense ISM may cause weak sources to radiate much longer than assumed in this work. This could potentially explain the remaining fraction of the smooth synchrotron component. Future, deeper observations of Arp 220 will probe the sources with lower luminosities and larger sizes. This will further constrain the evolution of SNe/SNRs in extreme environments and the presence of AGN activity.

Comment on “Characterizing the population of pulsars in the Galactic bulge with the Fermi large area telescope” [arXiv:1705.00009v1

Abstract The Fermi-LAT Collaboration recently presented a new catalog of gamma-ray sources located within the 40 ° × 40 ° region around the Galactic Center Ajello et al. (2017) — the Second Fermi Inner Galaxy (2FIG) catalog. Utilizing this catalog, they analyzed models for the spatial distribution and luminosity function of sources with a pulsar-like gamma-ray spectrum. Ajello et al. (2017) v1 also claimed to detect, in addition to a disk-like population of pulsar-like sources, an approximately 7 σ preference for an additional centrally concentrated population of pulsar-like sources, which they referred to as a “Galactic Bulge” population. Such a population would be of great interest, as it would support a pulsar interpretation of the gamma-ray excess that has long been observed in this region. In an effort to further explore the implications of this new source catalog, we attempted to reproduce the results presented by the Fermi-LAT Collaboration, but failed to do so. Mimicking as closely as possible the analysis techniques undertaken in Ajello et al. (2017), we instead find that our likelihood analysis favors a very different spatial distribution and luminosity function for these sources. Most notably, our results do not exhibit a strong preference for a “Galactic Bulge” population of pulsars. Furthermore, we find that masking the regions immediately surrounding each of the 2FIG pulsar candidates does not significantly impact the spectrum or intensity of the Galactic Center gamma-ray excess. Although these results refute the claim of strong evidence for a centrally concentrated pulsar population presented in Ajello et al. (2017), they neither rule out nor provide support for the possibility that the Galactic Center excess is generated by a population of low-luminosity and currently largely unobserved pulsars. In a spirit of maximal openness and transparency, we have made our analysis code available at https://github.com/bsafdi/GCE-2FIG .

A Mixture Evolution Scenario of the AGN Radio Luminosity Function. II. Do Low- and High-power Radio-loud AGNs Evolve Differently?

Abstract Following previous work, we further confirm that the cosmic evolution of steep-spectrum radio-loud AGNs (active galactic nuclei) can be reproduced by a simple combination of density evolution (DE) and luminosity evolution (LE). This mixture evolution scenario can naturally explain the luminosity-dependent evolution of radio-loud AGNs. Our models successfully fitted a large amount of data on radio luminosity functions of steep-spectrum sources and multi-frequency source counts. The modeling indicates that the DE slowly increases as out to , and then rapidly decreases as , while the LE rapidly increases as out to a higher redshift (at least ). We find a high-redshift decline (i.e., redshift cutoff) in the number density of steep-spectrum radio sources, but we cannot conclude whether such a decline is sharp or shallow. We believe that whether a redshift cutoff occurs or not depends mainly on DE, while its steepness is decided by LE, which, however, cannot be well constrained due to the lack of high-redshift samples. Most intriguingly, according to our mixture evolution scenario, there appears to be no need for different evolution for the low- and high-power radio-loud AGNs. Both types of sources experience the same combined evolution of DE and LE.

NONPARAMETRIC ESTIMATORS FOR INCOMPLETE SURVEYS

ABSTRACT Nonparametric estimators, such as the estimator and the estimator, have been applied extensively to estimate luminosity functions (LFs) of astronomical sources from complete, truncated survey data sets. Application of such estimators to incomplete data sets typically requires further truncation of data, separation into subsets of constant completeness, and/or correction for incompleteness-induced bias. In this paper, we derive generalizations of the above estimators designed for use with incomplete, truncated data sets. We compare these generalized nonparametric estimators, investigate some of their simple statistical properties, and validate them using Monte Carlo simulation methods. We apply a nonparametric estimator to data obtained from the extended Baryon Oscillation Spectroscopic Survey to estimate the QSO LF for redshifts .

CHANDRA ACIS SURVEY OF X-RAY POINT SOURCES IN NEARBY GALAXIES. II. X-RAY LUMINOSITY FUNCTIONS AND ULTRALUMINOUS X-RAY SOURCES

ABSTRACT Based on the recently completed Chandra/ACIS survey of X-ray point sources in nearby galaxies, we study the X-ray luminosity functions (XLFs) for X-ray point sources in different types of galaxies and the statistical properties of ultraluminous X-ray sources (ULXs). Uniform procedures are developed to compute the detection threshold, to estimate the foreground/background contamination, and to calculate the XLFs for individual galaxies and groups of galaxies, resulting in an XLF library of 343 galaxies of different types. With the large number of surveyed galaxies, we have studied the XLFs and ULX properties across different host galaxy types, and confirm with good statistics that the XLF slope flattens from lenticular (α ∼ 1.50 ± 0.07) to elliptical (∼1.21 ± 0.02), to spirals (∼0.80 ± 0.02), to peculiars (∼0.55 ± 0.30), and to irregulars (∼0.26 ± 0.10). The XLF break dividing the neutron star and black hole binaries is also confirmed, albeit at quite different break luminosities for different types of galaxies. A radial dependency is found for ellipticals, with a flatter XLF slope for sources located between D 25 and 2D 25, suggesting the XLF slopes in the outer region of early-type galaxies are dominated by low-mass X-ray binaries in globular clusters. This study shows that the ULX rate in early-type galaxies is 0.24 ± 0.05 ULXs per surveyed galaxy, on a 5σ confidence level. The XLF for ULXs in late-type galaxies extends smoothly until it drops abruptly around 4 × 1040 erg s−1, and this break may suggest a mild boundary between the stellar black hole population possibly including 30 M ⊙ black holes with super-Eddington radiation and intermediate mass black holes.

Binary black hole merger rates inferred from luminosity function of ultra-luminous X-ray sources

The Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) has detected direct signals of gravitational waves (GWs) from GW150914. The event was a merger of binary black holes whose masses are $36^{+5}_{-4}M_{\odot}$ and $29^{+4}_{-4}M_{\odot}$. Such binary systems are expected to be directly evolved from stellar binary systems or formed by dynamical interactions of black holes in dense stellar environments. Here we derive the binary black hole merger rate based on the nearby ultra-luminous X-ray source (ULX) luminosity function (LF) under the assumption that binary black holes evolve through X-ray emitting phases. We obtain the binary black hole merger rate as $5.8 ({t}_{\rm ULX}/{0.1 \ \rm Myr})^{-1} \lambda^{-0.6} \exp{(-0.30\lambda)} \ {\rm Gpc^{-3}\ yr^{-1}}$, where $t_{\rm ULX}$ is the typical duration of the ULX phase and $\lambda$ is the Eddington ratio in luminosity. This is coincident with the event rate inferred from the detection of GW150914 as well as the predictions based on binary population synthesis models. Although we are currently unable to constrain the Eddington ratio of ULXs in luminosity due to the uncertainties of our models and measured binary black hole merger event rates, further X-ray and GW data will allow us to narrow down the range of the Eddington ratios of ULXs. We also find the cumulative merger rate for the mass range of $5M_\odot\le M_{\rm BH}\le100M_\odot$ inferred from the ULX LF is consistent with that estimated by the aLIGO collaboration considering various astrophysical conditions such as the mass function of black holes.

Wavelet-based techniques for the gamma-ray sky

We demonstrate how the image analysis technique of wavelet decomposition can be applied to the gamma-ray sky to separate emission on different angular scales. New structures on scales that differ from the scales of the conventional astrophysical foreground and background uncertainties can be robustly extracted, allowing a model-independent characterization with no presumption of exact signal morphology. As a test case, we generate mock gamma-ray data to demonstrate our ability to extract extended signals without assuming a fixed spatial template. For some point source luminosity functions, our technique also allows us to differentiate a diffuse signal in gamma-rays from dark matter annihilation and extended gamma-ray point source populations in a data-driven way.

The high-redshift star formation history from carbon-monoxide intensity maps

Abstract We demonstrate how cosmic star formation history can be measured with one-point statistics of carbon-monoxide intensity maps. Using a P(D) analysis, the luminosity function of CO-emitting sources can be inferred from the measured one-point intensity PDF. The star formation rate density (SFRD) can then be obtained, at several redshifts, from the CO luminosity density. We study the effects of instrumental noise, line foregrounds, and target redshift, and obtain constraints on the CO luminosity density of the order of 10 per cent. We show that the SFRD uncertainty is dominated by that of the model connecting CO luminosity and star formation. For pessimistic estimates of this model uncertainty, we obtain an error of the order of 50 per cent on SFRD for surveys targeting redshifts between two and seven with reasonable noise and foregrounds included. However, comparisons between intensity maps and galaxies could substantially reduce this model uncertainty. In this case, our constraints on SFRD at these redshifts improve to roughly 5 − 10 per cent, which is highly competitive with current measurements.

XMM-Newtonstudy of the Draco dwarf spheroidal galaxy

Aims. We present the results of the analysis of five XMM-Newton observations of the Draco dwarf spheroidal galaxy (dSph). The aim of the work is the study of the X-ray population in the field of the Draco dSph.Methods. We classified the sources on the basis of spectral analysis, hardness ratios, X-ray-to-optical flux ratio, X-ray variability, and cross-correlation with available catalogues in X-ray, optical, infrared, and radio wavelengths.Results. We detected 70 X-ray sources in the field of the Draco dSph in the energy range of 0.2 − 12 keV and classified 18 AGNs, 9 galaxies and galaxy candidates, 6 sources as foreground stars, 4 low-mass X-ray binary candidates, 1 symbiotic star, and 2 binary system candidates. We also identified 9 sources as hard X-ray sources in the field of the galaxy. We derived the X-ray luminosity function of X-ray sources in the Draco dSph in the 2 − 10 keV and 0.5 − 2 keV energy bands. Using the X-ray luminosity function in the energy range of 0.5 − 2 keV, we estimate that ~10 X-ray sources are objects in the Draco dSph. We have also estimated the dark matter halo mass that would be needed to keep the low-mass X-ray binaries gravitationally bound to the galaxy.

Linking low- to high-mass young stellar objects withHerschel-HIFI observations of water

Water probes the dynamics in young stellar objects (YSOs) effectively, especially shocks in molecular outflows. It is a key molecule for exploring whether the physical properties of low-mass protostars can be extrapolated to massive YSOs. As part of the WISH key programme, we investigate the dynamics and the excitation conditions of shocks along the outflow cavity wall as function of source luminosity. Velocity-resolved Herschel-HIFI spectra of the H2O 988, 752, 1097 GHz and 12CO J=10-9, 16-15 lines were analysed for 52 YSOs with bolometric luminosities (L_bol) ranging from <1 to >10^5 L_sun. The profiles of the H2O lines are similar, indicating that they probe the same gas. We see two main Gaussian emission components in all YSOs: a broad component associated with non-dissociative shocks in the outflow cavity wall (cavity shocks) and a narrow component associated with quiescent envelope material. More than 60% of the total integrated intensity of the H2O lines (L_H2O) comes from the cavity shock component. The H2O line widths are similar for all YSOs, whereas those of 12CO 10-9 increase slightly with L_bol. The excitation analysis of the cavity shock component, performed with the non-LTE radiative transfer code RADEX, shows stronger 752 GHz emission for high-mass YSOs, likely due to pumping by an infrared radiation field. As previously found for CO, a strong correlation with slope unity is measured between log(L_H2O) and log(L_bol), which can be extrapolated to extragalactic sources. We conclude that the broad component of H2O and high-J CO lines originate in shocks in the outflow cavity walls for all YSOs, whereas lower-J CO transitions mostly trace entrained outflow gas. The higher UV field and turbulent motions in high-mass objects compared to their low-mass counterparts may explain the slightly different kinematical properties of 12CO 10-9 and H2O lines from low- to high-mass YSOs.

CHANDRAAND VERY LARGE ARRAY OBSERVATIONS OF THE NEARBY Sd GALAXY NGC 45

We present an analysis of high angular resolution observations made in the X-ray and the radio with the Chandra X-ray Observatory and the Karl Jansky Very Large Array (VLA), respectively, of the nearby spiral galaxy NGC 45. This galaxy is the third that we have considered in a study of the supernova remnant (SNR) populations of nearby spiral galaxies and the present work represents the first detailed analysis of the discrete X-ray and radio source populations of this galaxy. We analyzed data sets from the three pointed observations made of this galaxy with Chandra along with a merged data set obtained from combining these data sets: the total effective exposure time of the merged data set is 63515 s. A total of 25 discrete X-ray sources are found in the entire field of view of the ACIS-S3 chip, with 16 sources found within the visual extent of the galaxy. We estimate that as many as half of the sources detected in the entire field of view of the ACIS-S3 chip and seven of the sources detected in the optical extent of NGC 45 may be background sources. We analyzed the spectral properties of the discrete X-ray sources within the galaxy and conclude that the majority of these sources are X-ray binaries. We have searched for counterparts at different wavelengths to the discrete X-ray sources and we find two associations: one with a star cluster and the other with a background galaxy. We have found one source that is clearly variable within one observation and seven that are seen to vary from one observation to another. We also conduct a photometric analysis to determine the near-infrared fluxes of the discrete X-ray sources in Spitzer Infrared Array Camera channels. We constructed a cumulative luminosity function of the discrete X-ray sources seen toward NGC 45: taking into account simultaneously the luminosity function of background sources, the fitted slope of the cumulative luminosity function Γ = –1.3_(-1.6)^(+0.7) (all error bounds correspond to 90% confidence intervals). The VLA observations reveal seven discrete radio sources: we find no overlaps between these sources and the X-ray detected sources. Based on their measured spectral indices and their locations with respect to the visible extent of NGC 45, we classify one source as a candidate radio SNR associated with the galaxy and the others as likely background galaxies seen in projection toward NGC 45. Finally, we discuss the properties of a background cluster of galaxies (denoted as CXOU J001354.2–231254.7) seen in projection toward NGC 45 and detected by the Chandra observations. The fit parameters to the extracted Chandra spectra of this cluster are a column density N_H = 0.07(<0.14) × 10^(22) cm^(−2), a temperature kT = 4.22_(-1.42)^(+2.08) keV, an abundance Z = 0.30(<0.75) relative to solar and a redshift z = 0.28 ± 0.14. From the fit parameters we derive an electron number density n_e = 4(±1) × 10^(−3) cm^(−3), an unabsorbed X-ray luminosity L_(0.5-7.0keV) ~ 8.77(±0.96) × 10^(43) erg s^(−1) for the cluster and an X-ray emitting mass M = 2.32(±1.75) × 10^(12)M_☉.