Faint Compact Sources Research Articles

Exploring three faint source detections methods for aperture synthesis radio images

Wide-field radio interferometric images often contain a large population of faint compact sources. Due to their low intensity/noise ratio, these objects can be easily missed by automated detection methods, which have been classically based on thresholding techniques after local noise estimation. The aim of this paper is to present and analyse the performance of several alternative or complementary techniques to thresholding. We compare three different algorithms to increase the detection rate of faint objects. The first technique consists of combining wavelet decomposition with local thresholding. The second technique is based on the structural behaviour of the neighbourhood of each pixel. Finally, the third algorithm uses local features extracted from a bank of filters and a boosting classifier to perform the detections. The methods’ performances are evaluated using simulations and radio mosaics from the Giant Metrewave Radio Telescope and the Australia Telescope Compact Array. We show that the new methods perform better than well-known state of the art methods such as SExtractor, SAD and DUCHAMP at detecting faint sources of radio interferometric images.

X-ray emission from star-forming galaxies - II. Hot interstellar medium

We study the emission from the hot interstellar medium in a sample of nearby late type galaxies defined in Paper I. Our sample covers a broad range of star formation rates, from ~0.1 Msun/yr to ~17 Msun/yr and stellar masses, from ~3x10^8 Msun to ~6x10^10 Msun. We take special care of systematic effects and contamination from bright and faint compact sources. We find that in all galaxies at least one optically thin thermal emission component is present in the unresolved emission, with the average temperature of <kT>= 0.24 keV. In about ~1/3 of galaxies, a second, higher temperature component is required, with the <kT>= 0.71 keV. Although statistically significant variations in temperature between galaxies are present, we did not find any meaningful trends with the stellar mass or star formation rate of the host galaxy. The apparent luminosity of the diffuse emission in the 0.5-2 keV band linearly correlates with the star formation rate with the scale factor of Lx/SFR\approx 8.3x10^38 erg/s per Msun/yr, of which in average ~30-40% is likely produced by faint compact sources of various types. We attempt to estimate the bolometric luminosity of the gas and and obtained results differing by an order of magnitude, log(Lbol/SFR)\sim39-40, depending on whether intrinsic absorption in star-forming galaxies was allowed or not. Our theoretically most accurate, but in practice the most model dependent result for the intrinsic bolometric luminosity of ISM is Lbol/SFR\sim 1.5x10^40 erg/s per Msun/yr. Assuming that core collapse supernovae are the main source of energy, it implies that \epsilon_SN\sim5x10^-2 (E_SN/10^51)^-1 of mechanical energy of supernovae is converted into thermal energy of ISM.

Direct estimate of cirrus noise inHerschelHi-GAL images

In <i>Herschel<i/> images of the Galactic plane and many star forming regions, a major factor limiting our ability to extract faint compact sources is cirrus confusion noise, operationally defined as the “statistical error to be expected in photometric measurements due to confusion in a background of fluctuating surface brightness”. The histogram of the flux densities of extracted sources shows a distinctive faint-end cutoff below which the catalog suffers from incompleteness and the flux densities become unreliable. This empirical cutoff should be closely related to the estimated cirrus noise and we show that this is the case. We compute the cirrus noise directly, both on <i>Herschel<i/> images from which the bright sources have been removed and on simulated images of cirrus with statistically similar fluctuations. We connect these direct estimates with those from power spectrum analysis, which has been used extensively to predict the cirrus noise and provides insight into how it depends on various statistical properties and photometric operational parameters. We report multi-wavelength power spectra of diffuse Galactic dust emission from Hi-GAL observations at 70 to 500 <i>μ<i/>m within Galactic plane fields at <i>l<i/> = 30 °and <i>l<i/> = 59 °. We find that the exponent of the power spectrum is about –3. At 250 <i>μ<i/>m, the amplitude of the power spectrum increases roughly as the square of the median brightness of the map and so the expected cirrus noise scales linearly with the median brightness. For a given region, the wavelength dependence of the amplitude can be described by the square of the spectral energy distribution (SED) of the dust emission. Generally, the confusion noise will be a worse problem at longer wavelengths, because of the combination of lower angular resolution and the rising power spectrum of cirrus toward lower spatial frequencies, but the photometric signal to noise will also depend on the relative SED of the source compared to the cirrus.

Unresolved X-ray emission in M31 and constraints on progenitors of classical novae

We investigate unresolved X-ray emission from M31 based on an extensive set of archival XMM-Newton and Chandra data. We show that extended emission, found previously in the bulge and thought to be associated with a large number of faint compact sources, extends to the disk of the galaxy with similar X-ray to K-band luminosity ratio. We also detect excess X-ray emission associated with the 10-kpc star-forming ring. The L_X/SFR ratio in the 0.5-2 keV band ranges from zero to ~1.8 x 10^38 (erg/s)/(M_sun/yr), excluding the regions near the minor axis of the galaxy where it is ~1.5-2 times higher. The latter is likely associated with warm ionized gas of the galactic wind rather than with the star-forming ring itself. Based on this data, we constrain the nature of Classical Nova (CN) progenitors. We use the fact that hydrogen-rich material, required to trigger the explosion, accumulates on the white dwarf surface via accretion. Depending on the type of the system, the energy of accretion may be radiated at X-ray energies, thus contributing to the unresolved X-ray emission. Based on the CN rate in the bulge of M31 and its X-ray surface brightness, we show that no more than ~10 per cent of CNe can be produced in magnetic cataclysmic variables, the upper limit being ~3 per cent for parameters typical for CN progenitors. In dwarf novae, >~90-95 per cent of the material must be accreted during outbursts, when the emission spectrum is soft, and only a small fraction in quiescent periods, characterized by rather hard spectra.

The Nature of the Cluster of Radio Sources in GGD 14

We present sensitive radio continuum observations at 3.6 and 6 cm made quasi-simultaneously toward the star-forming region GGD 14. The observations reveal the presence of nine extremely compact (≤03) and faint radio sources in a region of ~30'' around the bright cometary H II region, VLA 1. Most of these sources show variability at radio wavelengths, and their spectral indices are characteristically negative. Four of the faint sources have an infrared counterpart. We propose that gyrosynchrotron radiation from an active magnetosphere explains the emission from almost all the faint compact sources, suggesting that the GGD 14 region harbors a cluster of low-mass, pre-main-sequence stars. Two sources that do not show time variability are VLA 1, the cometary H II region, and VLA 7. The derived spectral index for VLA 7 (+0.6 ± 0.3) fits the stellar wind model better than other possibilities, supporting the idea that VLA 7 is the powering source of the molecular outflow observed toward the GGD 14 region.

Dynamics of “Small Galaxies” in the Hubble Deep Field

We have previously found in the Hubble Deep Field (HDF) a significant angular correlation of faint, high-color redshift objects on scales below 1'', or several kiloparsecs in metric size. A correlation at this scale is most likely due to physical associations. We examine the correlation and nearest neighbor statistics to conclude that 38% of these objects in the HDF have a companion within 1'' (or ~6 kpc), 3 times the number expected in a random distribution with the same number of objects; the total approaches 1.5 objects by separations of 10''. We next examine three possible dynamical scenarios for these object multiplets: (1) the objects are star-forming regions within normal galaxies, whose disks have been relatively dimmed by K-correction and surface brightness dimming; (2) they are fragments merging into large galaxies; (3) they are satellites accreting onto parent normal L* galaxies. We find that hypothesis 1 is most tenable. First, large galaxies in the process of a merger formation would have accumulated too much mass in their centers (5 × 1012 M☉ inside 2 kpc) to correspond to any abundant category of present-day objects. Second, accretion by dynamical friction occurs with a predictable slope in density versus radius that is not seen among the faint HDF objects. Since the dynamical friction time is roughly 1 Gyr, a steady state should have been reached by redshift z 5. In the context of these two dynamical scenarios, we consider the possible effects of a gradient in mass-to-light ratio caused by induced star formation during infall. We note that star-forming regions within galaxies clearly present no dynamical problems, but also that large spirals would still appear as such in the HDF, which leads us to favor a scenario in which the faint compact sources in the HDF are giant star-forming regions within small normal galaxies, such as Magellanic irregulars. Last, we note that the excess number of correlated objects near a given faint source approaches 1.5, suggesting that the previous counts of objects have overestimated the number of galaxies by a factor of 2.5 while underestimating their individual luminosities by the same factor.