A Multiwavelength Study of Supernova Remnants in Six Nearby Galaxies. I. Detection of New X-ray-selected Supernova Remnants with Chandra (original) (raw)
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Advances in Space Research, 2006
Today's sensitive, high-resolution Chandra X-ray observations allow the study of many populations of X-ray sources. The traditional astronomical tools of photometric diagrams and luminosity functions are now applied to these populations, and provide the means for classifying the X-ray sources and probing their evolution. While overall stellar mass drives the amount of X-ray binaries in old stellar populations, the amount of sources in star forming galaxies is related to the star formation rate. Short-lived, luminous, high mass binaries (HMXBs) dominate these young X-ray populations. 1. Chandra observations of X-ray binary (XRB) populations It is well known that the Milky Way hosts both old and young X-ray source populations, reflecting its general stellar make up. In 1978, the Einstein Observatory, the first imaging X-ray telescope, opened up the systematic study of the X-ray emission of normal galaxies, and revealed populations of X-ray sources, at least in nearby spiral galaxies (Fabbiano 1989). With Chandra's sub-arcsecond angular resolution, combined with CCD photometric capabilities (Weisskopf et al. 2000), the study of normal galaxies in X-rays has taken a revolutionary leap: populations of individual X-ray sources, with luminosities comparable to those of the Galactic X-ray binaries, can be detected at the distance of the Virgo Cluster and beyond. We can now study these X-ray populations in galaxies of all morphological types, down to typical limiting luminosities in the 10 37 ergs s-1 range. At these luminosities, the old population X-ray sources are accreting neutron star or black-hole binaries with a lowmass stellar companion, the LMXBs (life-times ~10 8-9 yrs). The young population X-ray sources, in the same luminosity range, are dominated by neutron star or black hole binaries with a massive stellar companion, the HMXBs (life-times ~ 10 6-7 yrs; see Verbunt & van den Heuvel 1995 for a review on the formation and evolution of X-ray binaries), although a few young supernova remnants (SNRs) may also be expected. At lower luminosities, reachable with Chandra in Local Group galaxies, Galactic sources include accreting white dwarfs and more evolved SNRs. Fig. 1 shows two typical observations of galaxies with Chandra: the spiral M83 (Soria & Wu 2003) and the elliptical NGC4697 (Sarazin, Irwin & Bregman 2000), both observed with the ACIS CCD detector. The images are color coded to indicate the energy of the detected photons (red 0.3-1 keV, green 1-2 keV and blue 2-8 keV). Populations of point-like sources are easily detected above a generally cooler diffuse emission from the hot interstellar medium. Note that luminous X-ray sources are relatively sparse by comparison with the underlying stellar population, and can be detected individually with the Chandra subarcsecond resolution, with the exception of those in crowded circum-nuclear regions.
We present results from a study of optically emitting supernova remnants (SNRs) in six nearby galaxies (NGC 2403, 3077, 4214, 4395, 4449 and 5204) based on deep narrow-band Hα and [S II] images as well as spectroscopic observations. The SNR classification was based on the detected sources that fulfil the well-established emission-line flux criterion of [S II]/Hα > 0.4. This study revealed ∼400 photometric SNRs down to a limiting Hα flux of 10 −15 erg s −1 cm −2 . Spectroscopic observations confirmed the shock-excited nature of 56 out of the 96 sources with ([S II]/Hα) phot > 0.3 (our limit for an SNR classification) for which we obtained spectra. 11 more sources were spectroscopically identified as SNRs although their photometric [S II]/Hα ratio was below 0.3. We discuss the properties of the optically detected SNRs in our sample for different types of galaxies and hence different environments, in order to address their connection with the surrounding interstellar medium. We find that there is a difference in [N II]/Hα line ratios of the SNR populations between different types of galaxies which indicates that this happens due to metallicity. We cross-correlate parameters of the optically detected SNRs ([S II]/Hα ratio, luminosity) with parameters of coincident X-ray-emitting SNRs, resulted from our previous studies on the same sample of galaxies, in order to understand their evolution and investigate possible selection effects. We do not find a correlation between their Hα and X-ray luminosities, which we attribute to the presence of material in a wide range of temperatures. We also find evidence for a linear relation between the number of luminous optical SNRs (10 37 erg s −1 ) and star formation rate in our sample of galaxies.
2002
We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical (HST) data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray sources (ULXs), with L X ≥ 10 39 ergs s −1 , are not young compact Supernova Remnants (SNR), but accretion binaries. While their spectrum is consistent with those of ULXs studied in nearby galaxies, and interpreted as the counterparts of intermediate mass black-holes (M> 10 − 1000 M ⊙ ), comparison with the position of star-clusters suggests that some of the ULXs may be runaway binaries, thus suggesting lower-mass binary systems. The co-added spectrum of the sources in the 3 × 10 38 − 10 39 erg s −1 luminosity range is consistent with those of Galactic black-hole candidates. These sources are also on average displaced from neighboring star clusters. The softer spectrum of the less luminous sources suggests the presence of SNRs or of hot interstellar medium (ISM) in the Chandra source extraction area. Comparison with HI and CO observations shows that most sources are detected in the outskirts of large concentrations of gas. The absorbing columns inferred from these observations would indeed absorb X-rays up to 5 keV, so there may be several hidden X-ray sources. Associated with these obscured regions we find 6 sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range. We detect the nuclei of both galaxies with luminosities in the 10 39 ergs s −1 range and soft, possibly thermal, X-ray spectra.
The X-ray properties of luminous infrared galaxies
AIP Conference Proceedings, 2001
We present a study of the sample of luminous infrared galaxies (LIGs, LIR > 10 11 L⊙) observed in the hard (2-10 keV) X rays. The main results are: 1) most LIGs are powered both by AGN and starburst activity; 2) the AGNs in our sample are absorbed in the infrared by a lower NH than in the X-rays or, alternatively, the dust-togas ratio is lower than galactic; 3) the study of a subsample of sources observed in the 20-200 keV band indicates that most of the AGNs hosted by the LIGs are heavily obscured up to 100 keV and, therefore, their contribution to the X-ray background must be small.
The Cambridge-Cambridge x-ray serendipity survey. 2: Classification of x-ray luminous galaxies
We present the results of an intermediate-resolution (1.5 A) spectroscopic study of 17 X-ray luminous narrow emission-line galaxies previously identi ed in the Cambridge-Cambridge ROSAT Serendipity Survey and the Einstein Extended Medium Sensitivity Survey. Emission-line ratios reveal that the sample is composed of ten Seyfert and seven starburst galaxies. Measured linewidths for the narrow H emission lines lie in the range 170 460 km s 1. Five of the objects show clear evidence for asymmetry in the OIII] 5007 emission-line pro le. Broad H emission is detected in six of the Seyfert galaxies, which range in type from Seyfert 1.5 to 2. Broad H emission is only detected in one Seyfert galaxy. The mean full width at half maximum for the broad lines in the Seyfert galaxies is FWHM = 3900 1750 km s 1. Broad (FWHM = 2200 600 km s 1) H emission is also detected in three of the starburst galaxies, which could originate from stellar winds or supernovae remnants. The mean Balmer decrement for the sample is H /H = 3, consistent with little or no reddening for the bulk of the sample. There is no evidence for any trend with X-ray luminosity in the ratio of starburst galaxies to Seyfert galaxies. Based on our previous observations, it is therefore likely that both classes of object comprise 10 per cent of the 2 keV X-ray background.
Astronomy & Astrophysics, 2008
We use deep Chandra observations to measure the emissivity of the unresolved X-ray emission in the elliptical galaxy NGC 3379. After elimination of bright, low-mass X-ray binaries with luminosities > ∼ 10 36 erg s −1 , we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass L x /M * ∼ 8.2 × 10 27 erg s −1 M −1 in the 0.5−2 keV energy band. This value is in good agreement with those previosuly determined for the dwarf elliptical galaxy M 32, the bulge of the spiral galaxy M 31 and the Milky Way, as well as with the integrated X-ray emissivity of cataclysmic variables and coronally active binaries in the Solar neighborhood. This strongly suggests that i) the bulk of the unresolved X-ray emission in NGC 3379 is produced by its old stellar population; and ii) the old stellar populations in all galaxies can be characterized by a universal value of X-ray emissivity per unit stellar mass or per unit K band luminosity.
The Astronomical Journal, 2005
We have cross-correlated X-ray catalogs derived from archival Chandra ACIS observations with a Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2) galaxy catalog to form a sample of 42 serendipitously X-ray detected galaxies over the redshift interval 0.03 < z < 0.25. This pilot study will help fill in the "redshift gap" between local X-raystudied samples of normal galaxies and those in the deepest X-ray surveys. Our chief purpose is to compare optical spectroscopic diagnostics of activity (both star-formation and accretion) with X-ray properties of galaxies. Our work supports a normalization value of the X-ray-star-formation-rate (X-ray-SFR) correlation consistent with the lower values published in the literature. The difference is in the allocation of X-ray emission to high-mass X-ray binaries relative to other components such as hot gas, low-mass X-ray binaries, and/or AGN. We are able to quantify a few pitfalls in the use of lowerresolution, lower signal-to-noise optical spectroscopy to identify X-ray sources (as has necessarily been employed for many X-ray surveys). Notably, we find a few AGN that likely would have been misidentified as non-AGN sources in higher-redshift studies. However, we do not find any X-ray hard, highly X-ray-luminous galaxies lacking optical spectroscopic diagnostics of AGN activity. Such sources are members of the "X-ray Bright, Optically Normal Galaxy" (XBONG) class of AGN.
The X-ray Properties of Early-Type Galaxies in the Extended Chandra Deep Field-South
2006
We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ~ 0.7) of the average X-ray properties of early-type galaxies within the Extended Chandra Deep Field-South (E-CDF-S). Our early-type galaxy sample includes 539 objects with red-sequence colors and Sersic indices larger than n = 2.5, which were selected jointly from the COMBO-17 (Classifying Objects by Medium-Band Observations in 17 Filters) and GEMS (Galaxy Evolution from Morphologies and SEDs) surveys. We utilize the deep Chandra observations over the E-CDF-S and X-ray stacking analyses to constrain primarily the average X-ray emission from "normal" early-type galaxies (i.e., those that are not dominated by luminous active galactic nuclei [AGNs]). In our analyses, we study separately optically luminous (L_B ~ 10^[10-11] L_B,sol) and faint (L_B ~ 10^[9.3-10] L_B,sol) galaxy samples, which we expect to have soft (0.5-2.0 keV) X-ray emission dominated by hot (~1 keV) interstellar gas and low mass X-ray binary (LMXB) populations, respectively. We detect individually 49 (~9%) of our galaxies in the X-ray band, and classify these sources as either normal early-type galaxies (17 galaxies) or AGN candidates (32 galaxies). The AGN fraction of our optically luminous samples evolves with redshift in a manner consistent with the (1+z)^3 evolution observed in other investigations of X-ray-selected AGNs. After removing potential AGNs from our samples, we find that the X-ray-to-B-band mean luminosity ratio (L_X/L_B) for optically luminous early-type galaxies does not evolve significantly over the redshift range z~0.0-0.7. This lack of X-ray evolution implies a general balance between the heating and cooling of the hot interstellar gas [Abridged].
X-ray Source Populations in Galaxies
Chinese Journal of Astronomy and Astrophysics, 2003
Today's sensitive, high-resolution X-ray observations allow the study of populations of X-ray sources, in the luminosity range of Galactic X-ray binaries, in galaxies as distant as 20-30Mpc. The traditional astronomical tools of photometric diagrams and luminosity functions are now applied to these populations, providing a direct probe of the evolved binary component of different stellar populations. The study of the X-ray populations of E and S0 galaxies has revamped the debate on the formation and evolution of low-mass X-ray binaries (LMXBs) and on the role of globular clusters in these processes. While overall stellar mass drives the amount of X-ray binaries in old stellar populations, the amount of sources in star forming galaxies is related to the star formation rate. Short-lived, luminous, high mass binaries (HMXBs) dominate these young X-ray populations. The most luminous sources in these systems are the debated ULXs, which have been suggested to be ~100-1000 M black holes, but could alternatively include a number of binaries with stellar mass black holes. Very soft sources have also been discovered in many galaxies and their nature is currently being debated. Observations of the deep X-ray sky, and comparison with deep optical surveys are providing the first evidence of the X-ray evolution of galaxies. 1. CHANDRA: A NEW PARADIGM This review comes almost two decades after the 1989 Annual Review article on the X-ray emission from galaxies (Fabbiano 1989), and a few words on the evolution of this field are in order. In 1989, the Einstein Observatory (Giacconi et al. 1979), the first imaging X-ray telescope, had opened up the systematic study of the X-ray emission of normal galaxies. The Einstein images, in the ~0.3-4 keV range, with resolutions of ~5'' and ~45'' (see the Einstein Catalog and Atlas of Galaxies, Fabbiano, Kim & Trinchieri 1992) showed extended and complex X-ray emission, and gave the first clear detection of individual luminous X-ray sources in nearby spiral galaxies, other than the Milky Way. The first ultra-luminous (non-nuclear) Xray sources (ULXs) were discovered with Einstein, and the suggestion was advanced