A multiwavelength study of a nearby dust lane early-type galaxy IC 5063 is presented. The objectives are to investigate dust extinction properties and the association of interstellar dust with other phases of ISM. The color-index maps as well as the extinction maps derived from the analysis of deep CCD observations in optical passbands revealed a prominent dust lane along its optical major axis in the inner region. In addition, two more fainter and extended dust patterns are apparent in the color index map as well as extinction maps. These features are also evident in the smooth model subtracted residual maps. The extinction curve derived for this galaxy revealed that dust grains in it are identical to the canonical grains in the Milky Way with the dust grains little larger than the canonical grains. The total extinction measured in the V band extinction map enabled us to quantify the dust content of this galaxy to be equal to , an order of magnitude shorter than that estimated using the IRAS flux densities at 60 and 100 μm. A Multiphase ISM study revealed a surprising similarity in the morphologies of the Hα emitting ionized gas distribution and X-ray emitting gas. Systematic analysis of high resolution X-ray observations using Chandra telescope enabled us to detect 18 discrete X-ray sources within optical D 25 region of IC 5063, out of which 17 sources were separated out as the low mass X-ray binaries and one as the high mass X-ray binary source in the X-ray color-color plot.
Classically, early-type galaxies (ETGs, both elliptical and lenticular collectively) were considered to be old, passive systems with a very low or almost devoid of gas and dust. However, with the availability of highly sensitive, state-of-the-art observing facilities covering wide range of electromagnetic spectrum have revealed that a notable fraction of these galaxies does host a substantial amount of interstellar medium (ISM) including interstellar dust. In fact, presence of dust and its systematic studies in this class of galaxies has acted as the theoretical smoking gun to trace the evolution and formation history of their host. Particularly, the diffusely distributed component of interstellar dust within galaxies has been chosen as the component to explore histories of ETGs due to its direct link with the cold gas in ISM. As a result, study of dust properties in this class of galaxies has been the topic of extensive study in last few decades.
A large fraction (~50% - 80%) of ETGs, at least in the local universe, are now known to host interstellar dust in the form of lanes, filaments and patches, which has been confirmed through the careful analysis of high quality optical CCD imaging observations, thanks to the availability of sensitive array detectors [
The origin of dust in ETGs is one of the long debated issue. Three possible sources are supposed to yield the observed dust in ETGs: 1) formed in the atmospheres of the AGB stars, which then is shed into the ISM, 2) acquired through the merging or tidal capture from the neighbor, and 3) the same uncertain source as that in late-type galaxies e.g., growing continuously in the form of grains in ISM or supernovae, or left over from a vigorous star formation [
This paper presents dust extinction in a prominent dust lane early type galaxy IC 5063 (PKS 2048-57), which has been reported as a strong extended high-excitation optical line emitter and has also been detected as a powerful infrared emitter. The dust in this galaxy is found to extend out in the form of a lane till its edge [
The paper is organized as follows. Section 2 describes the optical and X-ray observations and data analysis processes, while Section 3 discusses the dust extinction properties and also compares its association with that of the ionized gas. Section 4 discusses results derived from the spatial and spectral analysis of X-ray emission from this galaxy and also presents observed properties of discrete X-ray binary sources that have been detected within the optical D25 region of this galaxy. The origin of dust and its association with other phases of ISM along with summary of the results are given in Section 5.
Parameter | Value |
---|---|
Alternate names | PKS 2048-57, IRAS 20481-5715, PGC 065600 |
ESO 204812-5715.5 | |
RA DEC | 20:52:02.3; −57:04:08 |
Morphology | SAO |
Mag BT | 12.89 |
Dimensions | 2’.1 × 1’.4 |
D25 | 12’ × 8’5 |
Distance (Mpc) | 45.3 |
Redshift (z) | 0.01114 |
Radial velocity (km/s) | 3402 ± 6 |
IRAS flux density (Jy) | 1.290 ± 0.024 (12 mm); 4.270 ± 0.030 (24 mm) |
5.87 ± 0.038(60 mm); 4.25 ± 0.207 (100 mm) | |
Radio flux density | 408 MHz: 6.200 Jy |
Deep, high S/N CCD images in broad band filters B, V, R, I and narrow band filter Hα on this dust lane galaxy IC 5063 observed with Danish 1.54-metre telescope with Instrument Adapter-No.17 at La Silla operated by ESO (European
Southern observatory) were acquired from NED [
Though X-ray emission properties of IC 5063 using Chandra observations have already been reported by LaMassa (2011) and LaMassa (2012) [
Though the presence of interstellar dust in IC 5063 has been known since long time [
Once morphology of the dust was confirmed in the color index maps, next step was to quantify the dust extinction in each of the passband. This require comparing the stellar light distribution in the presence and absence of interstellar dust in the target galaxy i.e., by comparing light distribution in the cleaned science frame with that of its 2D smooth model [
This code uses the Sersic model [
Dust free models thus generated were then used to quantify the wavelength dependent nature of the dust extinction, the extinction curve, using relation
A λ = − 2.5 log ( I λ , o b s I λ , m o d e l )
where, Aλ gives the amount of total extinction in a particular pass band (B,V,R,I) measured in magnitude scale, while I λ , o b s and I λ , m o d e l represent the observed (attenuated) and un-attenuated light intensities in a given pass band, respectively. Two of such extinction maps in V and R band for IC 5063 are shown in
wavelength dependent nature by measuring total extinction values by sliding a 5 × 5 box over the dust-extinguished regions. The numerical values of local extinctions in each of the passband (Aλ) measured using box method were then used to derive the extinctions (Rλ) by fitting linear regressions to the total extinction Aλ versus the selective extinction E ( B − V ) = A B − A V . Slopes of the best regression fits along with their associated uncertainties yielded the Rλ and hence the extinction curve. The extinction curve, the graph between the measured values of R λ ( = A λ E ( B − V ) ) versus inverse of the wavelength, for IC
5063 is shown in
〈 a a G a l 〉 = 1.008 ± 0.08 and RV equal to 4.02 ± 0.25.
Total dust content of IC 5063 was estimated using the total extinction values measured from within the V band extinction map. For this we integrate the dust column density Σ d over the dust occupied area (A). Here, we assume that the dust grains within IC 5063 have chemical composition and geometry similar to that of the canonical grains in the Milky Way. Assuming spherical grains of Mathis et al. (1977) [
M d = A × Σ d = A × l d × ∫ a min a max 4 3 π a 3 ρ d n ( a ) d a
where, n ( a ) d a ( = n H A i a − 3.5 d a ) gives particle size distribution of a, A i the overall abundance of silicate and graphite grains, n H the hydrogen number density; Σ d the dust column density; ρ d the specific grain size density (~3 g cm−3); l d the dust column length along the line of sight; and a max and a min representing the upper and lower cutoffs of the size distribution, respectively. The measured values of the average extinction in V band ( 〈 A V 〉 ) yielded the dust content of IC 5063 to be equal to 4.9 × 10 4 M ⊙ . However, this estimate provides the lower limit due to the reason that this method is based on the screening effect of dust grains. The grains that are distributed throughout the galaxy would not be accounted for in this method. This estimate is further affected by the uncertainties due to the lower and upper cutoffs of the grain sizes.
An alternative attempt was also made to quantify the true dust content of IC 5063 employing the far-IR emission flux densities of interstellar dust grains at 60 mm and 100 μm measured by the IRAS satellite. As a first step in this attempt, we
derive the dust grain temperature using relation T d = 49 ( S 60 S 100 ) 0.4 K [
which was found to be 58 K, hot relative to the grains evidenced in other dusty early-type galaxies [
With an analogy to examine morphology of the X-ray emitting hot gas and also to investigate its association with that of the interstellar dust we have derived X-ray emission maps of IC 5063 employing 34.1 ks Chandra observations. A 0.3 - 3 keV background subtracted, exposure corrected, point source removed, 3σ smoothed X-ray emission map for IC 5063 is shown in
Global properties of the X-ray emitting gas from within IC 5063 were also investigated by extracting a 0.3 - 7 keV combined spectrum of the X-ray photons from within its optical D25 region. Emission from central 1” was excluded from this analysis to avoid the harder component originating from the nuclear source. The spectrum was then imported to XSPEC v 12.7.1 and was fitted with an absorbed single temperature MEKAL model following the standard χ2 statistics and fixing the hydrogen column density at the Galactic value of 6.4 × 10 21 cm − 2 ) [
X-ray study of this galaxy enabled us to resolve a set of 18 discrete X-ray binary sources (XRBs) within optical D25 region of IC 5063 of the Chandra image. XRBs are believed to be associated with the star forming history and hence act as proxy to investigate formation scenario of the host galaxy [
As a result several attempts were made in the past to investigate population of these sources in early-type galaxies using the high spatial resolution data from Chandra observatory [
For this purpose following Vagshette et al. (2013) [
The plot between the measured values of X-ray hardness ratios X31 versus the soft color X21 is shown in
18 discrete sources 17 belongs to the region classified as low mass X-ray binaries (LMXBs), while only one source was found to be of high mass X-ray binary (HMXB). This analysis failed to detect any of the super-soft as well as AGN source in the field.
With an objective to examine the association of dust with ionized gas, we have derived Hα + [N II] emission map of IC 5063. Spatial distribution of the ionized gas within IC 5063 is shown in
This paper presents multiwavelength study of a dust lane early-type galaxy IC 5063 with an objective to investigate dust extinction properties and association of dust with other phases of ISM. The main results from this study are
summarized below.
• Imaging analysis of deep CCD observations of IC 5063 in optical passbands reveal a prominent dust lane along its optical major axis in the inner region. In addition to the prominent dust lane two more relatively fainter and extended dust patterns are apparent in color index map as well as extinction maps. These features were also evident in smooth model subtracted residual maps.
• To delineate the extinction characteristics of dust grains within IC 5063 we obtain the extinction curve as a function of wavelength by deriving its extinction maps in different passbands. Comparison of this curve with that derived for the canonical grain in the Milky Way reveals that the dust grains in the target galaxy exhibit identical characteristics.
• The total extinction measured in the V band extinction map enabled us to quantify the dust content of this galaxy to be equal to 4.9 × 10 4 M ⊙ , which was an order of magnitude shorter than that estimated using the IRAS flux densities at 60 and 100 μm.
• With an objective to investigate the presence of other phases of ISM in this galaxy we have also derived its nebular emission maps centered on the Ha emission and X-ray emission maps. A surprising similarity was evidenced in the morphologies of the Ha emitting ionized gas distribution and X-ray emitting gas.
• High resolution X-ray imaging analysis of Chandra observations of this galaxy also enabled us to detect 18 discrete X-ray binary sources within optical D25 region of IC 5063, out of which 17 sources were separated out as the low mass X-ray binaries and one as the high mass X-ray binary source in the X-ray color-color plot.
We are thankful to the referee for their valuable suggestions and comments to improve content of this paper. This work has made use of data from the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This publication has made use of data products from the Chandra, ESO (Danish), GALEX, 2MASS and IRAC archives.
Tate, B.T., Kyadampure, A.T., Pandey, S.K. and Patil, M.K. (2018) Multiphase ISM in Nearby Early Type Galaxy IC 5063. International Journal of Astronomy and Astrophysics, 8, 79-93. https://doi.org/10.4236/ijaa.2018.81006