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Register a new userSuzaku Observation of the Metallicity Distribution in the Elliptical Galaxy NGC 4636
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Authors
Katsuhiro Hayashi
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NGC 4636, an X-ray bright elliptical galaxy, was observed for 70 ks with Suzaku. The low background and good energy resolution of the XIS enable us to estimate the foreground Galactic emission accurately and hence measure, for the first time, the O, Mg, Si and Fe abundances out to a radius of ~28 arcmin ($simeq$ 140 kpc). These metal abundances are as high as $>$1 solar within the central 4 and decrease by ~50% towards the outer regions. Further, the O to Fe abundance ratio is about 0.60--1.0 solar in all regions analyzed, indicating that the products of both SNe II and SNe Ia have mixed and diffused to the outer regions of the galaxy. The O and Fe metal mass-to-light-ratios (MLR) of NGC 4636 are 2--3 times larger than those of NGC 1399 implying that metal distributions in NGC 4636 are less extended than those in NGC 1399, possibly due to environmental factors, such as frequency of galaxy interaction. We also found that the MLRs of NGC 4636 at 0.1 $r_{180}$ are $sim$5 times smaller than those of clusters of galaxies, possibly consistent with the correlation between temperature and MLR of other spherically symmetric groups of galaxies. We also confirmed a resonant scattering signature in the Fe$_{XV II}}$ line in the central region, as previously reported using the XMM-Newton RGS.
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The Suzaku X-ray satellite observed the nearby spiral galaxy NGC 4258 for a total good exposure time of 100 ks. We present an analysis of the Suzaku XIS data, in which we confirm that the 0.5--2 keV spectra of the interstellar medium (ISM) are well-represented by a two-temperature model. The cool and hot ISM temperatures are 0.23+-0.02 and 0.59 +-0.01 keV, respectively. Suzakus excellent spectral sensitivity enables us to measure the metal abundances of O, Ne, Mg, Si and Fe of the ISM for the first time. The resultant abundance pattern of O, Mg, Si, and Fe is consistent with that of the new solar abundance table of Lodders (2003), rather than Anders & Grevesse (1989). This suggests that the metal enrichment processes of NGC 4258 and of our Galaxy are similar.
High quality X-ray spectra of NGC 4636 are obtained with ASCA. Theoretical models are found unable to reproduce the data in the Fe-L line region. Spectral data above 1.4 keV indicate that Mg to Si abundance ratio is $sim 1$ solar. Assuming that the abundance ratios among $alpha$-elements are the same with the solar ratios, spectral fit with increased systematic error in the 0.4--1.6 keV range gives abundances of $alpha$-elements and Fe to be both $sim$ 1 solar by taking one solar of Fe to be 3.24$times 10^{-5}$ by number. These new abundance results solve discrepancy between stellar and hot-gas metallicity, but still a low supernova rate is implied. We also detect strong abundance gradients for both $alpha$-elements and iron in a similar fashion. The abundance is $sim 1$ solar within $4$, and decreases outerwards down to $0.2sim 0.3$ solar at $10$ from the galaxy center. Dilution due to an extended hot gas is suggested.
The metallicity distribution in the Fornax cluster was studied with the XIS instrument onboard the Suzaku satellite. K-shell lines of O and Mg were resolved clearly, and the abundances of O, Mg, Si, S and Fe were measured with good accuracy. The region within a 4 radius of NGC 1399 shows approximately solar abundances of Fe, Si and S, while the O/Fe and Mg/Fe abundance ratios are about 0.4--0.5 and 0.7 in solar units. In the outer region spanning radii between 6 and 23, the Fe and Si abundances drop to 0.4--0.5 solar and show no significant gradient within this region. The abundance ratios, O/Fe and Mg/Fe, are consistent with those in the central region. We also measured the Fe abundance around NGC 1404 to be approximately solar, and the O, Ne and Mg abundances to be 0.5--0.7 times the Fe level. The significant relative enhancement of Fe within 130 kpc of NGC 1399 and in NGC 1404 indicates an origin in SN Ia, in contrast to the species O, Ne, and Mg which reflect the stellar metallicity. The mass-to-light ratios for O and Fe within 130 kpc of NGC 1399 are over an order of magnitude lower than those in rich clusters, reflecting the metal enrichment history of this poor cluster.
We present a kinematic analysis of the globular cluster(GC) system in the giant elliptical galaxy (gE) NGC 4636 in the Virgo cluster. Using the photometric and spectroscopic database of 238 GCs, we have investigated the kinematics of the GC system. The NGC 4636 GC system shows weak overall rotation, which is dominated by the red GCs. However, both the blue GCs and red GCs show some rotation in the inner region at R<4.3. The velocity dispersion for all the GCs is derived to be sigma_p = 225{+12-9} km/s. The velocity dispersion for the blue GCs (sig=251 km/s) is slightly larger than that for the red GCs (sig=205 km/s). The velocity dispersions for the blue GCs about the mean velocity and about the best fit rotation curve have a significant variation depending on the galactocentric radius. Comparison of observed stellar and GC velocity dispersion profiles with the velocity dispersion profiles calculated from the stellar mass profile shows that the mass-to-light ratio should increase as the galactocentric distance increases, indicating the existence of an extended dark matter halo. From the comparison of the observed GC velocity dispersion profiles and the velocity dispersion profiles calculated for the X-ray mass profiles in the literature, we find that the orbit of the GC system is tangential, and that the orbit of the red GCs is slightly more tangential than that of the blue GCs. We compare the GC kinematics of NGC 4636 with those of other six gEs, finding that the kinematic properties of the GCs are diverse among gEs. We find several correlations between the kinematics of the GCs and the global parameters of their host galaxies. We discuss the implication of the results for the formation models of the GC system in gEs, and suggest a mixture scenario for the origin of the GCs in gEs.
We present a spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy NGC 4636 in the Virgo cluster. We selected target GC candidates using the Washington photometry derived from the deep CCD images taken at the KPNO 4m. Then we obtained the spectra of 164 target objects in the field of NGC 4636 using the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the SUBARU 8.2m Telescope. We have measured the velocities for 122 objects: 105 GCs in NGC 4636, the nucleus of NGC 4636, 11 foreground stars, 2 background galaxies, and 3 probable intracluster GCs in the Virgo cluster. The GCs in NGC 4636 are located in the projected galactocentric radius within 10arcmin (corresponding to 43 kpc). The measured velocities for the GCs range from 300km/s to 1600km/s, with a mean value of 932_{-22}^{+25} km/s, which is in good agreement with the velocity for the nucleus of NGC 4636, 928pm 45 km/s. The velocity dispersion of the GCs in NGC 4636 is derived to be 231_{-17}^{+15} km/s and the velocity dispersion of the blue GCs is slightly larger than that of the red GCs. Combining our results with data in the literature, we produce a master catalog of radial velocities for 238 GCs in NGC 4636. The velocity dispersion of the GCs in the master catalog is found to be 225_{-9}^{+12} km/s for the entire sample, 251_{-12}^{+18} km/s for 108 blue GCs, and 205_{-13}^{+11} km/s for 130 red GCs.
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