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Register a new userThe external origin of the polar gaseous disk of the S0 galaxy IC 5181
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Added by
Alessandro Pizzella Prof.
Publication date
2013
fields
Physics
and research's language is
English
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Galaxies accrete material from the environment through acquisition and merging events. We study the nearby S0 galaxy IC 5181 to address the origin of the ionized-gas component orbiting the galaxy on polar orbit ionized gas of IC 5181 from broad and narrow-band imaging. We measure the ionized-gas and stellar kinematics and the line strengths of the Lick indices of the stellar component along both the major and minor axis. The age, metallicity, and [alpha/Fe] enhancement of the stellar populations are derived using single stellar population models with variable element abundance ratios. The ionized-gas metallicity is obtained from the equivalent width of the emission lines. IC 5181 is a morphologically undisturbed S0 galaxy with a classical bulge made by old stars with super solar metallicity and overabundance. Stellar age and metallicity decrease in the disk region. The galaxy hosts a geometrically and kinematically decoupled component of ionized gas. It is elongated along the galaxy minor axis and in orthogonal rotation with respect to the galaxy disk. We interpret the kinematical decoupling as suggestive of a component of gas, which is not related to the stars and having an external origin. It was accreted by IC 5181 on polar orbits from the surrounding environment.
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The nearby S0 galaxy IC 5181 is studied to address the origin of the ionized gas component that orbits the galaxy on polar orbit. We perform detailed photometric and spectroscopic observations measuring the surface brightness distribution of the stars (I-band), ionized gas of IC 5181 (H-alpha narrow band), the ionized-gas and stellar kinematics along both the major and minor axis, and the corresponding line strengths of the Lick indices. We conclude that the galaxy hosts a geometrically and kinematically decoupled component of ionized gas. It is elongated along the galaxy minor axis and in orthogonal rotation with respect to the galaxy disk. The result is suggesting that the gas component is not related to the stars having an external origin. The gas was accreted by IC 5181 on polar orbits from the surrounding environment.
We present our results of the spectroscopic study of the lenticular galaxy NGC 4143 - an outskirt member of the Ursa Major cluster. Using the observations at the 6-m SAO RAS telescope with the SCORPIO-2 spectrograph and also the archive data of panoramic spectroscopy with the SAURON IFU at the WHT, we have detected an extended inclined gaseous disk which is traced up to a distance of about 3.5 kpc from the center, with a spin approximately opposite to the spin of the stellar disk. The galaxy images in the H-alpha and [NII]6583 emission lines obtained at the 2.5-m CMO SAI MSU telescope with the MaNGaL instrument have shown that the emission lines are excited by a shock wave. A spiral structure that is absent in the stellar disk of the galaxy is clearly seen in the brightness distribution of ionized-gas lines (H-alpha and [NII] from the MaNGaL data and [OIII] from the SAURON data). A complex analysis of both the Lick index distribution along the radius and of the integrated colors, including the ultraviolet measurements with the GALEX space telescope and the near-infrared measurements with the WISE space telescope, has shown that there has been no star formation in the galaxy, perhaps, for the last 10 Gyr. Thus, the recent external-gas accretion detected in NGC 4143 from its kinematics, was not accompanied by star formation, probably, due to an inclined direction of the gas inflow onto the disk.
We study the Kennicutt-Schmidt star formation law and efficiency in the gaseous disk of the isolated galaxy CIG 96 (NGC 864), with special emphasis on its unusually large atomic gas (HI) disk (r_HI/r_25 = 3.5, r_25 = 1.85). We present deep GALEX near and far ultraviolet observations, used as a recent star formation tracer, and we compare them with new, high resolution (16, or 1.6 kpc) VLA HI observations. The UV and HI maps show good spatial correlation outside the inner 1, where the HI phase dominates over H_2. Star-forming regions in the extended gaseous disk are mainly located along the enhanced HI emission within two (relatively) symmetric giant gaseous spiral arm-like features, which emulate a HI pseudo-ring at a r simeq 3 . Inside such structure, two smaller gaseous spiral arms extend from the NE and SW of the optical disk and connect to the previously mentioned HI pseudo-ring. Interestingly, we find that the (atomic) Kennicutt-Schmidt power law index systematically decreases with radius, from N simeq 3.0 +- 0.3 in the inner disk (0.8 - 1.7) to N = 1.6 +- 0.5 in the outskirts of the gaseous disk (3.3 - 4.2). Although the star formation efficiency (SFE), the star formation rate per unit of gas, decreases with radius where the HI component dominates as is common in galaxies, we find that there is a break of the correlation at r = 1.5 r_25. At radii 1.5 r_25 < r < 3.5 r_25, mostly within the HI pseudo-ring structure, there exist regions whose SFE remains nearly constant, SFE simeq 10^-11 yr^-1. We discuss about possible mechanisms that might be triggering the star formation in the outskirts of this galaxy, and we suggest that the constant SFE for such large radii r > 2 r_25 and at such low surface densities might be a common characteristic in extended UV disk galaxies.
We review our current knowledge about a particular case of decoupled gas kinematics -- inner ionized-gas polar disks. Though more difficult to be noticed, they seem to be more numerous than their large-scale counterparts; our recent estimates imply about 10 per cent of early-type disk galaxies to be hosts of inner polar disks. Since in the most cases the kinematics of the inner polar gaseous disks is decoupled from the kinematics of the outer large-scale gaseous disks and since they nested around very old stellar nuclei, we speculate that the inner polar disks may be relics of very early events of external gas accretion several Gyr ago. Such view is in agreement with our new paradigm of the disk galaxies evolution.
A review of the results of the Near-IR S0 galaxy Survey (NIRS0S) is presented. NIRS0S is a magnitude (mB 12.5 mag) and inclination (< 65o) limited sample of 200 nearby galaxies, mainly S0s. It uses deep Ks -band images, typically reaching a surface brightness of 23.5 mag arcsec^(-2) . Detailed visual and photometric classifications were made, for the first time coding also the lenses in a systematic manner. As a comparison sample, a similar sized spiral galaxy sample with similar image quality was used. The main emphasis were to study whether the S0s are former spirals in which star formation has been ceased, and also, how robust are bars in galaxies. Based on our analysis the Hubble sequence was revisited: following the early idea by van den Bergh we suggested that the S0s are spread throughout the Hubble sequence in parallel tuning forks as spirals (S0a, S0b, S0c etc.). This is evidenced by our improved bulge-to-total (B/T) flux ratios, reaching as small values as typically found in late-type spirals. The properties of bulges and disks in S0s were found to be similar to those in spirals. Also, the masses and scale parameters of the bulges and disks were found to be coupled. Bars were found to be fairly robust both in S0s and spirals, but inspite of that bars might evolve significantly within the Hubble sequence.
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