arXiv subjects

Heikki Salo

Publications and source records attributed to Heikki Salo.

59 records · Page 4Linked to original sources

arXiv2010

The Spitzer Survey of Stellar Structure in Galaxies (S^4G)

The Spitzer Survey of Stellar Structure in Galaxies S^4G is an Exploration Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc, |b| > 30 degrees, m_(Bcorr) < 15.5, D25>1') survey of 2,331 galaxies using IRAC at 3.6 and 4.5 microns. Each galaxy is observed for 240 s and mapped to > 1.5 x D25. The final mosaicked images have a typical 1 sigma rms noise level of 0.0072 and 0.0093 MJy / sr at 3.6 and 4.5 microns, respectively. Our azimuthally-averaged surface brightness profile typically traces isophotes at mu_3.6 (AB) (1 sigma) ~ 27 mag arcsec^-2, equivalent to a stellar mass surface density of ~ 1 Msun pc^-2. S^4G thus provides an unprecedented data set for the study of the distribution of mass and stellar structures in the local Universe. This paper introduces the survey, the data analysis pipeline and measurements for a first set of galaxies, observed in both the cryogenic and warm mission phase of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at mu_3.6 (AB) = 25.5 and 26.5 mag arcsec^-2 (equivalent to ~ mu_B (AB) =27.2 and 28.2 mag arcsec^-2, respectively). These measurements will form the initial S^4G catalog of galaxy properties. We also measure the total magnitude and the azimuthally-averaged radial profiles of ellipticity, position angle, surface brightness and color. Finally, we deconstruct each galaxy using GALFIT into its main constituent stellar components: the bulge/spheroid, disk, bar, and nuclear point source, where necessary. Together these data products will provide a comprehensive and definitive catalog of stellar structures, mass and properties of galaxies in the nearby Universe.

astro-ph.CO↗

arXiv2010

The opposition and tilt effects of Saturn's rings from HST observations

The two major factors contributing to the opposition brightening of Saturn's rings are i) the intrinsic brightening of particles due to coherent backscattering and/or shadow-hiding on their surfaces, and ii) the reduced interparticle shadowing. We utilize the Hubble Space Telescope observations for different elevation angles B to disentangle these contributions. We assume that the intrinsic contribution is independent of B, so that any B dependence of the phase curves is due to interparticle shadowing, which must also act similarly for all colors. We construct a grid of dynamical/photometric simulation models to fit the elevation-dependent part of opposition brightening. Eliminating the modeled interparticle component yields the intrinsic contribution to the opposition effect: for the B and A rings it is almost entirely due to coherent backscattering; for the C ring, an intraparticle shadow hiding contribution may also be present. Based on our simulations, the width of the interparticle shadowing effect is roughly proportional to B. This follows from the observation that as B decreases, the scattering is primarily from the rarefied low filling factor upper ring layers, whereas at larger $B$'s the dense inner parts are visible. The elevation angle dependence of interparticle shadowing also explains most of the B ring tilt effect (the increase of brightness with elevation). From comparison of the magnitude of the tilt effect at different filters, we show that multiple scattering can account for at most a 10% brightness increase as B -> 26^o, whereas the remaining 20% brightening is due to a variable degree of interparticle shadowing. The negative tilt effect of the middle A ring is well explained by the the same self-gravity wake models that account for the observed A ring azimuthal brightness asymmetry.

astro-ph.EP↗

arXiv2007

Kinematics and dynamics of the M51-type galaxy pair NGC 3893/96 (KPG 302)

We study the kinematics and dynamics of the M51-type interacting galaxy pair KPG 302 (NGC 3893/96). We analyse the distribution of the dark matter (DM) halo of the main galaxy in order to explore possible differences between DM halos of "isolated" galaxies and those of galaxies belonging to a pair. The velocity field of each galaxy was obtained using scanning Fabry-Perot interferometry. A two-dimensional kinematic and dynamical analysis of each galaxy and the pair as a whole is done emphasizing the contribution of circular and non-circular velocities. Non-circular motions can be traced on the rotation curves of each galaxy allowing us to differentiate between motions associated to particular features and motions that reflect the global mass distribution of the galaxy. For the main galaxy of the pair, NGC 3893, optical kinematic information is complemented with HI observations from the literature to build a multi-wavelength rotation curve. We try to fit this curve with a mass-distribution model using different DM halos. We find that the multi-wavelength rotation curve of NGC 3893, "cleaned" from the effect of non-circular motions, cannot be fitted neither by a pseudo-isothermal nor by a NFW DM halo.

astro-ph↗

arXiv2005

Multicomponent Decompositions for a Sample of S0 galaxies

We have estimated the bulge-to-total ($B/T$) light ratios in the $K_s$-band for a sample of 24 S0, S0/a and Sa galaxies by applying a 2-dimensional multicomponent decomposition method. For the disk an exponential function is used, the bulges are fitted by a Sérsic's $R^{1/n}$ function and the bars and ovals are described either by a Sérsic or a Ferrers function. In order to avoid non-physical solutions, preliminary characterization of the structural components is made by inspecting the radial profiles of the orientation parameters and the low azimuthal wavenumber Fourier amplitudes and phases. In order to identify also the inner structures, unsharp masks were created: previously undetected inner spiral arms were found in NGC 1415 and marginally in NGC 3941. Most importantly, we found that S0s have a mean $ _K$-ratio of 0.24 $\pm$ 0.11, which is significantly smaller than the mean $ _R$ = 0.6 generally reported in the literature. Also, the surface brightness profiles of the bulges in S0s were found to be more exponential-like than generally assumed, the mean shape parameter of the bulge being $ $ = 2.1 $\pm$ 0.7. We did not find examples of barred S0s lacking the disk component, but we found some galaxies (NGC 718, NGC 1452, NGC 4608) having a non-exponential disk in the bar region. To our knowledge our study is the first attempt to apply a multicomponent decomposition method for a moderately sized sample of early-type disk galaxies

astro-ph↗

arXiv2002

Bar strengths in spiral galaxies estimated from 2MASS images

Non-axisymmetric forces are presented for 107 spiral galaxies using the 2 Micron All Sky Survey (2MASS) images. We apply both Cartesian integration and a polar grid integration utilizing a limited number of azimuthal Fourier components of density. We found that bar strength is independent of the method used to evaluate the gravitational potential. However, the polar method is more suitable for weak and noisy images. Bar strength was found to be sensitive to the Hubble-type dependent scale height of the disk, which has been ignored in the previous studies. On the other hand, the method is rather insensitive to the vertical model of the disk, as long as a same vertical dispersion is assumed, or to the boxy/peanut shaped structure, studied in terms of non-constant vertical scale height along the disk. In the near-IR most galaxies in our sample show non-axisymmetric forces in some level, and 40 percent of them have bars in a sense that they have regions with a constant phase of the m=2 Fourier density component.

astro-ph↗