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Register a new userThe present-day galaxy population in spiral galaxies
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Reynier Peletier
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Although there are many more stellar population studies of elliptical and lenticular galaxies, studies of spiral galaxies are catching up, due to higher signal to noise data on one hand, and better analysis methods on the other. Here I start by discussing some modern methods of analyzing integrated spectra of spiral galaxies, and comparing them with traditional methods. I then discuss some recent developments in our understanding of the stellar content of spiral galaxies, and their associated dust content. I discuss star formation histories, radial stellar population gradients, and stellar populations in sigma drops.
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Todays galaxies experienced cosmic reionization at different times in different locations. For the first time, reionization ($50%$ ionized) redshifts, $z_R$, at the location of their progenitors are derived from new, fully-coupled radiation-hydrodynamics simulation of galaxy formation and reionization at $z > 6$, matched to N-body simulation to z = 0. Constrained initial conditions were chosen to form the well-known structures of the local universe, including the Local Group and Virgo, in a (91 Mpc)$^3$ volume large enough to model both global and local reionization. Reionization simulation CoDa I-AMR, by CPU-GPU code EMMA, used (2048)$^3$ particles and (2048)$^3$ initial cells, adaptively-refined, while N-body simulation CoDa I-DM2048, by Gadget2, used (2048)$^3$ particles, to find reionization times for all galaxies at z = 0 with masses $M(z=0)ge 10^8 M_odot$. Galaxies with $M(z=0) gtrsim 10^{11} M_odot$ reionized earlier than the universe as a whole, by up to $sim$ 500 Myrs, with significant scatter. For Milky-Way-like galaxies, $z_R$ ranged from 8 to 15. Galaxies with $M(z=0) lesssim 10^{11} M_odot$ typically reionized as late or later than globally-averaged $50%$ reionization at $langle z_Rrangle =7.8$, in neighborhoods where reionization was completed by external radiation. The spread of reionization times within galaxies was sometimes as large as the galaxy-to-galaxy scatter. The Milky Way and M31 reionized earlier than global reionization but later than typical for their mass, neither dominated by external radiation. Their most massive progenitors at $z>6$ had $z_R$ = 9.8 (MW) and 11 (M31), while their total masses had $z_R$ = 8.2 (both).
In this study, we have carried out a detailed, statistical analysis of isolated model galaxies, taking advantage of publicly available hierarchical galaxy formation models. To select isolated galaxies, we employ 2D methods widely used in the observational literature, as well as a more stringent 3D isolation criterion that uses the full 3D-real space information. In qualitative agreement with observational results, isolated model galaxies have larger fractions of late-type, star forming galaxies with respect to randomly selected samples of galaxies with the same mass distribution. We also find that the samples of isolated model galaxies typically contain a fraction of less than 15 per cent of satellite galaxies, that reside at the outskirts of their parent haloes where the galaxy number density is low. Projection effects cause a contamination of 2D samples of about 18 per cent, while we estimate a typical completeness of 65 per cent. Our model isolated samples also include a very small (few per cent) fraction of bulge dominated galaxies (B/T > 0.8) whose bulges have been built mainly by minor mergers. Our study demonstrates that about 65-70 per cent of 2D isolated galaxies that are classified as isolated at z = 0 have indeed been completely isolated since z = 1 and only 7 per cent have had more than 3 neighbours within a comoving radius of 1 Mpc. Irrespectively of the isolation criteria, roughly 45 per cent of isolated galaxies have experienced at least one merger event in the past (most of the mergers are minor, with mass ratios between 1:4 and 1:10). The latter point validates the approximation that isolated galaxies have been mainly influenced by internal processes.
We examine the reionization history of present-day galaxies by explicitly tracing the building blocks of halos from the Cosmic Reionization On Computers project. We track dark matter particles that belong to $z=0$ halos to trace the neutral fractions at corresponding positions during rapid global reionization. The resulting particle reionization histories allow us to explore different definitions of a halos reionization redshift and to account for the neutral content of the interstellar medium. Consistent with previous work, we find a systematic trend of reionization redshift with mass - present day halos with higher masses have earlier reionization times. Finally, we quantify the spread of reionization times within each halo, which also has a mass dependence.
We study the chemical and spectro-photometric evolution of galactic disks with detailed models calibrated on the Milky Way and using simple scaling relations, based on currently popular semi-analytic models of galaxy formation. We compare our results to a large body of observational data on present day galactic disks, including: disk sizes and central surface brightness, Tully-Fisher relations in various wavelength bands, colour-colour and colour-magnitude relations, gas fractions vs. magnitudes and colours, abundances vs. local and integrated properties, as well as spectra for different galactic rotational velocities. Despite the extremely simple nature of our models, we find satisfactory agreement with all those observables, provided the timescale for star formation in low mass disks is longer than for more massive ones. This assumption is apparently in contradiction with the standard picture of hierarchical cosmology. We find, however, that it is extremely successfull in reproducing major features of present day disks, like the change in the slope of the Tully-Fisher relation with wavelength, the fact that more massive galaxies are on average ``redder than low mass ones (a generic problem of standard hierarchical models) and the metallicity-luminosity relation for spirals. It is concluded that, on a purely empirical basis, this new picture at least as successful as the standard one. Observations at high redshifts could help to distinguish between the two possibilities.
We present results for X-ray point sources in the Sc galaxy NGC 2276, obtained by analyzing Chandra data. The galaxy is known to be very active in many wavelengths, possibly due to gravitational interaction with the central elliptical of the group, NGC 2300. However, previous XMM-Newton observations resulted in the detection of only one bright ULX and extended hot gas emission. We present here the X-ray population in NGC 2276 which comprises 17 sources. We found that 6 of them are new ULX sources in this spiral galaxy resolved for the first time by Chandra. We constructed the Luminosity Function that can be interpreted as mainly due of High Mass X-ray binaries, and estimate the Star Formation rate (SFR) to be SFR ~ 5-10 M_sun/yr.
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