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Register a new userMeasuring the fading of S0 galaxies using globular clusters
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Added by
Michael R. Merrifield
Publication date
2006
fields
Physics
and research's language is
English
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We test the hypothesis that S0 galaxies are the descendants of fading spirals whose star formation has been shut down, by using the properties of their globular cluster (GC) systems. We estimate the amount by which the GC specific frequency (number of GCs per unit V-band luminosity) is enhanced in S0s relative to spirals. If the transformation hypothesis is correct, and no GCs are created or destroyed in the process, then this difference provides a measure of the degree to which the S0s V-band luminosity has faded relative to that of its spiral progenitor. We also explore whether the degree to which the GC specific frequency is enhanced in S0s correlates with the colour of the stellar population, as also predicted by this hypothesis in which galaxies become redder as they fade. We find that, on average, the GC specific frequency is a factor ~3 larger for S0s than for spirals, which can be interpreted as meaning that passively-evolving S0s have faded by about a factor of 3 from their spiral progenitors. This value fits remarkably well with the predictions of stellar population synthesis calculations, and the offset between the S0 and spiral Tully-Fisher relations. We also find that the global colours of S0 galaxies are strongly correlated with their GC specific frequencies: the redder the stellar population of an S0, the larger its specific frequency, as we might expect if we are catching different S0s at different stages of passively fading and reddening. Comparison to the predictions of stellar population synthesis models show that this explanation works quantitatively as well as qualitatively. These tests strongly support the hypothesis that S0 galaxies were once normal spirals, whose star formation was cut off, presumably due to a change of environment.
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We test the theory that lenticular (S0) galaxies form from spirals whose star formation has been shut down. We use the globular cluster specific frequency S_N, defined as the number of globular clusters normalised by the galaxy luminosity as a diagnostic. NTT/EMMI long-slit spectroscopic observations of 11 S0 galaxies at z < 0.006 are used to measure the absorption-line indices, Hdelta, Hgamma, Mgb, Fe5270 and Fe5335 within the central r_e/8. By inverting single-stellar population models, luminosity-weighted mean ages, metallicities and alpha-element abundance ratios are derived. We estimate the amount of fading a galaxy has undergone by comparing each galaxys S_N with its deviation from the mean spiral S_N. Galaxies with higher S_N have older stellar populations. Moreover, we find that the zero-point and amount of fading is consistent with a scenario where lenticulars are formed by the quenching of star formation in spiral galaxies. Our data also rule out any formation method for S0s which creates a large number of new globular clusters. We confirm that previous results showing a relationship between S_N and color are driven by the S_N - Age relation. Five galaxies show detectable Hbeta, [OIII], Halpha or [NII] emission lines. However, only in the two youngest galaxies is this emission unambiguously from star formation. Our results are consistent with the theory that S0 galaxies are formed when gas in normal spirals is removed, possibly as a result of a change in environment. The on-going star formation in the youngest galaxies hints that the timescale of quenching is ~< 1 Gyr. We speculate, therefore, that the truncation of star formation is a rather gentle process unlikely to involve a rapid burst of star formation.
We quantify the evolution of the spiral, S0 and elliptical fractions in galaxy clusters as a function of cluster velocity dispersion ($sigma$) and X-ray luminosity ($L_X$) using a new database of 72 nearby clusters from the WIde-Field Nearby Galaxy-cluster Survey (WINGS) combined with literature data at $z=0.5-1.2$. Most WINGS clusters have $sigma$ between 500 and 1100 $rm km s^{-1}$, and $L_X$ between 0.2 and $5 times 10^{44} rm erg/s$. The S0 fraction in clusters is known to increase with time at the expense of the spiral population. We find that the spiral and S0 fractions have evolved more strongly in lower $sigma$, less massive clusters, while we confirm that the proportion of ellipticals has remained unchanged. Our results demonstrate that morphological evolution since $z=1$ is not confined to massive clusters, but is actually more pronounced in low mass clusters, and therefore must originate either from secular (intrinsic) evolution and/or from environmental mechanisms that act preferentially in low-mass environments, or both in low- and high-mass systems. We also find that the evolution of the spiral fraction perfectly mirrors the evolution of the fraction of star-forming galaxies. Interestingly, at low-z the spiral fraction anticorrelates with $L_X$. Conversely, no correlation is observed with $sigma$. Given that both $sigma$ and $L_X$ are tracers of the cluster mass, these results pose a challenge for current scenarios of morphological evolution in clusters.
Data are presently available on the luminosities and half-light radii of 101 globular clusters associated with low-luminosity parent galaxies. The luminosity distribution of globulars embedded in dwarf galaxies having $M_{v} > -16$ is found to differ dramatically from that for globular clusters surrounding giant host galaxies with $M_{v} < -16$. The luminosity distribution of globular clusters in giant galaxies peaks at $M_{v} sim -7.5$, whereas that for dwarfs is found to increases monotonically down to the completeness limit of the cluster data at $M_{v} sim -5.0$. Unexpectedly, the power law distribution of the luminosities of globular clusters hosted by dwarf galaxies is seen to be much flatter than the that of bright unevolved part of the luminosity distribution of globular clusters associated with giant galaxies. The specific frequency of globular clusters that are fainter than $M_{v} = -7.5$ is found to be particularly high in dwarf galaxies. The luminosity distribution of the LMC globular clusters is similar to that in giant galaxies, and differs from those of the globulars in dwarf galaxies. The present data appear to show no strong dependence of globular cluster luminosity on the morphological types of their parent galaxies. No attempt is made to explain the unexpected discovery that the luminosity distribution of globular clusters is critically dependent on parent galaxy luminosity (mass?), but insensitive to the morphological type of their host galaxy.
We present central velocity dispersions, masses, mass to light ratios ($M/L$s), and rotation strengths for 25 Galactic globular clusters. We derive radial velocities of 1951 stars in 12 globular clusters from single order spectra taken with Hectochelle on the MMT telescope. To this sample we add an analysis of available archival data of individual stars. For the full set of data we fit King models to derive consistent dynamical parameters for the clusters. We find good agreement between single mass King models and the observed radial dispersion profiles. The large, uniform sample of dynamical masses we derive enables us to examine trends of $M/L$ with cluster mass and metallicity. The overall values of $M/L$ and the trends with mass and metallicity are consistent with existing measurements from a large sample of M31 clusters. This includes a clear trend of increasing $M/L$ with cluster mass, and lower than expected $M/L$s for the metal-rich clusters. We find no clear trend of increasing rotation with increasing cluster metallicity suggested in previous work.
We have obtained deep photometry of NGC 1199 (in HCG 22) and NGC 6868 (in the Telescopium group). Both galaxies are the optically brightest galaxies of their groups. Our analysis of B and R images taken with the Keck II and the VLT/ESO telescopes, detected a population of globular clusters around both galaxies, with total specific frequencies S_N=1.7pm0.6 for NGC 1199 and S_N = 1.3pm0.6 for NGC 6868. The color distributions of the globular cluster systems shows bimodal peaks centered at (B-R)_0 = 1.13pm0.10 and 1.42pm0.10 (NGC 1199) and (B-R)_0=1.12pm0.10 and 1.42pm0.10 (NGC 6868).
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