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Register a new userThe imprint of the thick stellar disc in the mid-plane of three early-type edge-on galaxies in the Fornax cluster
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Galactic stellar discs, such as that of the Milky Way, have usually a complex structure consisting of a thin and a thick component. The study of galactic disc substructures and their differences can shed light on the galaxy assembling processes and their evolution. However, due to observational difficulties there is a lack of information about the stellar populations of the thick disc components in external galaxies. Here we investigate three edge-on early-type disc galaxies in the Fornax cluster IC335, NGC1380A, NGC1381 by using publicly available photometrical data and our new deep long-slit spectroscopy along galactic mid-planes obtained with the 10-m SALT telescope. We report that significant changes of the stellar population properties beyond the radius where photometrical profiles demonstrate a knee are caused by an increasing thick disc contribution. Stellar population properties in the outermost thick-disc dominated regions demonstrate remarkably old ages and a low metallicity. We interpret these findings as a consequence of star formation quenching in the outermost regions of the discs due to ram pressure gas stripping from the disc periphery at the beginning of the cluster assembly while subsequent star formation occurring in the inner discs being gradually extinguished by starvation.
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The Advanced Camera for Surveys (ACS) Fornax Cluster Survey is a Hubble Space Telescope program to image 43 early-type galaxies in the Fornax cluster, using the F475W and F850LP bandpasses of the ACS. We employ both 1D and 2D techniques to characterize the properties of the stellar nuclei in these galaxies, defined as the central luminosity excesses relative to a Sersic model fitted to the underlying host. We find 72+/-13% of our sample (31 galaxies) to be nucleated, with only three of the nuclei offset by more than 0.5 from their galaxy photocenter, and with the majority of nuclei having colors bluer than their hosts. The nuclei are observed to be larger, and brighter, than typical Fornax globular clusters, and to follow different structural scaling relations. A comparison of our results to those from the ACS Virgo Cluster Survey reveals striking similarities in the properties of the nuclei belonging to these different environments. We briefly review a variety of proposed formation models and conclude that, for the low-mass galaxies in our sample, the most important mechanism for nucleus growth is probably infall of star clusters through dynamical friction, while for higher mass galaxies, gas accretion triggered by mergers, accretions and tidal torques is likely to dominate, with the relative importance of these two processes varying smoothly as a function of galaxy mass. Some intermediate-mass galaxies in our sample show a complexity in their inner structure that may be the signature of hybrid nuclei that arose through parallel formation channels.
The globular cluster (GC) specific frequency ($S_N$), defined as the number of GCs per unit galactic luminosity, represents the efficiency of GC formation (and survival) compared to field stars. Despite the naive expectation that star cluster formation should scale directly with star formation, this efficiency varies widely across galaxies. To explore this variation we measure the z-band GC specific frequency ($S_{N,z}$) for 43 early-type galaxies (ETGs) from the Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) Fornax Cluster Survey. Combined with the homogenous measurements of $S_{N,z}$ in 100 ETGs from the HST/ACS Virgo Cluster Survey from Peng et al. (2008), we investigate the dependence of $S_{N,z}$ on mass and environment over a range of galaxy properties. We find that $S_{N,z}$ behaves similarly in the two galaxy clusters, despite the clusters order-of-magnitude difference in mass density. The $S_{N,z}$ is low in intermediate-mass ETGs ($-20<M_z<-23$), and increases with galaxy luminosity. It is elevated at low masses, on average, but with a large scatter driven by galaxies in dense environments. The densest environments with the strongest tidal forces appear to strip the GC systems of low-mass galaxies. However, in low-mass galaxies that are not in strong tidal fields, denser environments correlate with enhanced GC formation efficiencies. Normalizing by inferred halo masses, the GC mass fraction, $eta=(3.36pm0.2)times10^{-5}$, is constant for ETGs with stellar masses $mathcal{M}_star lesssim 3times10^{10}M_odot$, in agreement with previous studies. The lack of correlation between the fraction of GCs and the nuclear light implies only a weak link between the infall of GCs and the formation of nuclei.
Diffuse star clusters (DSCs) are old and dynamically hot stellar systems that have lower surface brightness and more extended morphology than globular clusters (GCs). Using the images from HST/ACS Fornax Cluster Survey, we find that 12 out of 43 early-type galaxies (ETGs) in the Fornax cluster host significant numbers of DSCs. Together with literature data from the HST/ACS Virgo Cluster Survey, where 18 out of 100 ETGs were found to host DSCs, we systematically study the relationship of DSCs with GCs, and their host galaxy environment. Two DSC hosts are post-merger galaxies, with most of the other hosts either having low mass or showing clear disk components. We find that while the number ratio of DSCs to GCs is nearly constant in massive galaxies, the DSC-to-GC ratio becomes systematically higher in lower mass hosts. This suggests that DSCs may be more efficient at forming (or surviving) in low density environments. DSC hosts are not special either in their position in the cluster, or in the galactic color-magnitude diagram. Why some disk and low-mass galaxies host DSCs while others do not is still a puzzle, however. The mean ages of DSC hosts and non-hosts are similar at similar masses, implying that formation efficiency, rather than survival, is the reason behind different DSC number fractions in early-type galaxies.
Three observables of early-type galaxies - size ($r_{e}$), surface brightness ($I_{e}$), and velocity dispersion ($sigma_{0}$) - form a tight planar correlation known as the fundamental plane (FP), which has provided great insights into the galaxy formation and the evolution processes. However, the FP has been found to be tilted against the simple virial expectation, prompting debates on its origin. In order to investigate the contribution of systematic stellar population variation to the FP tilt, we study here the FP relations of early-type galaxies in mid-infrared (MIR) which may represent the stellar mass well. We examined the wavelength dependence of the FP coefficients, $a$ and $b$ in $log r_{e}= alogsigma_{0} + blog< I >_{e} + c$, using a sample of 56 early-type galaxies for which visible (V-band), near-infrared (K-band), and MIR (Spitzer IRAC, 3.6--8.0$mu$m) data are available. We find that the coefficient $a$ increases as a function of wavelength as $da/dlambda=0.11pm0.04mu m^{-1}$, while the coefficient $b$ reaches the closest to -1 at 3.6--5.8$mu$m. When applied to the visible FP coefficients derived from a larger sample of nearby early-type galaxies, we get the FP relation with $(a,b) simeq $(1.6--1.8,-0.9) at 3.6$mu$m. Our result suggests that the stellar population effect can explain more than half of the FP tilt, closing the gap between the virial expectation and the optical FP. The reduction in the FP tilt is reflected in the dynamical mass-to-light ratio, $M_{dyn}/L$, dependence on $L$ which decreases toward 3.6--5.8$mu$m, suggesting that the MIR light better represents mass than the shorter wavelengths.
Disc truncations are the closest feature to an edge that galaxies have, but the nature of this phenomena is not yet understood. In this paper, we explore the truncations in two nearby (D ~15 Mpc) Milky Way-like galaxies: NGC 4565 and NGC 5907. We cover a wide wavelength range from the NUV and optical, to 3.6 {mu}m. We find that the radius of the truncation (26+/-0.5 kpc) is independent of wavelength. Surprisingly, we identify (at all wavelengths) the truncation at altitudes as high as 3 kpc above the mid-plane, which implies that the thin disc in those outer regions has a width of at least this value. We find the characteristic U-shape radial colour profile associated with a star formation threshold at the location of the truncation. Further supporting such an origin, the stellar mass density at the position of the truncation is ~1-2 M_sun pc^-2, in good agreement with the critical gas density for transforming gas into stars. Beyond the truncation, the stellar mass in the mid-plane of the disc drops to just 0.1-0.2% of the total stellar mass of the galaxies. The detection of the truncation at high altitude in combination with the U shape of the radial colour profile allows us to establish, for the first time, an upper limit to the present-day growth rate of galactic discs. We find that, if the discs of the galaxies are growing inside-out, their growth rate is less than 0.6-0.9 kpc Gyr^-1.
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