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Register a new userDo blue compact galaxies have red halos?
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Red halos are faint, extended and extremely red structures that have been reported around various types of galaxies since the mid-1990s. The colours of these halos are too red to be reconciled with any hitherto known type of stellar population, and instead indicative of a very bottom-heavy stellar initial mass function (IMF). Due to the large mass-to-light ratios of such stellar halos, they could contribute substantially to the baryonic masses of galaxies while adding very little to their overall luminosities. The red halos of galaxies therefore constitute potential reservoirs for some of the baryons still missing from inventories in the low-redshift Universe. While most studies of red halos have focused on disk galaxies, a red excess has also been reported in the faint outskirts of blue compact galaxies (BCGs). A bottom-heavy IMF can explain the colours of these structures as well, but due to model degeneracies, stellar populations with standard IMFs and abnormally high metallicities have also been demonstrated to fit the data. Here, we show that due to recent developments in the field of spectral synthesis, the metallicities required in this alternative scenario may be less extreme than previously thought. This suggests that the red excess seen in the outskirts of BCGs may stem from a normal, intermediate-metallicity host galaxy rather than a red halo of the type seen around disk galaxies. The inferred host metallicity does, however, still require the host to be more metal-rich than the gas in the central starburst of BCGs, in contradiction with current simulations of how BCGs form.
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The disk masses of four low surface brightness galaxies (LSB) were estimated using marginal gravitational stability criterion and the stellar velocity dispersion data which were taken from Pizzella et al., 2008 [1]. The constructed mass models appear to be close to the models of maximal disk. The results show that the disks of LSB galaxies may be significantly more massive than it is usually accepted from their brightnesses. In this case their surface densities and masses appear to be rather typical for normal spirals. Otherwise, unlike the disks of many spiral galaxies, the LSB disks are dynamically overheated.
We show evidence that the red giant star ksi Hya has an oscillation mode lifetime, tau, of about 2 days significantly shorter than predicted by theory (tau = 17 days, Houdek & Gough 2002). If this is a general trend of red giants it would limit the prospects of asteroseismology on these stars because of poor coherence of the oscillations.
We perform an exhaustive comparison among central galaxies from SDSS catalogs in different local environments at 0.01<=z<=0.08. The central galaxies are separated into two categories: group centrals (host halos containing satellites) and field centrals (host halos without satellites). From the latter, we select other two subsamples: isolated centrals and bright field centrals, both with the same magnitude limit. The stellar mass (Ms) distributions of the field and group central galaxies are different, which explains why in general the field central galaxies are mainly located in the blue cloud/star forming regions, whereas the group central galaxies are strongly biased to the red sequence/passive regions. The isolated centrals occupy the same regions as the bright field centrals since both populations have similar Ms distributions. At parity of Ms, the color and specific star formation rate (sSFR) distributions of the samples are similar, specially between field and group centrals. Furthermore, we find that the stellar-to-halo mass (Ms-Mh) relation of isolated galaxies does not depend on the color, sSFR and morphological type. For systems without satellites, the Ms-Mh relation steepens at high halo masses compared to group centrals, which is a consequence of assuming a one-to-one relation between group total stellar mass and halo mass. Under the same assumption, the scatter around the Ms-Mh relation of centrals with satellites increases with halo mass. Our results suggest that the mass growth of central galaxies is mostly driven by the halo mass, with environment and mergers playing a secondary role.
I highlight three results from cosmological hydrodynamic simulations that yield a realistic red sequence of galaxies: 1) Major galaxy mergers are not responsible for shutting off star-formation and forming the red sequence. Starvation in hot halos is. 2) Massive galaxies grow substantially (about a factor of 2 in mass) after being quenched, primarily via minor (1:5) mergers. 3) Hot halo quenching naturally explains why galaxies are red when they either (a) are massive or (b) live in dense environments.
The faint stellar halos of galaxies contain key information about the oldest stars and the process of galaxy formation. A previous study of stacked SDSS images of disk galaxies has revealed a halo with an abnormally red r-i colour, seemingly inconsistent with our current understanding of stellar halos. Here, we investigate the statistical properties of the faint envelopes of low surface brightness disk galaxies to look for further support for a red excess. 1510 edge-on low surface brightness galaxies were selected from the SDSS Data Release 5, rescaled to the same apparent size, aligned and stacked. This procedure allows us to reach a surface brightness of mu_g ~ 31 mag arcsec^-2. After a careful assessment of instrumental light scattering effects, we derive median and average radial surface brightness and colour profiles in g,r and i. The sample is then divided into 3 subsamples according to g-r colour. All three samples exhibit a red colour excess in r-i in the thick disk/halo region. The halo colours of the full sample, g-r = 0.60+-0.15 and r-i = 0.80+-0.15, are found to be incompatible with the colours of any normal type of stellar population. The fact that no similar colour anomaly is seen at comparable surface brightness levels along the disk rules out a sky subtraction residual as the source of the extreme colours. A number of possible explanations for these abnormally red halos are discussed. We find that two different scenarios -- dust extinction of extragalactic background light and a stellar population with a very bottom-heavy initial mass function -- appear to be broadly consistent with our observations and with similar red excesses reported in the halos of other types of galaxies.
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