Thursday, 30 October 2008
DISSECTING THE RED SEQUENCE—I. STAR FORMATION HISTORIES OF QUIESCENT GALAXIES: THE
COLOR-MAGNITUDE VS. THE COLOR-SIGMA RELATION
Genevieve J. Graves, S. M. Faber, & Ricardo P. Schiavon
From DR4 SDSS data with the NYU-VAGC, the authors select quiescent (i.e. emission line-free) galaxies. These lie on the red sequence. They measure luminosities, colors, velocity dispersion and a few element abundances (mainly Fe abundance and alpha-enrichment). In a plot that I don't show here, it shows that at fixed velocity dispersion there is no relation between luminosity and color (luminosities vary more than color, and contours of number density are largely horizontal). The red sequence is inclined, because higher sigma galaxies, have higher luminosities and are redder. Adding up different sigma bins results in the red sequence as we know it.
To investigate this further, the authors bin in L-sigma-color space and stack all spectra in a bin together to measure abundances and age. Here I will concentrate on age. In the plot you see six panels in the luminosity color plane, for six bins in velocity dispersion (these show no L-color relation!). The color coding is luminosity weighted mean age.
As you can see, low-sigma galaxies are younger and have a bigger spread in ages than luminous, high-sigma galaxies. Also, the variation in age is perpendicular to the red sequence, i.e. the width is set by the age distribution of the galaxies.
Quiescent galaxies are a multi-parameter family. Age (and Fe/H and alpha/Fe) all increase with sigma, and vary at fixed sigma depending on L. Age also varies as a function of color, at fixed sigma: brighter galaxies have lower age at fixed sigma and are bluer.
Friday, 24 October 2008
Recently Shen et al. (2007) found that high-z quasars from SDSS are very strongly clustered, with a bias of ~14 at z=4. White et al. (2008) used this result to show that there must be an extremely tight relation between quasar luminosity and halo mass, with an upper limit to the scatter of 0.3 dex. The basic reasoning behind this conclusion is that, if there were a larger scatter, then many quasars in (very abundant) low-mass halos would have high enough luminosity to make it into Shen's sample, however the observed number density of quasars is too low to allow for this. Wyithe & Loeb say that such a tight scatter is difficult to believe since the scatter in the relationship between black hole mass and bulge velocity dispersion is also 0.3 dex, and one might expect that this relationship is tighter and more direct then the relationship between halo mass and quasar luminosity.
Ok, I'll kick it off this week. This is a plot from Shen et al (2008, arXiv:0810.4144) who study the correlation of QSOs in the sky using the SDSS DR5. They look how clustering depends on luminosity, black hole mass, colour and radio loudness. What they find is perhaps a bit surprising but not entirely new - there is virtually no dependence on any of these parameters, except radio loudness. So - don't go around expecting a QSO to necessarily live in a massive halo, at least not at z<2.5
Friday, 17 October 2008
This paper presents another analysis of the sizes and surface densities of early-type galaxies at z=1-2; here they use a sample of 32 spectroscopically-confirmed galaxies with a mean redshift of 1.45 from several different surveys. Masses and ages are determined via SED fitting to the photometry. As has been reported before, these galaxies lie well off the z=0 size-luminosity relation. By evolving the galaxies (assuming pure luminosity evolution) from their measured redshifts to the present, the authors find that some galaxies would actually evolve to the z=0 relation in this manner, while some would not. These galaxies appear to be "young" and "old" respectively, and the authors appear to claim a bimodal age distribution in ETGs at this redshift, with typical ages ~1 Gyr and 3.5 Gyr. Young ETGs follow the local size-mass relation, old ETGs do not.
The authors conclude that the young objects have more or less completed their evolution (except for luminosity evolution), while the old galaxies still need some process to increase their effective radii. Dry merging cannot do this because it would create too many high-mass galaxies, so some other process must be at work. Much of this rests on the assumption that the relative ages can be accurately determined through photometry, of course.
Friday, 10 October 2008
The PN.S Elliptical Galaxy Survey is an ongoing survey to detect and measure the kinematics of planetary nebulae (PNe) in nearby early-type galaxies. Here they present 255 PNe measurements in the elliptical NGC 4494 out to 7 effective radii (Re). They construct mass models, where they include dark matter haloes to explain the observed kinematics at large radii. This plots shows the dark matter fraction they find in this galaxy and other galaxies in their sample, compared to results from numerical simulations. Overall, the dark matter fraction is lower than predicted in simulations, especially at smaller radii. This indicates a mismatch between observations and theory, with intermediate-luminosity galaxies having low concentration haloes.
From Napolitano et al., from http://arxiv.org/abs/0810.1291
This Nature paper describes integral field (OSIRIS on Keck) observations of a strongly lensed z=3.07 Lyman break galaxy. With AO corrections and the high magnification, these observations provide an effective physical resolution of 150 kpc. This figure shows (a) the reconstructed HST image with the lensing caustic overlaid, (b) [OIII] and Hbeta emission (bluescale and contours respectively), (c) velocity field and best-fit disk model, (d) [OIII] velocity dispersion, (e-h) 1-d profiles of the left panels taken along the "slit" shown in panel (d). The velocity field is well-fit by a disk model, so the authors conclude that this is a disk galaxy with v_r=67 km/s and M=2e9 Msun. However, the central velocity dispersion is large (v/sigma=1.2), so it's most likely still at an early stage of formation.
Another interesting sidenote: of the 2000ish subhalos that the authors tracked, only 51 became fully fledged satelite galaxies. This is still twice as many as are observed, but they reckon that optical selection effects can account for this. (Which you can kinda see from the plot.) This is essentially because they have completely supressed gas cooling in subhaloes with virial temperatures below 10^4 K.
Tuesday, 7 October 2008
Evidence for a Collision Between M86 and NGC 4438 and Implications for Collisional ISM Heating of Ellipticals
This is a color gri from SDSS, overlayed with narrowband Halpha+NII images (visible as the red and green filaments). The giant elliptical on the right is M86, which appears to be the brightest galaxy in a group or sub-cluster that is merging with Virgo. The galaxy on the left is NGC 4438 (also Arp 120), a highly-disturbed spiral. The red filaments are Halpha+NII emission that appear to link the two galaxies, suggesting that they have undergone a high-speed collision. The green filaments are Halpha+NII emission at a higher recessional velocity; it is not clear whether the galaxy in the lower right and the associated line emission are involved in this interaction.
NGC 4438 is very HI-deficient for a spiral galaxy of it's size. If it lost most of it's HI during the collision, then it is expected that a significant fraction of the kinetic energy of that gas went into heating the ISM of M86. This heating would be enough to prevent gas from cooling and forming stars in M86, possibly obviating the need for radio-mode AGN feedback. Thus this interacting system may be a nice example of the "gravitational quenching" mechanism discussed by Dekel & Birnboim (2008).