Friday, 12 November 2010

A Compact Early-type Galaxy at z = 0.6 Under a Magnifying Lens

By Auger et al.

Figure 1. Keck LGS-AO imaging of the EEL SDSSJ1347-0101 (left). The image is well modelled using a singular isothermal ellipsoid mass distribution for the lensing mass, a single Sersic component for the lensing galaxy, and a two-component de Vaucouleurs and Sersic profile for the background source (right), while a single-component fit to the source leaves a ring of residual flux (centre); the scaling of the two residual plots is set to saturate at 3 times the background noise level. The Kp-band imaging reveals an observed source magnitude of 17.6 but this is corrected to 20.3 when the magnification from lensing is taken into account; the inferred stellar mass of the source is 10^10.9M⊙ and the half-light radius of the two-component surface brightness model is 0.′′16 or 1.1 kpc at the redshift of the source.

Thursday, 4 November 2010

Rapid QSO shutdown in Hanny's Voorwerp

Schawinski et al (2010, presents interesting X-ray observations of Hanny's Voorwerp. They argue that the light from the Voorwerp (the green fuzz above) is caused by ionizing radiation from an AGN in the center of the host galaxy IC 2497. However their X-ray data show only a weak AGN, much too weak to explain the emission lines in the Voorwerp they argue (by 2-4 orders of magnitude). The inference is thus that the Voorwerp is a light echo of a QSO phase that terminated < 70,000 years ago. This is a rapid turn-off and more extreme than what you would expect by scaling up an X-ray binary for instance.

Friday, 15 October 2010

Title: Gas accretion as the origin of chemical abundance gradients in distant galaxies

In Cresci et al. (arXiv: 1010.2534) they find 'inverse' metallicity gradients for three rotationally supported star-forming galaxies at z=3. They conclude that the central gas has been diluted by the accretion of promordial gas, as predicted by 'cold flow' models. In Jones et al. (arXiv: 1010.1538) they find a 'normal' metallicity gradient for a lensed z=2 star-forming galaxy. While the physical gradient is considerably steeper than that observed in local galaxies, in terms of the effective radius at that epoch, the gradient is similar. They conclude that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.


Jones et al. present near-infrared imaging spectroscopy of the strongly-lensed z = 2.00 galaxy SDSSJ120601.69+514227.8 (‘the Clone arc’).

FIG. 1: (Left) Hubble Space Telescope color composite image of the Clone arc. A1-5 represent multiple images of the z=2.00 source; the critical curve is shown in red. The OSIRIS pointings are indicated by the two green rectangles offset by 1.8 arcseconds in the East-West direction. Foreground lensing galaxies are labeled as G1-4. (Right) Distribution of the key emission line fluxes in multiple images A3-5 in units of 10−18 erg/s/cm2 (see text for discussion of optimum samplig). North is up and East is to the left. The critical line passes through the A3 and A4 components such that a small part of the arc (including A5) is imaged 4 times in the OSIRIS field. The remainder of A3/A4 is imaged twice in the OSIRIS field.

From these and other results they conclude:

We find a strong radial gradient in both the [Nii]/H and [Oiii]/H ratios indicating a metallicity gradient of −0.27±0.05 dex kpc−1 with central metallicity close to solar. This suggests that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.

Friday, 17 September 2010

How does galaxy environment matter?

For some obscure reason blogger does not let me put on my text here fully, so see the first comment for that....

Friday, 10 September 2010

In 1009.1619 Nilsson et al ('Stellar populations of z=1 LBGs from ACS slitless grism spectra') the authors perform sliless spectroscopy on a small sample of z~1 LBGs and perform SED fitting on these. They compare the SED fitting results to broadband photometry SED fitting and find that the addition of slitless (high S/N, low spectral resolution) spectroscopy helps greatly in narrowing the allowed range of solutions of the stellar population.

The stellar populations are very well fit by a single SSP or by a continuous SFR, a second SSP has a best fit mass of ~0. Stellar masses, ages and extinction are very well constrained (~10^10 Msun, few-few tens of Myrs, 0.5-2 mag A_V), but the metallicity is not constrained at all.

Friday, 7 May 2010

No bulge in the Milky Way?

In 1005.0385 (Our Milky Way as a pure-disk galaxy - A challenge for galaxy formation), Shen et al. model Milky Way like galaxies with N-body only code. The set up disk galaxies with bars of various strengths and evolve them dynamically for a bit. Then they add realistic bulges of various masses and see what is still feasible, giving the kinetic data of the BRAVA kinetic bulge observations. What they find is that the puffing up of the initially quite thin bar mimicks the presence of a bulge already, if viewed from the location of the Sun. When they include bulges of various masses, they find that the bulge is at most as massive as ~.1 times the mass of the disk. This is, they say, much less then required by 'the classical galaxy formation scenario'.

Friday, 23 April 2010

An x-ray detected cluster at z=1.62

Tanaka et al. have independently identified the cluster at z=1.62 that I blogged about recently, and have confirmed the redshift using NIR spectroscopy.  They have also detected it in the x-ray, thereby upgrading its status from "protocluster" to "cluster" (according to the common usage of those terms), and making it the most distant cluster known.

Dust in DLAs

Frank & Peroux ( has done a study of dust attenuation in damped-Lya absorbers from the SDSS DR7. Using 676 absorbers and a comparison sample of QSOs they manage to put stringent constraints on the amount of dust attenuation in the Lya absorbers and find that on average they have <0.01 in E(B-V). What is nice is that they show that if they use the same selection techniques as others have used in the past, they do recover their detections - so their conclusion is that the average DLA has very little dust but that some specially selected subsamples do show some.

Friday, 16 April 2010

Peng et al (2010, take a data-driven approach to study how star-formation is turned off in different environments/different mass scales using SDSS and zCOSMOS. They combine a number of results but one of the fundaments for their work is the plot above which shows the relative quenching efficiency as a function of local density in several mass bins (top) and as a function of mass in several local density bins (bottom) - this plot is for the SDSS.

They define quenching here to be the number of objects on the red sequence that would have been on the blue sequence in the lowest density environment - so it is all done relative to the lowest density environment.

Their conclusion is that the effect of environment and mass seem to be decoupled and can be treated separately - ie. that there is one source of quenching that depends on stellar mass and one that depends on local density but that they do not interact significantly. They also find the same for zCosmos at z~0.8 and postulate that it extends somewhat higher in redshift.

They are able to fit a range of observables with a very simple model and quantify the relative importance of what they call 'mass quenching' and 'environmental quenching' for galaxies of different mass at different redshifts.

Thursday, 15 April 2010

A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies

This figure by Onodera et al. shows a velocity dispersion (red-filled circle) of a massive galaxy at z=1.82 in comparison to others (i.e. SDDS, Cappellari et al. 2009, van Dokkum et al. 2009). The velocity dispersion was measured from a 4.7 hours spectrum obtained with MOIRCS on Subaru. They find that their observations (morphology, size, and velocity dispersion) are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. I particularly like their conclusion: "It is clear that many more observations of similar galaxies are required to establish which kind of ETG is commonest at high redshift: either the compact/high-velocity dispersion objects like those found by van Dokkum et al. (2009), or the apparently normal, low- velocity dispersion objects presented in this paper."

Friday, 9 April 2010

The physical origins of the morphology-density relation: evidence for gas stripping from the SDSS

This figure by van der Wel et al. shows how the axis ratio of quiescent galaxies from SDSS depends on the halo mass and stellar mass. The left panel shows that at lower halo masses, quiescent galaxies with stellar mass (5-10)x10^10 Msun tend to be round, but at higher halo masses quiescent galaxies show a wider range of axis ratios. Most of the quiescent galaxies in high-mass halos are satellites, and so this additional satellite population (which is not present at lower halo masses) tends to have higher ellipticities.

The authors go on to present a simple model that shows how this extra population of quiescent satellites has an ellipticity distribution that is indistinguishable from ~L* spiral galaxies. If I understood it correctly, this model has no free parameters... which makes the agreement with data pretty impressive. So the interpretation is that the satellite galaxies may have been typical field spirals, which had their star formation shut off through some environmental process that did not affect the structural properties. The natural physical explanation is the gradual stripping of gas in the satellite galaxies and their (sub-) halos.

Which all sounds fine to me. But note that the difference in ellipticities only holds over an intermediate range of stellar mass, as shown in the right panel. My first thought is that perhaps the more massive galaxies were already quiescent and already had round profiles before they were accreted. And that at lower masses, all of the quiescent galaxies are satellites that became quiescent through environmental processes which operate with the same efficiency even in lower mass halos.

Friday, 26 March 2010

In 1003.4018 Schawinski et al take a sample of morphologically selected Early types from SDSS stripe 82 (deeper imaging), and by eye investigate whether they show signs of major mergers. The fraction of mergers is plotted against three measures of time: u-r colour, Emiisionline classification and post-starburst age.

They want to conclude from this seqence that mergers drive AGN, with a delay time of ~500 Myr and say this is one of very few observational indications of the theoretical understanding of AGN feeding by mergers.

The FIR SEDs of z~2 galaxies: evidence for scaled up cool galaxies

Last week we talked about a paper that calibrated the relationship between the observed 24um (= 8um in the restframe) flux and SFR for z~2 galaxies. This paper by Muzzin et al. looks at a similar issue, but takes a slightly different approach.

The Chary & Elbaz templates describe the infrared SEDs of local galaxies. These templates have a luminosity dependence, such that IR-luminous galaxies have templates that peak at shorter wavelengths, due to increased dust temperatures.

So one method of estimating the SFR of a galaxy is to see which template corresponds to the observed restframe 8um luminosity, assume that the template accurately describes the SED throughout the rest of the IR regime, and to convert the total IR luminosity of the template into an SFR.

The red dashed curves in the SEDs shown above illustrate the procedure. But Muzzin et al. have compared the templates to the observed fluxes at different IR wavelengths for these two galaxies, and obviously the templates don't work very well. But the solid curves, which are templates for low-luminosity galaxies that have been scaled up to match the data points, do provide good fits. This shows that the correlation between IR luminosity and dust temperature that we see in the local universe doesn't work at higher redshifts, and that even IR-luminous galaxies at z~2 can have relatively cool dust. Previous studies that relied on this correlation may have overestimated the SFR by a factor of several.

Muzzin et al. go on to show general agreement between the SFRs as derived from the IR luminosity and from the dust-corrected Halpha luminosity (where the extinction in Halpha is taken to be roughly twice the extinction to the stellar continuum, which in turn is estimated using stellar population synthesis modeling).

Friday, 12 March 2010

The mid-IR luminosities of normal galaxies

In the last few years there has been a lot of discussion about the effect of TP-AGB stars on stellar mass estimates for high redshift galaxies. Such stars produce a large fraction of the rest-frame NIR light from youngish (~1 Gyr old) stellar populations, so if observations at these wavelengths are used when fitting stellar population models, it is important to take these stars into account. However, given all of the modeling and observational uncertainties associated with our understanding of TP-AGB stars, we are a long way away from being able to robustly incorporate them into stellar population synthesis (SPS) models.

This letter by Dan Keslon and Brad Holden looks at another important effect of TP-AGB stars: their contribution to the MIR luminosity, which is frequently used as an indicator of the star formation rate. The authors estimate contribution to the MIR luminosity by adding the observed K-MIR colors for Galactic TP-AGB M and C stars to the expected contribution to the K-band luminosity that comes from Maraston's SPS model. This figure shows some of the results. The upper blue lines show the maximum contribution to the MIR, which occurs when the TP-AGB C stars dominate (although this comparison must depend somewhat on the star formation history assumed in the models). It thus appears that the MIR luminosity of galaxies is consistent with being entirely due to the TP-AGB stars.

There's a lot of stuff in this (very long) letter, but what is most striking is their figure 2d, which I haven't shown here. That figure shows that the authors are able to reproduce the observed correlation between 24um luminosity and star formation rate from Chary & Elbaz (2001) very well, but only when the SFR is averaged over the last 1.5 Gyr (which is when the TP-AGB stars are important). But I do find this result a little weird, since presumably that correlation was made against SFR indicators that are sensitive only to more recent SF. Also, it would suggest that very little of the MIR luminosity comes from dust that is not immediately surrounding TP-AGB stars.

Friday, 5 March 2010

The Merger-Driven Evolution of Massive Galaxies

Robaina et al. estimate the contribution of major mergers to the growth of the red sequence at M>10^11 Msun from z~1 to z~0.  To do this, they estimate the merger rate of all galaxies (i.e. both red and blue) more massive then 5x10^10 Msun using the fraction of galaxies in close pairs in COSMOS and COMBO-17, and make the assumption that the remnants of all such mergers lie on the red sequence (or that they move onto the RS very quickly).  The data points in this figure show the observed evolution in the number density of M>10^11 Msun RS galaxies, and the curve shows the predicted evolution due to their major merger estimates.

The agreement is impressively good.  But given the uncertainties (in the number density evolution, which is uncertain in part because of the uncertainty in the M/L evolution, as well as the uncertainties involved in getting a merger rate from a correlation function), it doesn't seem like you can draw very strong conclusions.  Additionally, as the authors mention, they somewhat underestimate the growth due to mergers because they would not count a merger between e.g. an 6x10^10 Msun galaxy and an 4x10^10 Msun galaxy.

The authors also find that a present day M>10^11 Msun galaxy has undergone 0.5 major mergers between M>5x10^10 Msun galaxies since z=0.6, and 0.7 such mergers since z=1.2.

Anyway, this is a nice work, and points to the importance in major mergers at the massive end.  I am somewhat curious about what the growth due to more minor mergers is, especially given that minor mergers are supposed to be what drives the size evolution of the RS galaxies.

Friday, 19 February 2010

A Spitzer-selected galaxy cluster at z=1.62

Papovich et al. have found a galaxy (proto-) cluster at z=1.62 by searching for overdensities of objects with red 3.6um-4.5um colors.  Both star-forming and quiescent galaxies at this redshift will have red colors in these passbands, so this is different than the red-sequence selection used by other groups.

Nonetheless, in the author's words, "this is the highest redshift, spectroscopically-confirmed clustering with a strong, well-defined red sequence" (but note that none of the RS galaxies has spectroscopic redshifts).