We present an analysis of the small-to-intermediate scale clustering of samples of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF-SDSS LRG and QSO Survey (2SLAQ) ...survey carefully matched to have the same rest-frame colours and luminosity. We study the spatial two-point autocorrelation function in both redshift space ξ(s) and real space ξ(r) of a combined sample of over 10 000 LRGs, which represent the most massive galaxies in the universe with stellar masses >1011h−1M⊙ and space densities ≃10−4h3Mpc−3. We find no significant evolution in the amplitude (r0) of the correlation function with redshift, but do see a slight decrease in the slope (γ) with increasing redshift over 0.19 < z < 0.55 and scales of 0.32 < r < 32 h−1Mpc. We compare our measurements with the predicted evolution of dark matter clustering and use the halo model to interpret our results. We find that our clustering measurements are inconsistent (>99.9 per cent significance) with a passive model whereby the LRGs do not merge with one another; a model with a merger rate of 7.5 ± 2.3 per cent from z= 0.55 to 0.19 (i.e. an average rate of 2.4 per cent Gyr−1) provides a better fit to our observations. Our clustering and number density measurements are consistent with the hypothesis that the merged LRGs were originally central galaxies in different haloes which, following the merger of these haloes, merged to create a single brightest cluster galaxy. In addition, we show that the small-scale clustering signal constrains the scatter in halo merger histories. When combined with measurements of the luminosity function, our results suggest that this scatter is sub-Poisson. While this is a generic prediction of hierarchical models, it has not been tested before.
We have used the Two-Degree Field (2dF) instrument on the Anglo-Australian Telescope (AAT) to obtain redshifts of a sample of z < 3 and 18.0 < g < 21.85 quasars selected from Sloan Digital Sky Survey ...(SDSS) imaging. These data are part of a larger joint programme between the SDSS and 2dF communities to obtain spectra of faint quasars and luminous red galaxies, namely the 2dF-SDSS LRG and QSO (2SLAQ) Survey. We describe the quasar selection algorithm and present the resulting number counts and luminosity function of 5645 quasars in 105.7 deg2. The bright-end number counts and luminosity functions agree well with determinations from the 2dF QSO Redshift Survey (2QZ) data to g~ 20.2. However, at the faint end, the 2SLAQ number counts and luminosity functions are steeper (i.e. require more faint quasars) than the final 2QZ results from Croom et al., but are consistent with the preliminary 2QZ results from Boyle et al. Using the functional form adopted for the 2QZ analysis (a double power law with pure luminosity evolution characterized by a second-order polynomial in redshift), we find a faint-end slope of β=−1.78 ± 0.03 if we allow all of the parameters to vary, and β=−1.45 ± 0.03 if we allow only the faint-end slope and normalization to vary (holding all other parameters equal to the final 2QZ values). Over the magnitude range covered by the 2SLAQ survey, our maximum-likelihood fit to the data yields 32 per cent more quasars than the final 2QZ parametrization, but is not inconsistent with other g > 21 deep surveys for quasars. The 2SLAQ data exhibit no well-defined ‘break’ in the number counts or luminosity function, but do clearly flatten with increasing magnitude. Finally, we find that the shape of the quasar luminosity function derived from 2SLAQ is in good agreement with that derived from Type I quasars found in hard X-ray surveys.
We have combined optical data from the 2dF-SDSS (Sloan Digital Sky Survey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ) redshift survey with radio measurements from the 1.4 GHz ...VLA (Very Large Array) FIRST (Faint Images of the Radio Sky at Twenty-cm) and NVSS (NRAO VLA Sky Survey) surveys to identify a volume-limited sample of 391 radio galaxies at redshift 0.4 < z < 0.7. By determining an accurate radio luminosity function for luminous early-type galaxies in this redshift range, we can investigate the cosmic evolution of the radio-galaxy population over a wide range in radio luminosity.
The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio luminosities in the range 1024 to 1025 W Hz−1, corresponding to Fanaroff-Riley I (FR I) radio galaxies in the local Universe) undergo significant cosmic evolution over the redshift range 0 < z < 0.7, consistent with pure luminosity evolution of the form (1 +z)
k
, where k= 2.0 ± 0.3. Our results appear to rule out (at the 6-7σ level) models in which low-power radio galaxies undergo no cosmic evolution. The most powerful radio galaxies in our sample (with radio luminosities above 1026 W Hz−1) may undergo more rapid evolution over the same redshift range.
The evolution seen in the low-power radio-galaxy population implies that the total energy input into massive early-type galaxies from active galactic nucleus (AGN) heating increases with redshift, and was at least 50 per cent higher at z∼ 0.55 (the median redshift of the 2SLAQ LRG sample) than in the local universe.
Is everything we know about the universe wrong? Sawangwit, Utane; Shanks, Tom
Astronomy & geophysics : the journal of the Royal Astronomical Society,
October 2010, Letnik:
51, Številka:
5
Journal Article
Recenzirano
Odprti dostop
In the first of a pair of articles, Utane Sawangwit and Tom Shanks play devil's advocate, wondering if the standard cosmological model is missing something.
We present new measurements of the luminosity function (LF) of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF SDSS LRG and Quasar (2SLAQ) survey. We have carefully ...quantified, and corrected for, uncertainties in the K and evolutionary corrections, differences in the colour selection methods, and the effects of photometric errors, thus ensuring we are studying the same galaxy population in both surveys. Using a limited subset of 6326 SDSS LRGs (with 0.17 < z < 0.24) and 1725 2SLAQ LRGs (with 0.5 < z < 0.6), for which the matching colour selection is most reliable, we find no evidence for any additional evolution in the LRG LF, over this redshift range, beyond that expected from a simple passive evolution model. This lack of additional evolution is quantified using the comoving luminosity density of SDSS and 2SLAQ LRGs, brighter than M0.2r− 5 log h0.7=−22.5, which are 2.51 ± 0.03 × 10−7 L⊙ Mpc−3 and 2.44 ± 0.15 × 10−7 L⊙ Mpc−3, respectively (<10 per cent uncertainty). We compare our LFs to the COMBO-17 data and find excellent agreement over the same redshift range. Together, these surveys show no evidence for additional evolution (beyond passive) in the LF of LRGs brighter than M0.2r− 5 log h0.7=−21 (or brighter than ∼L*). We test our SDSS and 2SLAQ LFs against a simple ‘dry merger’ model for the evolution of massive red galaxies and find that at least half of the LRGs at z≃ 0.2 must already have been well assembled (with more than half their stellar mass) by z≃ 0.6. This limit is barely consistent with recent results from semi-analytical models of galaxy evolution.
We have combined optical data from the 2dF-SDSS (Sloan Digital Sky Survey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ) redshift survey with radio measurements from the 1.4 GHz ...VLA (Very Large Array) FIRST (Faint Images of the Radio Sky at Twenty-cm) and NVSS (NRAO VLA Sky Survey) surveys to identify a volume-limited sample of 391 radio galaxies at redshift 0.4 < z < 0.7. By determining an accurate radio luminosity function for luminous early-type galaxies in this redshift range, we can investigate the cosmic evolution of the radio-galaxy population over a wide range in radio luminosity. The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio luminosities in the range 10 super(24) to 10 super(25) W Hz super(-1), corresponding to Fanaroff-Riley I (FR I) radio galaxies in the local Universe) undergo significant cosmic evolution over the redshift range 0 < z < 0.7, consistent with pure luminosity evolution of the form (1 +z) super(k), where k= 2.0 plus or minus 0.3. Our results appear to rule out (at the 6-7 sigma level) models in which low-power radio galaxies undergo no cosmic evolution. The most powerful radio galaxies in our sample (with radio luminosities above 10 super(26) W Hz super(-1)) may undergo more rapid evolution over the same redshift range.The evolution seen in the low-power radio-galaxy population implies that the total energy input into massive early-type galaxies from active galactic nucleus (AGN) heating increases with redshift, and was at least 50 per cent higher at z similar to 0.55 (the median redshift of the 2SLAQ LRG sample) than in the local universe.
We present newHubble Space Telescope(HST) Wide Field Planetary Camera images of the poststarburst quasar UN J1025−0040, which contains both an active galactic nucleus (AGN) and a 400 Myr old nuclear ...starburst of similar bolometric luminosity (∼1011.6
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). The F450W and F814W images resolve the AGN from the starburst and show that the bulk of the starlight (∼
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) is contained within a central radius of about 600 pc and lacks clear morphological structures at this scale. Equating the point‐source light in each image with the AGN contribution, we determined the ratio of AGN‐to‐starburst light. This ratio is 69% in the red F814W image, consistent with our previous spectral analysis, but ≤50% in the blue F450W image, whereas we had predicted 76%. TheHSTimages are consistent with previous photometry, ruling out variability (a fading AGN) as a cause for this result. We can explain the new data if there is a previously unknown young stellar population present, 40 Myr or younger, with as much as 10% of the mass of the dominant 400 Myr old population. This younger starburst may represent the trigger for the current nuclear activity. The multiple starburst ages seen in UN J1025−0040 and its companion galaxy indicate a complex interaction and star formation history.
We investigate the reddening and main-sequence-fitted distances to 11 young, Galactic open clusters that contain Cepheids. Each cluster contains or is associated with at least one Cepheid variable ...star. Reddening to the clusters is estimated using the U−B:B−V colours of the OB stars and the distance modulus to the cluster is estimated via B−V:V and V−K:V colour–magnitude diagrams. Our main-sequence fitting assumes that the solar-metallicity zero-age main sequence of Allen applies universally to all the open clusters, although this point is controversial at present. In this way we proceed to calibrate the Cepheid period–luminosity (PL) relation and find MV=−2.87 × log P− 1.243 ± 0.09, MK=−3.44 × log P− 2.21 ± 0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of 18.54 ± 0.10 from the V-band and 18.48 ± 0.10 from the K-band giving an overall distance modulus to the LMC of μ0= 18.51 ± 0.10. This is in good agreement with the previous Cepheid PL-K result of Laney & Stobie at μ0= 18.51 ± 0.09 and with the Hipparcos parallax-calibrated Cepheid PL-K estimate of Feast & Catchpole at μ0= 18.66 ± 0.10 when no account is taken of the LMC metallicity. We also find that the two-colour U−B:B−V diagrams of two important clusters are not well fitted by the standard main-sequence line. In one case, NGC 7790, we find that the F stars show a UV excess and in the second case, NGC 6664, they are too red in U−B. Previous spectroscopic estimates of the metallicity of the Cepheids in these clusters appear to suggest that the effects are not due to metallicity variations. Other possible explanations for these anomalies are positional variations in the dust reddening law and contamination by foreground or background stars.
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