The process that quenched star formation in galaxies at intermediate and high redshift is still the subject of considerable debate. One way to investigate this puzzling issue is to study the number density of quiescent galaxies at z ≃ 2, and its dependence on mass. Here we present the results of a new study based on very deep Ks-band imaging (with the HAWK-I instrument on the VLT) of two HST CANDELS fields (the UKIDSS Ultra-deep survey (UDS) field and GOODS-South). The new HAWK-I data (taken as part of the HUGSVLTLargeProgram)reach detection limits of Ks >26(ABmag).
We have combined this imaging with the other ground-based and HST data in the CANDELS fields to select a sample of passively-evolving galaxies in the redshift range 1.4 < z < 2.5 (via the pBzK colour-based selection criterion of Daddi et al. 2004). Thanks to the depth and large areal coverage of our imaging, we have been able to extend the selection of quiescent galaxies a magnitude fainter than previous analyses. This has enabled us to establish unambiguously that the number counts of quiescent galaxies at z ≃ 2 decline at magnitudes fainter than Ks ∼ 22 (AB mag.), in contrast to the number density of star-forming galaxies which continues to rise to fainter magnitudes. Through extensive simulations we demonstrate, for the first time, that this turn-over in the number of quiescent galaxies is not due to incompleteness, but is real. We show that this trend corresponds to a stellar mass threshold M∗ ≃ 1010.8 M⊙ below which the mechanism that halts the star formation in high-redshift galaxies seems to be inefficient. Finally we compare the observed pBzK number counts with a population of quiescent galaxies extracted from four different semi-analytic models. We find that only two of these models reproduce even qualitatively the observed trend in the number counts, and that none of the models provides a statistically acceptable description of the number density of quiescent galaxies at these redshifts. We conclude that the mass function of quiescent galaxies as a function of redshift continues to present a key and demanding challenge for proposed models of galaxy formation and evolution.
BzK plot for all CANDELS galaxies with a spectroscopic red- shift (35 for UDS and 87 for GOODS-S); each symbol refers to a differ- ent redshift range: red points 1.4 < z < 2.5, black crosses 1.0 < z < 1.4, and green triangles z > 2.5 (different colours in the electronic version). The spectroscopic redshift distribution for the BzK galaxies is shown in the box insert. Vertical lines delineate the standard anticipated redshift range of the BzK selection technique (1.4 < z < 2.5).
In the figure on the right we show the BzK plot for all CANDELS galaxies with a spectroscopic red- shift (35 for UDS and 87 for GOODS-S); each symbol refers to a differ- ent redshift range: red points 1.4 < z < 2.5, black crosses 1.0 < z < 1.4, and green triangles z > 2.5 (different colours in the electronic version). The spectroscopic redshift distribution for the BzK galaxies is shown in the box insert. Vertical lines delineate the standard anticipated redshift range of the BzK selection technique (1.4 < z < 2.5).
In the images below we show the number counts (left) of the CANDELS pBzK galaxies as a function of the observed K magnitude and the stellar mass distribution (right) in units of M⊙. The key breakthrough enabled here is that, by extending the study of BzK selected passive galaxies a magnitude fainter than was previously possible, this work has revealed a clear turnover in the number density of quiescent galaxies below a stellar mass threshold of M* ~ 1010.8 solar masses. This result confirms (and better quantifies) previous suggestions that passive galaxies are confined to the high‐mass regime at these redshifts, and comparison with the predictions of various current models of galaxy formation shows that none of them can properly reproduce this key feature of the evolving galaxy population. This work thus demonstrates that the evolving mass function of passive galaxies continues to present one of the key challenges for proposed models of galaxy formation and evolution.
The stellar mass distribution of the CANDELS pBzK galaxies in units of M⊙ (logarithmic scale). The black solid line shows the mass distribution computed using only the K-band (Daddi et al. 2004), while the purple dashed line is the result of using the masses obtained with the multi-band SED-fitting technique. For comparison we plot the galaxy stellar mass distribution of the pBzK derived by Grazian et al. (2007) from the GOODS-MUSIC sample (red dotted line).
The number counts of the CANDELS pBzK galaxies (black line with Poissonian errors, see the text.) as a function of the observed K magnitude. The upper x-axis shows the corresponding rest frame absolute magnitude in the I band at z ≃ 2. As highlighted in text, for completeness reason, we prefer to limit our analysis at K<24, however we plot the faint and of the distribution until K = 25 with a dotted line. The number counts corrected for incompleteness are shown in red. The blue, green, cyan, magenta and darkgreen lines show the number counts for pBzK galaxies from the literature, from Kong et al. (2006), Lane et al. (2007), Hartley et al. (2008), McCracken et al. (2010), and Arcila-Osejo et al. (2013) respectively.
For more details, the whole article can be found here…