Number Density Of Quiescent Galaxies Research Articles

Stellar populations of galaxies in the ALHAMBRA survey up toz ∼ 1

Aims.We aim at constraining the stellar population properties of quiescent galaxies. These properties reveal how these galaxies evolved and assembled sincez ∼ 1 up to the present time.Methods.Combining the ALHAMBRA multi-filter photo-spectra with the fitting code for spectral energy distribution MUFFIT (MUlti-Filter FITting), we built a complete catalogue of quiescent galaxies via the dust-corrected stellar mass vs. colour diagram. This catalogue includes stellar population properties, such as age, metallicity, extinction, stellar mass, and photometric redshift, retrieved from the analysis of composited populations based on two independent sets of simple stellar population (SSP) models. We developed and applied a novel methodology to provide, for the first time, the analytic probability distribution functions (PDFs) of mass-weighted age, metallicity, and extinction of quiescent galaxies as a function of redshift and stellar mass. We adopted different star formation histories to discard potential systematics in the analysis.Results.The number density of quiescent galaxies is found to increase sincez ∼ 1, with a more substantial variation at lower stellar mass. Quiescent galaxies feature extinctionAV < 0.6, with median values in the rangeAV = 0.15–0.3. At increasing stellar mass, quiescent galaxies are older and more metal rich sincez ∼ 1. A detailed analysis of the PDFs reveals that the evolution of quiescent galaxies is not compatible with passive evolution and a slight decrease of 0.1–0.2 dex is hinted at median metallicity. The intrinsic dispersion of the age and metallicity PDFs show a dependence on stellar mass and/or redshift. These results are consistent with both sets of SSP models and assumptions of alternative star formation histories explored. Consequently, the quiescent population must undergo an evolutive pathway including mergers and/or remnants of star formation to reconcile the observed trends, where the “progenitor” bias should also be taken into account.

Quiescent Galaxies at z ≳ 2.5: Observations versus Models

Abstract The presence of massive quiescent galaxies at high redshifts is still a challenge for most models of galaxy formation. The aim of this work is to compare the observed number density and properties of these galaxies with the predictions of state-of-the-art models. The sample of massive quiescent galaxies has been selected from the COSMOS2015 photometric catalog with z phot ≥ 2.5, , and a specific star formation rate (sSFR) of . The photometric spectral energy distributions (SEDs) of the selected galaxies have been thoroughly analyzed based on different stellar population synthesis models. The final sample includes only those galaxies qualified as quiescent in all SED fitting runs. The observed properties have been compared to theoretical models: the number density of quiescent galaxies with is reproduced by some models, although there is a large scatter in their predictions. Instead, very massive are underpredicted by most of the current models of galaxy formation: some of them, built on the CARNage simulation, are consistent with data up to z ∼ 4, while at higher redshifts the volume of the considered simulation is too small to find such rare objects. Simulated galaxies that match the observed properties in the sSFR–M * plane at z ∼ 3 have been analyzed by reconstructing their evolutionary paths: their merger trees suggest that AGN feedback could be the key process allowing for a rapid quenching of the star formation at z ≳ 4 and that its treatment should be improved in models.

Near infrared spectroscopy and star-formation histories of 3 ≤ z ≤ 4 quiescent galaxies

We present Keck–MOSFIRE H and K spectra for a sample of 24 candidate quiescent galaxies at 3 < z < 4, identified from their rest-frame UVJ colors and photometric redshifts in the ZFOURGE and 3DHST surveys. With median integration times of one hour in H and five in K, we obtain spectroscopic redshifts for half of the sample, using either Balmer absorption lines or nebular emission lines. We confirm the high accuracy of the photometric redshifts for this spectroscopically-confirmed sample, with a median |Zphot − Zspec|/(1 + Zspec) of 1.2%. Two galaxies turn out to be dusty Hα emitters at lower redshifts (z < 2.5), and these are the only two detected in the sub-mm with ALMA. High equivalent-width [O III] emission is observed in two galaxies, contributing up to 30% of the K-band flux and mimicking the UVJ colors of an old stellar population. This implies a failure rate of only 20% for the UVJ selection at these redshifts. Lastly, Balmer absorption features are identified in four galaxies, among the brightest of the sample, confirming the absence of OB stars. We then modeled the spectra and photometry of all quiescent galaxies with a wide range of star-formation histories. We find specific star-formation rates (sSFR) lower than 0.15 Gyr−1 (a factor of ten below the main sequence) for all but one galaxy, and lower than 0.01 Gyr−1 for half of the sample. These values are consistent with the observed Hβ and [O II] luminosities, and the ALMA non-detections. The implied formation histories reveal that these galaxies have quenched on average 300 Myr prior to being observed, between z = 3.5 and 5, and that half of their stars were formed by z ∼ 5.5 with a mean SFR ~ 300 M⊙ yr−1. We finally compared the UVJ selection to a selection based instead on the sSFR, as measured from the photometry. We find that galaxies a factor of ten below the main sequence are 40% more numerous than UVJ-selected quiescent galaxies, implying that the UVJ selection is pure but incomplete. Current models fail at reproducing our observations, and underestimate either the number density of quiescent galaxies by more than an order of magnitude, or the duration of their quiescence by a factor two. Overall, these results confirm the existence of an unexpected population of quiescent galaxies at z > 3, and offer the first insights on their formation histories.

A SPECTROSCOPIC SAMPLE OF MASSIVE, QUIESCENTz∼ 2 GALAXIES: IMPLICATIONS FOR THE EVOLUTION OF THE MASS-SIZE RELATION

We present deep, near-infrared HST/WFC3 grism spectroscopy and imaging for a sample of 14 galaxies at z~2 selected from a mass-complete photometric catalog in the COSMOS field. By combining the grism observations with photometry in 30 bands, we derive accurate constraints on their redshifts, stellar masses, ages, dust extinction and formation redshifts. We show that the slope and scatter of the z~2 mass-size relation of quiescent galaxies is consistent with the local relation, and confirm previous findings that the sizes for a given mass are smaller by a factor of two to three. Finally, we show that the observed evolution of the mass-size relation of quiescent galaxies between z=2 and 0 can be explained by quenching of increasingly larger star-forming galaxies, at a rate dictated by the increase in the number density of quiescent galaxies with decreasing redshift. However, we find that the scatter in the mass-size relation should increase in the quenching-driven scenario in contrast to what is seen in the data. This suggests that merging is not needed to explain the evolution of the median mass-size relation of massive galaxies, but may still be required to tighten its scatter, and explain the size growth of individual z=2 galaxies quiescent galaxies.

A mass threshold in the number density of passive galaxies atz~ 2

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 HUGS VLT Large Program) reach detection limits of Ks>26 (AB mag). We select a sample of passively-evolving galaxies in the redshift range 1.4<z<2.5. Thanks to the depth and large area coverage of our imaging, we have been able to extend the selection of quiescent galaxies a magnitude fainter than previous analyses. Through extensive simulations we demonstrate, for the first time, that the observed turn-over in the number of quiescent galaxies at K>22 is real. This has enabled us to establish unambiguously that the number counts of quiescent galaxies at z~2 flatten and slightly decline at magnitudes fainter than Ks~22(AB mag.). We show that this trend corresponds to a stellar mass threshold $M_*10^{10.8}\,{\rm M_{\odot}}$ 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 those of quiescent galaxies extracted from four different semi-analytic models. We find 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.

MOIRCS Deep Survey. X. Evolution of Quiescent Galaxies as a Function of Stellar Mass at 0.5 z 2.5

Abstract We studied the evolution of quiescent galaxies at 0.5 $\lt$z$\lt$ 2.5 as a function of the stellar mass, using very deep NIR imaging data from the MOIRCS Deep Survey in the GOODS-North region. The deep NIR data allowed us to construct a stellar mass-limited sample of quiescent galaxies down to $\sim\ $10$^{10}\ M_{\odot}$, even at z$\sim\ $ 2, for the first time. We selected quiescent galaxies with age / $\tau$$\gt$ 6 by performing a SED fitting of the multi broad-band photometry from the $U$ to Spitzer 5.8$\ \mu$m bands with the population synthesis model of Bruzual and Charlot (2003, MNRAS, 344, 1000), where exponentially decaying star-formation histories are assumed. The number density of quiescent galaxies increases by a factor of $\sim\ $3 from 1.0 $\lt$z$\lt$ 1.5 to 0.5 $\lt$z$\lt$ 1.0, and by a factor of $\sim\ $10 from 1.5 $\lt$z$\lt$ 2.5 to 0.5 $\lt$z$\lt$ 1.0, while that of star-forming galaxies with age / $\tau$$\lt$ 4 increases only by factors of $\sim\ $2 and $\sim\ $3 in the same redshift ranges. At 0.5 $\lt$z$\lt$ 2.5, the low-mass slope of the stellar mass function of quiescent galaxies is $\alpha$$\sim\ $ 0–0.6, which is significantly flatter than those of star-forming galaxies ($\alpha$$\sim\ $$-$1.3–$-$1.5). As a result, the fraction of quiescent galaxies in the overall galaxy population increases with stellar mass in the redshift range. The fraction of quiescent galaxies at 10$^{11}$–10$^{11.5}\ M_{\odot}$ increases from $\sim\ $20%–30% at z$\sim\ $ 2 to $\sim\ $40%–60% at z$\sim\ $ 0.75, while that at 10$^{10}$–10$^{10.5}\ M_{\odot}$ increases from $\lesssim\ $5% to $\sim\ $15% in the same redshift range. These results could suggest that the quenching of star formation had been more effective in more massive galaxies at 1 $\lesssim\ $z$\lesssim\ $ 2. Such a mass-dependent quenching could explain the rapid increase in the number density of $\sim\ M^*$ galaxies relative to lower-mass galaxies at z$\gtrsim$ 1–1.5.