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Register a new userCLEAR II: Evidence for Early Formation of the Most Compact Quiescent Galaxies at High Redshift
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Vicente Estrada-Carpenter
Publication date
2020
fields
Physics
and research's language is
English
Authors
Vicente Estrada-Carpenter
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The origin of the correlations between mass, morphology, quenched fraction, and formation history in galaxies is difficult to define, primarily due to the uncertainties in galaxy star-formation histories. Star-formation histories are better constrained for higher redshift galaxies, observed closer to their formation and quenching epochs. Here we use non-parametric star-formation histories and a nested sampling method to derive constraints on the formation and quenching timescales of quiescent galaxies at $0.7<z<2.5$. We model deep HST grism spectroscopy and photometry from the CLEAR (CANDELS Lyman$-alpha$ Emission at Reionization) survey. The galaxy formation redshifts, $z_{50}$ (defined as the point where they had formed 50% of their stellar mass) range from $z_{50}sim 2$ (shortly prior to the observed epoch) up to $z_{50} simeq 5-8$. editone{We find that early formation redshifts are correlated with high stellar-mass surface densities, $log Sigma_1 / (M_odot mathrm{kpc}^{-2}) >$10.25, where $Sigma_1$ is the stellar mass within 1~pkpc (proper kpc). Quiescent galaxies with the highest stellar-mass surface density, $logSigma_1 / (M_odot mathrm{kpc}^{-2}) > 10.25$, } show a textit{minimum} formation redshift: all such objects in our sample have $z_{50} > 2.9$. Quiescent galaxies with lower surface density, $log Sigma_1 / (M_odot mathrm{kpc}^{-2}) = 9.5 - 10.25$, show a range of formation epochs ($z_{50} simeq 1.5 - 8$), implying these galaxies experienced a range of formation and assembly histories. We argue that the surface density threshold $logSigma_1/(M_odot mathrm{kpc}^{-2})>10.25$ uniquely identifies galaxies that formed in the first few Gyr after the Big Bang, and we discuss the implications this has for galaxy formation models.
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Massive compact systems at 0.2<z<0.6 are the missing link between the predominantly compact population of massive quiescent galaxies at high redshift and their analogs and relics in the local volume. The evolution in number density of these extreme objects over cosmic time is the crucial constraining factor for the models of massive galaxy assembly. We select a large sample of ~200 intermediate-redshift massive compacts from the BOSS spectroscopic dataset by identifying point-like SDSS photometric sources with spectroscopic signatures of evolved redshifted galaxies. A subset of our targets have publicly available high-resolution ground-based images that we use to augment the dynamical and stellar population properties of these systems by their structural parameters. We confirm that all BOSS compact candidates are as compact as their high-redshift massive counterparts and less than half the size of similarly massive systems at z~0. We use the completeness-corrected numbers of BOSS compacts to compute lower limits on their number densities in narrow redshift bins spanning the range of our sample. The abundance of extremely dense quiescent galaxies at 0.2<z<0.6 is in excellent agreement with the number densities of these systems at high redshift. Our lower limits support the models of massive galaxy assembly through a series of minor mergers over the redshift range 0<z<2.
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We present an analysis of the galaxy population in XLSSC 122, an X-ray selected, virialized cluster at redshift $z=1.98$. We utilize HST WFC3 photometry to characterize the activity and morphology of spectroscopically confirmed cluster members. The quiescent fraction is found to be $88^{+4}_{-20}$ per cent within 0.5$r_{500}$, significantly enhanced over the field value of $20^{+2}_{-2}$ per cent at $zsim2$. We find an excess of bulge-like quiescent cluster members with Sersic index $n>2$ relative to the field. These galaxies are found to be larger than their field counterparts at 99.6 per cent confidence, being on average $63^{+31}_{-24}$ per cent larger at a fixed mass of $M_star = 5times10^{10} M_odot$. This suggests that these cluster member galaxies have experienced an accelerated size evolution relative to the field at $z>2$. We discuss minor mergers as a possible mechanism underlying this disproportionate size growth.
We present Gran-Telescopio-Canarias/OSIRIS optical spectra of 4 of the most compact and massive early-type galaxies in the Groth Strip Survey at redshift z~1, with effective radii Reff=0.5-2.4 kpc and photometric stellar masses Mstar=1.2-4x10^11 Msun. We find these galaxies have velocity dispersions sigma=156-236 km/s. The spectra are well fitted by single stellar population models with approximately 1 Gyr of age and solar metallicity. We find that: i) the dynamical masses of these galaxies are systematically smaller by a factor of ~6 than the published stellar masses using BRIJK photometry; ii) when estimating stellar masses as 0.7xMdyn, a combination of passive luminosity fading with mass/size growth due to minor mergers can plausibly evolve our objects to match the properties of the local population of early-type galaxies.
We have studied the evolution of high redshift quiescent galaxies over an effective area of ~1.7 deg^2 in the COSMOS field. Galaxies have been divided according to their star-formation activity and the evolution of the different populations has been investigated in detail. We have studied an IRAC (mag_3.6 < 22.0) selected sample of ~18000 galaxies at z > 1.4 with multi-wavelength coverage. We have derived accurate photometric redshifts (sigma=0.06) and other important physical parameters through a SED-fitting procedure. We have divided our sample into actively star-forming, intermediate and quiescent galaxies depending on their specific star formation rate. We have computed the galaxy stellar mass function of the total sample and the different populations at z=1.4-3.0. We have studied the properties of high redshift quiescent galaxies finding that they are old (1-4 Gyr), massive (log(M/M_sun)~10.65), weakly star forming stellar populations with low dust extinction (E(B-V) < 0.15) and small e-folding time scales (tau ~ 0.1-0.3 Gyr). We observe a significant evolution of the quiescent stellar mass function from 2.5 < z < 3.0 to 1.4 < z < 1.6, increasing by ~ 1 dex in this redshift interval. We find that z ~ 1.5 is an epoch of transition of the GSMF. The fraction of star-forming galaxies decreases from 60% to 20% from z ~ 2.5-3.0 to z ~ 1.4-1.6 for log(M/M_sun) > 11, while the quiescent population increases from 10% to 50% at the same redshift and mass intervals. We compare the fraction of quiescent galaxies derived with that predicted by theoretical models and find that the Kitzbichler & White (2007) model is the one that better reproduces the data. Finally, we calculate the stellar mass density of the star-forming and quiescent populations finding that there is already a significant number of quiescent galaxies at z > 2.5 (rho~6.0 MsunMpc^-3).
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