Stellar population properties of individual massive early-type galaxies at 1.4 < z < 2. (arXiv:1912.00084v1 [astro-ph.GA])

Stellar population properties of individual massive early-type galaxies at 1.4 < z < 2. (arXiv:1912.00084v1 [astro-ph.GA]) <a href="http://arxiv.org/find/astro-ph/1/au:+Lonoce_I/0/1/0/all/0/1">I. Lonoce</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maraston_C/0/1/0/all/0/1">C. Maraston</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Thomas_D/0/1/0/all/0/1">D. Thomas</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Longhetti_M/0/1/0/all/0/1">M. Longhetti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Parikh_T/0/1/0/all/0/1">T. Parikh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guarnieri_P/0/1/0/all/0/1">P. Guarnieri</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Comparat_J/0/1/0/all/0/1">J. Comparat</a>

We analyse publicly available, individual spectra of four, massive
($M>10^{11}M_{odot}$) early-type galaxies with redshifts in the range 1.4 < z < 2 to determine their stellar content, extending our previous work up to z~2. The wide wavelength range of the VLT/X-Shooter spectroscopic data in the UV-Optical-NIR arms along with the availability of spectro-photometry allows us to explore different techniques to obtain the stellar population properties, namely through age/metallicity sensitive spectral indices, full spectral fitting and broad-band photometric fitting. Moreover, together with the widely used optical Lick indices we consider further indices in the UV rest-frame, and demonstrate that UV indices significantly help the accuracy of the resulting population parameters.

We find galaxy ages ranging from 0.2 to 4 Gyr, where the oldest galaxy is
found at the lowest redshift, with an excellent agreement between ages
determined via indices, full spectral fitting or broad-band colours. These ages
are in perfect agreement with ages of local galaxies at the same velocity
dispersion when we assume pure passive evolution. Total metallicities derived
from indices show some scatter (between less than half-solar to very high
values, ([Z/H]~0.6]). We speculate on possible mechanisms explaining these
values, but given the sample size and low S/N of the spectra no conclusion can
be made.

Indices in the UV-rest frame generally lead to similar conclusions as optical
indices. For the oldest galaxy (4 Gyr) we show that its UV-indices can only be
explained by stellar population models including a UV contribution from old
stellar populations, suggesting that old, UV bright populations start to
inhabit mature galaxies of a few Gyr of age. This is the highest redshift
(z~1.4) detection of the UV-upturn up to date.

We analyse publicly available, individual spectra of four, massive
($M>10^{11}M_{odot}$) early-type galaxies with redshifts in the range 1.4 < z
< 2 to determine their stellar content, extending our previous work up to z~2.
The wide wavelength range of the VLT/X-Shooter spectroscopic data in the
UV-Optical-NIR arms along with the availability of spectro-photometry allows us
to explore different techniques to obtain the stellar population properties,
namely through age/metallicity sensitive spectral indices, full spectral
fitting and broad-band photometric fitting. Moreover, together with the widely
used optical Lick indices we consider further indices in the UV rest-frame, and
demonstrate that UV indices significantly help the accuracy of the resulting
population parameters.

We find galaxy ages ranging from 0.2 to 4 Gyr, where the oldest galaxy is
found at the lowest redshift, with an excellent agreement between ages
determined via indices, full spectral fitting or broad-band colours. These ages
are in perfect agreement with ages of local galaxies at the same velocity
dispersion when we assume pure passive evolution. Total metallicities derived
from indices show some scatter (between less than half-solar to very high
values, ([Z/H]~0.6]). We speculate on possible mechanisms explaining these
values, but given the sample size and low S/N of the spectra no conclusion can
be made.

Indices in the UV-rest frame generally lead to similar conclusions as optical
indices. For the oldest galaxy (4 Gyr) we show that its UV-indices can only be
explained by stellar population models including a UV contribution from old
stellar populations, suggesting that old, UV bright populations start to
inhabit mature galaxies of a few Gyr of age. This is the highest redshift
(z~1.4) detection of the UV-upturn up to date.

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