Pathways to quiescence: SHARDS view on the star formation histories of massive quiescent galaxies at 1.0 < z < 1.5

We present star formation histories (SFHs) for a sample of 104 massive (stellar mass M > 10^10 M_⊙) quiescent galaxies (MQGs) at z = 1.0–1.5 from the analysis of spectrophotometric data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) and HST/WFC3 G102 and G141 surveys of the GOODS-North field, jointly with broad-band observations from ultraviolet (UV) to far-infrared (far-IR). The sample is constructed on the basis of rest-frame UVJ colours and specific star formation rates (sSFRs = SFR/Mass). The spectral energy distributions (SEDs) of each galaxy are compared to models assuming a delayed exponentially declining SFH. A Monte Carlo algorithm characterizes the degeneracies, which we are able to break taking advantage of the SHARDS data resolution, by measuring indices such as MgUV and D4000. The population of MQGs shows a duality in their properties. The sample is dominated (85 per cent) by galaxies with young mass-weighted ages, t_M t_M < 2 Gyr, short star formation time-scales, 〈τ〉 ∼ 60–200 Myr, and masses log(M/M_⊙) ∼ 10.5. There is an older population (15 per cent) with t_M t_M = 2–4 Gyr, longer star formation time-scales, 〈τ〉∼ 400 Myr, and larger masses, log(M/M_⊙) ∼ 10.7. The SFHs of our MQGs are consistent with the slope and the location of the main sequence of star-forming galaxies at z > 1.0, when our galaxies were 0.5–1.0 Gyr old. According to these SFHs, all the MQGs experienced a luminous infrared galaxy phase that lasts for ∼500 Myr, and half of them an ultraluminous infrared galaxy phase for ∼100 Myr. We find that the MQG population is almost assembled at z ∼ 1, and continues evolving passively with few additions to the population.

Episodic star formation in a group of LAEs at z=5.07

We are undertaking a search for high-redshift low-luminosity Lyman Alpha sources in the SHARDS (Survey for High-z Absorption Red and Dead Sources) survey. Among the pre-selected Lyman Alpha sources two candidates were spotted, located 3.19 arcsec apart, and tentatively at the same redshift. Here, we report on the spectroscopic confirmation with Gran Telescopio Canarias of the Lyman Alpha emission from this pair of galaxies at a confirmed spectroscopic redshifts of z=5.07. Furthermore, one of the sources is interacting/merging with another close companion that looks distorted. Based on the analysis of the spectroscopy and additional photometric data, we infer that most of the stellar mass of these objects was assembled in a burst of star formation 100 Myr ago. A more recent burst (2 Myr old) is necessary to account for the measured Lyman Alpha flux. We claim that these two galaxies are good examples of Lyman Alpha sources undergoing episodic star formation. Besides, these sources very likely constitute a group of interacting Lyman Alpha emitters (LAEs).

The evolutionary paths among galaxy types on the red sequence at 0.3 < Z < 1.5

We have studied the main evolutionary paths among the galaxy types residing on the massive end of the Red Sequence and nearby locations on the Green Valley during the last ∼9 Gyr. The morphological and star formation properties of a sample of these galaxies at 0 . 3 < z < 1 .5 with stellar masses M_∗ > 5 × 10^10 M_⊙ have been analysed. We present direct observational evidence for the first time of the existence of two main evolutionary paths among the different red galaxy types since z ∼ 1 .5, which provide some clues on the nature of the processes that have governed the assembly of present-day massive quiescent galaxies. The results are in excellent agreement with the hierarchical evolutionary framework proposed in the Eliche-Moral et al. (2010) model. Data from SHARDS (one of the ESO/GTC Large Programmes approved in 2009A) will complement and improve the present findings, shedding some light into many of the still unsettled questions concerning the migration of galaxies from the Blue Cloud to the Red Sequence at z < 1 .5.

Star formation in a stellar mass-selected sample of galaxies to z=3 from the GOODS-NICMOS Survey

We present a study of the star-forming properties of a stellar mass-selected sample of galaxies in the GOODS (Great Observatories Origins Deep Survey) NICMOS Survey (GNS), based on deep Hubble Space Telescope (HST) imaging of the GOODS North and South fields. Using a stellar mass-selected sample, combined with HST/ACS and Spitzer data to measure both ultraviolet (UV) and infrared-derived star formation rates (SFRs), we investigate the star forming properties of a complete sample of ∼1300 galaxies down to log M_*= 9.5 at redshifts 1.5 < z < 3. Eight per cent of the sample is made up of massive galaxies with M_*≥ 10^11 M_⊙. We derive optical colours, dust extinctions and UV and infrared SFR to determine how the SFR changes as a function of both stellar mass and time. Our results show that SFR increases at higher stellar mass such that massive galaxies nearly double their stellar mass from star formation alone over the redshift range studied, but the average value of SFR for a given stellar mass remains constant over this ∼2 Gyr period. Furthermore, we find no strong evolution in the SFR for our sample as a function of mass over our redshift range of interest; in particular we do not find a decline in the SFR among massive galaxies, as is seen at z < 1. The most massive galaxies in our sample (log M_*≥ 11) have high average SFRs with values SFR_UV, corr= 103 ± 75 M_⊙ yr^−1, and yet exhibit red rest-frame (U−B) colours at all redshifts. We conclude that the majority of these red high-redshift massive galaxies are red due to dust extinction. We find that A_2800 increases with stellar mass, and show that between 45 and 85 per cent of massive galaxies harbour dusty star formation. These results show that even just a few Gyr after the first galaxies appear, there are strong relations between the global physical properties of galaxies, driven by stellar mass or another underlying feature of galaxies strongly related to the stellar mass.