Determining the stellar masses of submillimetre galaxies: the critical importance of star formation histories

Submillimetre (submm) galaxies are among the most rapidly star-forming and most massive high-redshift galaxies; thus, their properties provide important constraints on galaxy evolution models. However, there is still a debate about their stellar masses and their nature in the context of the general galaxy population. To test the reliability of their stellar mass determinations, we used a sample of simulated submm galaxies for which we derived stellar masses via spectral energy distribution (SED) modelling (with Grasil, Magphys, Hyperz and LePhare) adopting various star formation histories (SFHs). We found that the assumption of SFHs with two independent components leads to the most accurate stellar masses. Exponentially declining SFHs (tau) lead to lower masses (albeit still consistent with the true values), while the assumption of single-burst SFHs results in a significant mass underestimation. Thus, we conclude that studies based on the higher masses inferred from fitting the SEDs of real submm galaxies with double SFHs are most likely to be correct, implying that submm galaxies lie on the high-mass end of the main sequence of star-forming galaxies. This conclusion appears robust to assumptions of whether or not submm galaxies are driven by major mergers, since the suite of simulated galaxies modelled here contains examples of both merging and isolated galaxies. We identified discrepancies between the true and inferred stellar ages (rather than the dust attenuation) as the primary determinant of the success/failure of the mass recovery. Regardless of the choice of SFH, the SED-derived stellar masses exhibit a factor of ~2 scatter around the true value; this scatter is an inherent limitation of the SED modelling due to simplified assumptions. Finally, we found that the contribution of active galactic nuclei does not have any significant impact on the derived stellar masses.Comment: Accepted to A&A. 11 pages, 9 figures, 1 table. V2 main changes: 1) discussion of the stellar age as the main parameter influencing the success of an SED model (Fig. 4, 5, 7); 2) discussion of the age-dust degeneracy (Fig 9); 3) the comparison of real and simulated submm galaxies (Fig 1

The HiZELS/UKIRT large area survey for bright Lyman-alpha emitters at z~9

We present the largest area survey to date (1.4 deg2) for Lyman-alpha emitters (LAEs) at z~9, as part of the Hi-z Emission Line Survey (HiZELS). The survey, which primarily targets H-alpha emitters at z < 3, uses the Wide Field CAMera on the United Kingdom Infrared Telescope and a custom narrow-band filter in the J band to reach a Lyman-alpha luminosity limit of ~10^43.8 erg/s over a co-moving volume of 1.12x10^6 Mpc^3 at z = 8.96+-0.06. Two candidates were found out of 1517 line emitters, but those were rejected as LAEs after follow-up observations. This improves the limit on the space density of bright Lyman-alpha emitters by 3 orders of magnitude and is consistent with suppression of the bright end of the Lyman-alpha luminosity function beyond z~6. Combined with upper limits from smaller but deeper surveys, this rules out some of the most extreme models for high-redshift Lyman-alpha emitters. The potential contamination of narrow-band Lyman-alpha surveys at z>7 by Galactic brown dwarf stars is also examined, leading to the conclusion that such contamination may well be significant for searches at 7.7 < z < 8.0, 9.1 < z < 9.5 and 11.7 < z < 12.2.Comment: To appear in proceedings of "UKIRT at 30: A British Success Story

Dynamical and photometric imprints of feedback processes on the early evolution of E/S0 galaxies

We show that the observed Velocity Dispersion Function of E/S0 galaxies matches strikingly well the distribution function of virial velocities of massive halos virializing at z > 1.5, as predicted by the standard hierarchical clustering scenario in a \LambdaCDM cosmology, for a constant ratio sigma/V_vir = 0.55 \pm 0.05, close to the value expected at virialization if it typically occurred at z > 3. This strongly suggests that dissipative processes and later merging events had little impact on the matter density profile. Adopting the above sigma/V_vir ratio, the observed relationships between photometric and dynamical properties which define the fundamental plane of elliptical galaxies, such as the luminosity-sigma (Faber-Jackson) and the luminosity-effective radius relations, as well as the M_BH-sigma relation, are nicely reproduced. Their shapes turn out to be determined by the mutual feedback of star-formation (and supernova explosions)and nuclear activity, along the lines discussed by Granato et al. (2004). To our knowledge, this is the first semi-analytic model for which simultaneous fits of the fundamental plane relations and of the epoch-dependent luminosity function of spheroidal galaxies have been presented.Comment: 14 pages, 6 figures, submitted to Ap

HiZELS: the High Redshift Emission Line Survey with UKIRT

In these proceedings we report on HiZELS, the High-z Emission Line Survey, our successful panoramic narrow-band Campaign Survey using WFCAM on UKIRT to detect and study emission line galaxies at z~1-9. HiZELS employs the H2(S1) narrow-band filter together with custom-made narrow-band filters in the J and H-bands, with the primary aim of delivering large, identically-selected samples of H-alpha emitting galaxies at redshifts of 0.84, 1.47 and 2.23. Comparisons between the luminosity function, the host galaxy properties, the clustering, and the variation with environment of these H-alpha-selected samples are yielding unique constraints on the nature and evolution of star-forming galaxies, across the peak epoch of star-formation activity in the Universe. We provide a summary of the project status, and detail the main scientific results obtained so far: the measurement of the evolution of the cosmic star-formation rate density out to z > 2 using a single star-formation indicator, determination of the morphologies, environments and dust-content of the star-forming galaxies, and a detailed investigation of the evolution of their clustering properties. We also summarise the on-going work and future goals of the project.Comment: To appear in proceedings of "UKIRT at 30: A British Success Story"

Accurate Sky Continuum Subtraction with Fibre-fed Spectrographs

Fibre-fed spectrographs now have throughputs equivalent to slit spectrographs. However, the sky subtraction accuracy that can be reached has often been pinpointed as one of the major issues associated with the use of fibres. Using technical time observations with FLAMES-GIRAFFE, two observing techniques, namely dual staring and cross beam-switching, were tested and the resulting sky subtraction accuracy reached in both cases was quantified. Results indicate that an accuracy of 0.6% on sky subtraction can be reached, provided that the cross beam-switching mode is used. This is very encouraging with regard to the detection of very faint sources with future fibre-fed spectrographs, such as VLT/MOONS or E-ELT/MOSAIC.Comment: to appear in ESO Messenger, March 201

Optimizing high redshift galaxy surveys for environmental information

We investigate the performance of group finding algorithms that reconstruct galaxy groups from the positional information of tracer galaxies that are observed in redshift surveys carried out with multiplexed spectrographs. We use mock light-cones produced by the L-Galaxies semi-analytic model of galaxy evolution in which the underlying reality is known. We particularly focus on the performance at high redshift, and how this is affected by choices of the mass of the tracer galaxies (largely equivalent to their co-moving number density) and the (assumed random) sampling rate of these tracers. We first however compare two different approaches to group finding as applied at low redshift, and conclude that these are broadly comparable. For simplicity we adopt just one of these, "Friends-of-Friends" (FoF) as the basis for our study at high redshift. We introduce 12 science metrics that are designed to quantify the performance of the group-finder as relevant for a wide range of science investigations with a group catalogue. These metrics examine the quality of the recovered group catalogue, the median halo masses of different richness structures, the scatter in dark matter halo mass and how successful the group-finder classifies singletons, centrals and satellites. We analyze how these metrics vary with the limiting stellar mass and random sampling rate of the tracer galaxies, allowing quantification of the various trade-offs between different possible survey designs. Finally, we look at the impact of these same design parameters on the relative "costs" in observation time of the survey using as an example the potential MOONRISE survey using the MOONS instrument.ERC STF

The cosmic assembly of stellar haloes in massive Early-Type Galaxies

Using the exquisite depth of the Hubble Ultra Deep Field (HUDF12 programme) data set, we explore the ongoing assembly of the outermost regions of the most massive galaxies (Mstellar ≥ 5× 1010 M⊙) at z ≤ 1. The outskirts of massive objects, particularly early-type Galaxies (ETGs), are expected to suffer a dramatic transformation across cosmic time due to continuous accretion of small galaxies. HUDF imaging allows us to study this process at intermediate redshifts in six massive galaxies, exploring the individual surface brightness profiles out to ∼25 effective radii. We find that 5–20 per cent of the total stellar mass for the galaxies in our sample is contained within 10 1–2 per cent, very similar to predictions based on observed close pair counts. Therefore, the results for our small albeit meaningful sample suggest that the size and mass growth of the most massive galaxies have been solely driven by minor and major merging from z = 1 to today

New Constraints on Cosmic Reionization from the 2012 Hubble Ultra Deep Field Campaign

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign has acquired the deepest infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicates reionization was underway. The UDF12 campaign thus provides the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize the new UDF12 results with the most recent constraints from CMB observations to infer redshift-dependent ultraviolet (UV) luminosity densities, reionization histories, and electron scattering optical depth evolution consistent with the available data. Under reasonable assumptions about the escape fraction of hydrogen ionizing photons and the intergalactic medium clumping factor, we find that to fully reionize the universe by redshift z~6 the population of star-forming galaxies at redshifts z~7-9 likely must extend in luminosity below the UDF12 limits to absolute UV magnitudes of M_UV\sim -13 or fainter. Moreover, low levels of star formation extending to redshifts z~15-25, as suggested by the normal UV colors of z\simeq7-8 galaxies and the smooth decline in abundance with redshift observed by UDF12 to z\simeq10, are additionally likely required to reproduce the optical depth to electron scattering inferred from CMB observations.Comment: Version accepted by ApJ (originally submitted Jan 5, 2013). The UDF12 website can be found at http://udf12.arizona.ed

The identification of post-starburst galaxies at z∼1 using multiwavelength photometry: a spectroscopic verification

Despite decades of study, we still do not fully understand why some massive galaxies abruptly switch off their star formation in the early Universe, and what causes their rapid transition to the red sequence. Post-starburst galaxies provide a rare opportunity to study this transition phase, but few have currently been spectroscopically identified at high redshift (z > 1). In this paper, we present the spectroscopic verification of a new photometric technique to identify post-starbursts in high-redshift surveys. The method classifies the broad-band optical–nearinfrared spectral energy distributions (SEDs) of galaxies using three spectral shape parameters (supercolours), derived from a principal component analysis of model SEDs. When applied to the multiwavelength photometric data in the UKIDSS Ultra Deep Survey, this technique identified over 900 candidate post-starbursts at redshifts 0.5 5 angstrem) and Balmer break, characteristic of post-starburst galaxies.We conclude that photometric methods can be used to select large samples of recently-quenched galaxies in the distant Universe