Cosmological distance indicators

We review three distance measurement techniques beyond the local universe: (1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and (3) HI intensity mapping. We describe the principles and theory behind each method, the ingredients needed for measuring such distances, the current observational results, and future prospects. Time delays from strongly lensed quasars currently provide constraints on $H_0$ with < 4% uncertainty, and with 1% within reach from ongoing surveys and efforts. Recent exciting discoveries of strongly lensed supernovae hold great promise for time-delay cosmography. BAO features have been detected in redshift surveys up to z <~ 0.8 with galaxies and z ~ 2 with Ly-$\alpha$ forest, providing precise distance measurements and $H_0$ with < 2% uncertainty in flat $\Lambda$CDM. Future BAO surveys will probe the distance scale with percent-level precision. HI intensity mapping has great potential to map BAO distances at z ~ 0.8 and beyond with precisions of a few percent. The next years ahead will be exciting as various cosmological probes reach 1% uncertainty in determining $H_0$, to assess the current tension in $H_0$ measurements that could indicate new physics.Comment: Review article accepted for publication in Space Science Reviews (Springer), 45 pages, 10 figures. Chapter of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Ag

The stellar mass content of submillimeter-selected galaxies

We present a new study of stellar mass in a sample of ~70 submillimeter-selected galaxies (SMGs) with accurate spectroscopic redshifts. We fit combinations of stellar population synthesis models and power laws to the galaxies' observed-frame optical through mid-IR spectral energy distributions (SEDs) to separate stellar emission from non-stellar near-IR continuum. The availability of spectroscopic redshifts significantly enhances our ability to determine unambiguously not only the mass and luminosity of SMGs, but also the presence and contribution of non-stellar emission to their SEDs. By separating the stellar emission from the non-stellar near-IR continuum, we find that ~50% of our sample have non-stellar contributions of less than 10% in rest-frame H band and ~10% of our sample have non-stellar contributions greater than 50%. We find that the K-band luminosity of the non-stellar continuum emission is correlated with hard X-ray luminosity, indicating an active galactic nucleus (AGN) origin of the emission. Upon subtracting this AGN-contributed continuum component from all of the galaxies in our sample, we determine a lower median stellar mass for SMGs than previous studies, ~7 × 1010 M sun. We use constraints of the starburst timescale from molecular gas studies to estimate the amount of fading our sample would undergo if they passively evolve after the starburst terminates. The results suggest that typical SMGs, while among the most massive galaxies at z ~ 2, are likely to produce descendants of similar mass and luminosity to L* galaxies in the local universe

A mid-infrared imaging survey of submillimeter-selected galaxies with the Spitzer space telescope

We present Spitzer-IRAC and MIPS mid-IR observations of a sample of 73 radio-detected submillimeter-selected galaxies (SMGs) with spectroscopic redshifts, the largest such sample published to date. From our data, we find that IRAC colors of SMGs are much more uniform as compared with rest-frame UV and optical colors, and z>1.5 SMGs tend to be redder in their mid-IR colors than both field galaxies and lower-z SMGs. However, the IRAC colors of the SMGs overlap those of field galaxies sufficiently that color-magnitude and color-color selection criteria suggested in the literature to identify SMG counterparts produce ambiguous counterparts within an 8'' radius in 20%-35% of cases. We use a rest-frame J-H versus H-K color-color diagram and a S 24/S 8.0 versus S 8.0/S 4.5 color-color diagram to determine that 13%-19% of our sample are likely to contain active galactic nuclei which dominate their mid-IR emission. We observe in the rest-frame JHK colors of our sample that the rest-frame near-IR emission of SMGs does not resemble that of the compact nuclear starburst observed in local ultraluminous IR galaxies and is consistent with more widely distributed star formation. We take advantage of the fact that many high-z galaxy populations selected at different wavelengths are detected by Spitzer to carry out a brief comparison of mid-IR properties of SMGs to UV-selected high-z galaxies, 24 μm-selected galaxies, and high-z radio galaxies, and find that SMGs have mid-IR fluxes and colors which are consistent with being more massive and more reddened than UV-selected galaxies, while the IRAC colors of SMGs are most similar to powerful high-z radio galaxies

A Hubble Space Telescope NICMOS and ACS morphological study of z∼ 2 submillimetre galaxies.

We present a quantitative morphological analysis using HST NICMOS H160- and ACS I775- band imaging of 25 spectroscopically confirmed submillimetre galaxies (SMGs) which have redshifts between z=0.7-3.4. Our analysis also employs a comparison sample of more typical star-forming galaxies at similar redshifts (such as LBGs) which have lower far-infrared luminosities. This is the first large-scale study of the morphologies of SMGs in the near-infrared at ~0.1" resolution (<1kpc). We find that the half light radii of the SMGs (r_h=2.3+/-0.3 and 2.8+/-0.4kpc in the observed I- and H-bands respectively) and asymmetries are not statistically distinct from the comparison sample of star-forming galaxies. However, we demonstrate that the SMG morphologies differ more between the rest-frame UV and optical-bands than typical star-forming galaxies and interpret this as evidence for structured dust obscuration. We show that the composite observed H-band light profile of SMGs is better fit with a Sersic index with n~2, suggesting the stellar structure of SMGs is best described by a spheroid/elliptical galaxy light distribution. We also compare the sizes and stellar masses of SMGs to local and high-redshift populations, and find that the SMGs have stellar densities which are comparable to local early-type galaxies, as well as luminous, red and dense galaxies at z~1.5 which have been proposed as direct SMG descendants, although the SMG stellar masses and sizes are systematically larger. Overall, our results suggest that the physical processes occuring within the galaxies are too complex to be simply characterised by the rest-frame UV/optical morphologies which appear to be essentially decoupled from all other observables, such as bolometric luminosity, stellar or dynamical mass.Comment: Accepted for publication in MNRAS. 12 pages, 8 figs. High resolution version available at: http://astro.dur.ac.uk/~ams/papers/smg_morph.pd

Confirming a population of hot-dust dominated, star-forming, ultraluminous galaxies at high redshift

We identify eight z>1 radio sources undetected at 850um but robustly detected at 70um, confirming that they represent ultraluminous infrared galaxies (ULIRGs) with hotter dust temperatures (T_dust=52+-10 K) than Submillimetre Galaxies (SMGs) at similar luminosities and redshifts. These galaxies share many properties with SMGs: ultra-violet (UV) spectra consistent with starbursts, high stellar masses and radio luminosities. We can attribute their radio emission to star formation since high-resolution MERLIN radio maps show extended emission regions (with half light radii of 2-3kpc), which are unlikely to be generated by AGN activity. These observations provide the first direct confirmation of hot, dusty ULIRGs which are missed by current submillimetre surveys. They have significant implications for future observations from the Herschel Space Observatory and SCUBA2, which will select high redshift luminous galaxies with less selection biases.Comment: 9 pages, 6 figures; accepted for publication in MNRA

The properties of submm galaxies in hierarchical models

We use the combined GALFORM semi-analytical model of galaxy formation and GRASIL spectrophotometric code to investigate the properties of galaxies selected via their submillimetre (submm) emission. The fiducial model we use has previously been shown to fit the properties of local Ultra-Luminous Infrared Galaxies, as well as the number counts of faint submm galaxies. Here, we test the model in more detail by comparing the SEDs and stellar, dynamical, gas and halo masses of submm galaxies against observational data. We precisely mimic the submm and radio selection function of the observations and show that the predicted far-infrared properties of model galaxies with S850 > 5 mJy and S1.4 > 30 μJy are in good agreement with observations. Although the dust emission model does not assume a single dust temperature, the far-infrared SEDs are well described by single component modified blackbody spectrum with characteristic temperature 32 ± 5 K, in good agreement with observations. We also find evidence that the observations may have uncovered evolution in the far-infrared–radio relation in ULIRGs out to z∼ 2. We show that the predicted redshift distribution of submm galaxies provides a reasonable fit to the observational data with a median redshift z= 2.0. The radio-selected subset of submm galaxies are predicted to make up approximately 75 per cent of the population and peak at z= 1.7, in a reasonable agreement with the observed radio detected fraction and redshift distribution. However, the predicted K band and mid-infrared (3–8 μm) flux densities of the submm galaxies [and Lyman-Break Galaxies (LBGs)] are up to a factor of 10× fainter than observed. We show that including the stellar thermally pulsating asymptotic giant branch phase in the stellar population models does not make up for this deficit. This discrepancy may indicate that the stellar masses of the submm galaxies in the model are too low: M★∼ 1010M⊙, while observations suggest more massive systems, M★≳ 1011M⊙. However, if the predicted K- and 3–8-μm extinctions in the model could be dramatically reduced, then this would reduce, but not eliminate, this discrepancy. Finally, we discuss the potential modifications to the models which may improve the fit to the observational data, as well as the new observational tests which will be made possible with the arrival of new facilities, such as Submillimetre Common-User Bolometer Array2