Cosmology and the high redshift Universe
The details of the formation and evolution of galaxies is not completely understood, and remains one of the most important science goals of observational and theoretical cosmology today. ALMA will be a premier tool for studying various types of galaxies across the history of the universe. Understanding the true cosmic star formation history is important for constraining galaxy formation models. Multi-wavelength studies revealed that the cosmic star formation rate gradually rises by one order of magnitude from the present-day to redshift z=2 (~10 Gyr ago). However, at z>2, the contribution from the dusty star-forming galaxy population is still uncertain due to the lack of sensitivity of millimeter (mm) /submillimeter (submm) instruments before the ALMA-era. Searching for the first galaxies that emerged from the cosmic "dark ages" (at z>7) is also a big observational challenge for galaxy formation studies.
ALMA will trace the redshifted emission from near and beyond the peak of the dust emission. For high-z sources, negative k-correction overcomes the effect of the inverse square law and surface brightness dimming, making ALMA observations nearly independent of redshift for z ~ 0.5 – 10 (see Fig 1). The submm bright population is not the only target for ALMA, emission from fainter more normal (MW type) galaxies, selected through optical surveys (e.g. LBGs, LAEs etc.), will also be studied. The observed continuum emission can be used to trace the dust mass and temperature, the dusty star formation activity, and also the ISM mass. Because of its capability to make extremely deep, high-resolution images of high-redshift galaxies, ALMA will be an excellent and unique instrument for pinpointing SMGs selected from single-dish surveys with coarse beam-sizes, for studying gravitational lensing, and for resolving the mm / submm extragalactic background light which is likely originates from fainter normal galaxies.
Figure 1 - The predicted flux density of a dusty galaxy as a function of redshift in various submm / mm atmospheric windows (Blain et al. 2002).
In addition to studies of the dust continuum emission, ALMA’s strength is in its capability to detect and image line emission from molecular/atomic gas, which is an important diagnostic to trace the cold/warm component in distant galaxies. The cold gas component is thought to be the constituents for new stars, but its distribution and properties are likely regulated by the complex interplay between accretion (e.g. from major/minor mergers and cold mode accretion) and mass loss (e.g. from stellar/AGN outflows). Another advantage for ALMA, because of its superb sensitivity, is to determine the redshifts of dusty starburst galaxies by detecting one or more emission lines in the ALMA bands.
The FIR atomic lines are tracers of the neutral/ionized medium, and they are redshifted to the ALMA bands. Singly ionized carbon ([CII] 158 micron) is thought to be the dominant coolant in the ISM and the strongest fine structure line from galaxies. The [CI] line arises from similar regions as the CO emission. Other notable lines are [NII], [OI] and [OIII]; the ratios of these fine structure lines can be used to diagnose the strength of the radiation field or metallicity. This allows us to study very high-z sources from the reionization epoch, possibly enabling us to study the first galaxies.
Using the ALMA bands, we will be able to observe the CO emission from galaxies located at various redshifts across the Universe, with nearly continuous coverage in redshift. While the lowest-J CO line is redshifted out of ALMA Band 3 for z>0.37, higher-J rotational CO lines, which trace the the warm/dense gas directly associated with star-formation/AGN activity, are observable across a large redshift range. The very high-J rotational CO lines, observed with the Herschel satellite in nearby sources, can be observed with ALMA for high redshift galaxies. Example of the science that can be carried out using these lines are (to name a few): (1) the distribution and kinematics of the molecular emission, (2) molecular excitation, (3) gas/dynamical mass, (4) inflow/outflow, (5) properties of star formation, (6) metallicity. Other notable molecular lines include dense gas tracers HCN, HCO+, the shock tracer SiO, optically thin 13CO, and H2O (high critical densities (> 108 cm-3)), just to name a few.
Figure 2 – Redshifted frequencies of molecular and atomic lines observable in the ALMA bands. The filled circles correspond to lines detected at the time of publication (from Carilli & Walter 2013).
References: Blain et al. 2002, PhR, 369, 111; Carilli & Walter 2013, ARAA, 51, 105
Hodge et al., Karim et al., and Swinbank et al. observed 122 submm sources selected from the LABOCA Extended Chandra Deep Field South Submillimeter Survey (LESS) using Band 7 of ALMA. With 1.5” resolution, they were able to pinpoint the SMGs contributing the submillimeter emission in the LABOCA map, showing that the brightest sources in the original LESS sample comprise emission from multiple fainter SMGs. They also serendipitously detected bright emission lines in two of the SMG spectra which are likely [CII] 158 micron emission at z=4.42 and z=4.44, demonstrating that ALMA is able to detect the dominant fine-structure cooling lines from SMGs even with short (2 min) integrations. See Wang et al., Swinbank et al., Simpson et al., Thomson et al., and Chen et al. for related work on LESS.
Figure 3 – ALMA Band 7 observations reveal that the brightest submm sources (S870μm>12mJy) in the LESS survey comprise multiple, fainter galaxies (Karim et al. 2013).
Using ALMA Bands 3 and 7, Vieira et al., Weiss et al., and Hezaveh et al. observed strongly gravitationally lensed sources sampled originally from the South Pole Telescope (SPT) survey. They find that the sources are indeed composed of multiple components, indicative of gravitational lensing. Their gravitational lensing model suggests that the sources are amplified by factors of 4 – 22, which suggests that the lensed sources are ultra luminous starburst galaxies at high-z. Their blind redshift search in band 3 resulted in line detections in 23 sources, with 44 line features in the spectra, providing secure redshifts for ~70% of the sample. Their new analysis gave a mean redshift of z=3.5, and found that a significant portion of SMGs are indeed at high-z (z>4). These new findings will impact our current understanding of the formation of massive galaxies at high-z. Spilker et al. analyzed the stacked spectrum of 22 SPT sources from 250 to 770 GHz, and detected spectral features of 12CO, 13CO, HCN, HNC, HCO+, [CI], H2O.
Figure 4 – ALMA Band 3 redshift survey of 26 lensed galaxies selected from the South Pole Telescope (SPT) SZ survey (Vieira et al. 2013) [Reprinted by permission from Macmillan Publishers Ltd: Nature, Vieira J.D. et al. 2013, vol 495, Issue 7441, p. 344].
Using Band 6, Nagao et al. observed a z=4.8 SMG selected from the LESS survey. They detected the [NII] 205 micron emission line and assessed the metallicity of the SMG from the [NII] 205 micron and [CII] 158 micron flux ratio. They find that the metallicity in the SMG is consistent with solar, implying that the chemical evolution has progressed very rapidly in high-z SMGs.
A z=5.3 SMG called AzTEC-3 was observed in the [CII] and OH lines by Riechers et al. Their spatially resolved image allowed them to derive the surface density of the star formation rate (SFRD), and they find that it is comparable to the Eddington limit for radiation pressure supported disks. In addition to AzTEC-3, they detected the [CII] line in three Lyman Break Galaxies (LBGs) that are clustered around AzTEC-3 (Figure 5). These new measurements show that ALMA can detect the ISM in “typical” galaxies in the early universe. [CII] emission has been detected in other distant galaxies as well (De Breuck et al.).
Figure 5 - Artist's impression of the protocluster observed by ALMA. It shows the central starburst galaxy AzTEC-3 along with its labeled cohorts of smaller, less active galaxies. New ALMA observations suggest that AzTEC-3 recently merged with another young galaxy and that the whole system represents the first steps toward forming a galaxy cluster. | Credit: B. Saxton
References: Hodge et al. 2013, ApJ, 768, 91; Karim et al. 2013, MNRAS, 432, 2; Swinbank et al. 2012, 427, 1066; Nagao et al. 2012, A&A, 542, L34; Vieira et al. 2013, Nature, 495, 344; Weiss et al. 2013, ApJ, 767, 88; Hezaveh et al. 2013, ApJ, 767, 132; Riechers et al. ApJ, 2014, 796, 84; De Breuck et al. A&A, 2014, 565, 59; Wang et al. 2013, ApJ, 778, 179; Swinbank et al., 2014, MNRAS, 438, 1267; Simpson et al., 2014, ApJ, 788, 125; Thomson et al., 2014, MNRAS, 442, 577; Chen et al., 2015, ApJ, 299, 194; Spilker et al. 2014, ApJ, 785, 1492
Extragalactic Background Light
Hatsukade et al. serendipitously detected 15 faint "sub-mJy sources" in Band 6 data targeting 20 star-forming galaxies at z~1.4. They obtained source number counts at the faintest flux range among surveys at mm wavelengths, suggesting that ~80% of the extragalactic background light at mm /submm wavelengths come from such fainter galaxies. A similar result was obtained by Ono et al. 2014.
Figure 6 – ALMA Band 6 observations constrain the faint mm source number counts (Hatsukade et al. 2013).
References: Hatsukade et al. 2013, ApJ, 769, 27; Ono et al. 2014, ApJ, 795, 5
Using ALMA Band 7, Wang et al. observed the host galaxies of two gamma-ray bursts (GRBs) at z>2. The 345 GHz continuum observations show that the two host galaxies are at the faint end of the dusty galaxy population that gives rise to the submm extragalactic background light. Hatsukade et al. observed molecular gas and dust continuum in two GRB host galaxies, GRB 020819B and GRB 051022. Observations of the GRB 020819B revealed a remarkably dust-rich environment in the outskirts of the host galaxy, whereas molecular gas was found only around its centre.
References: Wang et al. 2012, ApJ, 761, L32; Hatsukade et al. 2014, Nature, 510, 247; Michalowski et al. 2014, A&A, 562, 70; Berger et al. 2014, ApJ, 796, 96
Wang et al. and Willott et al. observed z=5.8-6.4 QSOs using Band 6 and 7 of ALMA. They estimated the dynamical masses of the QSO host galaxies using the [CII] 158 micron emission line. They find that the dynamical masses are significantly lower than expected from the local black hole - velocity dispersion (or bulge mass) correlation, indicating that stellar mass growth lags black hole growth for high-z QSOs.
References: Willott et al. 2013, ApJ, 770, 13; Wang et al. 2013, ApJ, 773, 44
Ouchi et al. and Ota et al. observed z=6.6-7.0 LAEs using Band 6 of ALMA. They could not detect the 1.2mm continuum / [CII] 158 micron emission from these LAEs. The upper limit for the dust continuum and the [CII] emission flux suggest that these high-z galaxies in the reionization epoch have significantly lower gas and dust enrichment than SMGs and QSOs at similar/lower redshifts, as well as local star-forming galaxies.
References: Ouchi et al. 2013, ApJ, 778, 102; Ota et al. 2014, 792, 34