arXiv subjects

M. Pannella

Publications and source records attributed to M. Pannella.

At least 37 records · Page 2Linked to original sources

arXiv2018

The [C II] emission as a molecular gas mass tracer in galaxies at low and high redshift

We present ALMA Band 9 observations of the [C II]158um emission for a sample of 10 main-sequence galaxies at redshift z ~ 2, with typical stellar masses (log M*/Msun ~ 10.0 - 10.9) and star formation rates (~ 35 - 115 Msun/yr). Given the strong and well understood evolution of the interstellar medium from the present to z = 2, we investigate the behaviour of the [C II] emission and empirically identify its primary driver. We detect [C II] from six galaxies (four secure, two tentative) and estimate ensemble averages including non detections. The [C II]-to-infrared luminosity ratio (L[C II]/LIR) of our sample is similar to that of local main-sequence galaxies (~ 2 x 10^-3), and ~ 10 times higher than that of starbursts. The [C II] emission has an average spatial extent of 4 - 7 kpc, consistent with the optical size. Complementing our sample with literature data, we find that the [C II] luminosity correlates with galaxies' molecular gas mass, with a mean absolute deviation of 0.2 dex and without evident systematics: the [C II]-to-H2 conversion factor (alpha_[C II] ~ 30 Msun/Lsun) is largely independent of galaxies' depletion time, metallicity, and redshift. [C II] seems therefore a convenient tracer to estimate galaxies' molecular gas content regardless of their starburst or main-sequence nature, and extending to metal-poor galaxies at low and high redshifts. The dearth of [C II] emission reported for z > 6 - 7 galaxies might suggest either a high star formation efficiency or a small fraction of UV light from star formation reprocessed by dust.

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arXiv2018

Molecular gas in two companion cluster galaxies at z=1.2

We study the molecular gas properties of two star-forming galaxies separated by 6 kpc in the projected space and belonging to a galaxy cluster selected from the Irac Shallow Cluster Survey, at a redshift $z=1.2$, i.e., $\sim2$ Gyr after the cosmic star formation density peak. This work describes the first CO detection from $1<z<1.4$ star forming cluster galaxies with no reported clear evidence of AGN. We exploit observations taken with the NOEMA interferometer at $\sim3$ mm to detect CO(2-1) line emission from the two selected galaxies, unresolved by our observations. Based on the CO(2-1) spectrum we estimate a total molecular gas mass $M({\rm H_2})=(2.2^{+0.5}_{-0.4})\times10^{10}$ $M_\odot$ and dust mass $M_{\rm dust}<4.2\times10^8~M_\odot$ for the two blended sources. The two galaxies have similar stellar masses and a large relative velocity of $\sim$400 km/s estimated from the CO(2-1) line width. These findings tend to privilege a scenario where both sources contribute to the observed CO(2-1). By using the archival Spitzer MIPS flux at 24$μ$m we estimate an ${\rm SFR(24μm)}=(28^{+12}_{-8})~M_\odot$/yr for each of the two galaxies. Assuming that the two sources equally contribute to the observe CO(2-1) our analysis yields a depletion time scale $τ_{\rm dep}=(3.9^{+1.4}_{-1.8})\times10^8$ yr, and a molecular gas to stellar mass ratio $0.17\pm0.13$ for each of two sources, separately. Our results are in overall agreement with those of other distant cluster galaxies. The two target galaxies have molecular gas mass and depletion time that are marginally compatible with, but smaller than those of main sequence field galaxies, suggesting that the molecular gas has not been refueled enough. Higher resolution and higher frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms.

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arXiv2018

GOODS-ALMA: 1.1 mm galaxy survey - I. Source catalogue and optically dark galaxies

We present a 69 arcmin$^2$ ALMA survey at 1.1mm, GOODS-ALMA, matching the deepest HST-WFC3 H-band part of the GOODS-South field. We taper the 0"24 original image with a homogeneous and circular synthesized beam of 0"60 to reduce the number of independent beams - thus reducing the number of purely statistical spurious detections - and optimize the sensitivity to point sources. We extract a catalogue of galaxies purely selected by ALMA and identify sources with and without HST counterparts down to a 5$σ$ limiting depth of H=28.2 AB (HST/WFC3 F160W). ALMA detects 20 sources brighter than 0.7 mJy in the 0"60 tapered mosaic (rms sensitivity =0.18 mJy/beam) with a purity greater than 80%. Among these detections, we identify three sources with no HST nor Spitzer-IRAC counterpart, consistent with the expected number of spurious galaxies from the analysis of the inverted image; their definitive status will require additional investigation. An additional three sources with HST counterparts are detected either at high significance in the higher resolution map, or with different detection-algorithm parameters ensuring a purity greater than 80%. Hence we identify in total 20 robust detections. Our wide contiguous survey allows us to push further in redshift the blind detection of massive galaxies with ALMA with a median redshift of $z$=2.92 and a median stellar mass of M$_{\star}$ = 1.1 $\times 10^{11}$M$_\odot$. Our sample includes 20% HST-dark galaxies (4 out of 20), all detected in the mid-infrared with IRAC. The near-infrared based photometric redshifts of two of them $z\sim$4.3 and 4.8) suggest that these sources have redshifts $z$>4. At least 40% of the ALMA sources host an X-ray AGN, compared to 14% for other galaxies of similar mass and redshift. The wide area of our ALMA survey provides lower values at the bright end of number counts than single-dish telescopes

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arXiv2018

Deciphering the activity and quiescence of high-redshift cluster environments: ALMA observations of ClJ1449+0856 at z=2

We present ALMA observations of the 870$μ$m continuum and CO(4-3) line emission in the core of the galaxy cluster ClJ1449+0856 at z=2, a NIR-selected, X-ray detected system in the mass range of typical progenitors of today's massive clusters. The 870$μ$m map reveals six F$_{870μm}$ > 0.5 mJy sources spread over an area of 0.07 arcmin$^2$, giving an overdensity of a factor ~10 (6) with respect to blank field counts down to F$_{870μm}$ > 1 (0.5) mJy. On the other hand, deep CO(4-3) follow-up confirms membership of three of these sources, but suggests that the remaining three, including the brightest 870$μ$m sources in the field (F$_{870μm}\gtrsim$2 mJy), are likely interlopers. The measurement of 870$μ$m continuum and CO(4-3) line fluxes at the positions of previously-known cluster members provides a deep probe of dusty star formation occurring in the core of this high-redshift structure, adding up to a total SFR~700$\pm$100 M$_{\odot}$/yr and yielding an integrated star formation rate density of ~10$^4$ M$_{\odot}$/yr/Mpc$^3$, five orders of magnitude larger than in the field at the same epoch, due to the concentration of star-forming galaxies in the small volume of the dense cluster core. The combination of these observations with previously available HST imaging highlights the presence in this same volume of a population of galaxies with already suppressed star formation. This diverse composition of galaxy populations in ClJ1449+0856 is especially highlighted at the very cluster center, where a complex assembly of quiescent and star-forming sources is likely forming the future Brightest Cluster Galaxy.

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arXiv2018

Merger driven star-formation activity in Cl J1449+0856 at z=1.99 as seen by ALMA and JVLA

We use ALMA and JVLA observations of the galaxy cluster Cl J1449+0856 at z=1.99, in order to study how dust-obscured star-formation, ISM content and AGN activity are linked to environment and galaxy interactions during the crucial phase of high-z cluster assembly. We present detections of multiple transitions of $^{12}$CO, as well as dust continuum emission detections from 11 galaxies in the core of Cl J1449+0856. We measure the gas excitation properties, star-formation rates, gas consumption timescales and gas-to-stellar mass ratios for the galaxies. We find evidence for a large fraction of galaxies with highly-excited molecular gas, contributing $>$50% to the total SFR in the cluster core. We compare these results with expectations for field galaxies, and conclude that environmental influences have strongly enhanced the fraction of excited galaxies in this cluster. We find a dearth of molecular gas in the galaxies' gas reservoirs, implying a high star-formation efficiency (SFE) in the cluster core, and find short gas depletion timescales $τ$<0.1-0.4 Gyrs for all galaxies. Interestingly, we do not see evidence for increased specific star-formation rates (sSFRs) in the cluster galaxies, despite their high SFEs and gas excitations. We find evidence for a large number of mergers in the cluster core, contributing a large fraction of the core's total star-formation compared with expectations in the field. We conclude that the environmental impact on the galaxy excitations is linked to the high rate of galaxy mergers, interactions and active galactic nuclei in the cluster core.

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arXiv2018

The VANDELS ESO public spectroscopic survey

VANDELS is a uniquely-deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrograph on ESO's Very Large Telescope (VLT). The survey has obtained ultra-deep optical (0.48 < lambda < 1.0 micron) spectroscopy of ~2100 galaxies within the redshift interval 1.0 < z < 7.0, over a total area of ~0.2 sq. degrees centred on the CANDELS UDS and CDFS fields. Based on accurate photometric redshift pre-selection, 85% of the galaxies targeted by VANDELS were selected to be at z>=3. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the fundamental aim of the survey is to provide the high signal-to-noise ratio spectra necessary to measure key physical properties such as stellar population ages, masses, metallicities and outflow velocities from detailed absorption-line studies. Using integration times calculated to produce an approximately constant signal-to-noise ratio (20 < t_int < 80 hours), the VANDELS survey targeted: a) bright star-forming galaxies at 2.4 < z < 5.5, b) massive quiescent galaxies at 1.0 < z < 2.5, c) fainter star-forming galaxies at 3.0 < z < 7.0 and d) X-ray/Spitzer-selected active galactic nuclei and Herschel-detected galaxies. By targeting two extragalactic survey fields with superb multi-wavelength imaging data, VANDELS will produce a unique legacy data set for exploring the physics underpinning high-redshift galaxy evolution. In this paper we provide an overview of the VANDELS survey designed to support the science exploitation of the first ESO public data release, focusing on the scientific motivation, survey design and target selection.

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arXiv2018

The VANDELS ESO public spectroscopic survey: observations and first data release

This paper describes the observations and the first data release (DR1) of the ESO public spectroscopic survey "VANDELS, a deep VIMOS survey of the CANDELS CDFS and UDS fields". VANDELS' main targets are star-forming galaxies at 2.4<z<5.5 and massive passive galaxies at 1<z<2.5. By adopting a strategy of ultra-long exposure times, from 20 to 80 hours per source, VANDELS is designed to be the deepest ever spectroscopic survey of the high-redshift Universe. Exploiting the red sensitivity of the VIMOS spectrograph, the survey has obtained ultra-deep spectra covering the wavelength 4800-10000 A with sufficient signal-to-noise to investigate the astrophysics of high-redshift galaxy evolution via detailed absorption line studies. The VANDELS-DR1 is the release of all spectra obtained during the first season of observations and includes data for galaxies for which the total (or half of the total) scheduled integration time was completed. The release contains 879 individual objects with a measured redshift and includes fully wavelength and flux-calibrated 1D spectra, the associated error spectra, sky spectra and wavelength-calibrated 2D spectra. We also provide a catalog with the essential galaxy parameters, including spectroscopic redshifts and redshift quality flags. In this paper we present the survey layout and observations, the data reduction and redshift measurement procedure and the general properties of the VANDELS-DR1 sample. We also discuss the spectroscopic redshift distribution, the accuracy of the photometric redshifts and we provide some examples of data products. All VANDELS-DR1 data are publicly available and can be retrieved from the ESO archive. Two further data releases are foreseen in the next 2 years with a final release scheduled for June 2020 which will include improved re-reduction of the entire spectroscopic data set. (abridged)

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arXiv2017

Predicting emission line fluxes and number counts of distant galaxies for cosmological surveys

We estimate the number counts of line emitters at high redshift and their evolution with cosmic time based on a combination of photometry and spectroscopy. We predict the H$α$, H$β$, [OII], and [OIII] line fluxes for more than $35,000$ galaxies down to stellar masses of $\sim10^9$ $M_{\odot}$ in the COSMOS and GOODS-S fields, applying standard conversions and exploiting the spectroscopic coverage of the FMOS-COSMOS survey at $z\sim1.55$ to calibrate the predictions. We calculate the number counts of H$α$, [OII], and [OIII] emitters down to fluxes of $1\times10^{-17}$ erg cm$^{-2}$ s$^{-1}$ in the range $1.4 < z < 1.8$ covered by the FMOS-COSMOS survey. We model the time evolution of the differential and cumulative H$α$ counts, steeply declining at the brightest fluxes. We expect $\sim9,300-9,700$ and $\sim2,300-2,900$ galaxies deg$^{-2}$ for fluxes $\geq1\times10^{-16}$ and $\geq2\times10^{-16}$ erg cm$^{-2}$ s$^{-1}$ over the range $0.9<z<1.8$. We show that the observed evolution of the Main Sequence of galaxies with redshift is enough to reproduce the observed counts variation at $0.2<z<2.5$. We characterize the physical properties of the H$α$ emitters with fluxes $\geq2\times10^{-16}$ erg cm$^{-2}$ s$^{-1}$, including their stellar masses, UV sizes, [NII]/H$α$ ratios, and H$α$ equivalent widths. An aperture of $R\sim R_{\rm e}\sim0.5$" maximizes the signal-to-noise ratio for a detection, while causing a factor of $\sim2\times$ flux losses, influencing the recoverable number counts, if neglected. Our approach, based on deep and large photometric datasets, reduces the uncertainties on the number counts due to the selection and spectroscopic samplings, while exploring low fluxes. We publicly release the line flux predictions for the explored photometric samples.

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arXiv2017

ALMA constraints on star-forming gas in a prototypical z=1.5 clumpy galaxy: the dearth of CO(5-4) emission from UV-bright clumps

We present deep ALMA CO(5-4) observations of a main sequence, clumpy galaxy at z=1.5 in the HUDF. Thanks to the ~0.5" resolution of the ALMA data, we can link stellar population properties to the CO(5-4) emission on scales of a few kpc. We detect strong CO(5-4) emission from the nuclear region of the galaxy, consistent with the observed $L_{\rm IR}$-$L^{\prime}_{\rm CO(5-4)}$ correlation and indicating on-going nuclear star formation. The CO(5-4) gas component appears more concentrated than other star formation tracers or the dust distribution in this galaxy. We discuss possible implications of this difference in terms of star formation efficiency and mass build-up at the galaxy centre. Conversely, we do not detect any CO(5-4) emission from the UV-bright clumps. This might imply that clumps have a high star formation efficiency (although they do not display unusually high specific star formation rates) and are not entirely gas dominated, with gas fractions no larger than that of their host galaxy (~50%). Stellar feedback and disk instability torques funnelling gas towards the galaxy centre could contribute to the relatively low gas content. Alternatively, clumps could fall in a more standard star formation efficiency regime if their actual star-formation rates are lower than generally assumed. We find that clump star-formation rates derived with several different, plausible methods can vary by up to an order of magnitude. The lowest estimates would be compatible with a CO(5-4) non-detection even for main-sequence like values of star formation efficiency and gas content.

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arXiv2017

Insights on star formation histories and physical properties of $1.2 \leq z \lesssim 4 $ Herschel-detected galaxies

We test the impact of using variable star forming histories (SFHs) and the use of the IR luminosity (LIR) as a constrain on the physical parameters of high redshift dusty star-forming galaxies. We explore in particular the stellar properties of galaxies in relation with their location on the SFR-M* diagram. We perform SED fitting of the UV-NIR and FIR emissions of a large sample of GOODS-Herschel galaxies, for which rich multi-wavelength observations are available. We test different SFHs and imposing energy conservation in the SED fitting process, to face issues like the age-extinction degeneracy and produce SEDs consistent with observations. Our models work well for the majority of the sample, with the notable exception of the high LIR end, for which we have indications that our simple energy conservation approach cannot hold true. We find trends in the SFHs fitting our sources depending on stellar mass M* and z. Trends also emerge in the characteristic timescales of the SED models depending on the location on the SFR-M* diagram. We show that whilst using the same available observational data, we can produce galaxies less star-forming than usually inferred, if we allow declining SFHs, while properly reproducing their observables. These sources can be post-starbursts undergoing quenching, and their SFRs are potentially overestimated if inferred from their LIR. Fitting without the IR constrain leads to a strong preference for declining SFHs, while its inclusion increases the preference of rising SFHs, more so at high z, in tentative agreement with the cosmic star formation history. Keeping in mind that the sample is biased towards high LIR, the evolution shaped by our model appears as both bursty (initially) and steady-lasting (later on). The global SFH of the sample follows the cosmic SFH with a small scatter, and is compatible with the "downsizing" scenario of galaxy evolution.

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arXiv2017

EGG: hatching a mock Universe from empirical prescriptions

This paper introduces EGG, the Empirical Galaxy Generator, a tool designed within the ASTRODEEP collaboration to generate mock galaxy catalogs for deep fields with realistic fluxes and simple morphologies. The simulation procedure is based exclusively on empirical prescriptions -- rather than first principles -- to provide the most accurate match with observations at 0<z<7. In particular, we consider that galaxies can be either quiescent or star-forming, and use their stellar mass (M*) and redshift (z) as the fundamental properties from which all the other observables can be statistically derived. Drawing z and M* from the observed galaxy stellar mass functions, we associate a star formation rate (SFR) to each galaxy from the tight SFR-M* main sequence, while dust attenuation, optical colors and morphologies (including bulge-to-total ratios, sizes and aspect ratios) are obtained from empirical relations that we establish from the high quality Hubble and Herschel observations available in the CANDELS fields. Random scatter is introduced in each step to reproduce the observed distributions of each parameter. Based on these observables, a panchromatic spectral energy distribution (SED) is selected for each galaxy and synthetic photometry is produced by integrating the redshifted SED in common broad-band filters. Finally, the mock galaxies are placed on the sky at random positions with a fixed angular two-point correlation function to implement basic clustering. The resulting flux catalogs reproduce accurately the observed number counts in all broad bands from the ultraviolet up to the sub-millimeter, and can be directly fed to image simulators such as Skymaker. The images can then be used to test source extraction softwares and image-based techniques such as stacking. EGG is open-source, and is made available to the community together with a set of pre-generated catalogs and images.

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arXiv2017

The ALMA Redshift 4 Survey (AR4S): I. The massive end of the z=4 main sequence of galaxies

We introduce the ALMA Redshift 4 Survey (AR4S), a systematic ALMA survey of all the known galaxies with stellar mass (M*) larger than 5e10 Msun at 3.5<z<5 in the GOODS--south, UDS and COSMOS CANDELS fields. The sample we have analyzed in this paper is composed of 96 galaxies observed with ALMA at 890um (180um rest-frame) with an on-source integration time of 1.3 min per galaxy. We detected 32% of the sample at more than 3 sigma significance. Using the stacked ALMA and Herschel photometry, we derived an average dust temperature of 40+/-2 K for the whole sample, and extrapolate the Lir and SFR for all our galaxies based on their ALMA flux. We then used a forward modeling approach to estimate their intrinsic sSFR distribution, deconvolved of measurement errors and selection effects: we find a linear relation between SFR and M*, with a median sSFR=2.8+/-0.8 Gyr and a dispersion around that relation of 0.28+/-0.13 dex. This latter value is consistent with that measured at lower redshifts, which is proof that the main sequence of star-forming galaxies was already in place at z=4, at least among massive galaxies. These new constraints on the properties of the main sequence are in good agreement with the latest predictions from numerical simulations, and suggest that the bulk of star formation in galaxies is driven by the same mechanism from z=4 to the present day, that is, over at least 90% of the cosmic history. We also discuss the consequences of our results on the population of early quiescent galaxies. This paper is part of a series that will employ these new ALMA observations to explore the star formation and dust properties of the massive end of the z=4 galaxy population.

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arXiv2016

The red sequence at birth in the galaxy cluster ClJ1449+0856 at z=2

We use HST/WFC3 imaging to study the red population in the IR-selected, X-ray detected, low-mass cluster Cl J1449+0856 at z=2, one of the few bona-fide established clusters discovered at this redshift, and likely a typical progenitor of an average massive cluster today. This study explores the presence and significance of an early red sequence in the core of this structure, investigating the nature of red sequence galaxies, highlighting environmental effects on cluster galaxy populations at high redshift, and at the same time underlining similarities and differences with other distant dense environments. Our results suggest that the red population in the core of Cl J1449+0856 is made of a mixture of quiescent and dusty star-forming galaxies, with a seedling of the future red sequence already growing in the very central cluster region, and already characterising the inner cluster core with respect to lower density environments. On the other hand, the color-magnitude diagram of this cluster is definitely different from that of lower-redshift (z<1) clusters, as well as of some rare particularly evolved massive clusters at similar redshift, and it is suggestive of a transition phase between active star formation and passive evolution occurring in the proto-cluster and established lower-redshift cluster regimes.

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arXiv2016

VLA and ALMA Imaging of Intense, Galaxy-Wide Star Formation in z ~ 2 Galaxies

We present $\simeq$0$.\!\!^{\prime\prime}4$-resolution extinction-independent distributions of star formation and dust in 11 star-forming galaxies (SFGs) at $z = 1.3-3.0$. These galaxies are selected from sensitive, blank-field surveys of the $2' \times 2'$ Hubble Ultra-Deep Field at $λ= 5$ cm and 1.3 mm using the Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA). They have star-formation rates (SFRs), stellar masses, and dust properties representative of massive main-sequence SFGs at $z \sim 2$. Morphological classification performed on spatially-resolved stellar mass maps indicates a mixture of disk and morphologically disturbed systems; half of the sample harbor X-ray active galactic nuclei (AGN), thereby representing a diversity of $z \sim 2$ SFGs undergoing vigorous mass assembly. We find that their intense star formation most frequently occurs at the location of stellar-mass concentration and extends over an area comparable to their stellar-mass distribution, with a median diameter of $4.2 \pm 1.8$ kpc. This provides direct evidence for galaxy-wide star formation in distant, blank-field-selected main-sequence SFGs. The typical galactic-average SFR surface density is 2.5 M$_{\odot}$yr$^{-1}$kpc$^{-2}$, sufficiently high to drive outflows. In X-ray-selected AGN where radio emission is enhanced over the level associated with star formation, the radio excess pinpoints the AGN, which are found to be co-spatial with star formation. The median extinction-independent size of main-sequence SFGs is two times larger than those of bright submillimeter galaxies whose SFRs are $3-8$ times larger, providing a constraint on the characteristic SFR ($\sim300$ M$_{\odot}$yr$^{-1}$) above which a significant population of more compact star-forming galaxies appears to emerge.

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arXiv2016

A giant Ly$α$ nebula in the core of an X-ray cluster at $z=1.99$: implications for early energy injection

We present the discovery of a giant $\gtrsim$100~kpc Ly$α$ nebula detected in the core of the X-ray emitting cluster CL~J1449+0856 at $z=1.99$ through Keck/LRIS narrow-band imaging. This detection extends the known relation between Ly$α$ nebulae and overdense regions of the Universe to the dense core of a $5-7\times10^{13}$ M$_{\odot}$ cluster. The most plausible candidates to power the nebula are two Chandra-detected AGN host cluster members, while cooling from the X-ray phase and cosmological cold flows are disfavored primarily because of the high Ly$α$ to X-ray luminosity ratio ($L_{\mathrm{Lyα}}/L_{\mathrm{X}} \approx0.3$, $\gtrsim10-1000\times$ higher than in local cool-core clusters) and by current modeling. Given the physical conditions of the Ly$α$-emitting gas and the possible interplay with the X-ray phase, we argue that the Ly$α$ nebula would be short-lived ($\lesssim10$ Myr) if not continuously replenished with cold gas at a rate of $\gtrsim1000$ M$_{\odot}$ yr$^{-1}$. We investigate the possibility that cluster galaxies supply the required gas through outflows and we show that their total mass outflow rate matches the replenishment necessary to sustain the nebula. This scenario directly implies the extraction of energy from galaxies and its deposition in the surrounding intracluster medium, as required to explain the thermodynamic properties of local clusters. We estimate an energy injection of the order of $\thickapprox2$ keV per particle in the intracluster medium over a $2$ Gyr interval. In our baseline calculation AGN provide up to $85$% of the injected energy and 2/3 of the mass, while the rest is supplied by supernovae-driven winds.

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arXiv2016

ALMA resolves extended star formation in high-z AGN host galaxies

We present high resolution (0.3") Atacama Large Millimeter Array (ALMA) 870um imaging of five z~1.5-4.5 X-ray detected AGN with luminosities of L(2-8keV)>10^42 erg/s. These data provide a >~20x improvement in spatial resolution over single-dish rest-frame FIR measurements. The sub-millimetre emission is extended on scales of FWHM~0.2"-0.5", corresponding to physical sizes of 1-3 kpc (median value of 1.8 kpc). These sizes are comparable to the majority of z=1-5 sub-millimetre galaxies (SMGs) with equivalent ALMA measurements. In combination with spectral energy distribution analyses, we attribute this rest-frame far-infrared (FIR) emission to dust heated by star formation. The implied star-formation rate surface densities are ~20-200 Mo/yr/kpc^2, which are consistent with SMGs of comparable FIR luminosities (i.e., L(IR)~[1-5]x10^(12)Lo). Although limited by a small sample of AGN, which all have high FIR luminosities, our study suggests that the kpc-scale spatial distribution and surface density of star formation in high-redshift star-forming galaxies is the same irrespective of the presence of X-ray detected AGN.

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arXiv2015

Infrared color selection of massive galaxies at z > 3

We introduce a new color-selection technique to identify high-redshift, massive galaxies that are systematically missed by Lyman-break selection. The new selection is based on the H_{160} and IRAC 4.5um bands, specifically H - [4.5] > 2.25 mag. These galaxies, dubbed "HIEROs", include two major populations that can be separated with an additional J - H color. The populations are massive and dusty star-forming galaxies at z > 3 (JH-blue) and extremely dusty galaxies at z < 3 (JH-red). The 350 arcmin^2 of the GOODS-N and GOODS-S fields with the deepest HST/WFC3 and IRAC data contain 285 HIEROs down to [4.5] < 24 mag. We focus here primarily on JH-blue (z > 3) HIEROs, which have a median photometric redshift z ~4.4 and stellar massM_{*}~10^{10.6} Msun, and are much fainter in the rest-frame UV than similarly massive Lyman-break galaxies (LBGs). Their star formation rates (SFRs) reaches ~240 Msun yr^{-1} leading to a specific SFR, sSFR ~4.2 Gyr^{-1}, suggesting that the sSFRs for massive galaxies continue to grow at z > 2 but at a lower growth rate than from z=0 to z=2. With a median half-light radius of 2 kpc, including ~20% as compact as quiescent galaxies at similar redshifts, JH-blue HIEROs represent perfect star-forming progenitors of the most massive (M_{*} > 10^{11.2} Msun) compact quiescent galaxies at z ~ 3 and have the right number density. HIEROs make up ~60% of all galaxies with M_{*} > 10^{10.5} Msun identified at z > 3 from their photometric redshifts. This is five times more than LBGs with nearly no overlap between the two populations. While HIEROs make up 15-25% of the total SFR density at z ~ 4-5, they completely dominate the SFR density taking place in M_{*} >10^{10.5} Msun galaxies, and are therefore crucial to understanding the very early phase of massive galaxy formation.

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arXiv2015

Identification of z~>2 Herschel 500 micron sources using color-deconfusion

We present a new method to search for candidate z~>2 Herschel 500μm sources in the GOODS-North field, using a S500μm/S24μm "color deconfusion" technique. Potential high-z sources are selected against low-redshift ones from their large 500μm to 24μm flux density ratios. By effectively reducing the contribution from low-redshift populations to the observed 500μm emission, we are able to identify counterparts to high-z 500μm sources whose 24μm fluxes are relatively faint. The recovery of known z~4 starbursts confirms the efficiency of this approach in selecting high-z Herschel sources. The resulting sample consists of 34 dusty star-forming galaxies at z~>2. The inferred infrared luminosities are in the range 1.5x10^12-1.8x10^13 Lsun, corresponding to dust-obscured star formation rates (SFRs) of ~260-3100 Msun/yr for a Salpeter IMF. Comparison with previous SCUBA 850μm-selected galaxy samples shows that our method is more efficient at selecting high-z dusty galaxies with a median redshift of z=3.07+/-0.83 and 10 of the sources at z~>4. We find that at a fixed luminosity, the dust temperature is ~5K cooler than that expected from the Td-LIR relation at z<1, though different temperature selection effects should be taken into account. The radio-detected subsample (excluding three strong AGN) follows the far-infrared/radio correlation at lower redshifts, and no evolution with redshift is observed out to z~5, suggesting that the far-infrared emission is star formation dominated. The contribution of the high-z Herschel 500μm sources to the cosmic SFR density is comparable to that of SMG populations at z~2.5 and at least 40% of the extinction-corrected UV samples at z~4 (abridged).

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