The advent of the JWST has revolutionised our understanding of high-redshift galaxies. In particular, the NIRCam instrument on-board JWST has revealed a population of Hubble Space Telescope ...(HST)-dark galaxies that had previously evaded optical detection, potentially due to significant dust obscuration, quiescence, or simply extreme redshift. Here, we present the first NIRSpec spectra of 23 HST-dark galaxies (\(\mathrm{H-F444W>1.75}\)), unveiling their nature and physical properties. This sample includes both dusty and quiescent galaxies with spectroscopic data from NIRSpec/PRISM, providing accurate spectroscopic redshifts with \(\mathrm{\overline{z}_{spec} = 4.1 \pm 0.7}\). The spectral features demonstrate that, while the majority of HST-dark galaxies are dusty, a substantial fraction, \(\mathrm{13^{+9}_{-6} \%}\), are quiescent. For the dusty galaxies, we have quantified the dust attenuation using the Balmer decrement (\(\mathrm{H\alpha / H\beta}\)), finding attenuations \(\mathrm{A_{V} > 2\ mag}\). We find that HST-dark dusty galaxies are \(\mathrm{H\alpha}\) emitters with equivalent widths spanning the range \(\mathrm{ 68 A < EW_{H\alpha} < 550 A }\), indicative of a wide range of recent star-formation activity. Whether dusty or quiescent, we find that HST-dark galaxies are predominantly massive, with 85\% of the galaxies in the sample having masses \(\mathrm{log(M_{*}/M_{\odot}) > 9.8}\). This pilot NIRSpec program reveals the diverse nature of HST-dark galaxies and highlights the effectiveness of NIRSpec/PRISM spectroscopic follow-up in distinguishing between dusty and quiescent galaxies and properly quantifying their physical properties. Upcoming research utilising higher-resolution NIRSpec data and combining JWST with ALMA observations will enhance our understanding of these enigmatic and challenging sources.
We investigate the degree of dust obscured star formation in 49 massive (\({\rm log}_{10}(M_{\star}/{\rm M}_{\odot})>9\)) Lyman-break galaxies (LBGs) at \(z = 6.5\)-\(8\) observed as part of the ALMA ...Reionization Era Bright Emission Line Survey (REBELS) large program. By creating deep stacks of the photometric data and the REBELS ALMA measurements we determine the average rest-frame UV, optical and far-infrared (FIR) properties which reveal a significant fraction (\(f_{\rm obs} = 0.4\)-\(0.7\)) of obscured star formation, consistent with previous studies. From measurements of the rest-frame UV slope, we find that the brightest LBGs at these redshifts show bluer (\(\beta \simeq -2.2\)) colours than expected from an extrapolation of the colour-magnitude relation found at fainter magnitudes. Assuming a modified blackbody spectral-energy distribution (SED) in the FIR (with dust temperature of \(T_{\rm d} = 46\,{\rm K}\) and \(\beta_{\rm d} = 2.0\)), we find that the REBELS sources are in agreement with the local ''Calzetti-like'' starburst Infrared-excess (IRX)-\(\beta\) relation. By reanalysing the data available for 108 galaxies at \(z \simeq 4\)-\(6\) from the ALPINE ALMA large program using a consistent methodology and assumed FIR SED, we show that from \(z \simeq 4\)-\(8\), massive galaxies selected in the rest-frame UV have no appreciable evolution in their derived IRX-\(\beta\) relation. When comparing the IRX-\(M_{\star}\) relation derived from the combined ALPINE and REBELS sample to relations established at \(z < 4\), we find a deficit in the IRX, indicating that at \(z > 4\) the proportion of obscured star formation is lower by a factor of \(\gtrsim 3\) at a given a \(M_{\star}\). Our IRX-\(\beta\) results are in good agreement with the high-redshift predictions of simulations and semi-analytic models for \(z \simeq 7\) galaxies with similar stellar masses and SFRs.
We present results of dust continuum and CII\(\,158\,{\rm \mu m}\) emission line observations of a remarkably UV-luminous (\(M_{\rm UV}=-21.6\)) galaxy at \(z=10.603\): GN-z11. Using the Northern ...Extended Millimeter Array (NOEMA), observations have been carried out over multiple observing cycles. We achieved a high sensitivity resulting in a \(\lambda_{\rm rest}=160\,{\rm \mu m}\) continuum \(1\,\sigma\) depth of \(13.0\,\rm{\mu Jy/beam}\) and a CII emission line \(1\,\sigma\) sensitivity of \(31\,\rm{mJy/beam\,km/s}\) using \(50\,\rm{km/s}\) binning with a \(\sim 2\,{\rm arcsec}\) synthesized beam. Neither dust continuum nor CII\(\,158\,{\rm \mu m}\) line emission are detected at the expected frequency of \(\nu_{\rm CII} = 163.791\,\rm{GHz}\) and the sky location of GN-z11. The upper limits show that GN-z11 is neither luminous in \(L_{\rm IR}\) nor \(L_{\rm CII}\), with a dust mass \(3\,\sigma\) limit of \({\rm log}(M_{\rm dust}/{\rm M_{\odot}}) < 6.5-6.9\) and with a CII based molecular gas mass \(3\,\sigma\) limit of \({\rm log}(M_{\rm mol,CII}/{\rm M_{\odot}}) < 9.3\). Together with radiative transfer calculations, we also investigated the possible cause of the dust poor nature of the GN-z11 showed by the blue color in the UV continuum of GN-z11 (\(\beta_{\rm UV}=-2.4\)), and found that \(\gtrsim3\times\) deeper observations are crucial to study dust production at very high-redshift. Nevertheless, our observations show the crucial role of deep mm/submm observations of very high redshift galaxies to constrain multiple phases in the interstellar medium.
We present Atacama Large Millimeter/Submillimeter Array (ALMA) CII and \(\sim158\) \(\rm\mu m\) continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy ...(ULIRG; \(L_{\rm IR}=1.5^{+0.8}_{-0.5}\times10^{12}\) L\(_\odot\)) at \(z=7.31\), spectroscopically confirmed by the Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Programme. REBELS-25 has a significant stellar mass of \(M_{*}=8^{+4}_{-2}\times10^{9}\) M\(_\odot\). From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR \(=199^{+101}_{-63}\) M\(_\odot\) yr\(^{-1}\). This is about four times the SFR estimated from an extrapolated main-sequence. We also infer a CII-based molecular gas mass of \(M_{\rm H_2}=5.1^{+5.1}_{-2.6}\times10^{10}\) \(M_\odot\), implying a molecular gas depletion time of \( t_{\rm depl, H_2}=0.3^{+0.3}_{-0.2}\) Gyr. We observe a CII velocity gradient consistent with disc rotation, but given the current resolution we cannot rule out a more complex velocity structure such as a merger. The spectrum exhibits excess CII emission at large positive velocities (\(\sim500\) km s\(^{-1}\)), which we interpret as either a merging companion or an outflow. In the outflow scenario, we derive a lower limit of the mass outflow rate of 200 M\(_\odot\) yr\(^{-1}\), which is consistent with expectations for a star formation-driven outflow. Given its large stellar mass, SFR and molecular gas reservoir \(\sim700\) Myr after the Big Bang, we explore the future evolution of REBELS-25. Considering a simple, conservative model assuming an exponentially declining star formation history, constant star formation efficiency, and no additional gas inflow, we find that REBELS-25 has the potential to evolve into a galaxy consistent with the properties of high-mass quiescent galaxies recently observed at \(z\sim4\).
For a sample of star forming galaxies in the redshift interval
0.15$<$z$<$0.3, we study how both the relative strength of the AGN infra-red
emission, compared to that due to the star formation (SF), ...and the numerical
fraction of AGNs, change as a function of the total stellar mass of the hosting
galaxy group (M$^{*}_{\mathrm{group}}$), between $10^{10.25}$ and
$10^{11.9}$M$_{\odot}$. Using a multi-component SED fitting analysis, we
separate the contribution of stars, AGN torus and star formation to the total
emission at different wavelengths. This technique is applied to a new
multi-wavelength data-set in the SIMES field (23 not redundant photometric
bands), spanning the wavelength range from the UV (GALEX) to the far-IR
(Herschel) and including crucial AKARI and WISE mid-IR observations (4.5 \mu
m$<\lambda<$24 \mu m), where the BH thermal emission is stronger. This new
photometric catalog, that includes our best photo-z estimates, is released
through the NASA/IPAC Infrared Science Archive (IRSA). Groups are identified
through a friends of friends algorithm ($\sim$62% purity, $\sim$51%
completeness). We identified a total of 45 galaxies requiring an AGN emission
component, 35 of which in groups and 10 in the field. We find BHAR$\propto
($M$^{*}_{\mathrm{group}})^{1.21\pm0.27}$ and (BHAR/SFR)$\propto
($M$^{*}_{\mathrm{group}})^{1.04\pm0.24}$ while, in the same range of
M$^{*}_{\mathrm{group}}$, we do not observe any sensible change in the
numerical fraction of AGNs. Our results indicate that the nuclear activity
(i.e. the BHAR and the BHAR/SFR ratio) is enhanced when galaxies are located in
more massive and richer groups.
We analyse FIR dust continuum measurements for 14 galaxies (\(z\approx 7\)) in the ALMA REBELS LP to derive their physical properties. Our model uses three input data: (a) the UV spectral slope, ...\(\beta\), (b) the observed UV continuum flux at \(1500\)A, \(F_{\rm UV}\), (c) the observed continuum flux at \(\approx 158\mu\)m, \(F_{158}\), and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have (28-90.5)% of their star formation obscured; the total (UV+IR) star formation rates are in the range \(31.5 < {\rm SFR}/ (M_\odot {\rm yr}^{-1}) < 129.5\). The sample-averaged dust mass and temperature are \((1.3\pm 1.1)\times 10^7 M_\odot\) and \(52 \pm 11\) K, respectively. In some galaxies dust is abundant (REBELS-14, \(M'_d \approx 3.4 \times 10^7 M_\odot\)), or hot (REBELS-18, \(T'_d \approx 67\) K). The dust distribution is compact (\(<0.3\) kpc for 70% of the galaxies). The dust yield per supernova is \(0.1 \le y_d/M_\odot \le 3.3\), with 70% of the galaxies requiring \(y_d < 0.25 M_\odot\). Three galaxies (REBELS-12, 14, 39) require \(y_d > 1 M_\odot\). With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in CII nicely follow the local \(L_{\rm CII}-\)SFR relation, and are approximately located on the Kennicutt-Schmidt relation. The sample-averaged gas depletion time is of \(0.11\, y_P^{-2}\) Gyr, where \(y_P\) is the ratio of the gas-to-stellar distribution radius. For some systems a solution simultaneously matching the observed (\(\beta, F_{\rm UV}, F_{158}\)) values cannot be found. This occurs when the index \(I_m = (F_{158}/F_{\rm UV})/(\beta-\beta_{\rm int})\), where \(\beta_{\rm int}\) is the intrinsic UV slope, exceeds \(I_m^*\approx 1120\) for a MW curve. For these objects we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX-\(\beta\) relation.
ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature \(T_d\) remains mostly unconstrained due to the few available ...FIR continuum data at redshift \(z>5\). This introduces large uncertainties in several properties of high-\(z\) galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous CII 158\(\mu\)m line and underlying dust continuum measurements, we derive \(T_ d\) in the continuum and CII detected \(z\approx 7\) galaxies in the ALMA Large Project REBELS sample. We find \(39\ \mathrm{K} < T_d < 58\ \mathrm{K}\), and dust masses in the narrow range \(M_d = (0.9-3.6)\times 10^7 M_{\odot}\). These results allow us to extend for the first time the reported \(T_d(z)\) relation into the Epoch of Reionization. We produce a new physical model that explains the increasing \(T_ d(z)\) trend with the decrease of gas depletion time, \(t_{dep}=M_g/\mathrm{SFR}\), induced by the higher cosmological accretion rate at early times; this hypothesis yields \(T_d \propto (1+z)^{0.4}\). The model also explains the observed \(T_d\) scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, \(T_d\) only depends on the gas column density (metallicity), \(T_d \propto N_H^{1/6}\) (\(T_d \propto Z^{-1/6}\)). REBELS galaxies are on average relatively transparent, with effective gas column densities around \(N_H \simeq (0.03-1)\times 10^{21} \mathrm{cm}^{-2}\). We predict that other high-\(z\) galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated \(T_d \gg 60\) K, are significantly obscured, low-metallicity systems. In fact \(T_d\) is higher in metal-poor systems due to their smaller dust content, which for fixed \(L_{ IR}\) results in warmer temperatures.
We present the multi-wavelength counterparts of 850-\(\mu\)m selected submillimetre sources over a 2-deg\(^2\) field centred on the North Ecliptic Pole. In order to overcome the large beam size (15 ...arcsec) of the 850-\(\mu\)m images, deep optical to near-infrared (NIR) photometric data and arcsecond-resolution 20-cm images are used to identify counterparts of submillimetre sources. Among 647 sources, we identify 514 reliable counterparts for 449 sources (69 per cent in number), based either on probabilities of chance associations calculated from positional offsets or offsets combined with the optical-to-NIR colours. In the radio imaging, the fraction of 850-\(\mu\)m sources having multiple counterparts is 7 per cent. The photometric redshift, infrared luminosity, stellar mass, star-formation rate (SFR), and the AGN contribution to the total infrared luminosity of the identified counterparts are investigated through spectral energy distribution fitting. The SMGs are infrared-luminous galaxies at an average \(\langle z\rangle=2.5\) with \(\mathrm{log}_{10} (L_\mathrm{IR}/\mathrm{L}_\odot)=11.5-13.5\), with a mean stellar mass of \(\mathrm{log}_{10} (M_\mathrm{star}/\mathrm{M}_\odot)=10.90\) and SFR of \(\mathrm{log}_{10} (\mathrm{SFR/M_\odot\,yr^{-1}})=2.34\). The SMGs show twice as large SFR as galaxies on the star-forming main sequence, and about 40 per cent of the SMGs are classified as objects with bursty star formation. At \(z\ge4\), the contribution of AGN luminosity to total luminosity for most SMGs is larger than 30 per cent. The FIR-to-radio correlation coefficient of SMGs is consistent with that of main-sequence galaxies at \(z\simeq2\).
We present the average CII \(158\,\rm{\mu m}\) emission line sizes of UV-bright star-forming galaxies at \(z\sim7\). Our results are derived from a stacking analysis of CII \(158\,\rm{\mu m}\) ...emission lines and dust continua observed by ALMA, taking advantage of the large program Reionization Era Bright Emission Line Survey (REBELS). We find that the average CII emission at \(z\sim7\) has an effective radius \(r_e\) of \(2.2\pm0.2\,\rm{kpc}\). It is \(\gtrsim2\times\) larger than the dust continuum and the rest-frame UV emission, in agreement with recently reported measurements for \(z\lesssim6\) galaxies. Additionally, we compared the average CII size with \(4<z<6\) galaxies observed by the ALMA Large Program to INvestigate CII at Early times (ALPINE). By analysing CII sizes of \(4<z<6\) galaxies in two redshift bins, we find an average CII size of \(r_{\rm e}=2.2\pm0.2\,\rm{kpc}\) and \(r_{\rm e}=2.5\pm0.2\,\rm{kpc}\) for \(z\sim5.5\) and \(z\sim4.5\) galaxies, respectively. These measurements show that star-forming galaxies, on average, show no evolution in the size of the CII \(158\,{\rm \mu m}\) emitting regions at redshift between \(z\sim7\) and \(z\sim4\). This finding suggest that the star-forming galaxies could be morphologically dominated by gas over a wide redshift range.
For a sample of star forming galaxies in the redshift interval 0.15\(<\)z\(<\)0.3, we study how both the relative strength of the AGN infra-red emission, compared to that due to the star formation ...(SF), and the numerical fraction of AGNs, change as a function of the total stellar mass of the hosting galaxy group (M\(^{*}_{\mathrm{group}}\)), between \(10^{10.25}\) and \(10^{11.9}\)M\(_{\odot}\). Using a multi-component SED fitting analysis, we separate the contribution of stars, AGN torus and star formation to the total emission at different wavelengths. This technique is applied to a new multi-wavelength data-set in the SIMES field (23 not redundant photometric bands), spanning the wavelength range from the UV (GALEX) to the far-IR (Herschel) and including crucial AKARI and WISE mid-IR observations (4.5 \mu m\(<\lambda<\)24 \mu m), where the BH thermal emission is stronger. This new photometric catalog, that includes our best photo-z estimates, is released through the NASA/IPAC Infrared Science Archive (IRSA). Groups are identified through a friends of friends algorithm (\(\sim\)62% purity, \(\sim\)51% completeness). We identified a total of 45 galaxies requiring an AGN emission component, 35 of which in groups and 10 in the field. We find BHAR\(\propto (\)M\(^{*}_{\mathrm{group}})^{1.21\pm0.27}\) and (BHAR/SFR)\(\propto (\)M\(^{*}_{\mathrm{group}})^{1.04\pm0.24}\) while, in the same range of M\(^{*}_{\mathrm{group}}\), we do not observe any sensible change in the numerical fraction of AGNs. Our results indicate that the nuclear activity (i.e. the BHAR and the BHAR/SFR ratio) is enhanced when galaxies are located in more massive and richer groups.
