arXiv subjects

Laura Sommovigo

Publications and source records attributed to Laura Sommovigo.

48 records · Page 3Linked to original sources

arXiv2024

REBELS-25: Discovery of a dynamically cold disc galaxy at z = 7.31

We present high resolution ($\sim0.14$" = 710 pc) ALMA [CII] 158$μ$m and dust continuum follow-up observations of REBELS-25, a [CII]-luminous ($L_{\mathrm{[CII]}}=(1.7\pm0.2)\times 10^9 \mathrm{L_{\odot}}$) galaxy at redshift $z=7.3065\pm0.0001$. These high resolution, high signal-to-noise observations allow us to study the sub-kpc morphology and kinematics of this massive ($M_* = 8^{+4}_{-2} \times 10^9 \mathrm{M_{\odot}}$) star-forming (SFR$_{\mathrm{UV+IR}} = 199^{+101}_{-63} \mathrm{M_{\odot}} \mathrm{yr}^{-1}$) galaxy in the Epoch of Reionisation. By modelling the kinematics with $^{\mathrm{3D}}$BAROLO, we find it has a low velocity dispersion ($\barσ = 33 \pm 9$ km s$^{-1}$) and a high ratio of ordered-to-random motion ($V_{\mathrm{rot, ~max}}/\barσ = 11 ^{+8}_{-4}$), indicating that REBELS-25 is a dynamically cold disc. Additionally, we find that the [CII] distribution is well fit by a near-exponential disc model, with a Sérsic index, $n$, of $1.3 \pm 0.2$, and we see tentative evidence of more complex non-axisymmetric structures suggestive of a bar in the [CII] and dust continuum emission. By comparing to other high spatial resolution cold gas kinematic studies, we find that dynamically cold discs seem to be more common in the high redshift Universe than expected based on prevailing galaxy formation theories, which typically predict more turbulent and dispersion-dominated galaxies in the early Universe as an outcome of merger activity, gas accretion and more intense feedback. This higher degree of rotational support seems instead to be consistent with recent cosmological simulations that have highlighted the contrast between cold and warm ionised gas tracers, particularly for massive galaxies. We therefore show that dynamically settled disc galaxies can form as early as 700 Myr after the Big Bang.

astro-ph.GA↗

arXiv2024

Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Surveying the distant Universe

During the most active period of star formation in galaxies, which occurs in the redshift range 1<z<3, strong bursts of star formation result in significant quantities of dust, which obscures new stars being formed as their UV/optical light is absorbed and then re-emitted in the infrared, which redshifts into the mm/sub-mm bands for these early times. To get a complete picture of the high-z galaxy population, we need to survey a large patch of the sky in the sub-mm with sufficient angular resolution to resolve all galaxies, but we also need the depth to fully sample their cosmic evolution, and therefore obtain their redshifts using direct mm spectroscopy with a very wide frequency coverage. This requires a large single-dish sub-mm telescope with fast mapping speeds at high sensitivity and angular resolution, a large bandwidth with good spectral resolution and multiplex spectroscopic capabilities. The proposed 50-m Atacama Large Aperture Submillimeter Telescope (AtLAST) will deliver these specifications. We discuss how AtLAST allows us to study the whole population of high-z galaxies, including the dusty star-forming ones which can only be detected and studied in the sub-mm, and obtain a wealth of information for each of these up to z~7: gas content, cooling budget, star formation rate, dust mass, and dust temperature. We present worked examples of surveys that AtLAST can perform, both deep and wide, and also focused on galaxies in proto-clusters. In addition we show how such surveys with AtLAST can measure the growth rate and the Hubble constant with high accuracy, and demonstrate the power of the line-intensity mapping method in the mm/sub-mm wavebands to constrain the cosmic expansion history at high redshifts, as good examples of what can uniquely be done by AtLAST in this research field.

astro-ph.CO↗

arXiv2024

Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: The hidden circumgalactic medium

Our knowledge of galaxy formation and evolution has incredibly progressed through multi-wavelength observational constraints of the interstellar medium (ISM) of galaxies at all cosmic epochs. However, little is known about the physical properties of the more diffuse and lower surface brightness reservoir of gas and dust that extends beyond ISM scales and fills dark matter haloes of galaxies up to their virial radii, the circumgalactic medium (CGM). New theoretical studies increasingly stress the relevance of the latter for understanding the feedback and feeding mechanisms that shape galaxies across cosmic times, whose cumulative effects leave clear imprints into the CGM. Recent studies are showing that a -- so far unconstrained -- fraction of the CGM mass may reside in the cold (T < 1e4 K) molecular and atomic phase, especially in high-redshift dense environments. These gas phases, together with the warmer ionised phase, can be studied in galaxies from z ~ 0 to z ~ 10 through bright far-infrared and sub-millimeter emission lines such as [C II] 158$μ$m, [O III] 88 $μ$m, [C I] 609$μ$m, [C I] 370$μ$m, and the rotational transitions of CO. Imaging such hidden cold CGM can lead to a breakthrough in galaxy evolution studies but requires a new facility with the specifications of the proposed Atacama Large Aperture Submillimeter Telescope (AtLAST). In this paper, we use theoretical and empirical arguments to motivate future ambitious CGM observations with AtLAST and describe the technical requirements needed for the telescope and its instrumentation to perform such science.

astro-ph.GA↗

arXiv2023

Spatially resolved Kennicutt-Schmidt relation at z~7 and its connection with the interstellar medium properties

We exploit moderately resolved [OIII], [CII] and dust continuum ALMA observations to derive the gas density ($n$), the gas-phase metallicity ($Z$) and the deviation from the Kennicutt-Schmidt (KS) relation ($κ_s$) on ~sub-kpc scales in the interstellar medium (ISM) of five bright Lyman Break Galaxies at the Epoch of Reionization ($z\approx 7$). To do so, we use GLAM, a state-of-art, physically motivated Bayesian model that links the [CII] and [OIII] surface brightness ($Σ_{\rm [CII]}$, $Σ_{\rm [OIII]}$) and the SFR surface density ($Σ_{\rm SFR}$) to $n$, $κ_s$, and $Z$. All five sources are characterized by a central starbursting region, where the $Σ_{\rm gas}$ vs $Σ_{\rm SFR}$ align ~10x above the KS relation ($κ_s\approx10$). This translates into gas depletion times in the range $t_{\rm dep}\approx 80-250$ Myr. The inner starbursting centers are characterized by higher gas density ($\log (n/{\rm cm^{-3}}) \approx 2.5-3.0$) and higher metallicity ($\log (Z/Z_{\odot}) \approx -0.5$) than the galaxy outskirts. We derive marginally negative radial metallicity gradients ($\nabla \log Z \approx -0.03 \pm 0.07$dex/kpc), and a dust temperature ($T_d\approx$32-38 K) that anticorrelates with the gas depletion time.

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arXiv2023

The feedback of quasars on their galactic environment

Quasar outflows might either quench (negative) or enhance (positive feedback) star formation in galaxies located in the quasar environment. The possible outcome depends on 4 parameters: the quasar ($σ$) and satellite ($σ_*$) halo velocity dispersion, their relative distance, $d$, and satellite disk radius, $r_d$. We find that: (i) small satellites with $σ_* < 164\ σ_{200}^{2/3}\, \rm km\ s^{-1}$ have their star formation quenched; (ii) in larger satellites, star formation, and hence UV/FIR luminosity, is instead boosted by $>80$\% in a burst with a typical duration of $5-10$ Myr, if the following positive feedback criterion is met: ${d}/{r_d} < 15 (Q/η)^{1/2} σ_{200}$, where $Q \approx 1$ is the satellite disk Toomre parameter; the disruption parameter (see eq. 17) must be $η>1$ to prevent complete satellite gas removal. We compare our predictions with ALMA data finding that observed satellites of $z\simeq 6$ QSOs on average form stars at a $3\times$ higher rate with respect to field galaxies at the same redshift. Further tests of the model are suggested.

astro-ph.GA↗

arXiv2023

Cold Dust and Low [OIII]/[CII] Ratios: an Evolved Star-forming Population at Redshift 7

We present new ALMA Band 8 (rest-frame $90\,μ$m) observations of three massive ($M_\star \approx 10^{10}\,M_\odot$) galaxies at $z\approx7$ previously detected in [CII]$158\,μ$m and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect the dust continuum emission of two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through modified blackbody fitting we determine cold dust temperatures ($T_\mathrm{dust} \approx 30 - 35\,$K) in both of the dual-band detected targets, given a fiducial model of optically thin emission with $β= 2.0$. Their dust temperatures are lower than most $z\sim7$ galaxies in the literature, and consequently their dust masses are higher ($M_\mathrm{dust} \approx 10^{8}\,M_\odot$). Nevertheless, these large dust masses are still consistent with predictions from models of dust production in the early Universe. In addition, we target and detect [OIII]$88\,μ$m emission in both REBELS-12 and REBELS-25, and find $L_\mathrm{[OIII]} / L_\mathrm{[CII]}$ ratios of approximately unity, low compared to the $L_\mathrm{[OIII]} / L_\mathrm{[CII]} \gtrsim 2 - 10$ observed in the known $z\gtrsim6$ population thus far. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets. This low burstiness supports the cold dust temperatures and below average $\mathrm{[OIII]}λ\lambda4959,5007 + \mathrm{H}β$ equivalent widths of REBELS-25 and REBELS-38, compared to the known high-redshift population. Overall, this provides evidence for the existence of a massive, dust-rich galaxy population at $z\approx7$ which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner.

astro-ph.GA↗

arXiv2023

Blue Monsters. Why are JWST super-early, massive galaxies so blue?

The recent JWST tentative discovery of a population of super-early (redshift $z> 10$), relatively massive (stellar mass $M_* = 10^{8-9} M_{\odot}$) and evolved (metallicity $Z \approx 0.1 Z_{\odot}$) galaxies, which nevertheless show blue ($β\simeq -2.6$) spectra, and very small dust attenuation ($A_{\rm V} \leq 0.02$), challenges our interpretation of these systems. To solve the puzzle we propose two solutions in which dust is either (a) ejected by radiation pressure, or (b) segregated with respect to UV-emitting regions. We clarify the conditions for which the two scenarios apply, and show that they can be discriminated by ALMA observations, such as the recent non-detection of the $88μm$ dust continuum in GHZ2 ($z\simeq 12$) favouring dust ejection.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: The Dust-obscured Cosmic Star Formation Rate Density at Redshift 7

Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimeter wavelengths to resolve. Here we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [CII] and dust emission in UV-selected galaxies at $z\sim7$, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation $f_\mathrm{obs} = \mathrm{SFR}_\mathrm{IR} / \mathrm{SFR}_\mathrm{UV + IR}$ depends on stellar mass, similar to what is observed at lower redshift, and ranges from $f_\mathrm{obs} \approx 0.3 - 0.6$ for galaxies with $\log_{10}\left(M_\star / M_\odot\right) = 9.4 - 10.4$. We further adopt the $z\sim7$ stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between $3\lesssim z \lesssim 7$, with dust-obscured star formation still contributing $\sim30\%$ at $z\sim7$. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: Dust Continuum Detections at z > 6.5

We report 18 dust continuum detections ($\geq 3.3σ$) at $\sim88{\rm μm}$ and $158{\rm μm}$ out of 49 ultraviolet(UV)-bright galaxies ($M_{\rm UV} < -21.3$ mag) at $z>6.5$, observed by the Cycle-7 ALMA Large Program, REBELS and its pilot programs. This has more than tripled the number of dust continuum detections known at $z>6.5$. Out of these 18 detections, 12 are reported for the first time as part of REBELS. In addition, 15 of the dust continuum detected galaxies also show a [CII]$_{\rm 158{\rm μm}}$ emission line, providing us with accurate redshifts. We anticipate more line emission detections from six targets (including three continuum detected targets) where observations are still ongoing. The dust continuum detected sources in our sample tend to have a redder UV spectral slope than the ones without a dust continuum detection. We estimate that all of the sources have an infrared (IR) luminosity ($L_{\rm IR}$) in a range of $3-8 \times 10^{11} L_\odot$, except for one with $L_{\rm IR} = 1.5^{+0.8}_{-0.5} \times 10^{12}\,L_{\odot}$. Their fraction of obscured star formation is significant at $\gtrsim 50\%$. Some of the dust continuum detected galaxies show spatial offsets ($\sim 0.5-1.5''$) between the rest-UV and far-IR emission peaks. These separations appear to have an increasing trend against an indicator that suggests spatially decoupled phases of obscured and unobscured star formation. REBELS offers the best available statistical constraints on obscured star formation in UV-bright, massive galaxies at $z > 6.5$.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: Specific Star-Formation Rates in the Reionization Era

We present specific star-formation rates for 40 UV-bright galaxies at $z\sim7-8$ observed as part of the Reionization Era Bright Emission Line Survey (REBELS) ALMA large program. The sSFRs are derived using improved measures of SFR and stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [CII]-based spectroscopic redshifts. For each source in the sample, we derive stellar masses from SED fitting and total SFRs from calibrations of the UV and FIR emission. The median sSFR is $18_{-5}^{+7}$ Gyr$^{-1}$, significantly larger than literature measurements lacking constraints in the FIR. The increase in sSFR reflects the larger obscured SFRs we derive from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly-dependent on the assumed star formation history (SFH) in reionization-era galaxies. When large sSFR galaxies are modeled with non-parametric SFHs, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [CII] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses that are easily able to accommodate the dominant old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties in the derived parameters, we find that the sSFR increases rapidly toward higher redshifts for massive galaxies ($9.6<\log(\rm M_*/M_{\odot})<9.8$), with a power law that goes as $(1+z)^{1.7\pm0.3}$, broadly consistent with expectations from the evolving baryon accretion rates.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: Efficient Ly$α$ Transmission of UV-Bright z$\simeq$7 Galaxies from Large Velocity Offsets and Broad Line Widths

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly$α$ transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly$α$ profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at $z\sim7$ remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [CII]-based redshifts from ALMA. Here we report MMT/Binospec Ly$α$ spectroscopy of eight UV-bright ($\mathrm{M_{UV}}^{}\sim-22$) galaxies at $z\simeq7$ selected from the ALMA REBELS survey. We detect Ly$α$ in 4 of 8 galaxies and use the [CII] systemic redshifts to investigate the Ly$α$ velocity profiles. The Ly$α$ lines are significantly redshifted from systemic (average velocity offset=223 km/s) and broad (FWHM$\approx$300$-$650 km/s), with two sources showing emission extending to $\approx$750 km/s. We find that the broadest Ly$α$ profiles are associated with the largest [CII] line widths, suggesting a potential link between the Ly$α$ FWHM and the dynamical mass. Since Ly$α$ photons at high velocities transmit efficiently through the $z=7$ IGM, our data suggest that velocity profiles play a significant role in boosting the Ly$α$ visibility of the most UV-luminous reionization-era galaxies.

astro-ph.GA↗

arXiv2021

Accurate dust temperature determination in a $z = 7.13$ galaxy

We report ALMA Band 9 continuum observations of the normal, dusty star-forming galaxy A1689-zD1 at $z = 7.13$, resulting in a $\sim$4.6$σ$ detection at $702$ GHz. For the first time these observations probe the far infrared (FIR) spectrum shortward of the emission peak of a galaxy in the Epoch of Reionization (EoR). Together with ancillary data from earlier works, we derive the dust temperature, $T_{\rm d}$, and mass, $M_{\rm d}$, of A1689-zD1 using both traditional modified blackbody spectral energy density fitting, and a new method that relies only on the [CII] $158\ \mathrm{μm}$ line and underlying continuum data. The two methods give $T_{\rm d} = (42^{+13}_{-7}, 40^{+13}_{-7}$) K, and $M_{\rm d} = (1.7^{+1.3}_{-0.7}, 2.0^{+1.8}_{-1.0})\,\times{}\,10^{7} \,M_{\odot}$. Band 9 observations improve the accuracy of the dust temperature (mass) estimate by $\sim 50$% (6 times). The derived temperatures confirm the reported increasing $T_{\rm d}$-redshift trend between $z=0$ and $8$; the dust mass is consistent with a supernova origin. Although A1689-zD1 is a normal UV-selected galaxy, our results, implying that $\sim$85% of its star formation rate is obscured, underline the non-negligible effects of dust in EoR galaxies.

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