Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a ...starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5 < z < 2.5, i.e., at the cosmic peak of the star formation activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass are well described by Gaussians, with starburst galaxies representing only a relatively minor deviation that becomes apparent for SFRs more than four times higher than on the main sequence. Such starburst galaxies represent only 2% of mass-selected star-forming galaxies and account for only 10% of the cosmic SFR density at z ~ 2. Only when limited to SFR > 1000 M yr--1, off-sequence sources significantly contribute to the SFR density (46% ? 20%). We conclude that merger-driven starbursts play a relatively minor role in the formation of stars in galaxies, whereas they may represent a critical phase toward the quenching of star formation and morphological transformation in galaxies.
We take advantage of the sensitivity and resolution of the Herschel Space Observatory at 100 and 160 mum to directly image the thermal dust emission and investigate the infrared luminosities (LsubIR) ...and dust obscuration of typical star-forming (L*) galaxies at high redshift. The result is similar to that inferred from previous investigations of the UV, Halpha, 24 mum, radio, and X-ray properties of the same galaxies studied here. Stacking in bins of UV slope (beta) implies that L* galaxies with redder spectral slopes are also dustier and that the correlation between beta and dustiness is similar to that found for local starburst galaxies. Hence, the rest-frame ~30 and 50 mum fluxes validate on average the use of the local UV attenuation curve to recover the dust attenuation of typical star-forming galaxies at high redshift.
We present JWST/NIRCam observations of a strongly lensed, sub- L * , multiply imaged galaxy at z = 6.072, with magnification factors μ ≳ 20 across the galaxy. The galaxy has rich HST, MUSE, and ALMA ...ancillary observations across a broad wavelength range. Aiming to quantify the reliability of stellar mass estimates of high redshift galaxies, we performed a spatially resolved analysis of the physical properties at scales of ∼200 pc, inferred from spectral energy distribution (SED) modelling of five JWST/NIRCam imaging bands covering 0.16 μm < λ rest < 0.63 μm on a pixel-by-pixel basis. We find young stars surrounded by extended older stellar populations. By comparing H α +N II and O III +H β maps inferred from the image analysis with our additional NIRSpec integral field unit (IFU) data, we find that the spatial distribution and strength of the line maps are in agreement with the IFU measurements. We explore different parametric star formation history (SFH) forms with B AGPIPES on the spatially integrated photometry, finding that a double power-law (DPL) star formation history retrieves the closest value to the spatially resolved stellar mass estimate, and other SFH forms suffer from the dominant outshining emission from the youngest stars, thus underestimating the stellar mass – up to ∼0.5 dex. On the other hand, the DPL cannot match the IFU-measured emission lines. Additionally, the ionising photon production efficiency may be overestimated in a spatially integrated approach by ∼0.15 dex, when compared to a spatially resolved analysis. The agreement with the IFU measurements implies that our pixel-by-pixel results derived from the broadband images are robust, and that the mass discrepancies we find with spatially integrated estimates are not just an effect of SED-fitting degeneracies or the lack of NIRCam coverage. Additionally, this agreement points towards the pixel-by-pixel approach as a way to mitigate the general degeneracy between the flux excess from emission lines and underlying continuum, especially when lacking photometric medium-band coverage and/or IFU observations. This study stresses the importance of studying galaxies as the complex systems that they are, resolving their stellar populations when possible, or using more flexible SFH parameterisations. This can aid our understanding of the early stages of galaxy evolution by addressing the challenge of inferring robust stellar masses and ionising photon production efficiencies of high redshift galaxies.
We report on the discovery of a merger-driven starburst at z = 1.52, PACS-787, based on high signal-to-noise ALMA observations. CO(5-4) and continuum emission (850 m) at a spatial resolution of 0 3 ...reveal two compact (r1/2 ∼ 1 kpc) and interacting molecular gas disks at a separation of 8.6 kpc, indicative of an early stage in a merger. With an SFR of 991 M yr−1, this starburst event should occur closer to final coalescence, as is usually seen in hydrodynamical simulations. From the CO size, inclination, and velocity profile for both disks, the dynamical mass is calculated through a novel method that incorporates a calibration using simulations of galaxy mergers. Based on the dynamical mass, we measure (1) the molecular gas mass, independent from the CO luminosity, (2) the ratio of the total gas mass and the CO(1-0) luminosity ( ), and (3) the gas-to-dust ratio, with the latter two being lower than typically assumed. We find that the high star formation triggered in both galaxies is caused by a set of optimal conditions: a high gas mass/fraction, a short depletion time (τdepl = 85 and 67 Myr) to convert gas into stars, and the interaction of likely counter-rotating molecular disks that may accelerate the loss of angular momentum. The state of interaction is further established by the detection of diffuse CO and continuum emission, tidal debris that bridges the two nuclei and is associated with stellar emission seen by HST/WFC3. This observation demonstrates the power of ALMA to study the dynamics of galaxy mergers at high redshift.
Abstract
We present a joint analysis of the galaxy S04590 at
z
= 8.496 based on NIRSpec, NIRCam, and NIRISS observations obtained as part of the Early Release Observations program of the James Webb ...Space Telescope (JWST) and the far-infrared C
ii
158
μ
m emission line detected by dedicated Atacama Large Millimeter/submillimeter Array (ALMA) observations. We determine the physical properties of S04590 from modeling of the spectral energy distribution (SED) and through the redshifted optical nebular emission lines detected with JWST/NIRSpec. The best-fit SED model reveals a low-mass (
M
⋆
= 10
7.2
–10
8
M
⊙
) galaxy with a low oxygen abundance of
12
+
log
(
O
/
H
)
=
7.16
−
0.12
+
0.10
derived from the strong nebular and auroral emission lines. Assuming that C
ii
effectively traces the interstellar medium, we estimate the total gas mass of the galaxy to be
M
gas
= (8.0 ± 4.0) × 10
8
M
⊙
based on the luminosity and spatial extent of C
ii
. This yields an exceptionally high gas fraction,
f
gas
=
M
gas
/(
M
gas
+
M
⋆
) ≳ 90%, though one still consistent with the range expected for low metallicity. We further derive the metal mass of the galaxy based on the gas mass and gas-phase metallicity, which we find to be consistent with the expected metal production from Type II supernovae. Finally, we make the first constraints on the dust-to-gas (DTG) and dust-to-metal (DTM) ratios of galaxies in the epoch of reionization at
z
≳ 6, showing overall low mass ratios of logDTG < −3.8 and logDTM < −0.5, though they are consistent with established scaling relations and in particular with those of the local metal-poor galaxy I Zwicky 18. Our analysis highlights the synergy between ALMA and JWST in characterizing the gas, metal, and stellar content of the first generation of galaxies.
In the framework of a systematic study with the ALMA interferometer of IR-selected main-sequence and starburst galaxies at
z
∼ 1 − 1.7 at typical ∼1″ resolution, we report on the effects of mid-IR- ...and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find widespread detectable nuclear activity in ∼30% of the sample. The presence of dusty tori influences the IR spectral energy distribution of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (
f
AGN
=
L
IR, AGN
/
L
IR
), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (
S
1.2 mm
/
S
24 μm
) observed color, with evident consequences on the ensuing empirical star formation rate estimates. Nevertheless, we find only marginal effects of the presence and strength of AGN on the carbon monoxide CO (
J
= 2, 4, 5, 7) or neutral carbon (C I(
3
P
1
−
3
P
0
), C I(
3
P
2
−
3
P
1
)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full spectral line energy distributions. The C I and CO emission up to
J
= 5, 7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, our analysis highlights the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest
L
IR, AGN
/
L
IR, SFR
ratios, ≳3, that are more luminous in CO (5−4) than what is predicted by the
L
′
CO(5-4)
−
L
IR, SFR
relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and star formation efficiencies. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8 times larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with
L
IR
> 5 × 10
11
L
⊙
, typically underrepresented in surveys of quasars and submillimeter galaxies.
We present a multi-wavelength, UV-to-radio analysis for a sample of massive (M{sub *} {approx} 10{sup 10} M{sub sun}) IRAC- and MIPS 24 {mu}m detected Lyman break galaxies (LBGs) with spectroscopic ...redshifts z {approx} 3 in the GOODS-North field. For LBGs without individual 24 {mu}m detections, we employ stacking techniques at 24 {mu}m, 1.1 mm, and 1.4 GHz to construct the average UV-to-radio spectral energy distribution and find it to be consistent with that of a luminous infrared galaxy with L{sub IR} = 4.5{sup +1.1}{sub -2.3} x 10{sup 11} L{sub sun} and a specific star formation rate of 4.3 Gyr{sup -1} that corresponds to a mass doubling time {approx}230 Myr. On the other hand, when considering the 24 {mu}m detected LBGs we find among them galaxies with L{sub IR}>10{sup 12} L{sub sun}, indicating that the space density of z {approx} 3 UV-selected ultra-luminous infrared galaxies (ULIRGs) is {approx}(1.5 {+-} 0.5) x 10{sup -5} Mpc{sup -3}. We compare measurements of star formation rates from data at different wavelengths and find that there is tight correlation (Kendall's {tau}>99.7%) and excellent agreement between the values derived from dust-corrected UV, mid-IR, millimeter, and radio data for the whole range of L{sub IR} up to L{sub IR} {approx} 10{sup 13} L{sub sun}. This range is greater than that for which the correlation is known to hold at z {approx} 2, possibly due to the lack of significant contribution from polycyclic aromatic hydrocarbons to the 24 {mu}m flux at z {approx} 3. The fact that this agreement is observed for galaxies with L{sub IR}>10{sup 12} L{sub sun} suggests that star formation in UV-selected ULIRGs, as well as the bulk of star formation activity at this redshift, is not embedded in optically thick regions as seen in local ULIRGs and submillimeter-selected galaxies at z = 2.
ABSTRACT
We study a population of significantly sub-solar enrichment galaxies at z = 1.99, to investigate how molecular gas, dust, and star formation relate in low-metallicity galaxies at the peak ...epoch of star formation. We target our sample with several deep Atacama Large Millimeter/submillimeter Array and Very Large Array datasets, and find no individual detections of CO4–3, CO1–0, or dust, in stark contrast to the >60 per cent detection rate expected for solar-enrichment galaxies with these MS H α star formation rates (SFRs). We find that both low- and high-density molecular gas (traced by CO1–0 and CO4–3, respectively) are affected by the low enrichment, showing sample average (stacked) luminosity deficits >0.5–0.7 dex below expectations. This is particularly pertinent for the use of high-J CO emission as a proxy of instantaneous SFR. Our individual galaxy data and stacked constraints point to a strong inverse dependence ∝ Zγ of gas-to-dust ratios (G/D) and CO-to-H2 conversion factors (αCO) on metallicity at z ∼ 2, with γG/D <−2.2 and $\gamma _{\alpha _{\rm CO}}\lt $−0.8, respectively. We quantify the importance of comparing G/D and αCO versus metallicity trends from the literature on a common, suitably normalized metallicity scale. When accounting for systematic offsets between different metallicity scales, our z ∼ 2 constraints on these scaling relations are consistent with the corresponding relations for local galaxies. However, among those local relations, we favour those with a steep/double power-law dependence of G/D on metallicity. Finally, we discuss the implications of these findings for (a) gas mass measurements for sub-M* galaxies, and (b) efforts to identify the characteristic galaxy mass scale contributing most to the comoving molecular gas density at z = 2.
We present Keck spectroscopic observations and redshifts for a sample of 767 Herschel-SPIRE selected galaxies (HSGs) at 250, 350, and 500 mum, taken with the Keck I Low Resolution Imaging ...Spectrometer and the Keck II DEep Imaging Multi-Object Spectrograph. The redshift distribution of these SPIRE sources from the Herschel Multitiered Extragalactic Survey peaks at z = 0.85, with 731 sources at z < 2 and a tail of sources out to z ~ 5. By probing the dust spectral energy distribution (SED) at its peak, we estimate that the vast majority (72%-83%) of z < 2 Herschel-selected galaxies would drop out of traditional submillimeter surveys at 0.85-1 mm. We find that dust temperature traces infrared luminosity, due in part to the SPIRE wavelength selection biases, and partially from physical effects. As a result, we measure no significant trend in SPIRE color with redshift; if dust temperature were independent of luminosity or redshift, a trend in SPIRE color would be expected.
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