Abstract
How and when did galaxies form and assemble their stars and stellar mass? The answer to these questions, so crucial to astrophysics and cosmology, requires the full reconstruction of the ...so-called cosmic star formation rate density (SFRD), i.e., the evolution of the average star formation rate per unit volume of the universe. While the SFRD has been reliably traced back to 10–11 billion years ago, its evolution is still poorly constrained at earlier cosmic epochs, and its estimate is mainly based on galaxies luminous in the ultraviolet and with low obscuration by dust. This limited knowledge is largely due to the lack of an unbiased census of all types of star-forming galaxies in the early universe. We present a new approach to finding dust-obscured star-forming galaxies based on their emission at radio wavelengths coupled with the lack of optical counterparts. Here, we present a sample of 197 galaxies selected with this method. These systems were missed by previous surveys at optical and near-infrared wavelengths, and 22 of them are at very high redshift (i.e.,
z
> 4.5). The contribution of these elusive systems to the SFRD is substantial and can be as high as 40% of the previously known SFRD based on UV-luminous galaxies. The mere existence of such heavily obscured galaxies in the first two billion years after the Big Bang opens new avenues to investigate the early phases of galaxy formation and evolution, and to understand the links between these systems and the massive galaxies that ceased their star formation at later cosmic times.
Abstract
We present the characteristics of 2 mm selected sources from the largest Atacama Large Millimeter/submillimeter Array (ALMA) blank-field contiguous survey conducted to date, the Mapping ...Obscuration to Reionization with ALMA (MORA) survey covering 184 arcmin
2
at 2 mm. Twelve of 13 detections above 5
σ
are attributed to emission from galaxies, 11 of which are dominated by cold dust emission. These sources have a median redshift of
〈
z
2
mm
〉
=
3.6
−
0.3
+
0.4
primarily based on optical/near-infrared photometric redshifts with some spectroscopic redshifts, with 77% ± 11% of sources at
z
> 3 and 38% ± 12% of sources at
z
> 4. This implies that 2 mm selection is an efficient method for identifying the highest-redshift dusty star-forming galaxies (DSFGs). Lower-redshift DSFGs (
z
< 3) are far more numerous than those at
z
> 3 yet are likely to drop out at 2 mm. MORA shows that DSFGs with star formation rates in excess of 300
M
⊙
yr
−1
and a relative rarity of ∼10
−5
Mpc
−3
contribute ∼30% to the integrated star formation rate density at 3 <
z
< 6. The volume density of 2 mm selected DSFGs is consistent with predictions from some cosmological simulations and is similar to the volume density of their hypothesized descendants: massive, quiescent galaxies at
z
> 2. Analysis of MORA sources’ spectral energy distributions hint at steeper empirically measured dust emissivity indices than reported in typical literature studies, with
〈
β
〉
=
2.2
−
0.4
+
0.5
. The MORA survey represents an important step in taking census of obscured star formation in the universe’s first few billion years, but larger area 2 mm surveys are needed to more fully characterize this rare population and push to the detection of the universe’s first dusty galaxies.
We present a physical characterization of MM J100026.36+021527.9 (a.k.a. "Mambo-9"), a dusty star-forming galaxy (DSFG) at z = 5.850 0.001. This is the highest-redshift unlensed DSFG (and fourth most ...distant overall) found to date and is the first source identified in a new 2 mm blank-field map in the COSMOS field. Though identified in prior samples of DSFGs at 850 m to 1.2 mm with unknown redshift, the detection at 2 mm prompted further follow-up as it indicated a much higher probability that the source was likely to sit at z > 4. Deep observations from the Atacama Large Millimeter and submillimeter Array (ALMA) presented here confirm the redshift through the secure detection of 12CO(J = 6→5) and p-H2O (21,1 → 20,2). Mambo-9 is composed of a pair of galaxies separated by 6 kpc with corresponding star formation rates of 590 M yr−1 and 220 M yr−1, total molecular hydrogen gas mass of (1.7 0.4) × 1011M , dust mass of (1.3 0.3) × 109M , and stellar mass of ( ) × 109M . The total halo mass, (3.3 0.8) × 1012M , is predicted to exceed 1015M by z = 0. The system is undergoing a merger-driven starburst that will increase the stellar mass of the system tenfold in τdepl = 40−80 Myr, converting its large molecular gas reservoir (gas fraction of ) into stars. Mambo-9 evaded firm spectroscopic identification for a decade, following a pattern that has emerged for some of the highest-redshift DSFGs found. And yet, the systematic identification of unlensed DSFGs like Mambo-9 is key to measuring the global contribution of obscured star formation to the star formation rate density at z 4, the formation of the first massive galaxies, and the formation of interstellar dust at early times ( 1 Gyr).
Abstract
The 2 mm Mapping Obscuration to Reionization with ALMA (MORA) Survey was designed to detect high-redshift (
z
≳ 4), massive, dusty star-forming galaxies (DSFGs). Here we present two likely ...high-redshift sources, identified in the survey, whose physical characteristics are consistent with a class of optical/near-infrared (OIR)-invisible DSFGs found elsewhere in the literature. We first perform a rigorous analysis of all available photometric data to fit spectral energy distributions and estimate redshifts before deriving physical properties based on our findings. Our results suggest the two galaxies, called MORA-5 and MORA-9, represent two extremes of the “OIR-dark” class of DSFGs. MORA-5 (
z
phot
=
4.3
−
1.3
+
1.5
) is a significantly more active starburst with a star formation rate (SFR) of
830
−
190
+
340
M
⊙
yr
−1
compared to MORA-9 (
z
phot
=
4.3
−
1.0
+
1.3
), whose SFR is a modest
200
−
60
+
250
M
⊙
yr
−1
. Based on the stellar masses (
M
⋆
≈ 10
10−11
M
⊙
), space density (
n
∼ (5 ± 2) × 10
−6
Mpc
−3
, which incorporates two other spectroscopically confirmed OIR-dark DSFGs in the MORA sample at
z
= 4.6 and
z
= 5.9), and gas depletion timescales (<1 Gyr) of these sources, we find evidence supporting the theory that OIR-dark DSFGs are the progenitors of recently discovered 3 <
z
< 4 massive quiescent galaxies.
Abstract
The star formation rate density (SFRD) history of the universe is well constrained up to redshift
z
∼ 2. At earlier cosmic epochs, the picture has been largely inferred from UV-selected ...galaxies (e.g., Lyman-break galaxies; LBGs). However, the inferred star formation rates of LBGs strongly depend on the assumed dust extinction correction, which is not well constrained at high
z
, while observations in the radio domain are not affected by this issue. In this work we measure the SFRD from a 1.4 GHz selected sample of ∼600 galaxies in the GOODS-N field up to redshift ∼3.5. We take into account the contribution of active galactic nuclei from the infrared-radio correlation. We measure the radio luminosity function, fitted with a modified Schechter function, and derive the SFRD. The cosmic SFRD shows an increase up to
z
∼ 2 and then an almost flat plateau up to
z
∼ 3.5. Our SFRD is in agreement with those from other far-IR/radio surveys and a factor 2 higher than those from LBG samples. We also estimate that galaxies lacking a counterpart in the HST/WFC3
H
-band (
H-
dark) make up ∼25% of the
ϕ
-integrated SFRD relative to the full sample at
z
∼ 3.2, and up to 58% relative to LBG samples.
Abstract
About 12 billion years ago, the Universe was first experiencing light again after the dark ages, and galaxies filled the environment with stars, metals, and dust. How efficient was this ...process? How fast did these primordial galaxies form stars and dust? We can answer these questions by tracing the star formation rate density (SFRD) back to its widely unknown high-redshift tail, traditionally observed in the near-infrared (NIR), optical, and UV bands. Thus, objects with a large amount of dust were missing. We aim to fill this knowledge gap by studying radio-selected NIR-dark (RS-NIRdark) sources, i.e., sources not having a counterpart at UV-to-NIR wavelengths. We widen the sample of Talia et al. from 197 to 272 objects in the Cosmic Evolution Survey (COSMOS) field, including also photometrically contaminated sources, which were previously excluded. Another important step forward consists in the visual inspection of each source in the bands from
u
* to MIPS 24
μ
m. According to their “environment” in the different bands, we are able to highlight different cases of study and calibrate an appropriate photometric procedure for the objects affected by confusion issues. We estimate that the contribution of RS-NIRdark sources to the cosmic SFRD at 3 <
z
< 5 is ∼10%–25% of that based on UV-selected galaxies.
Abstract
In the last decades, an increasing scientific interest has been growing in the elusive population of
dark
(i.e., lacking an optical/near-IR, hereafter NIR, counterpart) dusty star-forming ...galaxies (DSFGs). Although extremely promising for their likely contribution to the cosmic star formation rate density (SFRD) and for their possible role in the evolution of the first massive and passive galaxies around
z
∼ 3, the difficulty in selecting statistically significant samples of dark DSFGs is limiting their scientific potentialities. This work presents the first panchromatic study of a sample of 263 radio-selected NIR-dark (RS-NIRdark) galaxies discovered in the COSMOS field following the procedure by Talia et al. These sources are selected as radio-bright galaxies (
S
3 GHz
> 12.65
μ
Jy) with no counterpart in the NIR-selected COSMOS2020 catalog (
Ks
≳ 25.5 mag). For these sources, we build a new photometric catalog including accurate photometry from the optical to the radio obtained with a new deblending pipeline (Photometry Extractor for Blended Objects, or
PhoEBO
). We employ this catalog to estimate the photo-
z
s and the physical properties of the galaxies through an spectral energy distribution-fitting procedure performed with two different codes (
Magphys
and
Cigale
). Finally, we estimate the active galactic nucleus contamination in our sample by performing a series of complementary tests. The high values of the median extinction (
A
v
∼ 4) and star formation rate (SFR ∼ 500
M
⊙
yr
−1
) confirm the likely DSFG nature of the RS-NIRdark galaxies. The median photo-
z
(
z
∼ 3) and the presence of a significant tail of high-
z
candidates (
z
> 4.5) suggest that these sources are important contributors to the cosmic SFRD and the evolutionary path of galaxies at high redshifts.
We present the detection of CO (5−4) with signal-to-noise ratio (S/N) > 7-13 and a lower CO transition with S/N > 3 (CO (4−3) for four galaxies, and CO (3−2) for one) with the Atacama Large ...Millimeter/submillimeter Array in bands 3 and 4 in five main-sequence (MS) star-forming galaxies with stellar masses (3-6) × 1010M at 3 < z < 3.5. We find a good correlation between the total far-infrared luminosity LFIR and the luminosity of the CO (5−4) transition , where increases with star formation rate (SFR), indicating that CO (5−4) is a good tracer of the obscured SFR in these galaxies. The two galaxies that lie closer to the star-forming MS have CO spectral line energy distribution (SLED) slopes that are comparable to other star-forming populations, such as local submillimeter galaxies and BzK star-forming galaxies; the three objects with higher specific star formation rates have far steeper CO SLEDs, which possibly indicates a more concentrated episode of star formation. By exploiting the CO SLED slopes to extrapolate the luminosity of the CO (1−0) transition and using a classical conversion factor for MS galaxies of , we find that these galaxies are very gas-rich, with molecular gas fractions between 60% and 80% and quite long depletion times, between 0.2 and 1 Gyr. Finally, we obtain dynamical masses that are comparable to the sum of stellar and gas mass (at least for four out of five galaxies), allowing us to put a first constraint on the CO parameter for MS galaxies at an unprecedented redshift.
We present the first C II 158 μm luminosity function (LF) at z ∼ 5 from a sample of serendipitous lines detected in the ALMA Large Program to INvestigate C II at Early times (ALPINE). A study of the ...118 ALPINE pointings revealed several serendipitous lines. Based on their fidelity, we selected 14 lines for the final catalog. According to the redshift of their counterparts, we identified eight out of 14 detections as C II lines at z ∼ 5, along with two as CO transitions at lower redshifts. The remaining four lines have an elusive identification in the available catalogs and we considered them as C II candidates. We used the eight confirmed C II and the four C II candidates to build one of the first C II LFs at z ∼ 5. We found that 11 out of these 12 sources have a redshift very similar to that of the ALPINE target in the same pointing, suggesting the presence of overdensities around the targets. Therefore, we split the sample in two (a “clustered” and “field” subsample) according to their redshift separation and built two separate LFs. Our estimates suggest that there could be an evolution of the C II LF between z ∼ 5 and z ∼ 0. By converting the C II luminosity to the star-formation rate, we evaluated the cosmic star-formation rate density (SFRD) at z ∼ 5. The clustered sample results in a SFRD ∼10 times higher than previous measurements from UV–selected galaxies. On the other hand, from the field sample (likely representing the average galaxy population), we derived a SFRD ∼1.6 higher compared to current estimates from UV surveys but compatible within the errors. Because of the large uncertainties, observations of larger samples will be necessary to better constrain the SFRD at z ∼ 5. This study represents one of the first efforts aimed at characterizing the demography of C II emitters at z ∼ 5 using a mm selection of galaxies.
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