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Register a new userStar formation and dust obscuration at z~2: galaxies at the dawn of downsizing
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M. Pannella
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We present first results of a study aimed to constrain the star formation rate and dust content of galaxies at z~2. We use a sample of BzK-selected star-forming galaxies, drawn from the COSMOS survey, to perform a stacking analysis of their 1.4 GHz radio continuum as a function of different stellar population properties, after removing AGN contaminants from the sample. Dust unbiased star formation rates are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: i) specific star formation rates are constant over about 1 dex in stellar mass and up to the highest stellar mass probed; ii) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects; iii) a single value of the UV extinction applied to all galaxies would lead to grossly underestimate the SFR in massive galaxies; iv) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provide results in very good agreement with the radio derived ones; v) the mean specific star formation rate of our sample steadily decreases by a factor of ~4 with decreasing redshift from z=2.3 to 1.4 and a factor of ~40 down the local Universe. These empirical SFRs would cause galaxies to dramatically overgrow in mass if maintained all the way to low redshifts, we suggest that this does not happen because star formation is progressively quenched, likely starting from the most massive galaxies.
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We provide a systematic measurement of the rest-frame UV continuum slope beta over a wide range in redshift (z~2-6) and rest-frame UV luminosity (0.1-2L*) to improve estimates of the SFR density at high redshift. We utilize the deep optical and infrared data (ACS/NICMOS) over the CDF-S and HDF-N GOODS fields, as well as the UDF for our primary UBVi dropout sample. We correct the observed distributions for selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z~2-4 than it is at z~5-6. Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z~2.5 and z~4. We do not find a large number of galaxies with betas as red as -1 in our dropout selections at z~4, and particularly at z>~5, even though such sources could be readily selected from our data. This suggests that star-forming galaxies at z>~5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z>~5 that are missed in dropout searches. Using the same relation between UV-continuum slope and dust extinction as found to be appropriate at z~0 and z~2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. We find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small (<~2x) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV, the overall dust extinction correction remains modest at all redshifts. We include the contribution from ULIRGs in our SFR density estimates at z~2-6, but find that they contribute only ~20% of the total at z~2.5 and <~10% at z>~4.
A substantial fraction of the stellar mass growth across cosmic time occurred within dust-enshrouded environments. Yet, the exact amount of star-forming activity that took place in high-redshift dusty galaxies currently missed by optical surveys has been barely explored. Using the Spitzer observations of COSMOS we determined the fraction of luminous star-forming galaxies at 1.5<z<3 escaping the traditional color selection techniques because of dust extinction, as well as their contribution to the cosmic star formation density at high redshift. We find that the BzK criterion offers an almost complete (~90%) identification of the 24mic sources at 1.4<z<2.5, while the BM/BX criterion miss 50% of the MIPS population. Similarly the criterion based on the presence of a stellar bump in massive sources (so-called IRAC peakers) miss up to 40% of the IR luminosity density and only 25% of the IR energy density at z~2 is produced by Optically-Faint IR-bright galaxies selected based on their extreme mid-IR to optical flux ratios. We conclude that color selections of distant star-forming galaxies must be used with lots of care given the substantial bias they can suffer. In particular, the effect of dust extinction strongly impacts the completeness of identifications at the bright end of the bolometric luminosity function, which implies large and uncertain extrapolations to account for the contribution of dusty galaxies missed by these selections. In the context of forthcoming facilities that will operate at long wavelengths (e.g., $JWST$, ALMA, SAFARI, EVLA, SKA), this emphasizes the importance of minimizing the extinction biases when probing the activity of star formation in the early Universe.
We present results from a survey of the internal kinematics of 49 star-forming galaxies at z$,sim,$2 in the CANDELS fields with the Keck/MOSFIRE spectrograph (SIGMA, Survey in the near-Infrared of Galaxies with Multiple position Angles). Kinematics (rotation velocity $V_{rot}$ and integrated gas velocity dispersion $sigma_g$) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by z$,sim,$2, massive star-forming galaxies ($log,M_*/M_{odot}gtrsim10.2$) have assembled primitive disks: their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully-Fisher relation. These massive galaxies have values of $V_{rot}/sigma_g$ which are factors of 2-5 lower than local well-ordered galaxies at similar masses. Such results are consistent with findings by other studies. We find that low mass galaxies ($log,M_*/M_{odot}lesssim10.2$) at this epoch are still in the early stages of disk assembly: their kinematics are often supported by gas velocity dispersion and they fall from the Tully-Fisher relation to significantly low values of $V_{rot}$. This kinematic downsizing implies that the process(es) responsible for disrupting disks at z$,sim,$2 have a stronger effect and/or are more active in low mass systems. In conclusion, we find that the period of rapid stellar mass growth at z$,sim,$2 is coincident with the nascent assembly of low mass disks and the assembly and settling of high mass disks.
In this work we analyze the physical properties of a sample of 153 star forming galaxies at z~0.84, selected by their H-alpha flux with a NB filter. B-band luminosities of the objects are higher than those of local star forming galaxies. Most of the galaxies are located in the blue cloud, though some objects are detected in the green valley and in the red sequence. After the extinction correction is applied virtually all these red galaxies move to the blue sequence, unveiling their dusty nature. A check on the extinction law reveals that the typical extinction law for local starbursts is well suited for our sample but with E(B-V)_stars=0.55 E(B-V)_gas. We compare star formation rates (SFR) measured with different tracers (H-alpha, UV and IR) finding that they agree within a factor of three after extinction correction. We find a correlation between the ratios SFR_FUV/SFR_H-alpha, SFR_IR/SFR_H-alpha and the EW(H-alpha) (i.e. weighted age) which accounts for part of the scatter. We obtain stellar mass estimations fitting templates to multi-wavelength photometry. The typical stellar mass of a galaxy within our sample is ~10^10 Msun. The SFR is correlated with stellar mass and the specific star formation rate (sSFR) decreases with it, indicating that massive galaxies are less affected by star formation processes than less massive ones. This result is consistent with the downsizing scenario. To quantify this downsizing we estimated the quenching mass M_Q for our sample at z~0.84, finding that it declines from M_Q ~10^12 Msun to M_Q ~8x10^10 Msun at the local Universe.
A large sample of spectroscopically confirmed galaxies at 1.4<z<3.7, with complementary imaging in the near- and mid-IR from the ground and from Hubble and Spitzer, is used to infer the average star formation histories (SFHs) of typical galaxies from z~7 to 2. For a subset of 302 galaxies at 1.5<z<2.6, we perform a comparison of star formation rates (SFRs) determined from SED modeling (SFRs[SED]) and those calculated from deep Keck UV and Spitzer/MIPS 24 micron imaging (SFRs[IR+UV]). Exponentially declining SFHs yield SFRs[SED] that are 5-10x lower on average than SFRs[IR+UV], indicating that declining SFHs may not be accurate for typical galaxies at z>2. The SFRs of z~2-3 galaxies are directly proportional to their stellar masses M*, with unity slope---a result that is confirmed with Spitzer/IRAC stacks of 1179 UV-faint (R>25.5) galaxies---for M*>5e8 Msun and SFRs >2 Msun/yr. We interpret this result in the context of several systematic biases that can affect determinations of the SFR-M* relation. The average specific SFRs at z~2-3 are similar within a factor of two to those measured at z>4, implying an average SFH where SFRs increase with time. A consequence of these rising SFHs is that (a) a substantial fraction of UV-bright z~2-3 galaxies had faint sub-L* progenitors at z>4; and (b) gas masses must increase with time from z=7 to 2, over which time the net cold gas accretion rate---as inferred from the specific SFR and the Kennicutt-Schmidt relation---is ~2-3x larger than the SFR . However, if we evolve to higher redshift the SFHs and masses of the halos that are expected to host L* galaxies at z~2, we find that <10% of the baryons accreted onto typical halos at z>4 actually contribute to star formation at those epochs. These results highlight the relative inefficiency of star formation even at early cosmic times when galaxies were first assembling. [Abridged]
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