We study the origin of high-redshift, compact, quenched spheroids (red nuggets) through the dissipative shrinkage of gaseous discs into compact star-forming systems (blue nuggets). The discs, fed by ...cold streams, undergo violent disc instability that drives gas into the centre (along with mergers). The inflow is dissipative when its time-scale is shorter than the star formation time-scale. This implies a threshold of ∼0.28 in the cold-to-total mass ratio within the disc radius. For the typical gas fraction ∼0.5 at z ∼ 2, this threshold is traced back to a maximum spin parameter of ∼0.05, implying that ∼half the star-forming galaxies
contract to blue nuggets, while the rest form extended stellar discs. Thus, the surface density of blue galaxies is expected to be bimodal about ∼109 M⊙ kpc−2, slightly increasing with mass. The blue nuggets are expected to be rare at low z when the gas fraction is low. The blue nuggets quench to red nuggets by complementary internal and external mechanisms. Internal quenching by a compact bulge, in a fast mode and especially at high z, may involve starbursts, stellar and active galactic nucleus feedback, or Q-quenching. Quenching due to hot-medium haloes above 1012 M⊙ provides maintenance and a slower mode at low redshift. These predictions are confirmed in simulations and are consistent with observations at z = 0-3.
We address the origin of the robust bimodality observed in galaxy properties about a characteristic stellar mass ∼3 × 1010 M⊙. Less massive galaxies tend to be ungrouped blue star forming discs, ...while more massive galaxies are typically grouped red old-star spheroids. Colour–magnitude data show a gap between the red and blue sequences, extremely red luminous galaxies already at z∼ 1, a truncation of today's blue sequence above L*, and massive starbursts at z∼ 2–4. We propose that these features are driven by the thermal properties of the inflowing gas and their interplay with the clustering and feedback processes, all functions of the dark matter halo mass and associated with a similar characteristic scale. In haloes below a critical shock-heating mass Mshock≲ 1012 M⊙, discs are built by cold streams, not heated by a virial shock, yielding efficient early star formation. It is regulated by supernova feedback into a long sequence of bursts in blue galaxies constrained to a ‘fundamental line’. Cold streams penetrating through hot media in M≥Mshock haloes preferentially at z≥ 2 lead to massive starbursts in L > L* galaxies. At z < 2, in M > Mshock haloes hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to feedback from energetic sources such as active galactic nuclei. This shuts off gas supply and prevents further star formation, leading by passive evolution to ‘red-and-dead’ massive spheroids starting at z∼ 1. A minimum in feedback efficiency near Mshock explains the observed minimum in M/L and the qualitative features of the star formation history. The cold flows provide a hint for solving the angular momentum problem. When these processes are incorporated in simulations they recover the main bimodality features and solve other open puzzles.
Abstract
We derive 2D dust attenuation maps at ∼1 kpc resolution from the UV continuum for 10 galaxies on the
z
∼ 2 star-forming main sequence (SFMS). Comparison with IR data shows that 9 out of 10 ...galaxies do not require further obscuration in addition to the UV-based correction, though our sample does not include the most heavily obscured, massive galaxies. The individual rest-frame
V
-band dust attenuation (
A
V
) radial profiles scatter around an average profile that gently decreases from ∼1.8 mag in the center down to ∼0.6 mag at ∼3–4 half-mass radii. We use these maps to correct UV- and H
α
-based star formation rates (SFRs), which agree with each other. At masses
, the dust-corrected specific SFR (sSFR) profiles are on average radially constant at a mass-doubling timescale of ∼300 Myr, pointing at a synchronous growth of bulge and disk components. At masses
, the sSFR profiles are typically centrally suppressed by a factor of ∼10 relative to the galaxy outskirts. With total central obscuration disfavored, this indicates that at least a fraction of massive
z
∼ 2 SFMS galaxies have started their inside-out star formation quenching that will move them to the quenched sequence. In combination with other observations, galaxies above and below the ridge of the SFMS relation have, respectively, centrally enhanced and centrally suppressed sSFRs relative to their outskirts, supporting a picture where bulges are built owing to gas “compaction” that leads to a high central SFR as galaxies move toward the upper envelope of the SFMS.
Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these ...galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids. Cosmological simulations and clustering theory are used to explore how these galaxies acquired their gas. Here we report that they are 'stream-fed galaxies', formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes. A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid. This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.
Most present-day galaxies with stellar masses ≥1011 solar masses show no ongoing star formation and are dense spheroids. Ten billion years ago, similarly massive galaxies were typically forming stars ...at rates of hundreds solar masses per year. It is debated how star formation ceased, on which time scales, and how this "quenching" relates to the emergence of dense spheroids. We measured stellar mass and star-formation rate surface density distributions in star-forming galaxies at redshift 2.2 with ∼1-kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on time scales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quenched bulges in their cores.
We measure new estimates for the galaxy stellar mass function and star formation rates for samples of galaxies at z ∼ 4, 5, 6 and 7 using data in the CANDELS GOODS South field. The deep near-infrared ...observations allow us to construct the stellar mass function at z ≥ 6 directly for the first time. We estimate stellar masses for our sample by fitting the observed spectral energy distributions with synthetic stellar populations, including nebular line and continuum emission. The observed UV luminosity functions for the samples are consistent with previous observations; however, we find that the observed M
UV-M
* relation has a shallow slope more consistent with a constant mass-to-light ratio and a normalization which evolves with redshift. Our stellar mass functions have steep low-mass slopes (α ≈ −1.9), steeper than previously observed at these redshifts and closer to that of the UV luminosity function. Integrating our new mass functions, we find the observed stellar mass density evolves from
$\log _{10} \rho _{*} = 6.64^{+0.58}_{-0.89}$
at z ∼ 7 to 7.36 ± 0.06 M⊙ Mpc− 3 at z ∼ 4. Finally, combining the measured UV continuum slopes (β) with their rest-frame UV luminosities, we calculate dust-corrected star formation rates (SFR) for our sample. We find the specific SFR for a fixed stellar mass increases with redshift whilst the global SFR density falls rapidly over this period. Our new SFR density estimates are higher than previously observed at this redshift.
ABSTRACT We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) 870 m dust continuum maps of six massive, compact, dusty star-forming galaxies at z ∼ 2.5. These galaxies ...are selected for their small rest-frame optical sizes ( kpc) and high stellar mass densities that suggest that they are direct progenitors of compact quiescent galaxies at z ∼ 2. The deep observations yield high far-infrared (FIR) luminosities of and star formation rates (SFRs) of SFR = 200-700 M yr−1, consistent with those of typical star-forming "main sequence" galaxies. The high spatial resolution (FWHM ∼ 0 12-0 18) ALMA and Hubble Space Telescope photometry are combined to construct deconvolved, mean radial profiles of their stellar mass and (UV+IR) SFR. We find that the dusty, nuclear IR-SFR overwhelmingly dominates the bolometric SFR up to r ∼ 5 kpc, by a factor of over 100× from the unobscured UV-SFR. Furthermore, the effective radius of the mean SFR profile ( kpc) is ∼30% smaller than that of the stellar mass profile. The implied structural evolution, if such nuclear starburst last for the estimated gas depletion time of Δt = 100 Myr, is a 4× increase of the stellar mass density within the central 1 kpc and a 1.6× decrease of the half-mass-radius. This structural evolution fully supports dissipation-driven, formation scenarios in which strong nuclear starbursts transform larger, star-forming progenitors into compact quiescent galaxies.
We present a self-consistent hydrodynamical simulation of a Milky Way-like galaxy at a resolution of 0.05 pc. The model includes star formation and a new implementation of stellar feedback through ...photoionization, radiative pressure and supernovae. The simulation resolves the structure of the interstellar medium at sub-parsec resolution for a few cloud lifetimes and at 0.05 pc for about a cloud-crossing time. The turbulence cascade and gravitation from kpc scales are de facto included in smaller structures like molecular clouds. We show that the formation of a bar influences the dynamics of the central ∼100 pc by creating resonances. At larger radii, the spiral arms host the formation of regularly spaced clouds: beads on a string and spurs. These instabilities pump turbulent energy into the gas, generally in the supersonic regime. Because of asymmetric drift, the supernovae explode outside their gaseous nursery, which diminishes the effect of feedback on the structure of clouds. The evolution of clouds is thus mostly due to fragmentation and gas consumption, regulated mainly by supersonic turbulence. The transition from turbulence-supported to self-gravitating gas is detected in the gas density probability distribution function at ∼2000 cm−3. The power-spectrum density suggests that gravitation governs the hierarchical organization of structures from the galactic scale down to a few pc.
We use machine learning to identify in color images of high-redshift galaxies an astrophysical phenomenon predicted by cosmological simulations. This phenomenon, called the blue nugget (BN) phase, is ...the compact star-forming phase in the central regions of many growing galaxies that follows an earlier phase of gas compaction and is followed by a central quenching phase. We train a convolutional neural network (CNN) with mock "observed" images of simulated galaxies at three phases of evolution- pre-BN, BN, and post-BN-and demonstrate that the CNN successfully retrieves the three phases in other simulated galaxies. We show that BNs are identified by the CNN within a time window of ∼0.15 Hubble times. When the trained CNN is applied to observed galaxies from the CANDELS survey at z = 1-3, it successfully identifies galaxies at the three phases. We find that the observed BNs are preferentially found in galaxies at a characteristic stellar mass range, 109.2-10.3 M at all redshifts. This is consistent with the characteristic galaxy mass for BNs as detected in the simulations and is meaningful because it is revealed in the observations when the direct information concerning the total galaxy luminosity has been eliminated from the training set. This technique can be applied to the classification of other astrophysical phenomena for improved comparison of theory and observations in the era of large imaging surveys and cosmological simulations.
We report high-quality, H or CO rotation curves (RCs) to several Re for 41 large, massive, star-forming disk galaxies (SFGs) across the peak of cosmic galaxy evolution (z ∼ 0.67-2.45), taken with the ...ESO-VLT, the LBT and IRAM-NOEMA. Most RC41 SFGs have reflection-symmetric RCs plausibly described by equilibrium dynamics. We fit the major axis position-velocity cuts using beam-convolved forward modeling generated in three dimensions, with models that include a bulge and turbulent disk component embedded in a dark matter (DM) halo. We include priors for stellar and molecular gas masses, optical light effective radii and inclinations, and DM masses from abundance-matching scaling relations. Two-thirds or more of the z ≥ 1.2 SFGs are baryon dominated within a few Re of typically 5.5 kpc and have DM fractions less than maximal disks (median 〈 f DM ( R e ) 〉 = 0.12 ). At lower redshift (z < 1.2), that fraction is less than one-third. DM fractions correlate inversely with the baryonic angular momentum parameter, baryonic surface density, and bulge mass. Inferred low DM fractions cannot apply to the entire disk and halo but more plausibly reflect a flattened, or cored, inner DM density distribution. The typical central "DM deficit" in these cores relative to Navarro-Frenk-White (NFW) distributions is ∼30% of the bulge mass. The observations are consistent with rapid radial transport of baryons in the first-generation massive gas-rich halos forming globally gravitationally unstable disks and leading to efficient build-up of massive bulges and central black holes. A combination of heating due to dynamical friction and AGN feedback may drive DM out of the initial cusps.
PDF
