Center Of Dark Matter Halos Research Articles

Novel structures and collapse of solitons in nonminimally gravitating dark matter halos

Ultralight dark matter simulations predict condensates with short-range correlation, known as solitons or boson stars, at the centers of dark matter halos. This paper investigates the formation and collapse of dark matter solitons influenced by nonminimal gravitational effects, characterized by gradient-dependent self-interactions of dark matter and an additional source in Poisson's equation for gravity. Our simulations suggest that the initial evolution of dark matter resembles that without nonminimal gravitational effects. However, regions with negative potential curvature may develop, and solitons will collapse when their densities reach certain critical values for both positive and negative coupling constants. With strong nonminimal gravitational effects, we verify that linear density perturbations could grow on both large and small scales, potentially enhancing structure formation.

Confronting fuzzy dark matter with the rotation curves of nearby dwarf irregular galaxies

Aims. In this paper, we carry out a phenomenological investigation of the viability of fuzzy dark matter, which is composed of coherent waves of non-interacting ultralight axions with a mass of ma ≈ 10−22 eV. We did so by confronting the predictions of the model, in particular, the formation of a solitonic core at the center of dark matter halos, with a homogeneous and robust sample of high-resolution rotation curves from the LITTLE THINGS in 3D catalog. This comprises a collection of isolated, dark matter-dominated dwarf-irregular galaxies that provides an optimal benchmark for cosmological studies. Our aim is to find evidence of fuzzy dark matter in the observations; alternatively, we seek to set exclusion ranges for its mass. Methods. We used a statistical framework based on a χ2 analysis of the rotation curves of the LITTLE THINGS in 3D catalog using a fuzzy dark matter profile as the theoretical model. This allows us to extract relevant parameters such as the axion mass and mass of the solitonic core, as well as the mass of the dark matter halo and its concentration parameter. We fit the data using current Markov chain Monte Carlo techniques with a rather loose set of priors, except for the implementation of a core-halo relation predicted by simulations. The results of the fits were then used to perform various diagnostics on the predictions of the model. Results. Fuzzy dark matter provides an excellent fit to the rotation curves of the LITTLE THINGS in 3D catalog, with axion masses determined from different galaxies clustering around ma ≈ 2 × 10−23 eV. However, we find two major problems from our analysis. First, the data follow scaling relations of the properties of the core, which are not consistent with the predictions of the soliton. This problem is particularly acute in the core radius-mass relation with a tension that (at face value) has a significance of ≳5σ. The second problem is related to the strong suppression of the linear power spectrum that is predicted by fuzzy dark matter for the axion mass preferred by the data. This can be constrained very conservatively by the galaxy counts in our sample, which leads to a tension that exceeds 5σ. We estimate the effects of baryons in our analysis and discuss whether they could alleviate the tensions of the model with observations.

Globular Clusters and Streaming Velocities: Testing the New Formation Channel in High-resolution Cosmological Simulations

Abstract The formation of globular clusters and their relation to the distribution of dark matter have long puzzled astronomers. One of the most recently proposed globular cluster formation channels ties ancient star clusters to the large-scale streaming velocity of baryons relative to dark matter in the early universe. These streaming velocities affect the global infall of baryons into dark matter halos, the high-redshift halo mass function, and the earliest generations of stars. In some cases, streaming velocities may result in dense regions of dark matter-free gas that becomes Jeans unstable, potentially leading to the formation of compact star clusters. We investigate this hypothesis using cosmological hydrodynamical simulations that include a full chemical network and the formation and destruction of H2, a process crucial for the formation of the first stars. We find that high-density gas in regions with significant streaming velocities is indeed somewhat offset from the centers of dark matter halos, but this offset is typically significantly smaller than the virial radius. Gas outside of dark matter halos never reaches Jeans-unstable densities in our simulations. We postulate that low-level (Z ≈ 10−3 Z ⊙) metal enrichment by Population III supernovae may enable cooling in the extra-virial regions, allowing gas outside of dark matter halos to cool to the cosmic microwave background temperature and become Jeans unstable. Follow-up simulations that include both streaming velocities and metal enrichment by Population III supernovae are needed to understand if streaming velocities provide one path for the formation of globular clusters in the early universe.

Design and Scaling of an Omega-EP Experiment to Study Cold Streams Feeding Early Galaxies

Abstract Galaxies form in the centers of dark matter halos, and grow by accreting gas from the halos. The gas supply to the galaxy is the bottleneck for star formation and ultimately sets its stellar mass and star formation rate. As gas falls from the cosmic web into the halos, the highly supersonic infalling gas (Mach ∼20 for Milky Way-type halos) eventually forms a shock. If cooling is inefficient, this shock quickly stabilizes as a quasi-static slowly expanding accretion shock at the outer edge of the halo. If, however, cooling is important, a recent theory suggests that filaments of gas from the cosmic web will freefall unshocked through the halos, and shock only at the outskirts of the central galaxies. These “cold streams” allow for a steady supply of gas into the galaxies and for more efficient star formation. The cold dense filament flowing into a hot less dense environment is potentially Kelvin–Helmholtz unstable. This instability may hinder the ability of the stream to deliver gas deeply enough into the halo. A design of a well-scaled laser experiment on Omega-EP meant for studying this phenomena is presented in the current work, with relevant theory informed by 2D hydrodynamic simulations of the experiment. We establish the hydrodynamic scaling analysis between the cosmological system and its experimental analog, presenting the experiment as an adiabatic upper limit to informing the role of mixing due to the Kelvin–Helmholtz instability.

Cusp or core? Revisiting the globular cluster timing problem in Fornax

ABSTRACT We use N-body simulations to revisit the globular cluster (GC) ‘timing problem’ in the Fornax dwarf spheroidal (dSph). In agreement with earlier work, we find that, due to dynamical friction, GCs sink to the centre of dark matter haloes with a cuspy inner density profile but ‘stall’ at roughly 1/3 of the core radius (rcore) in haloes with constant-density cores. The time-scales to sink or stall depend strongly on the mass of the GC and on the initial orbital radius, but are essentially the same for either cuspy (Navarro–Frenk–White) or cored haloes normalized to have the same total mass within rcore. Arguing against a cusp on the basis that GCs have not sunk to the centre is thus no different from arguing against a core, unless all clusters are today at $\sim(1/3)\,r_{\rm core}$. This would imply a core radius exceeding ∼3 kpc, much larger than seems plausible in any core-formation scenario. (The average projected distance of Fornax GCs is 〈RGC, Fnx〉 ∼ 1 kpc and its effective radius is ∼700 pc.) A simpler explanation is that Fornax GCs have only been modestly affected by dynamical friction, as expected if clusters started orbiting at initial radii of the order of ∼1–2 kpc, just outside Fornax’s present-day half-light radius but well within the tidal radius imprinted by Galactic tides. This is not entirely unexpected. Fornax GCs are significantly older and more metal-poor than most Fornax stars, and such populations in dSphs tend to be more spatially extended than their younger and more metal-rich counterparts. Contrary to some earlier claims, our simulations further suggest that GCs do not truly ‘stall’ at $\sim 0.3\, r_{\rm core}$, but rather continue decaying towards the centre, albeit at reduced rates. We conclude that dismissing the presence of a cusp in Fornax based on the spatial distribution of its GC population is unwarranted.

Brief Review on Scalar Field Dark Matter Models

The existence of dark matter in the Universe has been solidly established in the last decades, after the arrival of accurate cosmological and astrophysical observations, and some consider it the most challenging problem in modern physics. Given the magnitude of the problem, one cannot discard the existence of new particles with properties that may look exotic in comparison with our current understanding of ordinary matter. This is the case for the so-called scalar field dark matter model, which assumes the existence of a (probably fundamental) scalar field with a very tiny mass that can have observable consequences in the formation of cosmological structure. We present here a brief account of the main properties of an ultra-light scalar field (with masses of the order of $10^{-22} \, \mathrm{eV}/c^2$) and how different observations have been used in the last two decades to put constraints on its physical parameters. Among other topics, we review the cosmological solutions of the model, discuss the features of its self-gravitating equilibrium configurations, revisit the gravitational collapse for the formation of galaxies, and revise the possibility to find a soliton structure in the center of dark matter halos.

On the Origin of Star–Gas Counterrotation in Low-mass Galaxies

Abstract Stars in galaxies form from the cold rotationally supported gaseous disks that settle at the center of dark matter halos. In the simplest models, such angular momentum is acquired early on at the time of collapse of the halo and preserved thereafter, implying a well-aligned spin for the stellar and gaseous component. Observations, however, have shown the presence of gaseous disks in counterrotation with the stars. We use the Illustris numerical simulations to study the origin of such counterrotation in low-mass galaxies (M ⋆ = 2 × 109–5 × 1010 M ⊙), a sample where mergers have not played a significant role. Only ∼1% of our sample shows a counterrotating gaseous disk at z = 0. These counterrotating disks arise in galaxies that have had a significant episode of gas removal followed by the acquisition of new gas with misaligned angular momentum. In our simulations, we identify two main channels responsible for the gas loss: a strong feedback burst and gas stripping during a flyby passage through a more massive group environment. Once settled, counterrotation can be long-lived with several galaxies in our sample displaying misaligned components consistently for more than 2 Gyr. As a result, no major correlation with the present-day environment or structural properties might remain, except for a slight preference for early-type morphologies and a lower than average gas content at a given stellar mass.

Tau neutrinos from ultracompact dark matter minihalos and constraints on the primordial curvature perturbations

The observations and research on the neutrinos provide a kind of indirect way of revealing the properties of dark matter particles. For the detection of muon neutrinos, the main issue is the large atmospheric background, which is caused by the interactions between the cosmic rays and atoms within the atmosphere. Compared with muon neutrinos, tau neutrinos have a smaller atmospheric background especially for the downward-going direction. Except for the classical neutrino sources, dark matter particles can also annihilate into the neutrinos and are the potential high energy astrophysical sources. The annihilation rate of dark matter particles is proportional to the square of number density; therefore, the annihilation rate is large near the center of dark matter halos especially for the new kind of dark matter structures named ultracompact dark matter minihalos (UCMHs). In previous works, we have investigated the potential muon neutrino flux from UCMHs due to dark matter annihilation. Moreover, since the formation of UCMHs is related to the primordial density perturbations of small scales, we get the constraints on the amplitude of the primordial curvature perturbations of small scales, $1 \lesssim k \lesssim 10^{7} ~\rm Mpc^{-1}$. In this work, we focus on the downward-going tau neutrinos from UCMHs due to dark matter annihilation. Compared with the background of tau neutrino flux we get the constraints on the mass fraction of UCMHs. Then using the limits on the mass fraction of UCMHs we got the constraints on the amplitude of the primordial curvature perturbations which are extended to the scale $k \sim 10^{8} ~ \rm Mpc^{-1}$ compared with previous results.

The origin of the mass discrepancy–acceleration relation in ΛCDM

We examine the origin of the mass discrepancy--radial acceleration relation (MDAR) of disk galaxies. This is a tight empirical correlation between the disk centripetal acceleration and that expected from the baryonic component. The MDAR holds for most radii probed by disk kinematic tracers, regardless of galaxy mass or surface brightness. The relation has two characteristic accelerations; $a_0$, above which all galaxies are baryon-dominated; and $a_{\rm min}$, an effective minimum aceleration probed by kinematic tracers in isolated galaxies. We use a simple model to show that these trends arise naturally in $\Lambda$CDM. This is because: (i) disk galaxies in $\Lambda$CDM form at the centre of dark matter haloes spanning a relatively narrow range of virial mass; (ii) cold dark matter halo acceleration profiles are self-similar and have a broad maximum at the centre, reaching values bracketed precisely by $a_{\rm min}$ and $a_0$ in that mass range; and (iii) halo mass and galaxy size scale relatively tightly with the baryonic mass of a galaxy in any successful $\Lambda$CDM galaxy formation model. Explaining the MDAR in $\Lambda$CDM does not require modifications to the cuspy inner mass profiles of dark haloes, although these may help to understand the detailed rotation curves of some dwarf galaxies and the origin of extreme outliers from the main relation. The MDAR is just a reflection of the self-similar nature of cold dark matter haloes and of the physical scales introduced by the galaxy formation process.

Off-center dark matter halo leading to strong central disk lopsidedness

There is increasing evidence now from simulations and observations that the centre of dark matter halo in a Milky Way type galaxy could be off-centred by a few 100 pc w.r.t. the galactic disc. We study the effect of such an offset halo on the orbits and kinematics in the central few kpc of the disc. The equations of motion in the disc plane can be written in terms of the disc and halo potentials when these two are concentric and a perturbation term due to the offset halo. This perturbation potential shows an m=1 azimuthal variation, or is lopsided, and its magnitude increases at small radii. On solving these equations, we find that the perturbed orbit shows a large deviation of ~ 40 % in radius at R = 1.5 kpc, and also strong kinematical lopsidedness. Thus even a small halo offset of 350 pc can induce surprisingly strong spatial and kinematical lopsidedness in the central region within ~ 3 kpc radius. Further, the disc would remain lopsided for several Gyr, as long as the halo offset lasts. This would have important implications for the dynamical evolution of this region.

Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago.

In the cold dark matter cosmology, the baryonic components of galaxies-stars and gas-are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark-matter halo. In the local (low-redshift) Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius-a hallmark of the dark-matter model. Comparisons between the dynamical mass, inferred from these velocities in rotational equilibrium, and the sum of the stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon fractions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (owing to the chosen stellar initial-mass function and the calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves (showing rotation velocity as a function of disk radius) for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early (high-redshift) Universe efficiently condensed at the centres of dark-matter haloes when gas fractions were high and dark matter was less concentrated.

Dynamics of minimally coupled dark energy in spherical halos of dark matter

We analyse the evolution of scalar field dark energy in the spherical halos of dark matter at the late stages of formation of gravitationally bound systems in the expanding Universe. The dynamics of quintessential dark energy at the center of dark matter halo strongly depends on the value of effective sound speed $c_s$ (in units of speed of light). If $c_s\sim1$ (classical scalar field) then the dark energy in the gravitationally bound systems is only slightly perturbed and its density is practically the same as in cosmological background. The dark energy with small value of sound speed ($c_s<0.1$), on the contrary, is important dynamical component of halo at all stages of their evolution: linear, non-linear, turnaround, collapse, virialization and later up to current epoch. These properties of dark energy can be used for constraining the value of effective sound speed $c_s$ by comparison the theoretical predictions with observational data related to the large scale gravitationally bound systems.

Explaining the density profile of self-gravitating systems by statistical mechanics

AbstractA self-gravitating system usually shows a quasi-universal density profile, such as the NFW profile of a simulated dark matter halo, the flat rotation curve of a spiral galaxy, the Sérsic profile of an elliptical galaxy, the King profile of a globular cluster and the exponential law of the stellar disk. It will be interesting if all of the above can be obtained from first principles. Based on the original work of White &amp; Narayan (1987), we propose that if the self-bounded system is divided into infinite infinitesimal subsystems, the entropy of each subsystem can be maximized, but the whole system's gravity may just play the role of the wall, which may not increase the whole system's entropy St, and finally St may be the minimum among all of the locally maximized entropies (He &amp; Kang 2010). For spherical systems with isotropic velocity dispersion, the form of the equation of state will be a hybrid of isothermal and adiabatic (Kang &amp; He 2011). Hence this density profile can be approximated by a truncated isothermal sphere, which means that the total mass must be finite and our results can be consistent with observations (Kang &amp; He 2011b). Our method requires that the mass and energy should be conserved, so we only compare our results with simulations of mild relaxation (i.e. the virial ratio is close to -1) of dissipationless collapse (Kang 2014), and the fitting also is well. The capacity can be calculated and is found not to be always negative as in previous works, and combining with calculations of the second order variation of the entropy, we find that the thermodynamical stability still can be true (Kang 2012) if the temperature tends to be zero. However, the cusp in the center of dark matter halos can not be explained, and more works will continue.The above work can be generalized to study the radial distribution of the disk (Kang 2015). The energy constraint automatically disappears in our variation, because angular momentum is much more important than energy for the disk-shape system. To simplify this issue, a toy model is taken: 2D gravity is adopted, then at large scale it will be consistent with a flat rotation curve; the bulge and the stellar disk are studied together. Then with constraints of mass and angular momentum, the calculated surface density can be consistent with the truncated, up-bended or standard exponential law. Therefore the radial distribution of the stellar disk may be determined by both the random and orbital motions of stars. In our fittings the central gravity is set to be nonzero to include the effect of asymmetric components.

Are the most super-massive dark compact objects harbored at the center of dark matter halos?

We study an isothermal system of semi-degenerate self-gravitating fermions in general relativity (GR). The most general solutions present is mass density profiles with a central degenerate compact core governed by quantum statistics followed by an extended plateau, and ending in a power law behavior r-2. By fixing the fermion mass m in the keV regime, the different solutions depending on the free parameters of the model: the degeneracy and temperature parameters at the center, are systematically constructed along the one-parameter sequences of equilibrium configurations up to the critical point, which is represented by the maximum in a central density (ρ0) versus core mass (Mc) diagram. We show that for fully degenerate cores, the Oppenheimer–Volkoff (OV) mass limit [Formula: see text] is obtained, while instead for low degenerate cores, the critical core mass increases showing the temperature effects in a nonlinear way. The main result of this work is that when applying this theory to model the distribution of dark matter (DM) in big elliptical galaxies from miliparsec distance-scales up to 102Kpc, we do not find any critical core-halo configuration of self-gravitating fermions, able to explain both the most super-massive dark object at their center together with the DM halo simultaneously.

Cosmological simulations of multicomponent cold dark matter.

The nature of dark matter is unknown. A number of dark matter candidates are quantum flavor-mixed particles but this property has never been accounted for in cosmology. Here we explore this possibility from the first principles via extensive N-body cosmological simulations and demonstrate that the two-component dark matter model agrees with observational data at all scales. Substantial reduction of substructure and flattening of density profiles in the centers of dark matter halos found in simulations can simultaneously resolve several outstanding puzzles of modern cosmology. The model shares the "why now?" fine-tuning caveat pertinent to all self-interacting models. Predictions for direct and indirect detection dark matter experiments are made.

GALAXY HALO FORMATION IN THE ABSENCE OF VIOLENT RELAXATION AND A UNIVERSAL DENSITY PROFILE OF THE HALO CENTER

While N-body simulations testify to a cuspy profile of the central region of dark matter halos, observations favor a shallow, cored density profile of the central region of at least some spiral galaxies and dwarf spheroidals. We show that a central profile, very close to the observed one, inevitably forms in the center of dark matter halos if we make a supposition about a moderate energy relaxation of the system during the halo formation. If we assume the energy exchange between dark matter particles during the halo collapse is not too intensive, the profile is universal: it depends almost not at all on the properties of the initial perturbation and is very akin, but not identical, to the Einasto profile with a small Einasto index n ∼ 0.5. We estimate the size of the "central core" of the distribution, i.e., the extent of the very central region with a respectively gentle profile, and show that the cusp formation is unlikely, even if the dark matter is cold. The obtained profile is in good agreement with observational data for at least some types of galaxies but clearly disagrees with N-body simulations.

GALAXIES IN X-RAY GROUPS. II. A WEAK LENSING STUDY OF HALO CENTERING

Locating the centers of dark matter halos is critical for understanding the mass profiles of halos as well as the formation and evolution of the massive galaxies that they host. The task is observationally challenging because we cannot observe halos directly, and tracers such as bright galaxies or X-ray emission from hot plasma are imperfect. In this paper we quantify the consequences of miscentering on the weak lensing signal from a sample of 129 X-ray selected galaxy groups in the COSMOS field with redshifts 0<z<1 and halo masses in the range 10^13 - 10^14 M_sun. By measuring the stacked lensing signal around eight different candidate centers (such as the brightest member galaxy, the mean position of all member galaxies, or the X-ray centroid), we determine which candidates best trace the center of mass in halos. In this sample of groups, we find that massive galaxies near the X-ray centroids trace the center of mass to <~75 kpc, while the X-ray position and centroids based on the mean position of member galaxies have larger offsets primarily due to the statistical uncertainties in their positions (typically ~50-150 kpc). Approximately 30% of groups in our sample have ambiguous centers with multiple bright or massive galaxies, and these groups show disturbed mass profiles that are not well fit by standard models, suggesting that they are merging systems. We find that halo mass estimates from stacked weak lensing can be biased low by 5-30% if inaccurate centers are used and the issue of miscentering is not addressed.

CONSTRAINING HALO OCCUPATION PROPERTIES OF X-RAY ACTIVE GALACTIC NUCLEI USING CLUSTERING OFCHANDRASOURCES IN THE BOÖTES SURVEY REGION

We present one of the most precise measurement to date of the spatial clustering of X-ray selected AGNs using a sample derived from the Chandra X-ray Observatory survey in the Bootes field. The real-space two-point correlation function over a redshift interval from z=0.17 to z~3 is well described by the power law, xi(r)=(r/r0)^-gamma, for comoving separations r<~20h^-1 Mpc. We find gamma=1.84+-0.12 and r0 consistent with no redshift trend within the sample (varying between r0=5.5+-0.6 h^-1 Mpc for <z>=0.37 and r0=6.9+-1.0 h^-1 Mpc for <z>=1.28). Further, we are able to measure the projections of the two-point correlation function both on the sky plane and in the line of sight. We use these measurements to show that the Chandra/Bootes AGNs are predominantly located at the centers of dark matter halos with the circular velocity Vmax>320 km/s or M_200 > 4.1e12 h^-1 Msun, and tend to avoid satellite galaxies in halos of this or higher mass. The halo occupation properties inferred from the clustering properties of Chandra/Bootes AGNs --- the mass scale of the parent dark matter halos, the lack of significant redshift evolution of the clustering length, and the low satellite fraction --- are broadly consistent with the Hopkins et al. scenario of quasar activity triggered by mergers of similarly-sized galaxies.

Escape fraction of ionizing photons from high-redshift galaxies in cosmological SPH simulations

Combing the three-dimensional radiative transfer (RT) calculation and cosmological SPH simulations, we study the escape fraction of ionizing photons (f_esc) of high-redshift galaxies at z=3-6. Our simulations cover the halo mass range of M_h = 10^9 - 10^12 M_sun. We postprocess several hundred simulated galaxies with the Authentic Radiative Transfer (ART) code to study the halo mass dependence of f_esc. In this paper, we restrict ourselves to the transfer of stellar radiation from local stellar population in each dark matter halo. We find that the average f_esc steeply decreases as the halo mass increases, with a large scatter for the lower mass haloes. The low mass haloes with M_h ~ 10^9 M_sun have large values of f_esc (with an average of ~ 0.4), whereas the massive haloes with M_h ~ 10^11 M_sun show small values of f_esc (with an average of ~ 0.07). This is because in our simulations, the massive haloes show more clumpy structure in gas distribution, and star-forming regions are embedded inside these clumps, making it more difficult for the ionizing photons to escape. On the other hand, in low mass haloes, there are often conical regions of highly ionized gas due to the shifted location of young star clusters from the center of dark matter halo, which allows the ionizing photons to escape more easily than in the high-mass haloes. By counting the number of escaped ionizing photons, we show that the star-forming galaxies can ionize the intergalactic medium at z=3-6. The main contributor to the ionizing photons is the haloes with M_h < 10^10 M_sun owing to their high f_esc. The large dispersion in f_esc suggests that there may be various sizes of H{\sc ii} bubbles around the haloes even with the same mass in the early stages of reionization. We also examine the effect of UV background radiation field on f_esc using simple, four different treatment of UV background.

Ly�� blobs as an observational signature of cold accretion streams into galaxies

Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T ~ 1e4 K) gas flowing into the centers of dark matter halos as massive as 1e12-1e13.5 M_sun at redshifts z~1-3. In this paper we show that if > 20% of the gravitational binding energy of the gas is radiated away, then the simulated cold flows are spatially extended Lyman Alpha (Lya) sources with luminosities, Lya line widths, and number densities that are comparable to those of observed Lya blobs. Furthermore, the filamentary structure of the cold flows can explain the wide range of observed Lya blob morphologies. Since the most massive halos form in dense environments, the association of Lya blobs with overdense regions arise naturally. We argue that Lya blobs - even those which are clearly associated with starburst galaxies or quasars - provide direct observational support for the cold accretion mode of galaxies. We discuss various testable predictions of this association.