Discovery of Large Molecular Gas Reservoirs in Post-Starburst Galaxies

Post-starburst (or E+A) galaxies are characterized by low H$alpha$ emission and strong Balmer absorption, suggesting a recent starburst, but little current star formation. Although many of these galaxies show evidence of recent mergers, the mechanism for ending the starburst is not yet understood. To study the fate of the molecular gas, we search for CO (1-0) and (2-1) emission with the IRAM 30m and SMT 10m telescopes in 32 nearby ($0.01<z<0.12$) post-starburst galaxies drawn from the Sloan Digital Sky Survey. We detect CO in 17 (53%). Using CO as a tracer for molecular hydrogen, and a Galactic conversion factor, we obtain molecular gas masses of $M(H_2)=10^{8.6}$-$10^{9.8} M_odot$ and molecular gas mass to stellar mass fractions of $sim10^{-2}$-$10^{-0.5}$, comparable to those of star-forming galaxies. The large amounts of molecular gas rule out complete gas consumption, expulsion, or starvation as the primary mechanism that ends the starburst in these galaxies. The upper limits on $M(H_2)$ for the 15 undetected galaxies range from $10^{7.7} M_odot$ to $10^{9.7} M_odot$, with the median more consistent with early-type galaxies than with star-forming galaxies. Upper limits on the post-starburst star formation rates (SFRs) are lower by $sim10times$ than for star-forming galaxies with the same $M(H_2)$. We also compare the molecular gas surface densities ($Sigma_{rm H_2}$) to upper limits on the SFR surface densities ($Sigma_{rm SFR}$), finding a significant offset, with lower $Sigma_{rm SFR}$ for a given $Sigma_{rm H_2}$ than is typical for star-forming galaxies. This offset from the Kennicutt-Schmidt relation suggests that post-starbursts have lower star formation efficiency, a low CO-to-H$_2$ conversion factor characteristic of ULIRGs, and/or a bottom-heavy initial mass function, although uncertainties in the rate and distribution of current star formation remain.

The Properties of Lyman Alpha Nebulae: Gas Kinematics from Non-resonant Lines

[Abridged] With VLT/X-shooter, we obtain optical and NIR spectra of six Ly-alpha blobs at z~2.3. Using three measures --- the velocity offset between the Lya line and the non-resonant [OIII] or H-alpha line (Dv_Lya), the offset of stacked interstellar metal absorption lines, and the spectrally-resolved [OIII] line profile --- we study the kinematics of gas along the line of sight to galaxies within each blob center. These three indicators generally agree in velocity and direction, and are consistent with a simple picture in which the gas is stationary or slowly outflowing at a few hundred km/s from the embedded galaxies. The absence of stronger outflows is not a projection effect: the covering fraction for our sample is limited to <1/8 (13%). The outflow velocities exclude models in which star formation or AGN produce super or hyper winds of up to ~1000km/s. The Dv_Lya offsets here are smaller than typical of LBGs, but similar to those of compact LAEs. The latter suggests that outflow speed cannot be a dominant factor in driving extended Lya emission. For one Lya blob (CDFS-LAB14), whose Lya profile and metal absorption line offsets suggest no significant bulk motion, we use a simple radiative transfer model to make the first column density measurement of gas in an embedded galaxy, finding it consistent with a DLA system. Overall, the absence of clear inflow signatures suggests that the channeling of gravitational cooling radiation into Lya is not significant over the radii probed here. However, one peculiar system (CDFS-LAB10) has a blueshifted Lya component that is not obviously associated with any galaxy, suggesting either displaced gas arising from tidal interactions among blob galaxies or gas flowing into the blob center. The former is expected in these overdense regions, and the latter might signify the predicted but elusive cold gas accretion along filaments.

A Deep Narrowband Imaging Search for CIV and He II Emission from Ly$alpha$ Blobs

We conduct a deep narrow-band imaging survey of 13 Ly$alpha$ blobs (LABs) located in the SSA22 proto-cluster at z~3.1 in the CIV and HeII emission lines in an effort to constrain the physical process powering the Ly$alpha$ emission in LABs. Our observations probe down to unprecedented surface brightness limits of 2.1 $-$ 3.4 $times$ 10$^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ per 1 arcsec$^2$ aperture (5$sigma$) for the HeII$lambda$1640 and CIV$lambda$1549 lines, respectively. We do not detect extended HeII and CIV emission in any of the LABs, placing strong upper limits on the HeII/Ly$alpha$ and CIV/Ly$alpha$ line ratios, of 0.11 and 0.16, for the brightest two LABs in the field. We conduct detailed photoionization modeling of the expected line ratios and find that, although our data constitute the deepest ever observations of these lines, they are still not deep enough to rule out a scenario where the Ly$alpha$ emission is powered by the ionizing luminosity of an obscured AGN. Our models can accommodate HeII/Ly$alpha$ and CIV/Ly$alpha$ ratios as low as $simeq$0.05 and $simeq$0.07 respectively, implying that one needs to reach surface brightness as low as 1 $-$ 1.5 $times$ 10$^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ (at 5$sigma$) in order to rule out a photoionization scenario. These depths will be achievable with the new generation of image-slicing integral field units such as VLT/MUSE or Keck/KCWI. We also model the expected HeII/Ly$alpha$ and CIV/Ly$alpha$ in a different scenario, where Ly$alpha$ emission is powered by shocks generated in a large-scale superwind, but find that our observational constraints can only be met for shock velocities $v_{rm s} gtrsim$ 250 km s$^{-1}$, which appear to be in conflict with recent observations of quiescent kinematics in LABs.

Pinpointing the Molecular Gas within a Lyman Alpha Blob at z ~ 2.7

We present IRAM PdBI observations of the CO(3-2) and CO(5-4) line transitions from a Ly-alpha blob at z~2.7 in order to investigate the gas kinematics, determine the location of the dominant energy source, and study the physical conditions of the molecular gas. CO line and dust continuum emission are detected at the location of a strong MIPS source that is offset by ~1.5 from the Ly-alpha peak. Neither of these emission components is resolved with the 1.7 beam, showing that the gas and dust are confined to within ~7kpc from this galaxy. No millimeter source is found at the location of the Ly-alpha peak, ruling out a central compact source of star formation as the power source for the Ly-alpha emission. Combined with a spatially-resolved spectrum of Ly-alpha and HeII, we constrain the kinematics of the extended gas using the CO emission as a tracer of the systemic redshift. Near the MIPS source, the Ly-alpha profile is symmetric and its line center agrees with that of CO line, implying that there are no significant bulk flows and that the photo-ionization from the MIPS source might be the dominant source of the Ly-alpha emission. In the region near the Ly-alpha peak, the gas is slowly receding (~100km/s) with respect to the MIPS source, thus making the hyper-/superwind hypothesis unlikely. We find a sub-thermal line ratio between two CO transitions, I_CO(5-4)/I_CO(3-2)=0.97+/-0.21. This line ratio is lower than the average values found in high-z SMGs and QSOs, but consistent with the value found in the Galactic center, suggesting that there is a large reservoir of low-density molecular gas that is spread over the MIPS source and its vicinity.

Constraining Dust and Molecular Gas Properties in Lyman Alpha Blobs at z~3

In order to constrain the bolometric luminosities, dust properties and molecular gas content of giant Lyman alpha nebulae, the so-called Lyman alpha blobs, we have carried out a study of dust continuum and CO line emission in two well-studied representatives of this population at z ~ 3: a Lya blob discovered by its strong Spitzer MIPS 24um detection (LABd05; Dey et al. 2005) and the Steidel blob 1 (SSA22-LAB01; Steidel et al. 2000). We find that the spectral energy distribution of LABd05 is well described by an AGN-starburst composite template with L(FIR) = (4.0 +/- 0.5) x 10^12 Lsun, comparable to high-z sub-millimeter galaxies and ultraluminous infrared galaxies. New APEX/LABOCA 870um measurements rule out the reported SCUBA detection of the SSA22-LAB01 (S[850um] = 16.8 mJy) at the > 4sigma level. Consistent with this, ultra-deep Plateau de Bure Interferometer (PdBI) observations with ~2arcsec spatial resolution also fail to detect any 1.2mm continuum source down to ~0.45mJy per beam (3sigma). Combined with the existing (sub)mm observations in the literature, we conclude that the FIR luminosity of SSA22-LAB01 remains uncertain. No CO line is detected in either case down to integrated flux limits of (Snu dV) < 0.25--1.0 Jy km/s, indicating a modest molecular gas reservoir, M(H_2) < 1--3 x 10^10 Msun. The non-detections exclude, with high significance (12 sigma), the previous tentative detection of a CO(4-3) line in the SSA22-LAB01. The increased sensitivity afforded by ALMA will be critical in studying molecular gas and dust in these interesting systems.

Kinematics of Gas in Lyman Alpha Nebulae

Exploring the origin of Ly-alpha nebulae (blobs) requires measurements of their gas kinematics that are impossible with only the resonant, optically-thick LyA line. To define gas motions relative to the systemic velocity of the blob, the LyA line must be compared with an optically-thin line like Halpha, which is not much altered by radiative transfer effects. We obtain optical and NIR spectra of the two brightest LyA blobs from Yang et al. sample using the Magellan/MagE and VLT/SINFONI. Both the LyA and Halpha lines confirm that these blobs lie at the survey redshift, z~2.3. Within each blob, we detect several Halpha sources, which roughly correspond to galaxies seen in HST images. The Halpha detections show that these galaxies have large internal velocity dispersions (130 - 190km/s) and that, in the one system (LAB01), their velocity difference is ~440 km/s. The presence of multiple galaxies within the blobs, and those galaxies large velocity dispersions and large relative motion, is consistent with our previous finding that LyA blobs inhabit massive dark matter halos that will evolve into those typical of rich clusters today. To determine whether the gas near the embedded galaxies is predominantly infalling or outflowing, we compare the LyA and Halpha line centers, finding that LyA is not offset (Delta LyA = +0km/s) in LAB01 and redshifted by only +230 km/s in LAB02. These offsets are small compared to those of Lyman break galaxies, which average +450 km/s and extend to about +700 km/s. We test and rule out the simplest infall models and those outflow models with super/hyper-winds, which require large outflow velocities. Because of the unknown geometry of the gas distribution and the possibility of multiple sources of LyA emission embedded in the blobs, a larger sample and more sophisticated models are required to test more complex or a wider range of infall and outflow scenarios.

Strong Field-to-Field Variation of Lyman alpha Nebulae Populations at z~2.3

Understanding the nature of distant Ly-alpha nebulae (blobs) and connecting them to their present-day descendants requires constraining their number density, clustering, and large-scale environment. To measure these basic quantities, we conduct a deep narrowband imaging survey in four different fields, Chandra Deep Field South (CDFS), Chandra Deep Field North, and two COSMOS subfields, for a total survey area of 1.2deg^2. We discover 25 blobs at z=2.3 with Ly-alpha luminosities of 0.7-8x10^43 erg/s and isophotal areas of Aiso = 10-60 arcsec^2. The transition from compact Ly-alpha emitters (Aiso ~ a few arcsec^2) to extended blobs (Aiso > 10 arcsec^2) is continuous, suggesting a single family perhaps governed by similar emission mechanisms. Surprisingly, most blobs (16/25) are in one survey field, the CDFS. The six brightest, largest blobs with L > 1.5x10^43 erg/s and Aiso > 16 arcsec^2 lie only in the CDFS. These large, bright blobs have a field-to-field variance of sigma_v >~ 1.5 (150%) about their number density n ~ 1.0x10^-5 Mpc^-3. This variance is large, significantly higher than that of unresolved LAEs (sigma_v ~ 0.3 or 30%), and can adversely affect comparisons of blob number densities and luminosity functions among different surveys. We compare the statistics of our blobs with dark matter halos in a 1 Gpc/h cosmological N-body simulation. At z=2.3, the number density (n) implies that each bright, large blob could occupy a halo of M_halo > 10^13 Msun if most halos have detectable blobs. The predicted variance in n is consistent with that observed and corresponds to a bias of ~7. Blob halos lie at the high end of the halo mass distribution at z=2.3 and are likely to evolve into the ~10^14 Msun halos typical of galaxy clusters today. On larger scales of ~10 co-moving Mpc, blobs cluster where compact LAEs do, indicating that blobs lie in coherent, highly overdense structures.

The Detailed Evolution of E+A Galaxies into Early Types

Post-starburst, or E+A galaxies, are the best candidates for galaxies in transition from being gas-rich and star-forming to gas-poor and passively-evolving via galaxy-galaxy mergers. To determine what E+A galaxies become after their young stellar populations fade away, we present the detailed morphologies of 21 E+As using HST images. We find that E+As are similar to early types in that they have large bulge fractions (median B/T = 0.59), high Sersic index (n > 4), and high concentration indices (C > 4.3). The large fraction (70%) of E+As with positive color gradients (i.e., bluer nuclei) indicates that the young stellar populations are more concentrated than the old populations. We show that these positive color gradients can evolve into the negative gradients typical in E/S0s if the central parts of these galaxies are metal enhanced. E+A galaxies stand apart from the E/S0s in the edge-on projection of the Fundamental Plane, implying that E+As have, on average, a M/L that is ~3.8 times smaller than that of E/S0s. The tilt of the E+A FP indicates that the variation among stellar populations in these galaxies is closely tied to their structural parameters such that smaller or less massive galaxies have smaller M/L. We find a population of unresolved compact sources in nine E+As (45%). Their colors and luminosities are consistent with the hypothesis that these are newly formed star clusters. The bright end of the cluster LF is fainter in redder E+As, suggesting that the young star cluster systems have faded or been disrupted as the merger remnant aged. In summary, the morphologies, color profiles, scaling relations, and cluster populations are all consistent with the hypothesis that E+As galaxies are the results of mergers that evolve into early-type galaxies.

Extended Lyman Alpha Nebulae at z ~ 2.3: An Extremely Rare and Strongly Clustered Population?

To obtain an unbiased sample of bright LyA blobs [L(LyA) > 10^43 ergs/s], we have undertaken a blind, wide-field, narrow-band imaging survey in the NOAO Deep Wide Field Survey Bootes field with the Steward Bok-2.3m telescope. After searching over 4.82 sq. degrees at z=2.3, we discover four LyA blobs with L(LyA) = 1.6-5.3 x 10^43 ergs/s, isophotal areas of 28-57 sq. arcsec, and broad LyA line profiles (FWHM = 900-1250 km/s). In contrast with the extended Lyman alpha halos associated with high-z radio galaxies, none of our four blobs are radio-loud. The X-ray luminosities and optical spectra of these blobs are diverse. Two blobs (3 and 4) are X-ray-detected with L_X(2-7 keV) = 2-4 x 10^44 ergs/s and have broad optical emission lines (C IV) characteristic of AGN, implying that 50% of our sample blobs are associated with strong AGN. The other 50% of blobs (1 and 2) are not X-ray or optically-detected as AGN down to similar limits. The number density of the four blobs is ~3 x 10^{-6} Mpc^{-3}, comparable to that of galaxy clusters at similar redshifts and 3x lower than that found in the SSA22 proto-cluster at z=3.1, even after accounting for the over-density of that region. The two X-ray undetected blobs are separated by only 70 (550 kpc) and have almost identical redshifts (< 360 kpc along the line-of-sight), suggesting that they are part of the same system. Given the rarity of the blobs and our discovery of a close pair, we speculate that blobs occupy the highest density regions and thus may be precursors of todays rich cluster galaxies.