Rest-frame Optical Emission Lines Research Articles

THE REST-FRAME UV-TO-OPTICAL COLORS AND SPECTRAL ENERGY DISTRIBUTIONS OFz∼ 4-7 GALAXIES

We use the ultra-deep HUDF09 and the deep ERS data from the HST WFC3/IR camera, along with the wide area Spitzer/IRAC data from GOODS-S to derive SEDs of star-forming galaxies from the rest-frame UV to the optical over a wide luminosity range (M_1500 ~ -21 to M_1500 ~ -18) from z ~ 7 to z ~ 4. The sample contains ~ 400 z ~ 4, ~ 120 z ~ 5, ~ 60 z ~ 6, and 36 prior z ~ 7 galaxies. Median stacking enables the first comprehensive study of very faint high-z galaxies at multiple redshifts (e.g., [3.6] = 27.4 +/- 0.1 AB mag for the M_1500 ~ -18 sources at z ~ 4). At z ~ 4 our faint median-stacked SEDs reach to ~ 0.06 L*(z=4) and are combined with recently published results at high luminosity L > L* that extend to M_1500 ~ -23. We use the observed SEDs and template fits to derive rest frame UV-to-optical colors (U - V) at all redshifts and luminosities. We find that this color does not vary significantly with redshift at a fixed luminosity. The UV-to-optical color does show a weak trend with luminosity, becoming redder at higher luminosities. This is most likely due to dust. At z >~ 5 we find blue colors [3.6]-[4.5] ~ -0.3 mag that are most likely due to rest-frame optical emission lines contributing to the flux in the IRAC filter bandpasses. The scatter across our derived SEDs remains substantial, but the results are most consistent with a lack of any evolution in the SEDs with redshift at a given luminosity. The similarity of the SEDs suggests a self-similar mode of evolution over a timespan from 0.7 Gyr to 1.5 Gyr that encompasses very substantial growth in the stellar mass density in the universe (from ~ 4x10^6 to ~ 2x10^7 Msun Mpc^-3).

ALMA reveals a chemically evolved submillimeter galaxy atz= 4.76

The chemical properties of high-z galaxies provide important information to constrain galaxy evolutionary scenarios. However, widely-used metallicity diagnostics based on rest-frame optical emission lines are not usable for heavily dust-enshrouded galaxies (such as Sub-Millimeter Galaxies; SMGs), especially at z>3. Here we focus on the flux ratio of the far-infrared fine-structure emission lines [NII]205um and [CII]158um to assess the metallicity of high-z SMGs. Through ALMA cycle 0 observations, we have detected the [NII]205um emission in a strongly [CII]-emitting SMG, LESS J033229.4-275619 at z=4.76. The velocity-integrated [NII]/[CII] flux ratio is 0.043 +/- 0.008. This is the first measurement of the [NII]/[CII] flux ratio in high-z galaxies, and the inferred flux ratio is similar to the ratio observed in the nearby universe (~0.02-0.07). The velocity-integrated flux ratio and photoionization models suggest that the metallicity in this SMG is consistent with solar, implying the chemical evolution has progressed very rapidly in this system at z=4.76. We also obtain a tight upper limit on the CO(12-11) transition, which translates into CO(12-11)/CO(2-1) <3.8 (3 sigma). This suggests that the molecular gas clouds in LESS J033229.4-275619 are not affected significantly by the radiation field emitted by the AGN in this system.

CONSTRAINTS ON THE ASSEMBLY AND DYNAMICS OF GALAXIES. I. DETAILED REST-FRAME OPTICAL MORPHOLOGIES ON KILOPARSEC SCALE OFz∼ 2 STAR-FORMING GALAXIES

We present deep and high-resolution HST/NIC2 F160W imaging at 1.6micron of six z~2 star-forming galaxies with existing near-IR integral field spectroscopy from SINFONI at the VLT. The unique combination of rest-frame optical imaging and nebular emission-line maps provides simultaneous insight into morphologies and dynamical properties. The overall rest-frame optical emission of the galaxies is characterized by shallow profiles in general (Sersic index n<1), with median effective radii of ~5kpc. The morphologies are significantly clumpy and irregular, which we quantify through a non-parametric morphological approach, estimating the Gini (G), Multiplicity (Psi), and M_20 coefficients. The strength of the rest-frame optical emission lines in the F160W bandpass indicates that the observed structure is not dominated by the morphology of line-emitting gas, and must reflect the underlying stellar mass distribution of the galaxies. The sizes and structural parameters in the rest-frame optical continuum and Halpha emission reveal no significant differences, suggesting similar global distributions of the on-going star formation and more evolved stellar population. While no strong correlations are observed between stellar population parameters and morphology within the NIC2/SINFONI sample itself, a consideration of the sample in the context of a broader range of z~2 galaxy types indicates that these galaxies probe the high specific star formation rate and low stellar mass surface density part of the massive z~2 galaxy population, with correspondingly large effective radii, low Sersic indices, low G, and high Psi and M_20. The combined NIC2 and SINFONI dataset yields insights of unprecedented detail into the nature of mass accretion at high redshift. [Abridged]

THE HETDEX PILOT SURVEY. III. THE LOW METALLICITIES OF HIGH-REDSHIFT Lyα GALAXIES

We present Keck/NIRSPEC spectroscopic observations of three Lyman alpha emitting galaxies (LAEs) at z ~ 2.3 discovered with the HETDEX pilot survey. We detect Halpha, [OIII], and Hbeta emission from two galaxies at z = 2.29 and 2.49, designated HPS194 and HPS256, respectively, representing the first detection of multiple rest-frame optical emission lines in galaxies at high-redshift selected on the basis of their Lyman alpha emission. The redshifts of the Lyman alpha emission from these galaxies are offset redward of the systemic redshifts by Delta_v = 162 +/- 37 (photometric) +/- 42 (systematic) km/s for HPS194, and Delta_v = 36 +/- 35 +/- 18 km/s for HPS256. An interpretation for HPS194 is that a large-scale outflow may be occurring in its interstellar medium. The emission line ratios imply that neither LAE hosts an active galactic nucleus. Using the upper limits on the [NII] emission we place meaningful constraints on the gas-phase metallicities in these two LAEs of Z < 0.17 and < 0.28 Zsol (1 sigma). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (~ 10^10 and 6 x 10^8 Msol, respectively), we study the nature of LAEs in a mass-metallicity plane. At least one of these two LAEs appears to be more metal poor than continuum-selected star-forming galaxies at the same redshift and stellar mass, implying that objects exhibiting Lyman alpha emission may be systematically less chemically enriched than the general galaxy population. We use the SEDs of these two galaxies to show that neglecting the emission lines when fitting stellar population models to the observed photometry can result in overestimates of the population age by orders of magnitude, and the stellar mass by a factor of ~ 2. This effect is particularly important at z > 7, where similarly strong emission lines may masquerade in the photometry as a 4000 A break (abridged).

Metallicity diagnostics with infrared fine-structure lines

Although measuring the gas metallicity in galaxies at various redshifts is crucial to constrain galaxy evolutionary scenarios, only rest-frame optical emission lines have been generally used to measure the metallicity. This has prevented us to accurately measure the metallicity of dust-obscured galaxies, and accordingly to understand the chemical evolution of dusty populations, such as ultraluminous infrared galaxies. Here we propose diagnostics of the gas metallicity based on infrared fine structure emission lines, which are nearly unaffected by dust extinction even the most obscured systems. Specifically, we focus on fine-structure lines arising mostly from HII regions, not in photo-dissociation regions, to minimize the dependence and uncertainties of the metallicity diagnostics from various physical parameters. Based on photoionization models, we show that the emission-line flux ratio of ([OIII]51.80+[OIII]88.33)/[NIII]57.21 is an excellent tracer of the gas metallicity. The individual line ratios [OIII]51.80/[NIII]57.21 or [OIII]88.33/[NIII]57.21 can also be used as diagnostics of the metallicity, but they suffer a stronger dependence on the gas density. The line ratios [OIII]88.33/[OIII]51.80 and [NII]121.7/[NIII]57.21 can be used to measure and, therefore, account for the dependences on the of the gas density and ionization parameter, respectively. We show that these diagnostic fine-structure lines are detectable with Herschel in luminous infrared galaxies out z=0.4. Metallicity measurements with these fine-structure lines will be feasible at relatively high redshift (z=1 or more) with SPICA, the future infrared space observatory.

Galaxy counterparts of metal-rich damped Lyα absorbers - I. The case of the z= 2.35 DLA towards Q 2222−0946★

We have initiated a survey using the newly commissioned X-shooter spectrograph to target candidate relatively metal-rich damped Lyman-alpha absorbers (DLAs). The spectral coverage of X-shooter allows us to search for not only Lyman-alpha emission, but also rest-frame optical emission lines. We have chosen DLAs where the strongest rest-frame optical lines ([OII], [OIII], Hbeta and Halpha) fall in the NIR atmospheric transmission bands. In this first paper resulting from the survey, we report on the discovery of the galaxy counterpart of the z_abs = 2.354 DLA towards the z=2.926 quasar Q2222$-0946. This DLA is amongst the most metal-rich z>2 DLAs studied so far at comparable redshifts and there is evidence for substantial depletion of refractory elements onto dust grains. We measure metallicities from ZnII, SiII, NiII, MnII and FeII of -0.46+/-0.07, -0.51+/-0.06, -0.85+/-0.06, -1.23+/-0.06, and -0.99+/-0.06, respectively. The galaxy is detected in the Lyman-alpha, [OIII] lambda4959,5007 Halpha emission lines at an impact parameter of about 0.8 arcsec (6 kpc at z_abs = 2.354). We infer a star-formation rate of 10 M_sun yr^-1, which is a lower limit due to the possibility of slit-loss. Compared to the recently determined Halpha luminosity function for z=2.2 galaxies the DLA-galaxy counterpart has a luminosity of L~0.1L^*_Halpha. The emission-line ratios are 4.0 (Lyalpha/Halpha) and 1.2 ([OIII]/Halpha). The Lyalpha line shows clear evidence for resonant scattering effects, namely an asymmetric, redshifted (relative to the systemic redshift) component and a much weaker blueshifted component. The fact that the blueshifted component is relatively weak indicates the presence of a galactic wind. The properties of the galaxy counterpart of this DLA is consistent with the prediction that metal-rich DLAs are associated with the most luminous of the DLA-galaxy counterparts.

PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT

Increasingly large samples of galaxies are now being discovered at redshifts z~5-6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper we shed light on the physical conditions in a plausibly similar low mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z=2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (<100 Myr), low mass (M_star ~ 10^9 Msun), low in metallicity (Z ~ 1/6 Zsun), and unreddened (E(B-V)~0.02, UV continuum slope beta=-2.1). We infer a metallicity 12+log(O/H)=7.9 +/- 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio O III] 1661, 1666/[O III] 5007 to determine the electron temperature and finding 12+ log(O/H)=7.8 +/- 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum contains strong emission from Lya (with rest-frame equivalent width 54 A), He II 1640 (both stellar and nebular), C III] 1907, 1909 and O III] 1661, 1666. The C IV/C III] ratio indicates that the source of ionization is unlikely to be an AGN. Analysis of the He II, O III] and C III] line strengths indicates a very high ionization parameter log U ~ -1, while Lya and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418's unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.

THE SINS SURVEY: SINFONI INTEGRAL FIELD SPECTROSCOPY OFz∼ 2 STAR-FORMING GALAXIES

We present the Spectroscopic Imaging survey in the near-infrared (near-IR) with SINFONI (SINS) of high-redshift galaxies. With 80 objects observed and 63 detected in at least one rest-frame optical nebular emission line, mainly Hα, represents the largest survey of spatially resolved gas kinematics, morphologies, and physical properties of star-forming galaxies at z ~ 1-3. We describe the selection of the targets, the observations, and the data reduction. We then focus on the SINS Hα sample, consisting of 62 rest-UV/optically selected sources at 1.3 < z < 2.6 for which we targeted primarily the Hα and [N II] emission lines. Only ≈30% of this sample had previous near-IR spectroscopic observations. The galaxies were drawn from various imaging surveys with different photometric criteria; as a whole, the Hα sample covers a reasonable representation of massive M_* ≳ 10^(10) M_☉ star-forming galaxies at z ≈ 1.5-2.5, with some bias toward bluer systems compared to pure K-selected samples due to the requirement of secure optical redshift. The sample spans 2 orders of magnitude in stellar mass and in absolute and specific star formation rates, with median values ≈3 × 10^(10) M_☉, ≈70 M_☉ yr^(–1), and ≈3 Gyr^(–1). The ionized gas distribution and kinematics are spatially resolved on scales ranging from ≈1.5 kpc for adaptive optics assisted observations to typically ≈4-5 kpc for seeing-limited data. The Hα morphologies tend to be irregular and/or clumpy. About one-third of the Hα sample galaxies are rotation-dominated yet turbulent disks, another one-third comprises compact and velocity dispersion-dominated objects, and the remaining galaxies are clear interacting/merging systems; the fraction of rotation-dominated systems increases among the more massive part of the sample. The Hα luminosities and equivalent widths suggest on average roughly twice higher dust attenuation toward the H II regions relative to the bulk of the stars, and comparable current and past-averaged star formation rates.

REST-FRAME OPTICAL SPECTRA OF THREE STRONGLY LENSED GALAXIES ATz∼ 2

We present Keck II NIRSPEC rest-frame optical spectra for three recently discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z = 2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise ratio (S/N) from gravitational lensing provides an unusually detailed view of the physical conditions in these objects. A full complement of high S/N rest-frame optical emission lines is measured, spanning from rest frame 3600 to 6800 A, including robust detections of fainter lines such as Hγ, [S II]λ6717,6732, and in one instance [Ne III]λ3869. SDSS J090122.37+181432.3 shows evidence for active galactic nucleus activity, and therefore we focus our analysis on star-forming regions in the Cosmic Horseshoe and the Clone. For these two objects, we estimate a wide range of physical properties. Current lensing models for the Cosmic Horseshoe and the Clone allow us to correct the measured Hα luminosity and calculated star formation rate. Metallicities have been estimated with a variety of indicators, which span a range of values of 12+ log(O/H) = 8.3-8.8, between ~0.4 and ~1.5 of the solar oxygen abundance. Dynamical masses were computed from the Hα velocity dispersions and measured half-light radii of the reconstructed sources. A comparison of the Balmer lines enabled measurement of dust reddening coefficients. Variations in the line ratios between the different lensed images are also observed, indicating that the spectra are probing different regions of the lensed galaxies. In all respects, the lensed objects appear fairly typical of ultraviolet-selected star-forming galaxies at z ~ 2. The Clone occupies a position on the emission-line diagnostic diagram of [O III]/Hβ versus [N II]/Hα that is offset from the locations of z ~ 0 galaxies. Our new NIRSPEC measurements may provide quantitative insights into why high-redshift objects display such properties. From the [S II] line ratio, high electron densities (~1000 cm–3) are inferred compared to local galaxies, and [O III]/[O II] line ratios indicate higher ionization parameters compared to the local population. Building on previous similar results at z ~ 2, these measurements provide further evidence (at high S/N) that star-forming regions are significantly different in high-redshift galaxies, compared to their local counterparts.

The impact of nebular emission on the ages of z${\sf \approx}$ 6 galaxies

We examine the influence of nebular continuous and line emission in high redshift star forming galaxies on determinations of their age, formation redshift and other properties from SED fits. We include nebular emission consistently with the stellar emission in our SED fitting tool and analyse differentially a sample of 10 z~6 galaxies in the GOODS-S field studied earlier by Eyles et al. (2007). We find that the apparent Balmer/4000 Ang breaks observed in a number of z~6 galaxies detected at >~3.6 micron with IRAC/Spitzer can be mimicked by the presence of strong restframe optical emission lines, implying in particular younger ages than previously thought. Applying these models to the small sample of z~6 galaxies, we find that this effect may lead to a typical downward revision of their stellar ages by a factor ~3. In consequence their average formation redshift may drastically be reduced, and these objects may not have contributed to cosmic reionisation at z>6. Extinction and stellar mass estimates may also be somewhat modified, but to a lesser extent. Careful SED fits including nebular emission and treating properly uncertainties and degeneracies are necessary for more accurate determinations of the physical parameters of high-z galaxies.

Integral Field Spectroscopy of High‐Redshift Star‐forming Galaxies with Laser‐guided Adaptive Optics: Evidence for Dispersion‐dominated Kinematics

We present early results from an ongoing study of the kinematic structure of star-forming galaxies at redshift z similar to 2-3 using integral-field spectroscopy of rest-frame optical nebular emission lines in combination with Keck laser guide star adaptive optics (LGSAO). We show kinematic maps of three target galaxies Q1623-BX453, Q0449-BX93, and DSF 2237a-C2 located at redshifts z = 2.1820, 2.0067, and 3.3172, respectively, each of which is well resolved with a PSF measuring approximately 0.11-0.15 (similar to 900-1200 pc at z similar to 2-3) after cosmetic smoothing. Neither galaxy at z similar to 2 exhibits substantial kinematic structure on scales greater than or similar to 30 km s^(-1); both are instead consistent with largely dispersion-dominated velocity fields with sigma similar to 80 km s^(-1) along any given line of sight into the galaxy. In contrast, DSF 2237a-C2 presents a well-resolved gradient in velocity over a distance of similar to 4 kpc with peak-to-peak amplitude of 140 km s(-1). It is unlikely that DSF 2237a-C2 represents a dynamically cold rotating disk of ionized gas as the local velocity dispersion of the galaxy (sigma = 79 km s^(-1))is comparable to the observed shear. While some gas cooling models reproduce the observed kinematics better than a simple rotating disk model, even these provide a poor overall description of the target galaxies, suggesting that our current understanding of gas cooling mechanisms in galaxies in the early universe is (at best) incomplete.

Mass and kinematics of late-type galaxies (1.3 &lt; z &lt; 3.3) from the VVDS

AbstractGalaxy kinematics at early epochs give a key insight into the assembly of mass. Redshifts z ~ 1 − 3 appear to be the peak of the star formation rate of the Universe, possibly corresponding to the maximal merger activity. 2D velocity fields of late-type galaxies can be used to put important constraints on its total mass and hence on its dark matter halo. As dark halos are thought to govern the rate of galaxy evolution, securing reliable mass measurements for early systems at high redshift is a fundamental observational goal. In addition, accurate inner shape rotation curves, only drawed by 3D spectroscopic studies, should allow to disentangle cosmological scenarii. In the purpose of probing the evolution in masses and mass-to-light ratios of the galaxies during an epoch of expected strong evolution, and setting constrains on their formation and evolution, we present preliminary results obtained from Integral Field NIR Spectroscopy with SINFONI/VLT of a first sample of ten high-z (1.3 &lt; z &lt; 3.3) late-type galaxies selected in the VIMOS/VLT Deep Survey (VVDS). The SINFONI NIR-IFU mode allow to spatially resolve galaxy dynamics using bright rest-frame optical emission lines, in order to perform statistical studies of dynamical masses at this early epochs.

Spectroscopic Identification of Massive Galaxies at z ~ 2.3 with Strongly Suppressed Star Formation

We present first results of a spectroscopic survey targeting K-selected galaxies at z=2.0-2.7 using the GNIRS instrument on Gemini-South. We obtained near-infrared spectra with a wavelength coverage of 1.0-2.5 micron for 26 K-bright galaxies (K<19.7) selected from the MUSYC survey using photometric redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies using rest-frame optical emission lines or the redshifted Balmer/4000 Angstrom break. Twenty galaxies have spectroscopic redshifts in the range 2.0<z<2.7, for which bright emission lines like Halpha and [OIII] fall in atmospheric windows. Surprisingly, we detected no emission lines for nine of these 20 galaxies. The median 2 sigma upper limit on the rest-frame equivalent width of Halpha for these nine galaxies is ~10 Angstrom. The stellar continuum emission of these same nine galaxies is best fitted by evolved stellar population models. The best-fit star formation rate (SFR) is zero for five out of nine galaxies, and consistent with zero within 1 sigma for the remaining four. Thus, both the Halpha measurements and the independent stellar continuum modeling imply that 45% of our K-selected galaxies are not forming stars intensely. This high fraction of galaxies without detected line emission and low SFRs may imply that the suppression of star formation in massive galaxies occurs at higher redshift than is predicted by current CDM galaxy formation models. However, obscured star formation may have been missed, and deep mid-infrared imaging is needed to clarify this situation.

Lyman Break Galaxies under a Microscope: The Small‐Scale Dynamics and Mass of an Arc in the Cluster 1E 0657−56

Using the near-infrared integral field spectrograph SPIFFI on the VLT, we have studied the spatially resolved dynamics in the z = 3.2 strongly lensed galaxy 1E 0657-56 arc+core by observing the rest-frame optical emission lines [O III] λ5007 and Hβ. The lensing configuration suggests that the high surface brightness core is the ~ 20 magnified central ~1 kpc of the galaxy, whereas the fainter arc is the more strongly magnified peripheral region of the same galaxy at about a half-light radius, which otherwise appears to be a typical z ~ 3 Lyman break galaxy. The overall shape of the position-velocity diagram resembles the rotation curves of the inner few kpc of nearby ~ spiral galaxies. For = 20, our data have a spatial resolution of ~200 pc in the source plane. The projected velocities vrot rise rapidly to ~75 km s-1 within radii ~0.5 kpc from the center and asymptotically reach a velocity of ~190 km s-1 within the arc, at a projected radius of a few kpc radius. The rotation curve implies a dynamical mass of log Mdyn/M☉ ~ 9.3 within the central kpc and suggests that in this system the equivalent of the mass of a present-day ~ bulge at the same radius was already in place by z 3. Approximating the circular velocity of the halo by the measured asymptotic velocity of the rotation curve, we estimate a dark matter halo mass of log Mhalo/M☉ ~ 11.7 ± 0.3, in good agreement with large-scale clustering studies of Lyman break galaxies. The baryonic collapse fraction is low compared to z ~ 2 actively star-forming BX and low-redshift galaxies, perhaps implying comparatively less gas infall to small radii or efficient feedback. Even more speculatively, the high central mass density might indicate highly dissipative gas collapse in very early stages of galaxy evolution, in approximate agreement with what is expected for inside-out galaxy formation models.

On measuring the Tully-Fisher relation atz &gt; 1

The evolution of the line width - luminosity relation for spiral galaxies, the Tully-Fisher relation, strongly constrains galaxy formation and evolution models. At this moment, the kinematics of z > 1 spiral galaxies can only be measured using rest frame optical emission lines associated with star formation, such as Hα and [OIII]5007/4959 and [OII]3727. This method has intrinsic difficulties and uncertainties. Moreover, observations of these lines are challenging for present day telescopes and techniques. Here, we present an overview of the intrinsic and observational challenges and some ways way to circumvent them. We illustrate our results with the HST/NICMOS grism sample data of z ∼ 1.5 starburst galaxies. The number of galaxies we can use in the final Tully-Fisher analysis is only three. We find a ∼2 mag offset from the local rest frame B and R band Tully-Fisher relation for this sample. This offset is partially explained by sample selection effects and sample specifics. Uncertainties in inclination and extinction and the effects of star formation on the luminosity can be accounted for. The largest remaining uncertainty is the line width / rotation curve velocity measurement. We show that high resolution, excellent seeing integral field spectroscopy will improve the situation. However, we note that no flat rotation curves have been observed for galaxies with z > 1. This could be due to the described instrumental and observational limitations, but it might also mean that galaxies at z >1 have not reached the organised motions of the present day.

The first detection of [O III] emission from high-redshift damped Lyman- galaxies

We present the detection of [O iii] emission lines from the galaxies responsible for two high-redshift z > 1.75 damped Lyman-α (DLA) absorption lines. We find two sources of [O iii] emission corresponding to the z= 1.92 DLA absorber towards the quasar Q 2206−1958, and we also detect [O iii] emission from the galaxy responsible for the z= 3.10 DLA absorber towards the quasar 2233.9+1381. These are the first detections of rest-frame optical emission lines from high-redshift DLA galaxies. Unlike the Lyα line, the [O iii] line provides a measure of the systemic velocity of the galaxy. We compare the [O iii] redshifts with the velocity profile of the low-ionization metal lines in these two absorbers, with the goal of distinguishing between the model of Prochaska and Wolfe of DLA absorbers as large rapidly rotating cold thick discs, and the standard hierarchical cold dark matter model of structure formation, in which DLAs arise in protogalactic fragments. We find some discrepancies with the predictions of the former model. Furthermore, the image of the DLA galaxy towards Q 2206−1958 shows a complex disturbed morphology, which is more in accord with the hierarchical picture. We use the properties of the rest-frame optical emission lines to further explore the question posed by Moller et al.: are high-redshift DLA galaxies Lyman-break galaxies (LBGs) selected by gas cross-section? The measured velocity dispersions of the DLA galaxies are in agreement with this picture, while the data on the [O iii] luminosities and the velocity differences between the Lyα and [O iii] lines are inconclusive, as there are insufficient LBG measurements overlapping in luminosity. Finally, we estimate the star formation rates in these two DLA galaxies, using a variety of diagnostics, and include a discussion of the extent to which the [O iii] line is useful for this purpose.

Kinematics of Low‐zUltraluminous Infrared Galaxies and Implications for Dynamical Mass Derivations in High‐zStar‐forming Galaxies

The kinematic properties of the gaseous and stellar components of 11 ultraluminous infrared galaxies (ULIRGs; 14 nuclei) are investigated by means of integral field spectroscopy (IFS) with the INTEGRAL system and available IR and CO millimeter spectroscopy. The sample of ULIRGs cover different phases of the merging process and span all levels of activity from pure starbursts to Seyfert nuclei. The IFS data show that the ionized gas has a complex velocity structure with peak-to-peak velocity differences of a few to several hundred km s-1, detected in tidal tails or extranuclear star-forming regions. The velocity field of the ionized gas on scales of a few to several kiloparsecs is dominated by tidally induced flows and does not, in general, correspond to rotationally supported systems with a privileged orientation along the major rotating axis. The central velocity amplitude of the ionized gas and stars shows discrepancies in some galaxies but has, on average, a similar value (ratio of 0.92 ± 0.37) , while the velocity amplitude of the molecular gas is a factor of 2 larger (ratio of 1.9 ± 0.6) than that of the stars and ionized gas. The central velocity amplitude measured using different kinematic tracers should therefore not be used in ULIRGs as a reliable tracer of mass, in general. The IFS data also show that the velocity dispersion of the ionized gas maps the large-scale motions associated with tidal tails and extranuclear regions, with often the highest velocity dispersion not being associated with the nucleus galaxies. There is, however, a good agreement between the central ionized gas and stellar velocity dispersions (ratio of 1.01 ± 0.13), while the cold molecular gas velocity dispersion has lower values (average of about 0.8 that of the stellar and ionized gas). The central ionized gas velocity dispersion is therefore a robust and homogeneous observable and a good tracer of the dynamical mass in these systems. The IFS-based central ionized gas velocity dispersion measurements confirm that ULIRGs' hosts are moderate-mass (≤m*) galaxies, as previously concluded by Tacconi and coworkers. In general, velocity amplitudes should not be used to estimate the dynamical mass in high-z star-forming systems, such as Lyman break and in particular submillimeter galaxies, since they show irregular stellar and gaseous structures similar to those present in low-z merging systems such as ULIRGs, the subject of this study. A more reliable method is to measure the central velocity dispersion using the strong, high equivalent width, rest-frame optical emission lines, provided the location of the nucleus is independently established by high angular resolution red or near-IR rest-frame imaging. The kinematics derived from the millimeter CO line suggest that the cold gas in ULIRGs does not share the velocity field of the stars and ionized gas and seems to be more rotationally supported. This result needs to be investigated in more detail with a larger sample of low-z ULIRGs before using the millimeter CO line widths as a dynamical mass tracer in high-z submillimeter galaxies.

Lyman-break galaxies: high mass or low?

We reassess the hypothesis that Lyman-break galaxies (LBGs) at redshifts z~3 mark the centres of the most massive dark matter haloes at that epoch. First we reanalyse the kinematic measurements of Pettini et al., and of Erb et al., of the rest-frame optical emission lines of LBGs. We compare the distribution of the ratio of the rotation velocity to the central line width, against the expected distribution for galaxies with random inclination angles, modelled as singular isothermal spheres. The model fits the data well. On this basis we argue that the central line width provides a predictor of the circular velocity at a radius of several kpc. Assembling a larger sample of LBGs with measured line widths, we compare these results against the theoretical Lambda-CDM rotation curves of Mo, Mao & White, under the hypothesis that LBGs mark the centres of the most massive dark matter halos. We find that the circular velocities are over-predicted by a substantial factor, which we estimate conservatively as 1.8+/-0.4. This indicates that the model is probably incorrect. The model of LBGs as relatively low-mass starburst systems, of Somerville, Primack, and Faber (2001), provides a good fit to the data.

Physical properties of two low-luminosity $\mathsf{{\vec z} \sim 1.9}$ galaxies behind the lensing cluster AC 114

We present VLT/ISAAC near-infrared spectroscopy of two gravitationally-lensed z ~ 1.9 galaxies, A2 and S2, located behind the cluster AC 114. Thanks to large magnification factors, we have been successful in detecting rest-frame optical emission lines in star-forming galaxies 1 to 2 magnitudes fainter than in previous studies of Lyman break galaxies (LBGs) at z ~ 3. From the Ha luminosity, we estimate star formation rates (SFRs) which are 7 to 15 times higher than those inferred from the UV continuum flux at 1500 ang without dust extinction correction. The behavior of S2 and A2 in terms of O/H and N/O abundance ratios are very different, and they are also different from typical LBGs at z ~ 3. S2 is a low-metallicity object (Z ~ 0.03 Zsun) with a low N/O ratio, similar to those derived in the most metal-poor nearby HII galaxies. In contrast, A2 is a high-metallicity galaxy (Z ~ 1.3 Zsun) with a high N/O abundance ratio, similar to those derived in the most metal-rich starburst nucleus galaxies. The virial masses, derived from emission-line widths, are 0.5 and 2.4 x 10^10 Msun, for S2 and A2 respectively. Thanks to the gravitational amplification, the line profiles of S2 are spatially resolved, leading to a velocity gradient of +- 240 km/s, which yields a dynamical mass of ~ 1.3 x 10^10 Msun within the inner 1 kpc radius. Combining these new data with the sample of LBGs at z ~ 3, we conclude that these three galaxies exhibit different physical properties in terms of SFRs, abundance and mass-to-light ratios, and reddening. High-redshift galaxies of different luminosities could thus have quite different star formation histories (abridged version).

Near-ir integral field spectroscopy of high-Z galaxies

Here we summarize the exciting prospect of near-infrared spatially-resolved spectroscopy of highredshift galaxies. This is now possible with the new integral field units on 8 m-class telescopes, such as the Cambridge IR Panoramic Survey Spectrograph (CIRPASS, Parry et al.: 2001, SPIE 4008, 1193). Line emission should be an instantaneous tracer of star formation activity, but measurements based on Lyman-α are unreliable because of the selective extinction of this resonant UV line. Using more robust lines such as Hα forces a move to the near-infrared at z > 1. Great sensitivity can be achieved in the J- and H-bands (1.2 & 1.6 µm) by pre-selecting galaxies with redshifts for which prominent rest-frame optical emission lines fall into these near-infrared windows, between atmospheric lines. On an 8 m-class telescope, star formation rates of < 1 M ⊙ yr−1 will be reached at z ≈ 1.4 with Hα in the H-band. Such star formation rates are well below L* for the high-z Lyman-break population, and are comparable locally to the luminous giant H II complexes in M 101. Line ratio indices will reveal the metallicity and extinction in star forming regions within the galaxy disks. The line widths and velocity offsets will provide a sensitive probe of the kinematics of the galaxies, to explore the evolution of fundamental scaling relations. Combining this spatially-resolved spectroscopy with deep, multi-colour, high-resolution imaging (e.g. from HST) will explore fundamental structural parameters (2D rotation velocity profiles, stellar masses, sizes and disk scale lengths), luminosities and star formation rates, and study their evolution with redshift. This will provide a stringent test of current models for the formation and evolution of galactic disks.