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Nebular excitation and gas-phase abundances


"BPT diagrams" (Baldwin et al. 1981) relating the [OIII]λ5007Å/Hβ and [NII]λ6584Å/Hα line flux ratios, which provide diagnostics for the excitation mechanism of nebular gas in galaxies. The small grey dots show the distribution of the local galaxies taken from the SDSS survey, revealing the locus of purely star-forming galaxies on the left branch with decreasing gas-phase oxygen abundances towards lower [NII]/Hα and higher [OIII]/Hβ ratios, and of galaxies with shocks and AGN dominating the gas excitation on the right branch. In the left panel, the blue data points show source-integrated measurements from our SINS galaxies (and in green and orange, star-forming galaxies taken from selected published studies for comparison). The SINS galaxies tend to populate the region between the star-forming and AGN branches, suggesting that various excitation mechanisms contribute in different proportions to the global line emission or, possibly, different physical conditions are prevailing in non-AGN actively star-forming galaxies. Detailed case studies are needed to assess those in individual galaxies, as illustrated in the middle and right panels with D3a-15504, a large star-forming disk that hosts an AGN, and ZC-782941, another large disk with a small companion galaxy to the north-east. Maps of the Hα flux, [OIII]/Hβ, and [NII]/Hα (pixels with S/N < 5 are masked out) are shown at the bottom, and the distribution of ratios in individual pixels (colour coded as a function of spatial location as indicated in the top right insets) are plotted in the diagrams. For D3a-15504, the central AGN-dominated and outer star-forming disk regions separate clearly and the ratios suggest gas-phase oxygen abundances of ~ 1/3 to 1/2 solar in the outer disk. For ZC-782941, both integrated and spatially-resolved line ratios are consistent with pure photoionization in HII regions, with somewhat higher abundances. Interestingly, the [NII]/Hα peaks between the main part of the galaxy and the north-east companion, possibly reflecting a different ionization parameter and/or gas fraction.

For 15 galaxies from our SINS Hα sample, we observed the [OIII]λλ4959,5007 Å and Hβ line emission with SINFONI, complementing our Hα and [NII]λλ6548,6584 Å data obtained previously. Using in particular the [OIII]λ5007Å/Hβ and [NII]λ6584Å/Hα line flux ratios in the so-called "BPT diagram" (Baldwin et al. 1981; see figure above), we investigate the excitation mechanism of the nebular gas (photoionization by hot young stars in HII regions, shocks related to galactic outflows, and/or AGN) and the gas-phase oxygen abundances. Measurements of these ratios at z ~ 2, relying on four lines redshifted in the near-IR windows with many bright telluric emission lines throughout most of this wavelength regime, are very challenging and still scarce, and have been mostly obtained from integrated spectra. Results to date show that the integrated line ratios of high redshift galaxies tend to be offset from the locus of the local galaxy population in the "BPT diagram". This can be attributed to different physical conditions in distant star-forming galaxies, or to contributions from AGN and/or shocks. The global ratios of our SINS galaxies show in many cases such offsets. With the full spatial mapping afforded by SINFONI, we can take the next step and investigate the origin of the offsets using spatially resolved ratio maps in individual galaxies. Examples are shown in the figure above, illustrating the power of this approach.

These results will appear in a forthcoming paper by Buschkamp, P., et al.


SINS: largest survey of Hα kinematics and star formation at z ~ 2


Velocity fields for 30 of the 62 galaxies of the SINS H&\alpha; sample, derived from the observed shift in wavelength of the H&\alpha; emission line across the galaxies. Blue to red colours correspond to regions of the galaxies that are approaching towards us and receding from us relative to the systemic or bulk velocity of each galaxy as a whole. The minimum and maximum relative velocities are labeled for each galaxy (in $\rm km\,s^{-1}$). All sources are shown on the same angular scale; the white bars correspond to 1 arcsec, or about 8 kpc at z = 2. The galaxies are approximately sorted from left to right according to whether their kinematics are rotation-dominated or dispersion-dominated, and from top to bottom according to whether they are disk-like or merger-like as quantified by our kinemetry (Shapiro et al. 2008). Galaxies observed with the aid of adaptive optics, resolving details in the galaxies on scales as small as ~ 1–2 kpc, are indicated by the yellow rounded rectangles.

Upon completion of our SINFONI Guaranteed Time Observations at the ESO Very Large Telescope, we had collected spatially-resolved data of the ionized gas kinematics and star formation properties as traced by the Hα line emission of over 60 massive star-forming galaxies at z ~ 1.5 &ndash 2.5. This makes SINS the largest such survey to date based on near-infrared integral field spectroscopy. Our SINS Hα sample probes the z ~ 1.5 - 2.5 star-forming galaxy population over two orders of magnitude in stellar mass and star formation rates, with ranges of ≈ 3×109 – 3×1011 Msun and ≈ 10 – 800 Msun/yr. The ionized gas distribution and kinematics are resolved on spatial scales ranging from ≈ 1.5 kpc for adaptive optics (AO) assisted observations to ≈ 4 – 5 kpc$ for seeing-limited data. The Hα morphologies tend to be irregular and/or clumpy. About one-third of the SINS Hα sample galaxies are rotation-dominated yet turbulent disks, another 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 towards the HII regions relative to the bulk of the stars, and comparable current and past-averaged star formation rates. Adopting the relation between star formation rate and gas mass surface density we presented in Bouché et al. (2007; see the comparison of star formation properties, of different galaxy classes below), the Hα-derived star formation rates imply high fractions of gas to dynamical masses Mgas/Mdyn ~ 30% (or Mgas/[Mstars+Mgas] ~ 45%). Combining the stellar, gas, and dynamical mass estimates, we find also high baryonic mass fractions (Mstars+Mgas) /Mdyn ~ 60%-80% within the central ~ 10 kpc or our SINS galaxies.

These results appeared in Förster Schreiber, N. M., et al. 2009 ApJ, 706, 1364


Stacking SINS: Broad emission lines revealed in high z star-forming galaxies


Spatially-integrated average spectrum of 47 galaxies observed in our SINS program (left panel); the equivalent integration time of such a spectrum is 195 hours with VLT/SINFONI. High S/N detections are obtained on 5 important rest-frame optical diagnostic emission lines. Fitting the Hα-[NII] region (zoomed view in right panel, with horizontal axis in velocity units) reveals excess signal above the sum of three narrow lines (green, individual compoents are in blue). An additional broad velocity component is required to fit the spectrum (red, individual components are in blue).

Using a high S/N spectrum created by combining data from 47 SINS galaxies, we detect a broad emission component underneath the narrow Hα and [NII] lines. This feature is found in galaxies with and without a known active nucleus. It is preferentially found in the more massive and more rapidly star-forming galaxies, which tend to also be older and larger galaxies. The two possible explanations for such a feature are starburst-driven galactic winds and active supermassive black holes. If galactic winds are responsible for the broad emission, the luminosity and velocity of the emission line imply gas outflow rates comparable to the star formation rate ( = 72 Msun/yr for those 47 SINS galaxies). On the other hand, if the central disk of accreting gas associated with active black holes are powering the broad feature, we can use the dynamics of this gas (and therefore of the broad emission line) to probe the mass of the associated black hole. In this scenario, we find a black hole that is a factor of 10 less massive than in local galaxy bulges of similar mass, implying that bulges are assembled first and observed already at z ~ 2 (see the SINS "From rings to bulges" result below), with the black hole being somewhat delayed in its formation.

These results appeared in Shapiro, K. L., et al. 2009 ApJ, 701, 955

First determination of the stellar mass Tully-Fisher relation at z ~ 2


The stellar mass Tully-Fisher relation at z ~ 2 as derived from the dynamical modeling of 18 of our SINS galaxies with prominent rotational features. The filled triangles are the z ~ 1.5 galaxies from our sample, while the filled circles are at z ~ 2.2. The open circle shows the z = 2.03 disk galaxy F257 observed by van Starkenburg et al. (2007). The error bars in the lower right corner represent the average fitting uncertainties of the model maximum velocity and of the stellar mass from the SED fitting to multi-wavelength photometric data. The solid line is the z = 0 relation from Bell & de Jong (2001), while the dashed line is the best fitting zero point to the z ~ 2.2 observed galaxies. The measured offset in log(M*/Msun) is 0.41 dex for a given rotational velocity, with a significance of 3.7σ.

We have modeled the dynamics of 18 star-forming galaxies at z ~ 2 using the Hα line emission as observed with SINFONI. The galaxies were selected from the larger SINS "Hα sample," based on the prominence of ordered rotational motions with respect to more complex merger-induced kinematics. The quality of the data allowed us to carefully select systems with kinematics dominated by rotation, and to model the gas dynamics across the entire galaxies using suitable exponential disk models. We obtained a good correlation between the dynamical mass Mdyn and the stellar mass M*, finding that large gas mass fractions (Mgas ~ M*) are required to explain the difference between the two quantities. We used the derived maximum rotational velocity Vmax from the modeling together with the stellar mass to construct for the first time the stellar mass Tully-Fisher relation at z ~ 2. The tight Tully-Fisher relation connects the luminosity (or stellar mass) and maximum rotational velocity of disk galaxies, and was discovered for spirals in the nearby Universe by Tully & Fisher (1977). It is a key property for understanding the structure and evolution of these galaxies, as it links directly the luminosity (or mass) of the stars in disk galaxies with the angular momentum of the dark matter halos in which they reside. The relation obtained at high redshift shows a slope similar to what is observed at lower redshift, but we detected an evolution of the zero point, with galaxies at z ~ 2 rotating faster than those in the local Universe at a given stellar mass. This result is consistent with the predictions of some of the latest N-body/hydrodynamical simulations of disk formation and evolution, which invoke gas accretion onto the forming disk via "cold flows" associated with filaments in the dark matter cosmic web. This scenario is in agreement with other dynamical evidence obtained as part of our SINS survey, where relatively smooth but rapid gas accretion from the parent dark matter halo of galaxies is required to reproduce the observed properties of a significant fraction of the z ~ 2 massive star-forming galaxies.

These results appeared in the Astrophysical Journal: Cresci et al. 2009 ApJ, 697, 115


Millenium Simulation compared to observations of z ~ 2 galaxies
and the role of secular/internal dynamical evolution at high redshift


Distribution (shaded contours) of z ~ 2 halos in the dark matter accretion rate (left y-axis) versus halo mass plane. Major merger fractions are displayed (red contours), increasing as the specific dark matter accretion rate increases. Associated star formation rates (right y-axis) assume an effective star formation efficiency of 1 (between baryonic matter, taken to follow the assembly of dark matter, and star formation rate). SINS galaxies are indicated by blue symbols, grouped based on our disk/merger classification applying kinemetry. The star formation rates for submillimeter galaxies (SMGs) are shown as horizontal lines in the upper part of the plot (their halo masses cannot be observationally well constrained). When a high effective star formation efficiency is assumed, the host halos of SINS galaxies lie in the region where most halos of their mass are expected to be concentrated. Furthermore, the expected mass accretion rates are sufficient to account for the observed star formation rates, and the predicted major merger fraction is small (< ~ 0.5), consistent with observations. The halo mass of SMGs is inferred from this analysis to be typically 1012.5 Msun.

We have used the Millennium Simulation to show that in a Lambda-CDM universe, even dark matter halos not undergoing major mergers have mass accretion rates that are plausibly sufficient to account for the high star formation rates observed in z ~ 2 disk galaxies as studied in our SINS survey. On the other hand, the fraction of major mergers in the Millennium Simulation is sufficient to account for the number counts of submillimeter galaxies (SMGs), in support of observational evidence that these are frequently major mergers (e.g., from their dynamical properties). When following the fate of these two populations in the Millennium Simulation to z = 0, we find that subsequent mergers are not frequent enough to convert all z ~ 2 turbulent disks into elliptical galaxies at z = 0. Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at z ~ 2 into present-day massive cluster ellipticals. We argue therefore that secular and internal dynamical processes must play an important role in the evolution of a significant fraction of z ~ 2 rest-UV/optically and submillimeter selected galaxy populations.

These results appeared in the Astrophysical Journal: Genel et al. 2008 ApJ, 688, 789

From rings to bulges: evidence for rapid secular evolution
in massive star-forming disk galaxies at z ~ 2


Central mass concentration as a function of evolutionary stage for five z ~ 2 non-AGN disk galaxies from SINS (right panel). The central mass concentration is taken as the ratio of the dynamical masses within a radius of 3 kpc and of 10 kpc, derived from detailed modeling of the gas kinematics traced by the Hα line emission. The [NII]/Hα emission line ratio is a measure of the gas-phase chemical abundance, related to the evolutionary stage of galaxies. The properties of these disks suggest evolution driven by efficient secular processes in a globally unstable fragmenting disk, where massive clumps rapidly migrate inward due to dynamical friction and coalesce to form a young bulge. The trend between central mass concentration and [NII]/Hα is consistent with such a scenario. The maps at the top left show the Hα line flux and velocity field from our adaptive optics-assisted SINFONI data and the rest-frame 5000A continuum emission from HST/NICMOS H-band imaging of BX482 (all at a resolution of 0.18" or 1.5 kpc), which appears to be in early stages of this "clump-driven" dynamical evolution sequence. Despite irregular morphology, the Hα kinematics show compelling signatures of disk rotation on large scales, along with small-scale perturbations likely induced by the massive clumps. The maps at the bottom left show again the Hα line flux and velocity field as well as the K-band continuum (rest-frame 6600A) from SINFONI taken under excellent seeing conditions (0.5" or 4 kpc) of D3a-6004, which lies at the more evolved end of the sequence, with morphological and dynamical evidence for a nascent central bulge component.

In a detailed study of several of our best-resolved SINS galaxies, we found evidence for rapid secular/internal dynamical evolution taking place in massive early disks at z ~ 2 based on the morphologies and kinematics of the Hα line emission. Our Laser Guide Star AO and good seeing data show the presence of turbulent rotating star-forming outer rings/disks, plus central bulge/inner disk components, whose mass fractions relative to the total dynamical mass appear to scale with the [NII]/Hα line flux ratio (an indicator of the nebular gas chemical abundances) and the star formation age, both related to the global stellar evolutionary stage of galaxies. We propose that the buildup of the central disks and bulges of massive galaxies at z ~ 2 can be driven by the early secular evolution of gas-rich disks in formation. High-redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid "cold" accretion flows along the filaments of the dark matter cosmic web. As a result, dynamical friction and viscous processes proceed on a timescale of less than 1 billion years, at least an order of magnitude faster than in present-day disk galaxies. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without violent and dissipative major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short timescales of the stellar buildup observed in massive galaxies at z ~ 2.

These results appeared in the Astrophysical Journal: Genzel et al. 2008 ApJ, 687, 59

Kinemetry at high redshift:
confirmation of a majority of rotating disks among SINS galaxies


Diagnostic diagram used to distinguish between regularly-rotating "disk-like" galaxies and systems undergoing major mergers (right). Our templates for each group, shown in blue and red respectively, have been analyzed as if they were observed at redshift z ~ 2 with the VLT/SINFONI set-up used for the SINS survey observations. The total kinematic asymmetry is defined as the sum in quadrature of the measured asymmetry in the velocity and velocity dispersion fields, and the probability distribution functions of this parameter for non-merging and merging systems (inset) is used to identify the boundary between unperturbed and merging systems (black line). Performing this analysis on SINS galaxies (black points), we find that the majority (8/11) of our best resolved systems are disk-like. Visual analysis of the velocity fields of SINS galaxies (shown at left) confirms the efficacy of our classification. The centers of stellar continuum emission in each system is indicated with a black cross, and the maxima and minima of the Hα velocities are indicated (in km/s).

We have developed a set of quantitative kinematic criteria, based on templates from observations of nearby galaxies and from simulations, which enable us to differentiate between systems with and without recent major mergers in the SINS sample. Applying these criteria to our highest-quality data, we find that ~ 3/4 of the resolved systems (with half-light radius larger than 4 kpc) display no dynamical evidence of having had a recent major merger. This quantitatively confirms earlier results from our survey, which provided qualitative evidence that there is a significant population of rapidly star-forming systems (with star formation rates ~ 100 Msun/yr) in regularly-rotating, unperturbed configurations. Our detailed analysis of the kinematics showed that indeed the high star formation rates in these z ~ 2 systems are not driven by major mergers. Instead, these young (typical stellar ages of ~ 500 million years) but rapidly evolving galaxies must have formed via smoother accretion processes, such as gas inflow along cold filamentary streams, or rapid series of minor mergers.

These results appeared in the Astrophysical Journal: Shapiro et al. 2008 ApJ, 682, 231


First comparison of the dynamical and star formation properties of different galaxy classes at z ~ 1.4 - 3.4


Dynamical and star formation properties of galaxy samples at z ~ 1.4 - 3.4 : rest-frame UV- and optically-selected star-forming galaxies from the SINS survey (blue and red points, respectively), and bright submillimeter-selected galaxies (SMGs) observed as part of a program to map the CO molecular gas line emission with millimeter interferometry (black points). The left panel shows the location of these samples in the size vs rotation velocity diagram, along with nearby spiral galaxies revealing the local velocity-size relation (grey crosses, from Courteau 1997). The right panel shows the surface density of star formation rate and gas mass for the high-redshift galaxies compared to that for nearby galaxies, including normal as well as active starburst and (ultra)luminous infrared galaxies (grey plus symbols and crosses, from Kennicutt 1998). For SMGs, the gas masses are inferred directly from the CO line fluxes (adopting the so-called "ULIRGs CO-H2 conversion factor"). For the SINS galaxies, they are derived from the dynamical masses from Hα kinematics assuming the average gas mass fraction of 40% found for the SMGs.

Combining the results from our SINS survey with SINFONI at the VLT with those from a study carried out with the IRAM/Plateau de Bure millimeter interferometer, we made the first comparison of the dynamical and star formation properties of different classes of galaxies at redshift z ~ 1.4 - 3.4. Both surveys provide spatially-resolved information on the dynamics and distribution of gas closely related to star formation activity. The sample from SINS included 16 rest-frame UV and 16 rest-frame optically selected objects, probing the bulk of actively star-forming galaxies at the high mass end. The Hα emission line originating from HII regions was the tracer of gas kinematics and star formation. The millimeter interferometric observations targeted the CO line emission tracing molecular material from which stars form. This sample consisted of 13 bright submillimeter-selected galaxies (SMGs) at similar redshifts, which represent the most luminous and most intensely star-forming systems in the early universe. These data were taken as part of a long-term IRAM program involving several members of our team. The SMG sample is highly complementary to the SINS sample, as it probes a more extreme regime of star formation in systems that are also often so severely obscured by very large amounts of dust that they are difficult to observe at shorter wavelengths.

The main results from this first comparison are the following.
(i) We find that rest-frame UV- and optically-bright (K < 20 mag) z ~ 2 star-forming galaxies are dynamically similar, and follow the same velocity-size relation as spiral galaxies in the nearby universe (left panel of the figure above). In contrast, the bright SMGs (S850μm > mJy) have significantly larger velocity widths and are much more compact, implying higher central matter densities by nearly an order of magnitude and lower angular momenta than for the SINS galaxies. Together with the spatially-resolved CO line mapping obtained for several of them showing strongly perturbed kinematics on scales of ~ 1 – 2 kpc, these results suggest that dissipative gas-rich major mergers are more frequent among the bright SMG population compared to more "normal" star-forming galaxies at high redshift.
(ii) Because of their small sizes and high densities, SMGs lie at the high end of a "Schmidt-Kennicutt" relation between matter or gas surface density and star formation rate surface density. The best-fit relation implies that the star formation rate per unit area scales as the surface gas density to a power ~ 1.7, suggesting that a "universal" Scmidt-Kennicutt law holds out to z ~ 2.5) (see rightmost panels of the figure above).

These results appeared in Bouché al. 2007, ApJ, 671, 303
(see also Tacconi et al. 2006, ApJ, 640, 228;   Tacconi et al. 2008, ApJ, 680, 246


Detailed anatomy of a young massive star-forming disk at redshift z = 2.38


Hydrogen recombination line emission of Hα of the massive star-forming galaxy BzK-15504 eleven billion light-years away (redshift z = 2.38). The observations were carried out with SINFONI in adaptive-optics mode, resulting in an angular resolution of ≈ 0.15 arcsec, or a mere 1.2 kpc (4000 light-years; indicated by the grey filled circle) at the redshift of BzK-15504. The top left panel is a color-composite map of the integrated Hα line emission, showing from blue to red the ionized gas that is blueshifted to redshifted relative to the systemic velocity of the galaxy. The other panels are channel maps showing the spatial distribution of the Hα emitting gas moving at different velocities (given in km/s) relative to the systemic velocity.

Our finely resolved SINFONI data of BzK-15504 reveal a large galaxy about 16 kpc (53,000 light-years) across, with several prominent bright knots corresponding to luminous sites of active star formation. The galaxy appears to be a disk rotating with a maximum speed of 230 km/s, implying a large dynamical mass of ≈ 1011 Msun. The details of the kinematics further suggest that gas is being channeled via radial flows (outlined by the dotted line) towards a growing central bulge, and indicate the presence of a broad and high velocity component (bottom right panel) likely due to an outflow from the active galactic nucleus (AGN) powered by a massive accreting black hole. The high surface density of gas (~ 350 Msun/pc2), the high rate of star formation (~ 150 Msun/yr), and the moderately young stellar ages (~ 500 million years) suggest rapid assembly, fragmentation, and conversion to stars of an initially gas-rich protodisk. Surprisingly, there are no obvious signs for a recent major merger event, which would have led to the rapid mass assembly and triggerred the intense star formation activity. This may suggest that BzK-15504 assembled its mass via smoother infall such as in the "cold flow" accretion mechanism, or through a series of minor mergers. BzK-15504 could later evolve into a massive elliptical galaxy.

These results appeared in Genzel et al. 2006, Nature, 442, 786

Dynamical evidence for large massive rotating disks at z ~ 2


Spatial distribution (top row) and motions (bottom row) of the Hα line emitting gas in six large star-forming galaxies at cosmological redshifts z ~ 2. The observations were carried out with SINFONI in seeing-limited mode, under typical seeing conditions of ≈ 0.5–0.6 arcsec, corresponding to a spatial resolution of 4–5 kpc at the redshift of the sources. The maximum velocities, relative to the systemic velocity, are given in km/s for each source.

During the first year of the SINS survey, our SINFONI observations revealed many large star-forming galaxies with irregular and clumpy morphologies in Hα line emission but smooth and regular velocity fields. For the majority of the larger systems, the ionized gas kinematics exhibit monotonic variations across the galaxy with steepest gradient along the geometric/kinematic major axis (e.g., the four leftmost galaxies in the figure above) and in three of them the velocity profile flattens at large radii. These features are expected signatures of ordered rotation in a disk-like structure, and provided key dynamical evidence for the existence of large massive rotating disks at z ~ 2. The case of BzK-15504, observed with adaptive optics but otherwise similar in its overall properties, offers an unparalleled view into one such system, with 3–4 times finer detail. Interestingly, Q1623-BX663 is probably more consistent with an advanced merger or disturbed spiral hosting an AGN responsible for the high velocity dispersion measured at the location of the dominant Hα peak off the center. For Q1623-BX528, the reversal in velocities along the major axis could be indicative of a counter-rotating merger.
The discovery of so many massive rotating disks among our SINS sample was surprising. In view of the higher rate of major mergers at high redshift, we had expected that most of the larger systems would exhibit more complex gas motions. From a more detailed analysis of the best resolved cases, it appears that their disks are quite turbulent, probably fairly gas-rich, and likely unstable to global star formation and fragmentation. As some simulations of the evolution of gas-rich galactic disks suggest, the star-forming clumps could later sink towards the gravitational center by dynamical friction to form a central bulge on a timescale of order of 1 billion years. This could provide a mechanism whereby some of the young disks uncovered in the SINS survey evolve into elliptical galaxies or disk galaxies with a dominant massive bulge, as those observed in the present-day Universe.

These results appeared in Förster Schreiber et al. 2006, ApJ, 645, 1062

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