Near-Infrared spectroscopy of the super star cluster in NGC1705

We study the near-infrared properties of the super star cluster NGC1750-1 in order to constrain its spatial extent, its stellar population and its age. We use adaptive optics assisted integral field spectroscopy with SINFONI on the VLT. We estimate the spatial extent of the cluster and extract its K-band spectrum from which we constrain the age of the dominant stellar population. Our observations have an angular resolution of about 0.11", providing an upper limit on the cluster radius of 2.85+/-0.50 pc depending on the assumed distance. The K-band spectrum is dominated by strong CO absorption bandheads typical of red supergiants. Its spectral type is equivalent to a K4-5I star. Using evolutionary tracks from the Geneva and Utrecht groups, we determine an age of 12+/-6 Myr. The large uncertainty is rooted in the large difference between the Geneva and Utrecht tracks in the red supergiants regime. The absence of ionized gas lines in the K-band spectrum is consistent with the absence of O and/or Wolf-Rayet stars in the cluster, as expected for the estimated age.Comment: 5 pages, 4 figures. Research Note accepted in Astronomy and Astrophysic

Measures of galaxy dust and gas mass with Herschel photometry and prospects for ALMA

(Abridged) Combining the deepest Herschel extragalactic surveys (PEP, GOODS-H, HerMES), and Monte Carlo mock catalogs, we explore the robustness of dust mass estimates based on modeling of broad band spectral energy distributions (SEDs) with two popular approaches: Draine & Li (2007, DL07) and a modified black body (MBB). As long as the observed SED extends to at least 160-200 micron in the rest frame, M(dust) can be recovered with a >3 sigma significance and without the occurrence of systematics. An average offset of a factor ~1.5 exists between DL07- and MBB-based dust masses, based on consistent dust properties. At the depth of the deepest Herschel surveys (in the GOODS-S field) it is possible to retrieve dust masses with a S/N>=3 for galaxies on the main sequence of star formation (MS) down to M(stars)~1e10 [M(sun)] up to z~1. At higher redshift (z<=2) the same result is achieved only for objects at the tip of the MS or lying above it. Molecular gas masses, obtained converting M(dust) through the metallicity-dependent gas-to-dust ratio delta(GDR), are consistent with those based on the scaling of depletion time, and on CO spectroscopy. Focusing on CO-detected galaxies at z>1, the delta(GDR) dependence on metallicity is consistent with the local relation. We combine far-IR Herschel data and sub-mm ALMA expected fluxes to study the advantages of a full SED coverage.Comment: Accepted for publication in Astronomy and Astrophysics. Some figures have degraded quality for filesize reason

Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift

We determine the ensemble properties of z~5 Lyman break galaxies (LBGs) selected as V-band dropouts to i(AB)<26.3 in the Chandra Deep Field South using their rest-frame UV-to-visible SEDs. By matching the selection and performing the same analysis that has been used for z~3 samples, we show clear differences in the properties of two samples of LBGs which are separated by ~1Gyr in lookback time. We find that z~5 LBGs are typically much younger (<100Myr) and have lower stellar masses (10^9Msol) than their z~3 counterparts. The difference in mass is significant even when considering the presence of an older, underlying population in both samples. Such young and moderately massive systems dominate the luminous z~5 LBG population (>70%), whereas they comprise <30% of LBG samples at z~3. This result is robust under all reasonable modelling assumptions. These intense starbursts appear to be experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. Their dominance in luminous LBG samples suggests that z~5 witnesses a period of wide-spread, recent galaxy formation. As such, z~5 LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies. This is supported by their high stellar mass surface densities, their core phase-space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. Their high star formation rates per unit area suggest that these systems host outflows or winds that enrich the intra- and inter-galactic media with metals. Their estimated young ages are consistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies may contain a significant fraction of metal-free stars as has been proposed for z~3 LBGs (Jimenez & Haiman 2006). [Abridged]Comment: Accepted for publication in MNRAS. 21 pages, 9 postscript figures. For a PDF file with high resolution figures, see http://www-astro.physics.ox.ac.uk/~averma

Gemini H-band imaging of the field of a z=10 candidate

We present a deep H-band image of the field of a candidate z=10 galaxy magnified by the foreground (z=0.25) cluster A1835. The image was obtained with NIRI on Gemini North to better constrain the photometry and investigate the morphology of the source. The image is approximately one magnitude deeper and has better spatial resolution (seeing was 0.4-0.5 arcsec) than the existing H-band image obtained with ISAAC on the VLT by Pello' et al. 2004. The object is not detected in our new data. Given the published photometry (H(AB)=25.0), we would have expected it to have been detected at more than ~7 sigma in a 1.4 arcsec diameter aperture. We obtain a limit of H(AB)>26.0 (3 sigma) for the object. A major part of the evidence that this object is at z=10 was the presence of a strong continuum break between the J and H band, attributed to absorption of all continuum shortward of 1216 Ang in the rest-frame of the object. Our H-band non-detection substantially reduces the magnitude of any break and therefore weakens the case that this object is at z=10. Without a clear continuum break, the identification of an emission line at 1.33745um as Ly-alpha at z~10 is less likely. We show that the width and flux of this line are consistent with an alternative emission line such as [OIII]5007 from an intermediate redshift HII/dwarf galaxy.Comment: accepted for publication in ApJ Letter

The metallicity dependence of the CO {\rightarrow} H_2 conversion factor in z>1 star forming galaxies

We use the first systematic samples of CO millimeter emission in z>1 'main-sequence' star forming galaxies (SFGs) to study the metallicity dependence of the conversion factor {\alpha}CO, from CO line luminosity to molecular gas mass. The molecular gas depletion rate inferred from the ratio of the star formation rate (SFR) to CO luminosity, is ~1 Gyr-1 for near-solar metallicity galaxies with stellar masses above M_S~1e11 M_sun. In this regime the depletion rate does not vary more than a factor of two to three as a function of molecular gas surface density, or redshift between z~0 and 2. Below M_S the depletion rate increases rapidly with decreasing metallicity. We argue that this trend is not caused by starburst events, by changes in the physical parameters of the molecular clouds, or by the impact of the fundamental metallicity-SFR-stellar mass relation. A more probable explanation is that the conversion factor is metallicity dependent and that star formation can occur in 'CO-dark' gas. The trend is also expected theoretically from the effect of enhanced photodissociation of CO by ultraviolet radiation at low metallicity. From the available z~0 and z~1-3 samples we constrain the slope of the log({\alpha}CO) -log (metallicity) relation to range between -1 and -2, fairly insensitive to the assumed slope of the gas-star formation rate relation. Because of the lower metallicities near the peak of the galaxy formation activity at z~1-2 compared to z~0, we suggest that molecular gas masses estimated from CO luminosities have to be substantially corrected upward for galaxies below M_S.Comment: ApJ in pres

Molecular and Ionized Gas Phases of an AGN-driven Outflow in a Typical Massive Galaxy at z ≈ 2

International audienc

Dust Attenuation, Bulge Formation, and Inside-out Quenching of Star Formation in Star-forming Main Sequence Galaxies at z similar to 2

We derive 2D dust attenuation maps at similar to 1 kpc resolution from the UV continuum for 10 galaxies on the z similar to 2 star-forming main sequence (SFMS). Comparison with IR data shows that 9 out of 10 galaxies do not require further obscuration in addition to the UV-based correction, though our sample does not include the most heavily obscured, massive galaxies. The individual rest-frame V-band dust attenuation (A(V)) radial profiles scatter around an average profile that gently decreases from similar to 1.8 mag in the center down to similar to 0.6 mag at similar to 3-4 half-mass radii. We use these maps to correct UV-and H alpha-based star formation rates (SFRs), which agree with each other. At masses less than or similar to 10(11) M circle dot, the dust-corrected specific SFR (sSFR) profiles are on average radially constant at a mass-doubling timescale of similar to 300. Myr, pointing at a synchronous growth of bulge and disk components. At masses greater than or similar to 10(11) M circle dot, the sSFR profiles are typically centrally suppressed by a factor of similar to 10 relative to the galaxy outskirts. With total central obscuration disfavored, this indicates that at least a fraction of massive z similar to 2 SFMS galaxies have started their inside-out star formation quenching that will move them to the quenched sequence. In combination with other observations, galaxies above and below the ridge of the SFMS relation have, respectively, centrally enhanced and centrally suppressed sSFRs relative to their outskirts, supporting a picture where bulges are built owing to gas &quot;compaction&quot; that leads to a high central SFR as galaxies move toward the upper envelope of the SFMS

Evidence for mature bulges and an inside-out quenching phase 3 billion years after the Big Bang

Most present-day galaxies with stellar masses &gt;= 10(11) solar masses show no ongoing star formation and are dense spheroids. Ten billion years ago, similarly massive galaxies were typically forming stars at rates of hundreds solar masses per year. It is debated how star formation ceased, on which time scales, and how this &quot;quenching&quot; relates to the emergence of dense spheroids. We measured stellar mass and star-formation rate surface density distributions in star-forming galaxies at redshift 2.2 with similar to 1-kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on time scales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quenched bulges in their cores

THE SINS/zC-SINF SURVEY OF z similar to 2 GALAXY KINEMATICS: REST-FRAME MORPHOLOGY, STRUCTURE, AND COLORS FROM NEAR-INFRARED HUBBLE SPACE TELESCOPE IMAGING

We present the analysis of Hubble Space Telescope (HST) J- and H-band imaging for 29 galaxies on the star-forming main sequence at z similar to 2, which have adaptive optics Very Large Telescope SINFONI integral field spectroscopy from our SINS/zC-SINF program. The SINFONI H alpha data resolve the ongoing star. formation and the ionized gas kinematics on scales of 1-2 kpc; the near-IR images trace the galaxies&apos; rest-frame optical morphologies and distributions of stellar mass in old stellar populations at a similar resolution. The global light profiles of most galaxies show disk-like properties well described by a single Sersic profile with n similar to 1, with only similar to 15% requiring a high n &gt; 3 Sersic index, all more massive than 10(10) M-circle dot. In bulge+disk fits, about 40% of galaxies have a measurable bulge component in the light profiles, with similar to 15% showing a substantial bulge-to-total ratio (B/T) B/T greater than or similar to 0.3. This is a lower limit to the frequency of z similar to 2 massive galaxies with a developed bulge component in stellar mass because it could be hidden by dust and/or outshined by a thick actively star-forming disk component. The galaxies&apos; rest-optical half-light radii range between 1 and. 7 kpc, with a median of 2.1 kpc, and lie slightly above the size-mass relation at these epochs reported in the literature. This is attributed to differences in sample selection and definitions of size and/or mass measurements. The (u - g)(rest) color gradient and scatter within individual z similar to 2 massive galaxies with greater than or similar to 10(11) M-circle dot are as high as in z = 0 low-mass, late-type galaxies. and are consistent with the high star. formation rates of massive z similar to 2 galaxies being sustained at large galactocentric distances

Kiloparsec Scale Properties of Star Formation Driven Outflows at z similar to 2.3 in the SINS/zC-SINF AO Survey

We investigate the relationship between star formation activity and outflow properties on kiloparsec scales in a sample of 28 star-forming galaxies at z similar to 2-2.6, using adaptive optics assisted integral field observations from SINFONI on the Very Large Telescope. The narrow and broad components of the H alpha emission are used to simultaneously determine the local star formation rate surface density (Sigma(SFR)), and the outflow velocity v(out) and mass outflow rate M-out, respectively. We find clear evidence for faster outflows with larger mass loading factors at higher Sigma(SFR). The outflow velocities scale as v(out) proportional to Sigma(0.34 +/- 0.10)(SFR), which suggests that the outflows may be driven by a combination of mechanical energy released by supernova explosions and stellar winds, as well as radiation pressure acting on dust grains. The majority of the outflowing material does not have sufficient velocity to escape from the galaxy halos, but will likely be re-accreted and contribute to the chemical enrichment of the galaxies. In the highest Sigma(SFR) regions the outflow component contains an average of similar to 45% of the H alpha flux, while in the lower Sigma(SFR) regions only similar to 10% of the H alpha flux is associated with outflows. The mass loading factor, eta = M-out/SFR, is positively correlated with Sigma(SFR) but is relatively low even at the highest Sigma(SFR): eta less than or similar to 0.5 x (380 cm(-3)/n(e)). This may be in tension with the eta greater than or similar to 1 required by cosmological simulations, unless a significant fraction of the outflowing mass is in other gas phases and has sufficient velocity to escape the galaxy halos