High-Redshift Protoclusters Traced by Submillimeter Galaxies: Tracing Star Formation Activity out to z > 4

Clustering analysis indicate that at z ~ 2 submm-selected galaxies (SMGs) reside in very massive halos (M_(DM) > 5 × 10^(13)), suggesting that SMGs trace high-density environments that evolve into rich galaxy clusters. Conversely, recent work suggests that SMGs are tracers of a broader range of environments, including structures with more modest masses caught in highly active periods; since galaxies in these structures are likely caught during episodes of peak starbursts, SMGs may be tracers of a wider range of environments beyond the progenitors of todays very rich clusters, opening a window for a more complete exploration of the details underpinning the process of galaxy evolution in concert with the assembly of the large scale structure (LSS). We have undertaken a large observing program comprising deep narrow-band Ly-alpha imaging and multi-object spectroscopy using Palomar/Keck/Magellan/Gemini telescopes to probe for galaxy overdensities in SMG environments at z ~ 1 − 5. With ~200 spectroscopically-confirmed Ly-alpha emitters, we are in a position to gauge the level of galaxy overdensity in these regions

Quenching Star Formation at Intermediate Redshifts: Downsizing of the Mass Flux Density in the Green Valley

The bimodality in galaxy properties has been observed at low and high redshifts, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence; the absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. In this paper, we present a study of over 100 transiting galaxies in the so-called green valley at intermediate redshifts (z ~ 0.8). By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope we are able to infer the star formation histories of these objects and measure the stellar mass flux density transiting from the blue cloud to the red sequence when the universe was half its current age. Our results indicate that the process happened more rapidly and for more massive galaxies in the past, suggesting a top-down scenario in which the massive end of the red sequence is forming first. This represents another aspect of downsizing, with the mass flux density moving toward smaller galaxies in recent times

Profile comparison of the 6–9 μm polycyclic aromatic hydrocarbon bands in starburst-dominated galaxies

Polycyclic aromatic hydrocarbons (PAHs) are of great astrochemical and astrobiological interest due to their potential to form prebiotic molecules. We analyse the 7.7 and 8.6 μm PAH bands in 126 pre-dominantly starburst-dominated galaxies extracted from the Spitzer/IRS ATLAS project. Based on the peak positions of these bands, we classify them into the different A, B, and C Peeters’ classes, which allows us to address the potential characteristics of the PAH-emitting population. We compare this analysis with previous work focused on the 6.2 μm PAH band for the same sample. For the first time in the literature, this statistical analysis is performed on a sample of galaxies. In our sample, the 7.7 μm complex is equally distributed in A and B object’s class while the 8.6 μm band presents more class B sources. Moreover, 39 per cent of the galaxies were distributed into A class objects for both 6.2 and 7.7 μm bands and only 18 per cent received the same A classification for the three bands. The ‘A A A’ galaxies presented higher temperatures and less dust in their interstellar medium. Considering the redshift range covered by our sample, the distribution of the three bands into the different Peeters’ classes reveals a potential cosmological evolution in the molecular nature of the PAHs that dominate the interstellar medium in these galaxies, where B class objects seem to be more frequent at higher redshifts and, therefore, further studies have to be addressed

Detections of CO Molecular Gas in 24um-Bright ULIRGs at z~2 in the Spitzer First Look Survey

We present CO observations of 9 ULIRGs at z~2 with S(24\mu m)>1mJy, previously confirmed with the mid-IR spectra in the Spitzer First Look Survey. All targets are required to have accurate redshifts from Keck/GEMINI near-IR spectra. Using the Plateau de Bure millimeter-wave Interferometer (PdBI) at IRAM, we detect CO J(3-2) [7 objects] or J(2-1) [1 object] line emission from 8 sources with integrated intensities Ic ~(5-9)sigma. The CO detected sources have a variety of mid-IR spectra, including strong PAH, deep silicate absorption and power-law continuum, implying that these molecular gas rich objects at z~2 could be either starbursts or dust obscured AGNs. The measured line luminosity L'[CO] is (1.28-3.77)e+10[K km/s pc^2]. The averaged molecular gas mass M(H2) is 1.7e+10Msun, assuming CO-to-H2 conversion factor of 0.8Msun/[K km/s pc^2]. Three sources (33%) -- MIPS506, MIPS16144 & MIPS8342 -- have double peak velocity profiles. The CO double peaks in MIPS506 and MIPS16144 show spatial separations of 45kpc and 10.9kpc, allowing the estimates of the dynamical masses of 3.2e+11*sin^(-2)(i)Msun and 5.4e+11*sin^{-2}(i)Msun respectively. The implied gas fraction, M(gas)/M(dyn), is 3% and 4%, assuming an average inclination angle. Finally, the analysis of the HST/NIC2 images, mid-IR spectra and IR SED revealed that most of our sources are mergers, containing dust obscured AGNs dominating the luminosities at (3-6)um. Together, these results provide some evidence suggesting SMGs, bright 24um z~2 ULIRGs and QSOs could represent three different stages of a single evolutionary sequence, however, a complete physical model would require much more data, especially high spatial resolution spectroscopy.Comment: 15 pages, 8 figures, accepted for publication in ApJ

Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

We present $^{12}$CO(1-0) and $^{12}$CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 $\mu$m emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses <~ 10$^9$ M$_{\odot}$, contrary to the atomic hydrogen fraction, M$_{HI}$/M$_*$, which increases inversely with M$_*$. The flattening of the M$_{H_2}$/M$_*$ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, $\tau_{dep}$, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between HI deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity.Comment: 19 pages, 11 figures, accepted for publication in Astronomy & Astrophysic

A CO emission line from the optical and near-IR undetected submillimeter galaxy GN10

We report the detection of a CO emission line from the submillimiter galaxy (SMG) GN10 in the GOODS-N field. GN10 lacks any counterpart in extremely deep optical and near-IR imaging obtained with the Hubble Space Telescope and ground-based facilities. This is a prototypical case of a source that is extremely obscured by dust, for which it is practically impossible to derive a spectroscopic redshift in the optical/near-IR. Under the hypothesis that GN10 is part of a proto-cluster structure previously identified at z~4.05 in the same field, we searched for CO[4-3] at 91.4 GHz with the IRAM Plateau de Bure Interferometer, and successfully detected a line. We find that the most likely redshift identification is z=4.0424+-0.0013, based on: 1) the very low chance that the CO line is actually serendipitous from a different redshift; 2) a radio-IR photometric redshift analysis; 3) the identical radio-IR SED, within a scaling factor, of two other SMGs at the same redshift. The faintness at optical/near-IR wavelengths requires an attenuation of A_V~5-7.5 mag. This result supports the case that a substantial population of very high-z SMGs exists that had been missed by previous spectroscopic surveys. This is the first time that a CO emission line has been detected for a galaxy that is invisible in the optical and near-IR. Our work demonstrates the power of existing and planned facilities for completing the census of star formation and stellar mass in the distant Universe by measuring redshifts of the most obscured galaxies through millimeter spectroscopy.Comment: 5 pages, 4 figures. ApJ Letters in pres

Mid-IR Spectroscopy of High-z SMGs: First Results

We present mid-infrared spectra of 5 submmillimeter galaxies at redshifts z = 0.65 − 2.38 taken with the Infrared Spectrograph aboard the Spitzer Space Telescope. Four of these sources have strong PAH features and the strength of these features are consistent with these galaxies being dominated by star formation. The other source displays a Mrk 231-type broad emission feature at restframe ~8 μm that does not conform to the typical 7.7/8.6 μm PAH complex in starburst galaxies, suggesting a more substantial AGN contribution

Reexamination of the Radial Abundance Gradient Break in NGC 3359

In this contribution, we reexamine the radial oxygen abundance gradient in the strongly barred spiral galaxy NGC 3359, for which, using an imaging spectrophotometric technique, Martin & Roy detected a break near the effective radius of the galaxy. We have new emission line flux measurements of HII regions in NGC 3359 from spectra obtained with the Subaru telescope to further investigate this claim. We find that there are small systematic variations in the line ratios determined from narrow-band imaging as compared to our spectroscopic measurements. We derive and apply a correction to the line ratios found by Martin & Roy and statistically examine the validity of the gradient break proposed for NGC 3359 using recently developed metallicity diagnostics. We find that, with a high degree of confidence, a model with a break fits the data significantly better than one without it. This suggests that the presence of a strong bar in spiral galaxies can generate measurable changes in the radial distribution of metals.Comment: Accepted to A