Ayesha Begum

The Spitzer Local Volume Legacy: Survey Description and Infrared Photometry

The survey description and the near-, mid-, and far-infrared flux properties are presented for the 258 galaxies in the Local Volume Legacy (LVL). LVL is a Spitzer Space Telescope legacy program that surveys the local universe out to 11 Mpc, built upon a foundation of ultraviolet, Hα, and Hubble Space Telescope imaging from 11HUGS (11 Mpc Hα and Ultraviolet Galaxy Survey) and ANGST (ACS Nearby Galaxy Survey Treasury). LVL covers an unbiased, representative, and statistically robust sample of nearby star-forming galaxies, exploiting the highest extragalactic spatial resolution achievable with Spitzer. As a result of its approximately volume-limited nature, LVL augments previous Spitzer observations of present-day galaxies with improved sampling of the low-luminosity galaxy population. The collection of LVL galaxies shows a large spread in mid-infrared colors, likely due to the conspicuous deficiency of 8 μm polycyclic aromatic hydrocarbon emission from low-metallicity, low-luminosity galaxies. Conversely, the far-infrared emission tightly tracks the total infrared emission, with a dispersion in their flux ratio of only 0.1 dex. In terms of the relation between the infrared-to-ultraviolet ratio and the ultraviolet spectral slope, the LVL sample shows redder colors and/or lower infrared-to-ultraviolet ratios than starburst galaxies, suggesting that reprocessing by dust is less important in the lower mass systems that dominate the LVL sample. Comparisons with theoretical models suggest that the amplitude of deviations from the relation found for starburst galaxies correlates with the age of the stellar populations that dominate the ultraviolet/optical luminosities.

The calibration of monochromatic far-infrared star formation rate indicators

Spitzer data at 24, 70, and 160 μm and ground-based Hα images are analyzed for a sample of 189 nearby star-forming and starburst galaxies to investigate whether reliable star formation rate (SFR) indicators can be defined using the monochromatic infrared dust emission centered at 70 and 160 μm. We compare recently published recipes for SFR measures using combinations of the 24 μm and observed Hα luminosities with those using 24 μm luminosity alone. From these comparisons, we derive a reference SFR indicator for use in our analysis. Linear correlations between SFR and the 70 μm and 160 μm luminosity are found for L(70) ≳ 1.4 × 10^(42) erg s^(–1) and L(160) ≳ 2 × 10^(42) erg s^(–1), corresponding to SFR ≳ 0.1-0.3 M_☉ yr^(–1), and calibrations of SFRs based on L(70) and L(160) are proposed. Below those two luminosity limits, the relation between SFR and 70 μm (160 μm) luminosity is nonlinear and SFR calibrations become problematic. A more important limitation is the dispersion of the data around the mean trend, which increases for increasing wavelength. The scatter of the 70 μm (160 μm) data around the mean is about 25% (factor ~2) larger than the scatter of the 24 μm data. We interpret this increasing dispersion as an effect of the increasing contribution to the infrared emission of dust heated by stellar populations not associated with the current star formation. Thus, the 70 (160) μm luminosity can be reliably used to trace SFRs in large galaxy samples, but will be of limited utility for individual objects, with the exception of infrared-dominated galaxies. The nonlinear relation between SFR and the 70 and 160 μm emission at faint galaxy luminosities suggests a variety of mechanisms affecting the infrared emission for decreasing luminosity, such as increasing transparency of the interstellar medium, decreasing effective dust temperature, and decreasing filling factor of star-forming regions across the galaxy. In all cases, the calibrations hold for galaxies with oxygen abundance higher than roughly 12 +log(O/H) ~ 8.1. At lower metallicity, the infrared luminosity no longer reliably traces the SFR because galaxies are less dusty and more transparent.

Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way

We announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the color-magnitude diagram, it lies at a distance of ~420 kpc and has an intermediate-age stellar population with a metallicity of [Fe/H] = -1.6, together with a young population of blue stars of age ~200 Myr. There is a compact cloud of neutral hydrogen with mass ~105 M☉ and radial velocity +35 km s-1 coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.

FIGGS: Faint Irregular Galaxies GMRT Survey – overview, observations and first results

The Faint Irregular Galaxies GMRT Survey (FIGGS) is a Giant Metrewave Radio Telescope (GMRT) based HI imaging survey of a systematically selected sample of extremely faint nearby dwarf irregular galaxies. The primary goal of F IGGS is to provide a comprehensive and statistically robust characterization of the neut ral inter-stellar medium properties of faint, gas rich dwarf galaxies. The FIGGS galaxies represent the extremely low-mass end of the dwarf irregular galaxies population, with a median MB � 13:0 and median HI mass of � 3�10 7 M� , extending the baseline in mass and luminosity space for a comparative study of galaxy properties. The HI data is supplemented with observations at other wavelengths. In addition, distances accurate to � 10% are available for most of the sample galaxies. This paper gives an introduction to FIGGS, describe the GMRT observations and presents the first results from the HI observations. From the FIGGS data we confi rm the trend of increasing HI to optical diameter ratio with decreasing optical luminosi ty; the median ratio of DH I/DH o for the FIGGS sample is 2.4. Further, on comparing our data with aperture synthesis surveys of bright spirals, we find at best marginal evidence for a decrea se in average surface density with decreasing HI mass. To a good approximation the disks of gas rich galaxies, ranging over 3 orders of magnitude in HI mass, can be described as being drawn from a family with constant HI surface density.

Baryonic Tully-Fisher relation for extremely low mass Galaxies

We study Tully-Fisher (TF) relations for a sample that combines extremely faint (M B < -14) galaxies along with bright (i.e. L.) galaxies. Accurate (∼10 per cent) distances, I-band photometry and B - V colours are known for the majority of the galaxies in our sample. The faint galaxies are drawn from the Faint Irregular Galaxy GMRT survey (FIGGS), and we have H I rotation velocities derived from aperture synthesis observations for all of them. For the faint galaxies, we find that even though the median HI and stellar masses are comparable, the H I mass correlates significantly better with the circular velocity indicators than the stellar mass. We also find that the velocity width at the 20 per cent level (W 20 ) correlates better with mass than the rotation velocity, although the difference is not statistically significant. The faint galaxies lie systematically below the I-band TF relation defined by bright galaxies, and also show significantly more intrinsic scatter. This implies that the integrated star formation in these galaxies has been both less efficient and also less regulated than in large galaxies. We estimate the intrinsic scatter of the faint galaxies about the I-band TF to be ∼1.6 mag. We find that while the faint-end deviation is greatly reduced in Baryonic Tully-Fisher (BTF) relations, the existence of a break at the faint end of the BTF is subject to systematics such as the assumed stellar mass-to-light ratio. If we assume that there is an intrinsic BTF and try to determine the baryonic mass by searching for prescriptions that lead to the tightest BTF, we find that scaling the H I mass leads to a much more significant tightening than scaling the stellar mass-to-light ratio. The most significant tightening that we find, however, is if we scale the entire baryonic mass of the faint (but not the bright) galaxies. Such a scenario would be consistent with models where dwarf (but not large) galaxies have a large fraction of dark or missing baryons. In all cases, however, the minimum in the Χ 2 curve is quite broad and the corresponding parameters are poorly constrained.

The Local Group dwarf Leo T: H i on the brink of star formation

We present Giant Meterwave Radio Telescope (GMRT) andWesterbork Synthesis Radio Telescope (WSRT) observations of the recently discovered Local Group dwarf galaxy, Leo T. The peak HI column density is measured to be 7 x 1020 cm(-2), and the total HI mass is 2.8 x 10(5) M-circle dot,, based on a distance of 420 kpc. Leo T has both cold (similar to 500 K) and warm (similar to 6000 K) HI at its core, with a global velocity dispersion of 6.9 km s(-1), from which we derive a dynamical mass within the HI radius of 3.3 x 10(6) M-circle dot, and a mass-to-light ratio of > 50. We calculate the Jeans mass from the radial profiles of the HI column density and velocity dispersion, and predict that the gas should be globally stable against star formation. This finding is inconsistent with the half light radius of Leo T, which extends to 170 pc, and indicates that local conditions must determine where star formation takes place. Leo T is not only the lowest luminosity galaxy with on-going star formation discovered to date, but it is also the most dark matter-dominated, gas-rich dwarf in the Local Group.

A study of interstellar medium of dwarf galaxies using H i power spectrum analysis

We estimate the power spectrum of H Iintensity fluctuations for a sample of eight galaxies (seven dwarf and one spiral). The power spectrum can be fitted to a power-law P HI (U) = AU α for six of these galaxies, indicating turbulence is operational. The estimated best-fitting value for the slope ranges from ~ -1.5 (AND IV, NGC 628, UGC 4459 and GR 8) to ~ -2.6 (DDO 210 and NGC 3741). We interpret this bi-modality as being due to having effectively 2D turbulence on length-scales much larger than the scale-height of the galaxy disc and 3D otherwise. This allows us to use the estimated slope to set bounds on the scale-heights of the face-on galaxies in our sample. We also find that the power-law slope remains constant as we increase the channel thickness for all these galaxies, suggesting that the fluctuations in H I intensity are due to density fluctuations and not velocity fluctuations, or that the slope of the velocity structure function is ~0. Finally, for the four galaxies with 2D turbulence we find that the slope α correlates with the star formation rate (SFR) per unit area, with larger SFRs leading to steeper power laws. Given our small sample size, this result needs to be confirmed with a larger sample.

H i power spectrum of the spiral galaxy NGC 628

We have measured the H I power spectrum of the nearly face-on spiral galaxy NGC 628 (M74) using a visibility-based estimator. The power spectrum is well fitted by a power law P(U) =AU α, with α=−1.6 ± 0.2 over the length-scale 800 pc to 8 kpc. The slope is found to be independent of the width of the velocity channel. This value of the slope is a little more than one in excess of what has been seen at considerably smaller length-scales in the Milky Way, the Small Magellanic Cloud, the Large Magellanic Cloud (LMC) and the dwarf galaxy DDO 210. We interpret this difference as indicating a transition from three-dimensional turbulence at small scales to two-dimensional turbulence in the plane of the galaxy disc at length-scales larger than the H i scaleheight of the galaxy. The slope measured here is similar to that found at large scales in the LMC. Our analysis also places an upper limit to the galaxy scaleheight at 800 pc.

Life in the last lane: star formation and chemical evolution in an extremely gas rich dwarf

ABSTRACT We present an analysis of HI, Hα, and oxygen abundance data for NGC 3741. This galaxyhas a very extended gas disk (∼ 8.8 times the Holmberg radius), and a dark to luminous (i.e.stellar) mass ratio of ∼ 149, which makes it one of the “darkest” dwarf irregular galaxie sknown. However its ratio of baryons (i.e. gas + stellar) mass to dark mass is typical of thatin galaxies. Our new high-resolution HI images of the galaxyshow evidence for a large scale(purely gaseous) spiral arm and central bar. From our HI data, a rotation curve can be derivedout to ∼ 37− 44 disk scale lengths in the J and B band respectively. This is just slightlyshort of the radius at which one would expect an NFW type rotation curve to start falling.The galaxy has an integrated star formation rate of ∼ 0.0034 M ⊙ yr −1 , while the average starformation rate within the optical disk is ∼ 0.0049 M ⊙ yr −1 kpc −2 . Despite the gaseous spiralfeature and the on-going star formation, we find that the global gas density in NGC 3741 issignificantly lower than the Toomre instability criterion. This is consistent with the behaviourseen in other dwarf galaxies. We also find that the star format ion rate is consistent with thatexpected from the observed correlations between HI mass and SFR and the global Kennicutt-Schmidt law respectively.We measure the oxygenabundanceto be 12+ log(O/H)=7.66±0.10,which is consistent with that expected from the metallicity-luminosity relation, despite itsextreme gas mass ratio. We also examine the issue of chemical evolution of NGC 3741 in thecontext of closed-box model of chemical evolution. The effective oxygen yield of NGC 3741is consistent with recent model estimates of closed-box yields, provided one assumes that thegas has been efficiently mixed all the way to edge of the HI disk (i.e. > 8 times the opticalradius). This seems a priori unlikely. On the other hand, using a sample of galaxies with bothinterferometric HI maps and chemical abundance measurements, we find that the effectiveyield is anti-correlated with the total dynamical mass, as expected in leaky box models.Key words: galaxies: dwarf - galaxies: kinematics and dynamics – galaxies: individual:NGC 3741 radio lines: galaxies

The Local Group dwarf Leo T: H i on the brink of star formation: H i in Leo T

We present Giant Meterwave Radio Telescope (GMRT) andWesterbork Synthesis Radio Telescope (WSRT) observations of the recently discovered Local Group dwarf galaxy, Leo T. The peak HI column density is measured to be 7 x 1020 cm(-2), and the total HI mass is 2.8 x 10(5) M-circle dot,, based on a distance of 420 kpc. Leo T has both cold (similar to 500 K) and warm (similar to 6000 K) HI at its core, with a global velocity dispersion of 6.9 km s(-1), from which we derive a dynamical mass within the HI radius of 3.3 x 10(6) M-circle dot, and a mass-to-light ratio of > 50. We calculate the Jeans mass from the radial profiles of the HI column density and velocity dispersion, and predict that the gas should be globally stable against star formation. This finding is inconsistent with the half light radius of Leo T, which extends to 170 pc, and indicates that local conditions must determine where star formation takes place. Leo T is not only the lowest luminosity galaxy with on-going star formation discovered to date, but it is also the most dark matter-dominated, gas-rich dwarf in the Local Group.