The dependence of H II region properties on global and local surface brightness within galaxy discs (original) (raw)

Stellar Populations and the Star Formation Histories of LSB Galaxies—Part I: Optical and Hα Imaging

Advances in Astronomy, 2011

This paper presents optical and Hα imaging for a large sample of LSB galaxies selected from the PSS-II catalogs . As noted in previous work, LSB galaxies span a range of luminosities (−10 > M V > −20) and sizes (0.3 kpc < R V 25 < 10 kpc), although they are consistent in their irregular morphology. Their Hα luminosities (L(Hα) range from 10 36 to 10 41 ergs s −1 (corresponding to a range in star formation, using canonical prescriptions, from 10 −5 to 1 M yr −1 ). Although their optical colors are at the extreme blue edge for galaxies, they are similar to the colors of dwarf galaxies (van Zee 2001) and gas-rich irregulars . However, their star formation rates per unit stellar mass are a factor of ten less than other galaxies of the same baryonic mass, indicating that they are not simply quiescent versions of more active star forming galaxies. This paper presents the data, reduction techniques and new philosophy of data storage and presentation. Later papers in this series will explore the stellar population and star formation history of LSB galaxies using this dataset. arXiv:1109.2360v1 [astro-ph.CO] 11 Sep 2011 -2 -4449 (Huchra et al. 1983). The advent of newer all-sky surveys in the 1980's/90's demonstrated the importance of low surface brightness (LSB) galaxies to the galaxy population, and opened up a wider range of irregular late-type galaxies for study. Although initial suggestions were that LSB galaxies dominate the total galaxy population of the Universe over their higher surface brightness (HSB) cousins, it was later found to be untrue , Hayward, Irwin & Bergman 2005. Nonetheless, LSB galaxies offer a new avenue for the study of galaxy evolution, having low stellar densities and recent star formation rates. Their study, as a class of galaxies, has merit for stellar population work.

STAR FORMATION IN H i–SELECTED GALAXIES. II. H ii REGION PROPERTIES

A sample of 69 galaxies with radial velocities less than 2500 km s À1 was selected from the H i Parkes All Sky Survey (HiPASS) to deduce details about star formation in nearby disk galaxies selected with no bias to optical surface brightness selection effects. Broadband (B and R) and narrowband (H) images were obtained for all of these objects. More than half of the sample galaxies are late-type, dwarf disks (mostly Sc and Sm galaxies). We have measured the properties of the H ii regions on H continuum–subtracted images, using the HIIphot package developed by Thilker et al. All but one of the galaxies contained at least one detectable H ii region. Examination of the properties of the H ii regions in each galaxy revealed that the brightest regions in higher surface brightness galaxies tend to be more luminous than those in lower surface brightness galaxies. A higher fraction (referred to as the diffuse fraction) of the H emission from lower surface brightness galaxies comes from diffuse ionized gas. H ii region luminosity functions (LFs) co-added according to surface brightness show that the shapes of the LFs for the lowest surface brightness galaxies are different from those for typical spiral galaxies. This discrepancy could be caused by the lowest surface brightness galaxies having somewhat episodic star formation or by them forming a relatively larger fraction of their stars outside of dense, massive molecular clouds. In general, the results imply that the conditions under which star formation occurs in lower surface brightness galaxies are different than in more typical, higher surface brightness spiral galaxies.

PHANGS–MUSE: The H II region luminosity function of local star-forming galaxies

Astronomy & Astrophysics

We use an unprecedented sample of about 23 000 H II regions detected at an average physical resolution of 67 pc in the PHANGS–MUSE sample to study the extragalactic H II region Hα luminosity function (LF). Our observations probe the star-forming disk of 19 nearby spiral galaxies with low inclination and located close to the star formation main sequence at z = 0. The mean LF slope, α, in our sample is =1.73 with a σ of 0.15. We find that α decreases with the galaxy’s star formation rate surface density, ΣSFR, and argue that this is driven by an enhanced clustering of young stars at high gas surface densities. Looking at the H II regions within single galaxies, we find that no significant variations occur between the LF of the inner and outer part of the star-forming disk, whereas the LF in the spiral arm areas is shallower than in the inter-arm areas for six out of the 13 galaxies with clearly visible spiral arms. We attribute these variations to the spiral arms increasing the molecu...

STAR FORMATION IN H i–SELECTED GALAXIES. I. SAMPLE CHARACTERISTICS

A sample of 69 galaxies with radial velocities of less than 2500 km s À1 was selected from the H i Parkes All-Sky Survey (HIPASS) and imaged in broadband B and R and narrowband H, to deduce details about star formation in nearby disk galaxies while avoiding surface brightness selection effects. The sample is dominated by late-type, dwarf disks (mostly Sc and Sm galaxies) with exponential disk scale lengths of 1–5kpc.TheHIPASSgalaxies,onaverage,havelowerstarformationrates(SFRs),arebluer,andhavelowersurfacebrightnessthananopticallyselectedsample.Hiiregionsweredetectedinallbutoneofthegalaxies.ManygalaxieshadasfewastwotofiveHiiregions.Thegalaxies′Hequivalentwidths,colors,andSFRsperunitofHimassarebestexplainedbyyoungmeanages(1–5 kpc. The HIPASS galaxies, on average, have lower star formation rates (SFRs), are bluer, and have lower surface brightness than an optically selected sample. H ii regions were detected in all but one of the galaxies. Many galaxies had as few as two to five H ii regions. The galaxies' H equivalent widths, colors, and SFRs per unit of H i mass are best explained by young mean ages (1–5kpc.TheHIPASSgalaxies,onaverage,havelowerstarformationrates(SFRs),arebluer,andhavelowersurfacebrightnessthananopticallyselectedsample.Hiiregionsweredetectedinallbutoneofthegalaxies.ManygalaxieshadasfewastwotofiveHiiregions.Thegalaxies′Hequivalentwidths,colors,andSFRsperunitofHimassarebestexplainedbyyoungmeanages(3–5 Gyr, according to Schmidt-law models) with star formation histories in which the SFRs were higher in the past. Comparison of the surface brightness coverage of the HIPASS galaxies with that of an optically selected sample shows that such a sample may miss $10% of the local galaxy number density and could possibly miss as much as 3%–4% of the SFR density. The amount lower surface brightness galaxies contribute to the total luminosity density may be insignificant, but this conclusion is somewhat dependent on how the fluxes of these objects are determined.

The Morphology of Low Surface Brightness Disk Galaxies

The Astronomical Journal, 1995

We present U BV I and Hα images of a sample of Low Surface Brightness (LSB) disk galaxies. These galaxies are generally late types, if they can be sensibly classified at all. However, they are not dwarfs, being intrinsically large and luminous.

Star Formation Properties of a Large Sample of Irregular Galaxies

The Astronomical Journal, 2004

We present the results of Hα imaging of a large sample of irregular galaxies. Our sample includes 94 galaxies with morphological classifications of Im, 26 Blue Compact Dwarfs (BCDs), and 20 Sm systems. The sample spans a large range in galactic parameters including integrated absolute magnitude (M V of −9 to −19), average surface brightness (20 to 27 mag/arcsec 2), current star formation activity (0 to 1.3 M ⊙ yr −1 kpc −2), and relative gas content (0.02 to 5 M ⊙ /L B). The Hα images were used to measure the integrated star formation rates, determine the extents of star formation in the disks, and compare azimuthallyaveraged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 10 4 with 10% Im galaxies and one Sm system having no measureable star formation at the present time. The BCDs fall, on average, at the high star formation rate end of the range. We find no correlation between star formation activity and proximity to other catalogued galaxies. Two galaxies located in voids are similar in properties to the Sm group in our sample. The H ii regions in these galaxies are most often found within the Holmberg radius R H , although in a few systems H ii regions are traced as far as 1.7R H. Similarly, most of the star formation is found within 3 disk scale-lengths R D , but in some galaxies H ii regions are traced as far as 6R D. A comparison of Hα surface photometry with V-band surface photometry shows that the two approximately follow each other with radius in Sm galaxies, but in most BCDs there is an excess of Hα emission in the centers that drops

Clustered star formation as a natural explanation for the Hα cut-off in disk galaxies

Nature, 2008

Star formation is mainly determined by the observation of Hα radiation which is related to the presence of short lived massive stars. Disc galaxies show a strong cutoff in Hα radiation at a certain galactocentric distance which has led to the conclusion that star formation is suppressed in the outer regions of disc galaxies. This is seemingly in contradiction to recent UV observations 1 that imply disc galaxies to have star formation beyond the Hα cutoff and that the star-formation-surface density is linearly related to the underlying gas surface density being shallower than derived from Hα luminosities 2 . In a galaxy-wide formulation the clustered nature of star formation has recently led to the insight that the total galactic Hα luminosity is non-linearly related to the galaxy-wide star formation rate 3 . Here we show that a local formulation of the concept of clustered star formation naturally leads to a steeper radial decrease of the Hα surface luminosity than the star-formation-rate surface density in quantitative agreement with the observations, and that the observed Hα cutoff arises naturally.

Hα Imaging of Early-Type Sa-Sab Spiral Galaxies. II. Global Properties

The Astronomical Journal, 2005

New results, based on one of the most comprehensive Hα imaging surveys of nearby Sa-Sab spirals completed to date, reveals early-type spirals to be a diverse group of galaxies that span a wide range in massive star formation rates. While the majority of Sa-Sab galaxies in our sample are forming stars at a modest rate, a significant fraction (∼ 29%) exhibit star formation rates greater than 1 M ⊙ yr −1 , rivaling the most prolifically star forming late-type spirals. A similar diversity is apparent in the star formation history of Sa-Sab spirals as measured by their Hα equivalent widths. Consistent with our preliminary results presented in the first paper in this series, we find giant H II regions (L(Hα ) ≥ 10 39 ergs −1 ) in the disks of ∼ 37% of early-type spirals. We suspect that recent minor mergers or past interactions are responsible for the elevated levels of Hα emission and perhaps, for the presence of giant H II regions in these galaxies. Our results, however, are not in total agreement with the Hα study of Kennicutt & Kent who did not find any early-type spirals with Hα equivalent widths >14Å. A close examination of the morphological classification of galaxies, however, suggests that systematic differences between the Revised Shapley Ames catalog and the Second Reference Catalog may be responsible for the contrasting results.

A spectroscopic study of the Hα surface brightness profiles in the outer discs of galaxies

Monthly Notices of the Royal Astronomical Society, 2010

The surface brightness profile of Hα emission in galaxies is generally thought to be confined by a sharp truncation, sometimes speculated to coincide with a star formation threshold. Over the past years, observational evidence for both old and young stellar populations, as well as individual H II regions, has demonstrated that the outer disc is an actively evolving part of a galaxy. To provide constraints on the origin of the aforementioned Hα truncation and the relation of Hα emission in the outer disc to the underlying stellar population, we measure the shape of the outer Hα surface brightness profile of 15 isolated, edge-on late-type disc galaxies using deep, long-slit spectroscopy. Tracing Hα emission up to 50 per cent beyond the optical radius, R 25 , we find a composite Hα surface brightness profile, well described by a brokenexponential law, that drops more steeply in the outer disc, but which is not truncated. The stellar continuum and Hα surface brightness both exhibit a break at ∼0.7R 25 , but the Hα emission drops more steeply than the stellar continuum beyond that break. Although profiles with truncations or single exponential laws correctly describe the Hα surface brightness profiles of some individual galaxies, flexible broken exponentials are required in most cases and are therefore the more appropriate generic description. The common existence of a significant second surface brightness component beyond the Hα break radius disfavours the hypothesis that this break is a purely stochastic effect.

STAR FORMATION IN GALAXIES WITH LARGE LOWER SURFACE BRIGHTNESS DISKS

We present B, R, and H imaging data of 19 large disk galaxies whose properties are intermediate between classical low surface brightness galaxies and ordinary high surface brightness galaxies. We use data taken from the Lowell 1.8 m Perkins telescope to determine the galaxies' overall morphology, color, and star formation properties. Morphologically, the galaxies range from Sb through Irr and include galaxies with and without nuclear bars. The colors of the galaxies vary from B À R ¼ 0:3 to 1.9, and most show at least a slight bluing of the colors with increasing radius. The H images of these galaxies show an average star formation rate lower than is found for similar samples with higher surface brightness disks. In addition, the galaxies studied have both higher gas mass-to-luminosity ratios and more diffuse H emission than is found in higher surface brightness samples.